6. LATE-STAGE MELT EVOLUTION AND TRANSPORT IN THE SHALLOW MANTLE BENEATH THE EAST PACIFIC RISE 1

Transcription

1 Mével, C, Gilüs, K.M., Allan, J.F., and Meyer, P.S. (Eds.), 99 Prceedings f the Ocean Drilling Prgram, Scientific esults, Vl.. LATE-STAGE MELT EVOLUTION AND TANSPOT IN THE SHALLOW MANTLE BENEATH THE EAST PACIFIC ISE Henry J.B. Dick and James H. Natland ABSTACT This paper reprts the igneus petrlgy and gechemistry f harzburgite-tectnites, crsscutting dunites, and lcal gabbric segregatins drilled frm a tectnically expsed utcrp f shallw East Pacific ise mantle at Ocean Drilling Prgram Site 89 near Hess Deep. Melting f the Hess Deep mantle sectin, as fr mst abyssal peridtites, ccurred in the fur-phase field livine-enstatite-dipside-spinel limited by the dipside-ut phase bundary. The Site 89 peridtites are clinpyrxenepr harzburgites at the mst depleted end f the range fr abyssal peridtites. Unlike similar highly depleted peridtites frm the Atlantic and Indian Oceans, hwever, these are nt spatially assciated with a mantle ht spt. Abyssal basalts frm the walls f Hess Deep near Site 89, believed t h erupted at the East Pacific ise, are N-type mid-cean-ridge basalt (MOB). These indicate that the Site 89 peridtites are residues f melting f an N-type MOB surce. Like ridge basalts near ht spts, hwever, the Hess Deep basalts, and Hle 89G gabbrs nearby, require crystallizatin frm very lw sdium primary liquids cnsistent with the very depleted Hess Deep residual mantle peridtites. Given seismic data that indicate a smewhat thin East Pacific ise crust in the regin, and an absence f any chemical r physical evidence fr a mantle ht spt, we suggest that the high degree f depletin f the Hess Deep peridtites and the inferred primary basalt cmpsitins, reflect mderate degrees f melting f a refractry mantle sectin pr in basaltic cmpnents under a nrmal gethermal gradient. This regin f the East Pacific ise, then, des nt fit the prpsed glbal crrelatin f Klein and Langmuir (98) between ridge basalt and crustal thickness. ather, it implies that crustal thickness at cean ridges may be as much a functin f variatins f initial mantle cmpsitin as initial mantle temperature (e.g., Dick et al., 98). Primary melts transprted thrugh the Site 89 mantle sectin, as inferred frm the gechemistry f gabbric segregatins in dunites crsscutting the harzburgite tectnites, were fully aggregated MOB. They were nt, therefre, in equilibrium with the shallw mantle since clinpyrxenes in the Hess Deep harzburgites h trace element cmpsitins in equilibrium with highly refractry melts previusly nly reprted as inclusins in phencrysts in Atlantic MOBs. This supprts, and may require, highly fcused flw and aggregatin f melt thrugh the underlying mantle-melting-clumn. The crystallizatin sequence f MOB in the Hess Deep shallw mantle sectin is the same as fr nrmal abyssal thleiites. Hwever, high-magnesium dipside and high-calcium Plagiclase (up t An 99 ), present in sme f the gabbric segregatins, are believed t be the prduct f late-stage reactin between melt and the harzburgite tectnite t prduce high calcium and alumina melts. Similar spradic xencrysts widely distributed in MOBs might then be explained by mixing f MOB with stagnated highly reacted melts in shallw mantle cnduits. The dike-like frm and cmpund nature f sme gabbric segregatins in the dunites crsscutting the residual mantle harzburgite tectnites at Site 89 demnstrate that late-stage melt transprt was, at least in part, fracture cntrlled. The segregatins themselves, and small amunts f late-magmatic-textured intergranular clinpyrxene impregnating the harzburgite tectnites, als demnstrate that magma mass was decreasing as melt was transprted thrugh the transitin zne in the mantle immediately beneath the crust. This requires that the mantle temperature there is depressed belw the mantle adiabat due t cnductive heat lss. The presence f this cnductive lid is a significant factr in the evlutin f MOB and impacts estimates f the cmpsitin f the ceanic crust. Such a lid limits the extent f melting f the upwelling mantle, traps late melt fractins generated at the tp f the mantle melting clumn, and creates a zne where melt-rck reactin prduces small, but apparently significant, quantities f high alumina and calcic MOB magmas. INTODUCTION The cmpsitin and structure f the cean crust and mantle are as yet prly knwn. This is largely a cnsequence f the availability f little direct stratigraphic infrmatin fr anything ther than the shallwest levels f the mdern cean crust. Mdels fr the frmatin f the cean crust, accrdingly h relied heavily n inference frm marine gephysical data and analgy t philites f ambiguus ceanic prvenance. As a cnsequence, the nature f the lwer crust and the mechanisms f melt transprt thrugh the mantle and 'Mével, C, Gillis, K.M., Allan, J.F., and Meyer, P.S. (Eds.), 99. Prc. ODP, Sci. esults, : Cllege Statin, TX (Ocean Drilling Prgram). Wds Hle Oceangraphic Institutin, Wds Hle, MA 0, U.S.A. hdick@whi.edu senstiel Schl f Marine and Atmspheric Science, University f Miami, 00 ickenbacker Causeway, Miami, FL 9, U.S.A. jnatland@umigw.miami.edu the crust and hw these vary with spreading rate and magma supply are widely debated (Cannat, 99; Sintn and Detrick, 99; Swift and Stephen, 99). This is a study f the igneus petrlgy and gechemistry f residual harzburgites and related crsscutting dunites and gabbric segregatins frm six hles drilled at Ocean Drilling Prgram Site 89 in an utcrp f the shallw East Pacific ise (EP) mantle expsed n a tectnic blck n the flr f Hess Deep in the equatrial eastern Pacific (Table ). These rcks, and m f gabbrs drilled at Hle 89G nearby, represent the first sectins f plutnic rcks frm Pacific lwer crust and mantle with enugh key stratigraphic relatinships preserved t allw a reasnable direct recnstructin f prtins f the riginal stratigraphy. The results f shipbard studies (Gillis, Mével, Allan, et al., 99), prir wrk n dredge and dive samples frm Hess Deep (Hekinian et al., 99), and this paper find a similarity between the Site 89 mantle sectins and znes f melt transprt expsed in the shallw mantle sectins f philites such as Oman (Ceuleneer and abinwicz, 99). There, as at Hess Deep, evidence f late-stage melt migratin is seen where 0

2 H.J.B. DICK, J.H. NATLAND depleted harzburgite tectnite is crsscut by irregular t tabular dunite bdies. These mark znes where melts passed thrugh the shallw mantle, reacted ut pyrxene, and precipitated livine in prus melt flw channels (Budier and Niclas, 9;Cassardetal., 98; Dick, 9a, 9b; Quick, 98a, 98b). Lcally, these dunites cntain gabbric segregatins, ranging frm trctlite, where Plagiclase impregnates disaggregated dunite, t livine gabbr and gabbr. These segregatins then allw the cmpsitin and evlutin f the primary magmas migrating thrugh the shallw mantle beneath the East Pacific ise t be directly inferred. The tabular character and cmpund nature f sme gabbric segregatins suggest that the last stages f shallw melt transprt were fracture dminated. The initial cmpsitin f the migrating melt was N-type mid-cean-ridge basalt (MOB), based n the trace element cmpsitin f dipside in the gabbric segregatins within the dunites. In sme cases the riginal melt appears t h reacted with the wall-rck harzburgite t make extreme melt cmpsitins in equilibrium with high-calcium Plagiclase and high-magnesium dipside. The latter minerals als ccur as ccasinal, but widely distributed, xencrysts in MOB and h previusly been used t suggest high-pressure fractinal crystallizatin (Elthn, 989; Elthn and Scarfe, 98), r the frmatin f ultra-depleted melts high in the melting clumn (Duncan and Green, 980; Sblev and Shimizu, 99). The Hess Deep residual harzburgites exhibit ne f the greatest majr- and trace-element depletins yet fund fr abyssal peridtites, and wuld be in equilibrium with nly extremely depleted melts, nt MOB. Such extreme depletin is cnsistent with fractinal melting in the mantle beneath cean ridges (Jhnsn and Dick, 99; Jhnsn et al., 990). The harzburgites h EE patterns unlike any fund previusly assciated with N-MOB far frm mantle ht spts. The shape f the EE pattern is characterized by a strng depletin f the middle rare earths relative t the heavy rare earths, a feature that is similar t patterns fund fr abyssal peridtites dredged near the Marin and Buvet ht spts n the Suthwest Indian idge (Jhnsn et al., 990). Because seismic data fr the EP in the Hess Deep regin suggest a nrmal r smewhat thin cean crust (Znenshain et al., 980), we speculate that this unusual pattern is best explained if the mantle beneath this regin f the EP was initially mre depleted in majr- and trace-element cmpsitin than typical fr mantle regins beneath the Atlantic and Indian Oceans away frm ht spts. This wuld be cnsistent with the cmpsitin f N- MOB and gabbrs frm Hess Deep that require crystallizatin frm very sda-pr primary melts generally believed t reflect very high degrees f mantle melting (Dick et al., 98; Klein and Langmuir, 98), but is incnsistent with the crrelatin between crustal thickness and basalt chemistry pstulated by Klein and Langmuir (98). Hle, cre, sectin, interval (cm) -89A- -, -0-89B- -, - -89C- -,8- -, -9 -, 0- -,-8 -, -8 -, - -, -8 -, 08- -, 08- -, 08- -, - -a, 0-0a -b, 0-0b -c, 0-0c -- -, ^ -, 8- -, -0 -,- -, -0 -, 9-0 -,- -, , 0-0 -,-9 -l,99-0a -l,99-0b -, 8- -, - -, - -, 9- -, - -, - 8-, -9 8-, 9-9-a, 0-9-b, 0-9-C, E- -, - -, - -,- -,-9 -, - -, - -, - -, -8 -,- -, , - -89F- -, - -, -80 Table. Samples used in this study. Piece 9b? 9 b b b a d a b b Depth (mbsf) ck type Trctlitic dunite Trctlitic dunite Trctlite Trctlitic dunite Trctlite Olivine gabbrnrite Trctlite Plag. dunite Trctlite Gabbrnrite Trctlite Trctlite Trctlite c. gr. Trctlite m. gr. Trctlite c. gr. Trctlitic gabbrnrite Trctlite m. gr. Olivine gabbr Olivine gabbr Dunite Gabbr Olivine gabbr Olivine gabbr Dunite Olivine gabbr Olivine gabbr Dunite Trctlite Olivine gabbr Gabbr Olivine gabbr Dunite Surce Allan, I Allan, I Allan Allan Allan I Allan I I I Allan Allan I Allan ANALYTICAL TECHNIQUES Mdal Analysis Mdal analysis was dne by pint cunting primary relict phases and their pseudmrphs. Talc, talc-amphible, and serpentine pseudmrphs f each primary phase were cunted separately frm the primary mineral relicts, and the primary mde was calculated withut a crrectin fr vlume expansin. Typically between 800 and 00 pints were cunted at a -mm interval. This yields estimates f precisin ( sigma) fr analyzing a peridtite with. vl% livine 0% enstatite, % clinpyrxene, and % spinel f: +, +, +9, and +%, respectively (Chayes, 9; Chayes, pers. cram., 990). Orientatin prblems culd arise if samples were cut parallel t layering, which can radically bias a mde. Here, hwever, spinel lineatins, cntacts, and layering in the peridtites, where bserved, dip apprximately 0, and as ur chips were all cut in the vertical plane rthgnal t bservable fliatin, bias is unlikely. Ntes: Depth = meters belw seaflr based n expanding the curated recvery t fill the drilled interval. Surce = data fr samples frm surces ther than this paper: I = Initial eprts Vl. (Gillis, Mével, Allan, et al., 99); Allan = Allan et al., this vlume, c. gr. = carse grained; m. gr. = medium grained. Anther surce f ptential errr is differential expansin during serpentinizatin. In thery, up t % vlume expansin can ccur during ischemical hydratin f peridtite (Kmr et al., 98b; O'Hanley, 99). If livine r pyrxene is preferentially serpentinized, this may intrduce errrs in estimating the primary mde by cunting pseudmrphs. The data in Table shw that livine is generally but nt always preferentially altered relative t pyrxene, and the degree f serpentinizatin generally increases with increasing mdal livine. The amunt f serpentine ges up by as much as 0% with nly a 0% increase in verall livine cntent, suggesting that the presence f pyrxene may h a mechanical r chemical effect which stabilizes livine. As a cnsequence f preferential alteratin, the vlume percentage f primary livine in ur samples tends t be verestimated. Since enstatite is als heavily altered, hwever, gen- 0

3 Table. Mdal analyses f Site 89 harzburgites. Sample Cunts elict 0 Serp Ol TcAm Oi elict En Serp En TcAm En elict Di Serp Di TcAm Di Pg Ttal Serp Ol En Primary mde Di Sp Pg Ol Crrected fr vlume expansin En Di Sp Pg -89A- -l,-0-89b- -, - -89C- -, 8- -,-9 -, , - -, 8- -, -0 -, -0 -, 9-0 -,- -, , 0-0 -, E- -, - -, F- -, - -, Average St. deviatin Ntes: Cunts = ttal pints cunted ( mm spacing). elict = primary unaltered mineral: Ol = livine, En = enstatite, Di = dipside, Pg = Plagiclase (includes alteratin after Plagiclase), Sp = spinel. Serp and TcAm = serpentine, talc and/r amphible pseudmrphs replacing the phase listed immediately belw. Analyses where the number f cunts and pseudmrph prprtins are nt shwn are frm the Initial eprts vlume fr Leg (Gillis, Mével, Allan, et al., 99). Primary mde = sum f relict primary phase and all its pseudmrphs. Numbers in italics are the rage mde, uncrrected fr the effects f serpentinizatin.

4 H.J.B. DICK, J.H. NATLAND erally arund 0%, the verall effect is typically small. Shwn fr cmparisn in Table are primary mdes recalculated assuming a 0% vlume expansin during serpentinizatin. As can be seen by cmparisn t the uncrrected rage mde given in italics, n rage, using uncrrected mdes results in a fairly small errrs in estimating mineral abundance, and wuld h little effect n the cnclusins drawn in this paper. As we wish t use the additinal data cllected n bard ship, and t cmpare ur results t thse fr ther abyssal peridtite lcalities, fr which alteratin phases were nt cunted separately, we use uncrrected mdes in ur plts and in the discussin. Any vilatin f the ischemical serpentinizatin assumptin als means that actual errrs in pint cunting primary mineral mdes are likely t be less than the theretical maximum. Overall, mdal mineral trends, where the mineral data are nt crrected fr changes in vlume due t serpentinizatin, will h increased scatter and will be smewhat cmpressed tward the livine apex f the mdal pyrxene-livine ternary. Electrn Micrprbe Majr element analyses f silicates and xides was dne with the MIT -spectrmeter JEOL Superprbe, using an accelerating vltage f KeV and a beam current f 0 na fr all phases. Data were reduced using Bence-Albee matrix crrectins (Albee and ay, 90; Bence and Albee, 98) (Tables -). A beam spt size f 0 µm was used fr analysis f pyrxenes, with 0 spts each analyzed alng apprximately a 00-µm trrse in rder t btain bulk analyses f grains incrprating any slid-slutin lamellae. The analytical standard deviatins bserved when a 00-µm trrse using 0 spts was made were essentially identical t thse fr the 00-µm trrses (Table ; Sample ---, -0 cm), suggesting that the technique accurately determined pyrxene bulk cmpsitin in ur samples fr the plane we trrsed. A -µm-beam spt size was used fr analysis f all ther phases. Three- t-seven-pint analyses were made n chrmian spinel, representing cre t rim trrses. Little zning was bserved, and these analyses were then raged t give a bulk cmpsitin fr each grain. In Micrprbe are earth and ther trace element analyses (Table 8) were made using the Wds Hle Oceangraphic Institutin Cameca IMS-f in micrprbe facility under the directin f Dr. Nbu Shimizu. Analytical techniques are the same as thse used by Jhnsn et al. (990). Overall accuracy and precisin f the data are generally believed t be: ±%-0% fr Ti, Cr, and V, ±0%-% fr Sr and Zr, and ±0%-0% fr light rare earth elements (LEE) and ±0%- Table. Site 89 livine analyses. Hle Cre, sectin, interval (cm) ck type Pts. Gr. MgO AO SiO CaO TiO CrO MnO FeO NiO Ttal Mg# 89E 89C 89C 89C 89C 89C 89E 89E 89E 89B 89C 89C 89E 89F 89F -,- -, 08- -, 08- -, - -, 0-0 -, 0-0 -, , , 8- -, - 8-, 9- -, - -,- 8-, - -, - -,8- -,-9 -,8^ -, - -, 9-0 -, , 9- -, , - -, -80 Olivine gabbr Olivine gabbrnrite Olivine gabbrnrite Trctlitic gabbrnrite Trctlite Trctlite Trctlite, m. gr. Trctlite, c. gr. Trctlite c. gr. Plagiclase dunite Dunite Dunite Dunite Dunite 0 8 If If g f f : l l , Ntes: Pts. = number f pints analyzed; = ne standard deviatin. Gr. = number f grain analyzed. Mg# = mlecular rati: (Mg 00)/(Mg+Fe). m. gr. = medium grained; c. gr. carse grained. 0

5 LATE-STAGE MELT EVOLUTION AND TANSPOT % fr the heavy rare earth elements (HEE) (Jhnsn et al., 990). In ur mre depleted harzburgites, hwever, relatively large errrs are assciated with analysis f the LEE, Sr and Zr, which ccur clse t, r belw, the analytical detectin limit. In a few cases La and Ce are deleted frm Table 8 fr the mst questinable analyses, and as we are nt certain f the reliability f La cncentratins fr all the harzburgite dipside analyses, and thus it is nt used in the figures. Analytical uncertainties at the very lw levels f analysis f the harzburgites varied with the electrnic stability f the instrument, which, in turn, varied ver the curse f the year-lng analytical prgram. Fr this reasn, almst all the analyses quted in Table 8 fr the harzburgites represent multiple analyses which h been raged, rejecting bviusly bad data. The rage cunting errrs fr these analyses are als given in Table 8 as a guide t interpreting the data. Any data with an rage cunting errr f mre than 0% must be regarded as unreliable. TECTONIC SETTING Hess Deep (Fig. ) is a lcal basin near the largely amagmatic tip f a V-shaped rift valley breaking int -.0 Ma crust n the eastern flank f the EP ahead f the westward prpagating Ccs- Nazca idge (Lnale, 988). Frm its tip, abut 0 km frm the EP, the "Ccs-Nazca" rift valley deepens eastward t >00 m at Hess Deep, while its walls rise t abut 00 m. The EP west f Hess Deep is spreading at a half-rate f. cm/yr ( km/m.y.; Lnale, 988). Apprximately 0 km east f the EP, the Ccs-Nazca idge begins t build a vlcanic ridge n the rift valley flr, which matures t the east t a. cm/yr half-rate spreading center (Lnale, 988). Multichannel seismic reflectin data acrss Hess Deep and nt nrmally nrth-suth lineated smth Pacific crust frmed at the EP cllected by Znenshain et al. (980) shw highly disturbed crust beneath the valley flr. Mre typical, but variable thickness, crust ccurs nrth and suth f the edge f the V-shaped rift, with crustal thickness decreasing smewhat twards the flanks f the rift. The EP crust is generally smewhat thinner than that nrmally calculated using simple layered crustal mdels (~ km) raging abut km, and ranging frm t km. French and U.S. submersibles (P. Lnale, unpubl. data, 99; Francheteau et al., 990; Hekinian et al., 99; Karsn et al., 99) fund a dismembered but apparently nearly cmplete cmpsite sectin f EP crust and shallw mantle expsed n the nrth wall f the rift valley and n an intrarift ridge between the nrthern rift valley wall and Hess Deep. A 00-m-thick subvertical nrth-suth striking sheeted dike cmplex, with 0-00 m f mixed extrusives and intrusives capped by m f pillw lava is expsed n the nrth wall. Gabbrs are als lcally expsed beneath the sheeted dikes alng this wall. The intrarift ridge, n the ther hand, represents a fundered tectnic blck f largely plutnic rck including dikes gabbrs and massive serpentinized peridtites, expsed and lcally uplifted during the pening f the rift. The intrarift ridge is an elngated east-west riented tectnic blck-lying parallel t the axis f the Ccs-Nazca idge with eastern and western highs and an intervening saddle (Fig. ). The Nautile made a series f suth t nrth transects up and ver the western and eastern highs. On the western transect, the rift valley slped -0 gently up frm Hess Deep fr - km. Gabbr and rare dunite crp ut frm 00 t 00 m, and diabase ledges ccur between 00 and 000 m. Frm 000 t 900 m the slpe steepens t frm the suth wall f the intrarift ridge. Gabbrs crp ut n bth the nrth and suth walls f the ridge, with an islated basalt utcrp at the crest. Alng the eastern transect mstly dunite and harzburgite are expsed in subhrizntal nrthward-dipping ledges with scattered utcrps f gabbric rck alng a gently dipping slpe frm 00 t 00 m. Pillw lavas and dikes are expsed ver the crest f the ridge, and lwtemperature hydrthermal activity was bserved. In situ nrth-suth dikes crp ut n the nrth slpe f the summit. Gabbrs were dredged between the east and west Nautile transects. Large land-slip scars n the suth wall f the intrarift ridge demnstrate significant lcal mass wasting. A disrupted magnetic stratigraphy, lw recvery, and pr drilling cnditins at at least ne hle drilled during ODP Leg (Hle ) likely reflect minr lcal slip and disruptin f the sectin. The distributin f rcks ver the intrarift ridge indicates a cmplex internal structure (Hekinian et al., 99): the result f unrfing and blck emplacement during the pening f Hess Deep. Alternative mdels explaining the pening f Hess Deep and the expsure f Table. Site 89 rthpyrxene cmpsitins. Hle Cre, sectin, interval (cm) Gr. Lc. Pts. Na O MgO AO SiO CaO TiO Cr O MnO FeO Ttal Mg# W En Fs s 89B -, - 89B 89C 89C 89E 89F -, - -, 8- -, -9 -, 8^ -,8- -, -0 -, 9-0 -, , 9- -, , - Olivine gabbrnrite 89C -, 08- C Ntes: Gr. = grain number. Lc. = lcatin in grain (C = cre r = rim). Pts. = number f pints analyzed; = ne standard deviatin. Mg# = mlecular rati: (Mg 00)/(Mg+Fe). Wllastnite, enstatite, and ferrsilite prprtins cmputed with all irn as FeO. 0

6 H.J.B. DICK, J.H. NATLAND Table. Site 89 clinpyrxene analyses. Hle Gabbr 89E 89E 89E Cre, sectin, interval (cm) -, -8 -, -8 -, -8 Olivine gabbr 89E 89E 8-, -9 8-, -9 9-, 0-B 9-, 0-B -, - -,- Olivine gabbrnrite 89C 89C -, 08- -, 08- Trctlitic gabbrnrite -, - -, - -, - Trctlite 89C 89C 89C -, - -, - -, - Plagiclase-bearing dunite 89C 89C -, - -, - 89B -,- 89B 89C 89C 89C 89C 89C 89C 89E -,- -,8- -,8- -,8- -,-9 -,-9 -,-9 -, 8- -, -0 -, -0 -,-0 -, 9-0 -, 9-0 -, , , 9- -, 9- -, 9- -, 8-89 Gr. Lc. C I C C C C C C C C C C B C C Size Pts I0 I I Na O MgO A O SiO CaO TiO Cr O MnO FeO Ttal Mg# W En Fs

7 LATE-STAGE MELT EVOLUTION AND TANSPOT Table (cntinued). Hle Cre, sectin, interval (cm) Gr. Lc. Size Pts. Na O MgO AO SiO CaO TiO Cr O MnO FeO Ttal Mg# W En Fs 89E 89F 89F 89F -, , - -, - -, -80 C Ntes: Gr. = grain number. Lc. = lcatin in grain (C = cre, = rim, I = interir). Size = maximum length and width f grain analyzed given in millimeters; Pts. = number f pints analyzed. Mg# = mlecular rati: (Mg 00)/(Mg+Fe). Wllastnite, enstatite, and ferrsilite prprtins cmputed with all irn as FeO. plutnic rcks alng the intrarift ridge emphasize vertical mvement f mantle hrsts r serpentine diapirs, and lw-angle faulting, respectively (Francheteau et al., 990). On-bttm gravity and seismic data frm the intrarift ridge, hwever, indicate that it is a high-density high-velcity blck (L. Drman and J. Hildebrand, pers. cmm., 99), which seems t preclude emplacement by serpentine diapirism. Site 89 is lcated n the intrarift ridge 9 km t the east f Hle 89G between 8 and 9 m, near the eastern Nautile transect (Fig. ). tectnites were recvered with dunite and related gabbric rcks frm six hles n the suth flank f the high there (Fig. ). In all,.9 m was drilled, with an rage recvery f.%, ranging frm.% in Hle 89F t.% in Hle 89E. Hles 89 A t are within 0 m f ne anther. Significant penetratins ccurred nly in Hles 89C and (. and 9. m). Hles 89E and 89F were drilled 0 and 0 m, respectively, t the nrth, directly up slpe, penetrating 8. and. m. These rcks are similar t the mantle transitin zne in many philite cmplexes, and were interpreted by the Leg scientific party as lying clse t the crust-mantle bundary (Gillis, Mével, Allan, et al., 99). The JOIDES eslutin (SEDCO/BP ) als drilled at Site 89 n the crest f the intrarift ridge near the western Nautile transect during Leg (Fig. ). Althugh many test hles were drilled at Site 89, nly Hle 89G attempted and made significant penetratin int the blck ( m) recvering.8 m f fine- t medium-grained gabbric rck, mstly gabbrnrite. These rcks, thugh gechemically cumulates (nt representing melt cmpsitins), are nt layered. Carse-grained intervals were generally fund as small irregular patches, subvertical lenses and pipes, ften cntaining abundant ilmenite, apatite, and zircn. The sectin clsely resembles gabbric rcks seen in the upper parts f many plutnic sequences in philites (Gillis, Mével, Allan, et al., 99). Several dikes were drilled at Site 89 in chilled cntact with gabbr, a feature als frequently seen in high-level gabbr suites in philites. The Hle 89G gabbrs, hwever, entirely lack evidence fr high-temperature ductile defrmatin and dynamic alteratin which is prminent in gabbrs frmed at slw-spreading ridges (Dick et al., 99a; Dick et al., 99; Stakes et al., 99). ather, the Site 89 gabbrs exhibit an initial phase f pervasive static amphiblitizatin beginning at temperatures arund 0 C, and a later phase f lwer temperature greenschist facies alteratin attributed t hydrthermal circulatin accmpanying the prpagatin f the Ccs-Nazca ift and the pening f Hess Deep (Gillis, Mével, Allan, et al., 99; Manning and MacLed, this vlume). Natland and Dick (this vlume) interpret the Site 89 gabbric sequence as representing a series f small intrusins emplaced immediately beneath the shallw "melt lens" believed t exist beneath the sheeted dikes at the EP (e.g., Sintn and Detrick, 99). It is imprtant t nte that the EP crust that extends t the edge f the Ccs-Nazca rift sme 9 km nrth f Site 89 is nrmally nrth-suth lineated parallel t the present-day EP, as is much f the present-day seaflr immediately west f the tip f the amagmatic Ccs Nazca rift valley. As there has been cnsiderable nrth-suth extensin f the riginal EP crustal sectin due t pening f Hess Deep, the present-day wall f the rift valley nrth f the intrarift ridge likely represents a pint n the pale-ep clse t that at which the Site 89 peridtites were riginally emplaced. This suggests that the initial frmatin f the EP crust brken pen at Hess Deep reflects nly the stress field f the pale EP, and was nt directly and immediately influenced by the Ccs-Nazca prpagatr. SITE 89 STATIGAPHY Hle stratigraphies are reprted here n the basis f "expandedrecvery." When there is nly partial recvery frm a drill hle, there are tw appraches t reprting the relative abundances f lithlgies. One is t assume that the partial recvery frm each cred interval is representative f the interval as a whle, and that by treating each cre as a separate sampling interval, the verall distributin and abundance f lithlgies can be best interplated. Fr this apprach, the actual recvery is expanded statistically t fill the entire cred interval, and each interval is then weighted based n its length in recnstructing the stratigraphy f the entire hle. The ther apprach makes the assumptin that the recvery frm the entire hle is representative f all the lithlgies drilled and disregards the sampling infrmatin prvided by stpping drilling and retrieving cre at regularly spaced intervals. Many factrs cntrl recvery. These include external mechanical factrs such as fluid pressure, bit type, and bit life at the time a sectin f the hle is drilled; gelgic cnditins including the nature and grain size f the lithlgy and the extent f alteratin and frmatin cementatin; and drilling cnditins such as verall hle stability and sea-state. It is unrealistic, therefre t assume cnstant cnditins ver the curse f drilling an entire hle, r even that recvery frm the same lithlgy will be cnstant with time. Accrdingly we believe that utilizing the sampling infrmatin prvided by individual drilling intervals, and weighing the recvery by the length f the cred interval prvides the best apprximatin t the actual stratigraphy. In particular, we draw attentin t the similarity f the expanded recveries in Hles 89A t, all f which were drilled within 0 m f each ther at the same lcatin. Despite depths ranging frm less than 0 m t 9 m, the relative prprtins f the rck types as calculated frm expanded recveries varies little between these hles. This wuld appear t justify t sme degree ur use f expanded recveries in recnstructing the stratigraphy. There is, hwever, n substitute fr 00% recvery, and it is realistic t cnsider the data prvided in Figure as nly a starting pint fr interpreting the gelgic relatinships at Site 89. In all, 0 dunites, trctlites, 0 livine gabbrs, and gabbrs, intercalated with harzburgite tectnite, were drilled at Site 89. The thickness f individual dunites rages m in Hle and.9 m in Hle 89E, ccurring in units up t m, r if gabbric segregatins are ignred, up t 8 m in Hle 89E. Gabbrs and trctlites range frm t m thick, with a few intervals up t m thick. Actual unit thicknesses, given the steep rientatin f the few intact cntacts, are likely half that measured vertically. The harzburgite in- 09

8 Table. Site 89 spinel cmpsitins. Hle 89E 89E 89C 89E 89E 89C 89C 89C 89E 89E 89E 89E 89B 89C 89C 89E 89F 89F Cre, sectin, interval (cm) -,- -,- -, 08- -,- -,- -, - -, - -l,99-0a -l,99-0a -l,99-0b -, 8- -, 0-0a -, 0-0c -, - -,-9 8-, 9- -, - -, - -,- 8-, - -, - -,8- -,-9 -, 8 -, -0 -, 9-0 -, 9-0 -, , 9- -, , - -, -80 ck type Gabbr? Gabbr? Olivine gabbrnrite Olivine gabbr Olivine gabbr Trctlitic gabbrnrite Trctlitic gabbrnrite Trctlite, m. gr. Trctlite, m. gr. Trctlite, c. gr. Trctlite, c. gr. Trctlite, c. gr. Trctlite, c. gr. Plagiclase dunite Dunite Dunite Dunite Dunite Dunite Dunite Gr. Size Pts. 9 8 MgO AO SiO CaO 0. TiO Cr O MnO FeO* NiO FeO Fe O :s Ttal* Ttal Fe + # Mg# Cr# b u ' i t X > > D Ntes: All analyses represent trrses frm cre t rim. Gr. = grain number. Size = maximum length and width f grain analyzed, given in millimeters. Pts. = number f spts analyzed and raged alng the trrse; = ne standard deviatin. FeO and Ttal* = ttal irn as FeO. Fe + # = mlecular (Fe loo/cr+al+fe ). Mg# = (Mg 00)/(Mg+Fe ). Cr# = (Cr 00)/(Cr+Al). m. gr. = medium grained; c. gr. = carse grained.

9 LATE-STAGE MELT EVOLUTION AND TANSPOT Table. Site 89 Plagiclase analyses. Hle Cre, sectin, interval (cm) ck type Gr. Size Pts. Na O MgO A O SiO CaO FeO K O Ttal An Ab Or 89E 89C 89E 89E 89E 89C 89C 89E 89C 89C 9-a, 0- -, -8 -, , -9 8-, -9 9-b, 0-9-C, 0- -, - -,- 9-b, 0- -, - -, 08- -, - -, - -, , - -,-9 -,99-0 -, - -, - Gabbr Gabbr Gabbrnrite Olivine gabbr Olivine gabbr Olivine gabbr Olivine gabbr Olivine gabbr Olivine gabbr Olivine gabbr? Olivine? gabbr Olivine gabbrnrite Trtlitic gabbrnrite Trctlite Trctlite c. gr. Trctlite c. gr. Trctlite m. gr. Trctlite m. gr. Plagiclase dunite Plagiclase dunite le le li c le e le le le lc le If le le e le le le le f Ntes: Gr. = grain number. Size = maximum length and width f grain analyzed, given in millimeters. Pts. = number f pints analyzed; = ne standard deviatin, c. gr. grained; m. gr. = medium grained. An = mlecular rati f Ca/(Ca+Na+K). Ab = mlecular rati Na/(Ca+Na+K). Or = mlecular rati K/(Ca+Na+K). tervals vary cnsiderably in thickness, frming screens f depleted mantle peridtite between crsscutting subvertical dunite and gabbric "dikes." Cntacts between harzburgite and dunite, between dunite and the gabbric rcks, and between different gabbrs are either gradatinal ver 0 cm r less, r sharp with a sutured igneus cntact. Where rientatin was preserved, they are generally steep, raging clse t 0. Cntacts that culd be reriented palemagnetically were subvertical with a strike similar t the subvertical magmatic fliatins f Hle 89G gabbrnrites (Gillis, Mével, Allan, et al., 99). These rientatins differ frm thse assciated with the Hess Deep pening, which is east-west, nearly rthgnal t the EP, and can be related directly t the nrth-suth EP stress field (MacLed, Célérier, et al., this vlume). Thus, the Site 89 peridtite gabbr sequence, and the Site 89 gabbrnrites, despite very different stratigraphic psitins and a 9-km ffset alng the intrarift ridge, reflect deep and shallw melt transprt, respectively, cntrlled by the same stress field beneath the EP. The recnstructed stratigraphy at each lcatin drilled at Site 89 is strikingly different (Fig. ). Hles 89A t h expanded recveries il.9% harzburgite-tectnite,.% dunite,.% gabbric rcks, and.% rdingitized basaltic dikes f prbable Ccs Nazca rigin. Hle 89E, n the ther hand, recvered nly.% harzburgite,.% dunite, and.% gabbrs, and Hle 89F recvered nly harzburgite. The recvery is high enugh t recnstruct the likely riginal relatinships between lithlgies. Where preserved, bth mdal and lithlgic cntacts appear planar and are ften quite steep. Gabbric segregatins were almst always recvered in assciatin with dunite, which ften is fund as cre fragments bth abve and belw the gabbric rcks. Where cntacts are preserved, gabbr is generally in cntact with dunite, nt harzburgite. Only ne -cm gabbr seam was fund directly in cntact with harzburgite; therwise, harzburgite ccurs next t gabbric rcks nly where there is a missing interval in the recvery. A cnsistent sequence was bserved during descriptin f the cre n bard ship, which may be present in all r part at any ne lcatin. This sequence cnsists f harzburgite tectnite enclsing dunite, fllwed by dunite enclsing trctlite, fllwed by livine-gabbr, then gabbr and in sme cases gabbrnrite (Fig. ). Many dunites crsscutting the harzburgite, hwever, d nt cntain gabbric segregatins (Fig. ). Dunite frmatin in harzburgites is cmmnly assciated with late fcused flw f melt thrugh the mantle. As will be discussed later, the similarity f these dunites t thse in many philites, the structure and chemistry f the gabbric segregatins, and the large lateral variatin in lithlgy ver the half-kilmeter mantle sectin drilled at Site 89, suggest that Leg drilled acrss a majr cnduit fr melt-migratin in the shallw mantle. HAZBUGITES Petrgraphy The Hess Deep harzburgites are petrgraphically typical f residual mantle peridtites sampled previusly at Hess Deep (Girardeau and Francheteau, 99; Hekinian et al., 99), at the Garret Fracture Zne n the EP t the suth (Hebert et al., 98), and at slwspreading ridges (e.g., Dick, 989). The harzburgite tectnites cntain abundant relict livine enclsing Prphyrclastic enstatite and dipside with accessry chrmian spinel. Spinel is generally highly

10 H.J.B. DICK, J.H. NATLAND Table 8. Site 89 clinpyrxene rare earth and ther trace element abundances. Hle Cre, sectin, interval (cm) Gr. Pts. La Ce Nd Sm Eu Dy Er Yb Gr.* Pis. Ti V Cr Sr Y Zr Ti/Zr Gabbr 89E 89E -, -8 -, -8 Olivine gabbrnrite 89C -, 08- Olivine gabbr 8-, -9 8-, -9 9-, 0-89E -,- Trctlitic gabbr -, - -, - -, - Trctlite 89C -, - 89C -, - Plagiclase dunite 89C -, - 89B -,- 89C -,8-89C -, 8-89C -, -9 -, -0 -, 9-0 -, 9-89E -, ^0 Gabbr 89E 89E -, -8 -, -8 Olivine g; ibbrnrite 89C -, 08- Olivine gabbr 8-, -9 8-, -9 9-, 0-89E -,- Trctlitic gabbr -, - -, - -, - Trctlite 89C -, - 89C -, - Plagiclase dunite 89C -, - 89B -, - 89C -, 8-89C -, 8-89C -,-9 -, -0 -, 9-0 -, 9-89E -, -0 amp C amp C Cncentratin in ppm Percent mean deviatin C cre Ntes: Gr. = grain number and psitin n grain ( = rim, C = cre); amp = amphible patch in clinpyrxene; = rage f several spts n grain. Pts. = number f pints analyzed. Percent mean deviatin given is the rage percent mean deviatin fr the pints analyzed determined frm the cunting statistics. Gr.* = grain number and lcatin if different than fr EE analysis. irregular, ranging frm vermifrm t hlly leaf textured, thugh in sme cases it is equant subhedral. Orthpyrxene ranges frm carse granular textured t Prphyrclastic. In the frmer, relatively carse - t -mm grains are characterized by smth, curved bundaries, deep embayments, and prjecting lbes where the pyrxene interlcks with the enclsing livine t frm a prtgranular texture. The prtgranular texture is generally mdified by later slid-state defrmatin, and individual pyrxenes h been variusly stretched, runded, defrmed, and recrystallized at grain bundaries. Clinpyrxene cnsistently has three mrphlgies: () subequant anhedral granular, with smth, curved, ften deeply embayed r prjecting lbes, similar t the earliest frmed rthpyrxene textures, () intergranular, ranging frm thin films and selvages between livine grains and adjacent t rthpyrxene, and () small equant neblasts ccurring in a matrix f recrystallized rthpyrxene. The clinpyrxene neblasts are generally interpreted as the result f exslutin f dipside frm the enstatite during mechanical recrystallizatin accmpanying late-stage slid-state defrmatin and flw (Medaris, 9; Medaris and Dtt, 90). Intergranular clinpyrxene and spinel are similar in their ccurrence, are ften intergrwn, and may frm small stringers in the livine. We interpret these trains f aligned mineral grains as the result

11 LATE-STAGE MELT EVOLUTION AND TANSPOT Figure. Map f the eastern Pacific seaflr shwing the relevant prtins f the EP, the Ccs-Nazca idge, and Hess Deep, mdified frm Lnale, 988. f crystallizatin f clinpyrxene and spinel frm late-melts migrating thrugh the peridtite. In many cases, the stringers cnsist nly f spinel enclsed in livine, and therefre cannt represent slidstate breakdwn f recrystallized aluminus-pyrxene t a spinelbearing assemblage due t the lack f apprpriate reactin prducts. At the same time, the clinpyrxene trains preserve cmplex intergranular textures unlikely t survive plastic defrmatin. We als bserve a gradatin frm the intergranular clinpyrxene t the relatively carse granular clinpyrxene. While the granular clinpyrxene may be residual, given its depleted cmpsitin and mφhlgical similarity t residual rthpyrxene, the textural gradatin t intergranular clinpyrxene als suggests late vergrwth frm melt migrating thrugh the harzburgite. Girardeau and Francheteau (99) and Hekinian et al. (99) als reached similar cnclusins fr the rigin f "wehrlitic" clinpyrxene patches in Hess Deep harzurgites sampled by dredging and submersible. Melts migrating upward thrugh the mantle are generally believed t disslve pyrxene and precipitate livine and spinel in rder t maintain equilibrium with peridtite due t the expansin f the field f livine n the basalt liquidus with decreasing pressure (Dick, 9a; Dick, 9b; Dickey at al., 9; Kelemen et al., 990; O'Hara, 98; Quick, 98a). This is generally the case where the mantle is melting and melt mass is increasing. The situatin can be quite different where the mantle is n lnger melting, melt mass is decreasing, and mantle mass is increasing due t crystallizatin f minerals frm melts perclating thrugh it (Kelemen et al., 99). The cprecipitatin f spineland clinpyrxene in the Hess Deep harzburgites indicates such a situatin. While late-state melts may h been disslving enstatite as they perclated thrugh the Hess Deep peridtites, their cmpsitins must h reached saturatin with respect t clinpyrxene. Based n the knwn liquidus phase relatins fr mantle peridtites, this indicates a significant decrease in melt mass due t crystallizatin f the migrating magma reacting with the hst peridtite. This in turn requires that the getherm immediately beneath the East Pacific ise was cler than the mantle adiabat, and prvides the first direct evidence fr a significant cnductive lid n the upwelling mantle there. The spinel trains and tectnically elngated pyrxenes define a fliatin recgnizable in mst f the harzburgites. This fliatin, when palemagnetically reriented has a highly variable dip, but a strng preferred nrth-nrtheast strike rughly parallel t the EP (Gillis, Mével, Allan, et al., 99). This fliatin is mre weakly seen defined by spinel trains in the dunite, but is absent t rare in the trctlites, which shw little evidence f the late high-temperature defrmatin seen in the peridtites. Taken tgether, the petrgraphy and micrstructures in the harzburgite indicate high temperature-lw deviatric stress cnditins expected fr slid-state flw in the asthensphere (Niclas et al., 9). Estimates f the temperature f emplacement f the Hess Deep harzburgites based n their fabrics are clse t the mantle slidus near 00 C (Gillis, Mével, Allan, et al., 99; Budier et al, this vlume; Kennedy et al., this vlume). The harzburgites are heavily altered, cntaining n rage % seφentine (Table ). The percentages f seφentine pseudmφhs f livine and enstatite are given in Table. These data shw that in mst cases, livine is preferentially seφentinized, althugh in ne instance enstatite is mre altered, while in ther cases enstatite and livine are equally altered. This suggests variable fluid chemistries and cnditins f alteratin. Unlike dredged samples, the Site 89 harzburgites are relatively little weathered, cntaining little r n clay replacing livine, except fr thse frm the shallwest levels. There is als an additinal.% pseudmφhs f talc and/r amphible after enstatite and % talc and/r amphible pseudmrphs after livine n rage. The talc-amphible pseudmφhs include three different assemblages: talc, talc-amphible, and amphible. These assemblages indicate hydratin ver a brad range f cnditins, likely up t lwer amphiblite facies cnditins. The amphibles in thin sectin appear t be mstly tremlite, thugh cummingtnite is als lcally present. In additin, the assemblage talcsecndary livine ccurs in sme veins crss-cutting enstatite, with very-fine-grained livine ccurring enclsed in the talc at the center f the vein. The presence f these higher temperature assemblages is nt, in itself, unusual, and they are ubiquitus in small quantities in the several hundred different suites f abyssal peridtites previusly examined by the senir authr. The vlume f this alteratin in the Hess Deep peridtites, hwever, is atypically high in ur experience, and suggests extensive late hydrthermal circulatin during emplacement at the time f Hess Deep pening. The rage f 8 primary mdes is very refractry, cntaining 8. vl.% livine,.% enstatite,.0% dipside and % spinel. Fr cmparisn, the rage plagiclase-free abyssal peridtite cntains % livine, % enstatite,.% dipside and % spinel (Dick, 989). The mst depleted Suthwest Indian idge peridtites frm the Buvet Fracture Zne, which cuts the Buvet ht spt, cntain an rage f 8.% livine,.% enstatite,.% dipside, and % spinel (. f mdes). While errrs due t variable degrees f seφentinizatin and differential vlume expansin may be significant in cmparing individual samples, they are nt large enugh t accunt fr the 0% greater rage dipside cntent f the Buvet harzburgites, particularly as the verall degree f seφentinizatin f the Buvet and Hess Deep harzburgites is similar. The difference in mdal dipside cntent, hwever, might reflect utcrp scale hetergeneities, and frtuitus sampling near a dunite cmplex at Hess Deep and far frm a dunite cmplex at the Buvet Fracture Zne, rather than reflecting a reginal difference in the degree f mantle melting. Despite the presence f the gabbric segregatins, we bserve virtually n relict r pseudmφhed Plagiclase in the thin sectins cunted, thugh it culd always be present in samples we h nt examined. Individual mdes are pltted in Figure. Similar t many abyssal peridtite suites, these mdes shw a strng tendency fr di-

12 H.J.B. DICK, J.H. NATLAND km Cntur interval 00 m.,' Lwer crust/ ' "- v mantle utcrp Yung lavas f -;;- Ccs-Nazca Axial idge 8'N " Samples Basalt Dlerite 0 Nncumulate gabbr ^ Cumulate gabbr Peridtite Breccia Hydrthermal Ccs-Nazca Axial idge 0 'W 0 8' Figure. Bathymetric map f the western end f the Ccs-Nazca idge shwing the V-shaped amagmatic rift valley and the lcatins f Hess Deep and the intrarift ridge where Sites 89 and 89 are situated. pside and enstatite t decrease tgether as livine increases in abundance (e.g., Dick, 989; Dick et al., 98; Jhnsn and Dick, 99; Michael and Bnatti, 98b). Mineralgy Majr element analyses f the primary phases are given in Tables thrugh. Olivine is frsteritic relative t mst abyssal peridtites, raging F 90, with a standard deviatin f nly % frsterite. Peridtites frm near the Buvet ht spt, fr cmparisn, h n rage F 908, while the rage abyssal peridtite has F 90, and peridtites dredged far frm mantle ht spts rage abut F 90, (Dick, 989; Dick et al., 98). Dipside and enstatite cmpsitins are given in Tables and and are pltted in the pyrxene quadrilateral in Figure. The principal variatin exhibited by pyrxenes frm the Site 89 harzburgites and gabbrs in the quadrilateral is a simple variatin in wllastnite cntent lying directly n a jin between dipside and enstatite. This likely reflects varying degrees f exslutin f dipside frm enstatite, and f enstatite frm dipside, during subslidus reequilibratin and recrystallizatin that accmpanied cling and emplacement t the base f the crust prir t unrfing f the mantle sectin at Hess Deep. Any significant chemical fractinatin related t either varying degrees f melting r fractinal crystallizatin during frmatin f the peridtites and gabbrs, respectively, wuld cause a small but significant spread in the data parallel t the enstatite-ferrsilite jin. The rage cmpsitin f enstatite and dipside is refractry cmpared t pyrxenes in ther abyssal peridtites suites, including thse assciated with ht-spts in the Atlantic and Indian ceans. This is clearest fr the cncentratins f the mre incmpatible elements such as sda, titanium, and alumina. Sdium and titanium are virtually belw the detectin limit fr the electrn micrprbe in all the pyrxenes we analyzed in the harzburgites. Alumina cntent f enstatite has been shwn experimentally and theretically t be a gd mnitr f mantle melting, decreasing systematically with increasing depletin f peridtite (e.g., Dick, 9b; Dick et al., 98; Jaques and Green, 980; Jhnsn and Dick, 99; Michael and Bnatti, 98a; Mysen and Bettcher, 9). The rage alumina cntent f enstatite in Hess Deep harzburgites is nly.9 ± %. By cmparisn, undepleted harzburgites far frm mantle ht spts cntain % alumina r mre, while the peridtites frm near the Suthwest Indian idge near the Buvet ht spt cntain.% alumina, and thse frm N n the Mid-Atlantic idge near the Azres ht spt cntain. wt%.

13 LATE-STAGE MELT EVOLUTION AND TANSPOT. 00 -" "" *Nautile gabbrqnrite '.0'.9" J O.8".'.'.' 0.0" ' West lngitude 0.' 0.' SeaBeam depth Drill-pipe depth Pipe depth pint 00 m Figure. A. Bathymetric map cntured fr depth frm gridded SeaBeam data (dashed lines) and fr sundings with the drill pipe (slid lines) shwing lcatins f Hles 89A-89F. Frm Gillis, Mével, Allan, et al. (99). B. Expanded stratigraphies f Hles and 89E. The expanded recvery is based n the assumptin that the partial recvery frm each drilled interval is representative f the lithlgic interval drilled fr each cre. Fr example, the percentages f different rck types recvered fr an entire hle may differ smewhat frm the percentages calculated based n expanded recvery, as the recvery frm each interval drilled (cre) is ften quite different. The first clumn shws the psitin f each cred interval, the secnd clumn shws the amunt f rck r sediment recvered (slid black bar), and the third clumn shws the stratigraphy after expanding the recvered rck prprtinately t fill the cred interval. The rientatin f the cntacts between units is generally steep, thugh variable, and hrizntal bars represent nly the vertical dimensin f individual rck units. C. Expanded recvery stacked by rck type fr Hles 89A-89F. Hles 89A- were drilled within several tens f meters f ne anther, whereas Hle 89E was drilled 0 m upslpe and Hle 89F an additinal 00 m upslpe. The plt des nt reflect the stratigraphic rder f these rck types, which are cmplexly intercalated with ne anther as shwn graphically in (B). The plt des shw that the sectin drilled is laterally very hetergeneus, and that the gabbric rcks are strngly assciated with the dunites.

14 I Hle Igneus Stratigraphy Interval Q) O m. * > 8 ö ü DC Hle 89E Igneus Lithstratigraphy Interval Dunite 0 O <D c Olivine Gabbr Fresh Glassy Basalt 0 ~ Dunite Olivine Gabbr Dunite Olivine Gabbr Gabbr 0 - Gabbr Dunite Gabbr 0 - dingitized Basalt Dike 0-0- Dunite Gabbr Trctlite Dunite 8 9 Dunite 0 Trctlitic Gabbr Dunite 8 Trctlite 0 Olivine Gabbr 0- d8 Dunite 9 Olivine Gabbr 0 Dunite Trctlite Dunite Gabbr Dunite Trctlite Dunite 0-0- Dunite 8 Dunite 9 0 dingitized Basalt Dike Gabbr 8 Dunite Gabbr ^ Trctlite Dunite Trctlite 9 Dunite 0 Gabbr Dunite Gabbr 8 Dunite 9 0 Dunite " Dunite Dunite Olivine Gabbr 8 9 Olivine Gabbr 0 Dunite - Dunite 0-0 Olivine Gabbr Trctlite Dunite Trctlite Dunite 8 9 Dunite 0 Trctlite Dunite 8 Trctlite Dunite 0 Dunite Dunite Dunite Dunite 8 Olivine Gabbr 9 Dunite Figure (cntinued).

15 LATE-STAGE MELT EVOLUTION AND TANSPOT C A 8 m D Basalt Site 89 Lithlgic Prprtins Gabbr 0 Olivine Gabbr E Trctlite π Dunite E B 8 m C 80 m D m Hle/Depth t Sea Flr E m F 9 m Dipside Figure (cntinued). Olivine Enstatite Dipside 0% Olivine 80% 0% Enstatite Figure. Mineral mdes pltted in the livine-dipside-ternary. A blw-up f the livine crner f the ternary is shwn at the bttm f the figure, with the mdal trend fr Suthwest Indian idge peridtites frm Dick et al. (98) shwn fr reference. At the upper right is a cartn illustrating pseud-eutectic melting f mantle peridtite t illustrate hw varying degrees f mantle melting prduce linear mdal trends. A full discussin f this can be fund in Dick et al. (98).,^ = sequential residues f melting; L = liquid prduced at the reactin pint. Curved lines shw the inferred ctectic bundaries fr the system A-B-C; the arrw shws the trend fr prgressive melting and melt remval frm the parent cmpsitin. Figure. Pssible lithstratigraphic relatinships between harzburgite-tectnite screens and crsscutting dunites at Site 89 based n features f the recvered cre and the utcrp-scale relatinships f similar features fund in philite mantle sectins. Dark shaded area is harzburgite-tectnite; light shaded regin is crsscutting dunite. Dunite is inferred t enclse sequentially: trctlite (hashed), livine gabbr (crsses), and gabbr (hrizntal lines). There are n evident crrelatins between mineral mde and pyrxene r livine cmpsitin. Alumina and calcium in enstatite pltted against mdal livine is shwn, fr example, in Figure. These analyses are believed t apprximate their bulk cmpsitin prir t cling and subslidus exslutin. Given the relatively unifrm degree f serpentinizatin f the livine ( ± 0%; Table ), the large range in livine mde (8.-9.%) is undubtedly quite real. There is, hwever, n significant variatin in the calcium r alumina cntents f enstatite that relates t mdal prprtins (Fig. B). In cntrast, the glbal data set fr the Mid-Atlantic, Suthwest Indian, and American-Antarctic idges has a strng negative crrelatin between alumina in enstatite and mdal livine (Fig. A) (Bnatti, 99; Dick, 989; Dick et al., 98; Michael and Bnatti, 98a). This systematic glbal variatin in mineral mde and chemistry exists als fr the trace element cmpsitin f dipside and spinel Cr/ (Cr+Al), and crrelates directly with the cmpsitin f spatially as- sciated abyssal thleiites, demnstrating a systematic increase in the degree f mantle melting beneath cean ridges near mantle ht spts (Dick et al., 98). The lack f a systematic variatin in pyrxene cmpsitin with mineral mde, which is als the case fr spinel cmpsitin, demnstrates that the Site 89 harzburgites h last equilibrated with a relatively hmgeneus pervasively perclating melt. Mdal variatins in the Site 89 harzburgites, then, d nt arise frm variatins in the degree f batch-equilibrium r fractinal melting, but rather, reflect ther physical-chemical prcesses such as melt-rck reactin, r the frmatin f mdal layering by mechanical prcesses (Dick and Sintn, 99). Spinel in the Hess Deep harzburgites lies at the extreme Cr-rich end f the range fr abyssal peridtites (Fig. 8). The abyssal spinel peridtites pltted in Figure 8 define the field fr the Atlantic and Indian Oceans, and h been shwn t define a mantle-melting array (Dick and Bullen, 98), with a sharp cutff at Cr/(Cr+Al) (Cr#) f arund, which is believed t crrespnd t the activity f Cr and Al in the mantle at the pint at which dipside disappears frm the mantle residue during melting (Dick and Fisher, 98). Cr# sharply increases during partial melting f peridtite reflecting the cntrasting behavirs f Cr and Al. In cntrast, even thugh bulk rck Mg/ (Mg+Fe) als increases with melting, Mg/(Mg+Fe) in spinel shws a slight irn-enrichment due t the changing partitin cefficients fr Mg and Fe between spinel and livine with increasing spinel Cr# (Dick and Bullen, 98; Irvine, 9). Dick et al. (98) h shwn that the upper limit fr abyssal peridtite spinel Cr# is exceeded in many alpine-type peridtites, which

16 H.J.B. DICK, J.H. NATLAND Hess Deep, Site 89 & Hle 89G Pyrxenes a Nrth Slpe Gabbr π Gabbr Gabbrnrite Olivine Gabbrnrite 0% Olivine Gabbr * Trctlite Plagiclase Dunite + 0%, Atlantic & Indian Ocean Peridtites 0% En 90% 80% 0% 0% Figure. Pyrxene quadrilateral shwing the cmpsitin f all pyrxenes frm Sites 89 and 89. Cmpsitins with ferrsilite (Fs) greater than 0% are all Hle 89G gabbrnrites and livine gabbrnrites; thse with less than 0% ferrsilite are frm Site 89 harzburgites and gabbrs. Hle 89G analyses are frm Natland and Dick (this vlume). Shwn fr cmparisn are pyrxene cmpsitin istherms fr cexisting augite enstatite assemblages frm Lindsley and Andersen (98). The authrs estimate that the psitins f these istherms h an uncertainty f ±0-0 C plus abut. Using these t directly estimate temperatures, hwever, requires recalculatin f the analyses based n mineral stichimetry t exclude nnquadrilateral cmpnents. As we h nt dne this, the pyrxene istherms are pltted here nly t illustrate the behavir f cexisting pyrxenes during crystallizatin and subslidus reequilibratin. Inset shws the trend fr cexisting pyrxenes in the Skaergaard intrusin frm Wager and Brwn (9). may cntain extremely Cr-rich spinel (up t a Cr# = ; Mutte, 99). Many philites, such as Papua (England and Davies, 9), als cntain n reprted primary clinpyrxene, and extremely alumina- and calcium-pr enstatite undersaturated with respect t dipside. This is cnsistent with melting well int the dipside-ut three phase field where the residue cnsists nly f livine, enstatite, and spinel. Up until the pint at which dipside disappears frm the residue, the calcium cntent f cexisting enstatite is buffered by the enstatite-dipside slvus and by the mantle temperature t a narrw range near wt% (Fig. ; Dick et al, 98; Hess, 99). The cexistence f dipside and enstatite during melting als apparently buffers the minimum alumina cntent f enstatite, and therefre the upper limit f Cr# in cexisting spinel as well. Experimental petrlgy shws that additinal melting beynd the dipside-ut pint requires substantially mre heat per unit vlume f melt prduced, and this phase bundary, accrdingly, prvides a natural limit fr dry mantle melting beneath cean ridges (Dick and Fisher, 98; Hess, 99). Thus, despite the high degree f depletin in trace elements cmpared t ther abyssal peridtites, melting f the mantle beneath the EP appears t h been limited t the fur phase field, and represents thermal cnditins nt t different than beneath ther cean ridges. By cntrast, many philites must represent substantially different melting envirnments. Since it seems unlikely that the mantle beneath an island-arc wuld be htter than that beneath an cean ridge, the extremely refractry cmpsitins f pyrxene and spinel mst likely reflects hydrus mantle-melting in a supra-subductin zne envirnment, with the frmatin f hydrus, and high silica 80 8 Mdal livine (vl.%) Figure. Alumina and calcium cntents f enstatite pltted against mdal livine in vl% fr Hess Deep Site 89 harzburgites. A. Cmpsitins f enstatite in Atlantic and Indian Ocean abyssal peridtites mdified frm Dick and Fisher (98). B. Cmpsitin f enstatite frm Site 89 harzburgites as reprted in this paper. magmas (Dick and Bullen, 98), and mre extensive melt-rck reactin in respnse t a thicker lithsphere than in the ceans. Gechemistry are earth and trace elements analyses fr dipsides frm Hess Deep harzburgites are given in Table 8. Hess Deep harzburgites h extreme depletins in incmpatible trace elements as demnstrated by the Zr and Ti plt in Figure 9. Shwn fr cmparisn are data frm Jhnsn et al. (990) fr the Suthwest Indian and American-Antarctic idges. A feature f this plt, which may nt be real, is the extreme depletin in Ti relative t Zr, which ffsets the Hess Deep peridtites frm the mantle array f Jhnsn et al. (990). T the degree that Zr/Ti ratis define a mantle surce regin, this wuld indicate that the EP mantle is cmpsitinally different frm that beneath the Indian and Atlantic Oceans as represented by the array f Jhnsn et al. (990). Our analyses fr Zr, hwever, are near the knwn detectin limit f the in prbe, and the abundances measured may be mre a functin f undetected randm electrnic spiking in the backgrund than the actual abundance. The latter cnclusin is strngly suggested by the fact that all the Site 89 gabbric segregatin, including thse knwn t h undergne extensive melt-rck reactin with the Site 89 harzburgites, as well as the Hle 89G high-level gabbrs, lie alng the trend f Jhnsn et al. (990) in Figure 8 and prject dwn tward the mst depleted harzburgites analyzed by Jhnsn et al. (990). This suggests that the Zr cntent f the Site 89 harzburgite dipsides was in fact belw ur detectin limit, and shuld lie alng the extensin f the Jhnsn et al. trend. are earth elements are als extremely depleted relative t ther abyssal peridtites (Fig. 0). Of particular interest is the extreme depletin in HEE, which gives the Site 89 patterns a smewhat higher Ce/Yb rati than sme, but nt all, f the peridtites dredged near the Marin and Buvet ht spts n the Suthwest Indian idge (Fig. 8

17 LATE-STAGE MELT EVOLUTION AND TANSPOT Site 89 Spinels O Dunite O Plagiclase dunite HI Trctlite + Olivine gabbr IS"*" - 0 Site 89 s ffl T C ^ 0 < ü ü - 0 h 0 - * : i"t f ffl Abyssal Spinel Peridtites - - O ü 0. B 0 Hess Deep * Hle 89 C Hle 89 D Φ Hle 89 E SWI (ht spt related) a near Marin Is. «near Buvet Is. Ce Nd Sm Eu Dy Er Yb Mg*00/(Mg+Fe + ) Figure 8. Cmpsitins f Hess Deep Site 89 spinels pltted n the Cr-Al- Mg-Fe + face f the spinel cmpsitin prism (Irvine, 9, 9). The Y- axis length is equal t tw times the X-axis, reflecting the relative mlecular prprtins f + and + catins. Cmpsitin f abyssal chrmites frm Dick and Bullen (98). Q. Q. <D I0 CD O >, 000 Q. 00 "T Site 89 D * A Gabbr Olivine Gabbr Trctlitic Gabbr Trctlite 8 ^ B n G E D? /.. Hle 0B phencrysts \ \ 0 / / ' $ <~~ Zr in clinpyrxene (ppm) / i i i i i i i i i i Hle S0B / grundmass / / / - + Site 89G Gabbrs Abyssal Peridtites Surce Cmpsitin Figure 9. Ti vs. Zr cntent fr dipsides and augite frm Hess Deep Site 89 and Hle 89G peridtites and gabbrs. Data fr Hle 89G gabbrs frm Natland and Dick (this vlume). epresentative abyssal peridtite data (pen circles) is fr Suthwest Indian and American-Antarctic idge peridtites frm Jhnsn et al. (990). Arrw shws prjectin f data fr Site 89 gabbric segregatins alng the trend fr abyssal peridtites t Zr cntents apprpriate fr Hess Deep residual harzburgites, assuming that the measured values represent nise at the detectin limit f the in prbe. Dashed line shws a cnstant Ti/Zr rati f 0 passing thrugh an assumed surce cmpsitin used by Jhnsn et al. (990). 0A). In additin t the HEE-depletin, the secnd defining characteristic f the Site 89 harzburgite EE patterns is the strng depletin in middle rare earths, which cntrasts sharply t peridtites assciated with N-type r "nrmal" MOB. The latter are characterized by a cnvex upward pattern with strng enrichment in the middle EE relative t the LEE. A strng depletin in MEEs is als a O O 0. La Ce Figure 0. A. EE in dipsides frm Site 89 harzburgites nrmalized t chndrites (slid symbls). Shwn fr cmparisn is the field f Suthwest Indian and American-Antarctic idge harzburgites sampled far frm mantle ht spts frm Jhnsn et al. (990). EE patterns fr Buvet and Discvery Fracture Zne harzburgites (pen symbls), frm near the Buvet and Marin ht spts, respectively, are shwn frm Jhnsn et al. (990). B. Calculated equilibrium liquid cmpsitins fr Site 89 harzburgites and representative l - EP basalts frm Lnale et al. (99). characteristic feature f the Buvet and Discvery ht spt peridtite dipsides (Fig. 0A), which has been attributed t mre extensive melting in the garnet stability field than peridtites far frm mantle ht spts (Jhnsn et al., 990). DISCUSSION OF THE SITE 89 HAZBUGITES Mantle Cmpsitin and Melting Anmalies Beneath the EP In terms f majr elements, the primary mineral phases in the Hess Deep harzburgites were previusly recgnized as depleted relative t mst abyssal peridtites and sme philites (Girardeau and Francheteau, 99; Hekinian et al., 99). This includes peridtites frm the Garret Fracture Zne n the EP at O'S, as indicated by their significantly mre aluminus spinel (Cr# = 0-). The abslute cncentratin f incmpatible elements in Hess Deep peridtites, hwever, is even lwer than that assciated with peridtites dredged near the Buvet and Marin ht spts. Peridtites with strng depletins in the middle rare earths h previusly been fund nly in assciatin with enriched-mob dredged near mantle ht spts n the Suthwest Indian idge. This pattern, and the highly depleted majr element cmpsitins f these peridtites, then, h been attributed t anmalusly high mantle temperatures prducing high degrees f mantle in assciatin with mantle plumes n r near 9

18 H.J.B. DICK, J.H. NATLAND cean ridges (Dick et al., 98; Michael and Bnatti, 98a). EP basalts frm t N t the west f Hess Deep, hwever, h nrmal LEE-depleted patterns characteristic f N-MOB (Fig. 0B; Lnale et al., 99), and there is n knwn bathymetric expressin f a mantle ht spt alng this regin f the EP. This shws, therefre, that the characteristic shape f the Buvet and Marin peridtites is nt a functin f a plume vs. MOB surce mantle, but is related t ther envirnmental factrs. Fllwing the ratinale f Jhnsn et al. (990), then, mantle melting culd h begun anmalusly deep cmpared t that in N- MOB regins f the Atlantic and Indian Oceans. If this is the case, it wuld require a htter mantle beneath this regin f the EP. We nte, hwever, that the EP crust near Hess Deep is smewhat thinner than nrmal (Znenshain et al., 980), which indicates that despite the middle EE-depletin, and lw HEE cmpared t the rest f the glbal data set, that the ttal amunt f melting beneath the EP was less than that assciated with nrmal sectins f slwspreading ridges. This suggests t us that the mantle beneath the EP in this regin was initially mre depleted in bth majr and trace elements, perhaps due t an earlier mantle melting event. The latter event culd h ccurred in the garnet peridtite field, whereas the event assciated with the EP may h largely ccurred in the spinel field, cnsistent with a mre nrmal mantle getherm. This might explain the shape f the EE pattern in the Site 89 harzburgite clinpyrxenes as an inherited feature; the relatively lw Ce/Yb rati and lw HEE abundances wuld then reflect additinal melting beneath the EP in the spinel field. A mre critical examinatin f the melting histry fr these peridtites, hwever, must await detailed numerical mdeling. Tw-dimensinal vs. Three-dimensinal Mantle Flw Anther, nt necessarily cnflicting, hypthesis is that the unusual depletin f the Site 89 harzburgites may be a characteristic f a zne f fcused melt flw in the mantle. Certainly the presence f abundant dunite demnstrates the presence f a majr flw channel thrugh the Site 89 mantle sectin. In cntrast, dunite is rare in dredge cllectins frm slw-spreading cean ridges (Dick, 989) fr which mst f the existing data fr abyssal peridtites t which we are cmparing the Hess Deep data cmes. Bth basalts and gabbrs frm the Hess Deep area, hwever, reflect crystallizatin frm refractry primary liquids nrmally assciated with unusually high degrees f melting f spatially assciated mantle peridtites, and thus indicate derivatin frm a highly depleted surce. As shwn in Figure, the gabbrs frm Hle 89G and frm the nrth wall f Hess Deep tgether define a rugh trend in a plt f Plagiclase Ca/ (Ca+Na) vs. clinpyrxene Mg/(Mg+Fe) that is significantly ffset t mre calcic Plagiclase at a given Mg/(Mg+Fe) value (Fig. A). A similar ffset can be seen in a plt f Ca/(Ca+Na) vs. Mg/(Mg+Fe) fr EP basalt glasses dredged frm the walls f Hess Deep (Fig. IB). This upward shift f the crystallizatin trends in these figures is analgus t very high basalt Na 8 values and is expected fr gabbrs (Meyer et al., 989) and basalts crystallized frm liquids reflecting very high degrees f melting f the mantle surce (Dick et al., 98; Klein and Langmuir, 98; Schuten et al., 98), r alternatively fr nrmal degrees f melting f an initially mre refractry surce mantle cmpsitin (Dick et al., 98). The unusual depletin f the Hess Deep harzburgites is apparently nt a lcal feature f the mantle sectin beneath the EP, but is a reginal characteristic reflected in the cmpsitin f the verlying basalts and gabbrs ver a wide area. Thus, if the unusual cmpsitinal characteristics f the Site 89 harzburgites cmpared t mst Atlantic and Indian Ocean abyssal peridtites are a reflectin f fcused melt flw, then it is likely that such flw channels are rather bradly distributed in the mantle sectin beneath the EP cmpared t slw-spreading ridges. A greater dispersal f such flw channels beneath a ridge axis might be expected if mantle flw beneath fast-spreading ridges is mre tw Ja) O O % t cl en t D..0 ^ t ü Hess Deep, & selected MA & SWI gabbrs l. A E ~O Nrth Slpe 89G Gabbrnrite α 89G Gabbr -O 89G Olivine gabbrnrite 0 89G Olivine gabbr 89 Plagiclase dunite ' * ' * 89 Trctlite _ -Φ 89 Olivine gabbr C ^"i - f f" Q 89 Gabbr a G O &j- ~V 89 Olivine gabbrnrite 0 Cayman Trugh ffl Qα9 SWI Hle B * ^ * MA Hle _ ffi. Clinpyrxene Mg/(Mg+Fe) Mantle w -. melting A Hess Deep & selected MA & SWI basalt glasses FAMOUS (near Azres Htspt). Speiss idge (Buvet Htspt) Atlantis II F.Z. & Site B. O EP Basalts, Hess Deep rift Wall Ccs-Nazca ift tip Mantle melting Basalt Glass Mg/(Mg+Fe) Figure. A. Plagiclase Ca/(Ca+Na) pltted against Mg/(Mg+Fe) fr Hess Deep gabbrs frm Hle 89G (pen large symbls), Site 89 (clsed large symbls), and the nrth wall (slpe) f the Ccs-Nazca ift nrth f Hess Deep (divided squares). Shwn fr cmparisn are the cmpsitin f gabbrs spatially assciated with basalts and peridtites reflecting lw degrees f mantle melting (Dick et al., 98), including: the Cayman Trugh (Elthn, 98) and Hle B in the Indian Ocean (Ozawa et al., 99). Fr cntrast, gabbr suites frm Hle near the Azres ht spt (Hdges and Papike, 9) are shwn. The latter are assciated with basalts and mantle peridtites reflecting sme f the highest degrees f melting anywhere in the Atlantic and Indian Oceans (Dick et al., 98; Klein and Langmuir, 98). Lightly shaded arrws indicate the general directin f crystal fractinatin trends; the darkly shaded arrw shws the cntrasting general effect f prgressive mantle melting n primary melt cmpsitins. B. Plt f Ca/(Ca+Na) vs. Mg/(Mg+Fe) fr basalt glasses frm the Hess Deep (data frm Nilssn, 99). Shwn fr cmparisn are suites frm the Atlantis II Fracture Zne near Hle B (Dick et al., 99b), which are believed t reflect lw degrees f melting and mantle depletin (Dick et al., 98; Dick et al., 99b), and suites frm Speiss idge n the Suthwest Indian idge near the Buvet ht spt (Dick, unpubl. data) and the FAMOUS regin f the MA near Hle and the Azres ht spt (Bryan and Mre, 9), which reflect high degrees f melting and mantle depletin. Arrws as in (A). dimensinal than at slw-spreading ridges as suggested by Lin and Phipps Mrgan (99). Evidence fr Fractinal Melting and Ultra-depleted Melts Petrgraphically, clinpyrxene and spinel mrphlgies in the Hess Deep harzburgites appear t reflect late-stage perclative melt flw, with much f the clinpyrxene ccurring in lineated trains f 0

19 LATE-STAGE MELT EVOLUTION AND TANSPOT intergranular grains believed t h crystallized at the very end f melting in the zne f cnductive heat lss beneath the crust at the ridge axis. In Figure 0B we shw calculated liquid cmpsitins in equilibrium with the Site 89 harzburgite clinpyrxenes using partitin cefficients prvided by Sblev and Shimizu, with values fr Cr and V frm Hart and Dunn (Hart and Dunn, 99; Sblev and Shimizu, 99). As fund fr ther abyssal peridtites (Jhnsn et al., 990), calculated liquid cmpsitins in equilibrium with Hess Deep harzburgites are ultra-depleted cmpared t MOB and, except fr the absence f the characteristically cnvex upward shape, are similar t the ultra-depleted melt inclusins reprted by Sblev and Shimizu (Sblev and Shimizu, 99). These authrs suggest that the ultra-depleted inclusins wuld be in equilibrium with the mantle at lw pressures (-0 kb), and mdel them as an instantaneus fractinal melt prduced at % melting with a retained melt fractin f. wt%. The analyzed clinpyrxene cmpsitins we h used fr ur calculatins are thse f the hst dipside after any subslidus rthpyrxene exslutin, nt actual cmpsitins at magmatic temperatures. Althugh we h nt seen much evidence fr extensive exslutin in the dipsides in these peridtites, any exslutin f enstatite with cling wuld likely increase the EE abundances in the hst dipside. This wuld make the actual liquids that were last in equilibrium with these harzburgites even mre refractry than thse we calculate. The depleted cmpsitin f the Hess Deep harzburgite tectnites, then, are cnsistent with ther abyssal peridtites in seeming t require near-fractinal melting f the mantle beneath bth fastand slw-spreading ridges. Thus, the Hess Deep harzburgites represent the end-pint f a near-fractinal melting sequence, whereas the spatially assciated basalts represent the aggregatin f many melt fractins frm the sequence presumably smewhere in the underlying mantle melting clumn. It is difficult t assess the prprtin f ultra-depleted melts, representing the later stages f sequential fractinal fusin f the upwelling mantle, which need t be added int the equatin fr the bulk cmpsitin f the cean crust. Such melts, generated in the upper levels f the melting clumn might well escape the principal zne f melt aggregatin and perclate directly t the base f the crust. Sblev and Shimizu (99) reprt such refractry melt inclusins in livine phencrysts in MOB, demnstrating that such liquids intrude the crust. Hwever, numerical mdeling f aggregate melts by Jhnsn et al. (990) des shw that rdinary MOB may incrprate much f this melt fractin. Petrgraphically, there is evidence that the residual harzburgites at Hess Deep cntain sme cmpnent f intergranular clinpyrxene precipitated frm such melts trapped in the mantle in the zne f cnductive cling beneath the crust. The amunt f such pyrxene is small but significant, prbably in the range f %. This wuld crrespnd t perhaps as much as % trapped ultra-depleted melt, back-reacted with livine t frm clinpyrxene and enstatite (e.g., Kelemen et al., 99). It is therefre pssible that relatively little f the shallwest, mst extreme fractinal melts escape frm the mantle, but remain trapped in the residue by the "lithspheric" r cnductive lid at the tp f the mantle melting clumn. Where such melts d escape t the crust, perhaps due t lcal fracturing in the shallw mantle, they culd accunt fr wehrlitic intrusins in the lwer crust, because clinpyrxene with livine is likely t precede Plagiclase n their liquidus at lw pressure. A bimdal distributin f ultra-depleted and MOB primary liquids intruded t the crust is cnsistent with mst melting mdels. The incmpatible-element-rich early melt fractins required t make MOB are generated deep within the mantle-melting clumn, and are mst likely t be affected by any mechanism fcusing melt flw in the mantle. Later melt fractins, generated at mderate and high levels, wuld h highly depleted cmpsitins quite distinct frm MOB. Such melts wuld likely nly preserve their depleted character, hwever, where they escaped the principal zne f melt aggregatin where incmpatible-element-rich melt fractins flwing frm greater depth are cncentrated. DUNITES AND GABBOIC SEGEGATIONS Petrgraphy The dunite, ver large intervals, is cmpsed entirely f serpentinized livine with varying amunts f Cr-spinel, ranging frm nne t several percent. Dunite is generally mre altered than harzburgite, cnsistent with the verall increase in serpentinizatin with riginal livine cntent seen in the harzburgites (Table ). Frequently livine is cmpletely replaced, thugh many samples cntain relict livine and spinel. Textures vary, but mst cmmnly are anhedral granular. Kink bands and evidence f plastic defrmatin are cmmn. Spinel is typically euhedral r subhedral. Lcally, ne small centimeterscale pdifrm spinel segregatin was fund. Euhedral, hlly-leaf, and vermifrm spinel ccur, ften tgether, in the dunites. Euhedral spinel is ften taken t indicate crystallizatin at high melt percentages, whereas vermifrm and hlly-leaf spinel may represent crystallizatin and vergrwth f spinel during late-stage melt perclatin thrugh a nearly slid dunite. Tw kinds f trctlites are present (Fig. ). The first cntains % t 8% Plagiclase and appears t be an autclastic igneus breccia cnsisting f disaggregated plycrystalline dunite clasts impregnated by Plagiclase and minr intergranular clinpyrxene (Fig. C). The cntacts f these trctlites with dunite may be either gradatinal (Fig. A) r sharp (Fig. F). Lcally the dunite clasts may assume a near idimrphic texture, evidently reflecting ctectic crystallizatin f livine with the Plagiclase accmpanied by disslutin f the angular dunite fragments at their crners (Fig. F). Similar textures h been reprted frmed in peridtites disaggregated during partial melting experiments (Budier and Niclas, 9). There is an abrupt transitin frm trctlites frmed by dunite disaggregatin and impregnatin t the secnd kind f trctlite thse with gabbric prprtins f Plagiclase t livine (Fig. B, E). Gabbric prprtins f Plagiclase relative t mafic minerals [- 0 Plag/(Plag+Ol)] are thse expected fr ctectic crystallizatin f livine and Plagiclase directly frm a basaltic melt based n the psitin f the natural ctectics defined by MOB glasses. Cnsistent with this, the prprtin f Plagiclase t livine and clinpyrxene are relatively cnstant in mst crustal gabbrs (e.g., Blmer et al., 99). This nearly cnstant prprtin f Plagiclase is due t the lack f curvature f the Plagiclase livine ctectic in the plagiclase-livine-clinpyrxene ternary, its fixed psitin parallel t the dipside-frsterite jin, and its relatively cnstant psitin with pressure (Presnall et al., 99; Presnall et al., 98; Dick and Bryan, 98). This argues that the prprtin f melt lcally exceeded the limit fr a grain-supprted matrix, allwing direct crystallizatin f "gabbric" trctlites within dikes and fractures in the dunites. Many f these trctlitic gabbrs h extreme inequigranular textures and may cntain bvius xenliths f disaggregated wall-rck dunite (Fig. C, D). Lcally gabbric trctlites cut disaggregatin trctlites as dikes parallel t their cntacts with the adjacent dunite (Fig. F). Individual dunite clasts alng these cntacts are cut by the gabbric trctlite, thugh the cntact, n a thin sectin scale, is sutured rather than sharp, suggesting lcal disslutin and reprecipitatin f livine alng it during intrusin (Fig. F). These features demnstrate that many f the dunites and their gabbric segregatins represent cmpund dikes and flw channels in the peridtite, with mre than ne phase f intrusin and melt flw. Within the trctlites, clinpyrxene may frm selvages r narrw crnas arund livine grains. These may lcally carsen t frm islated, carse, anhedral, subphitic t phitic pyrxene grains (Fig. B). Lcally, such pyrxene may ccur within the trctlites in sufficient quantity t cnstitute pegmatitic patches f livine-gabbr, livine gabbrnrite, and gabbr (Fig. A). Elsewhere, much thicker units f livine-gabbr and livine gabbrnrite are enclsed within the trctlites, apparently frming true gabbr dikes. All these gabbrs h typical anhedral granular and intergranular gabbric textures. Chrmian spinel is a cmmn accessry phase in the trct-

20 HJ.B. DICK, J.H. NATLAND cm Figure. See captin n facing page.

21 LATE-STAGE MELT EVOLUTION AND TANSPOT lites and gabbric segregatins, ranging frm virtually absent t several vlume percent, and is generally euhedral r subhedral. Spinel generally ccurs as small euhedra enclsed in either plagiclase r livine. In Sectin f Cre -89C- (Fig. A), hwever, the trctlite cntains unique carse skeletal spinel intergrwn with Plagiclase. The trctlitic gabbric segregatins in the Site 89 dunites h usually undergne substantial calc-silicate metasmatism (rdingitizatin), but typically cntain cres f fresh Plagiclase and relict livine and pyrxene. elict Plagiclase is ften entirely replaced by chlrite, while prehnite and varius amphibles are als fund as replacements. Olivine is cmmnly replaced by talc and talc-amphible. A characteristic feature f the alteratin is that either nly plagiclase- r nly livine-relicts ccur in mst samples, and samples with relicts f bth are nt cmmn. The gabbric segregatins may be lcally verprinted by hightemperature slid-state defrmatin textures riented bliquely t the cntacts between the gabbrs, dunites, and harzburgites (Gillis, Mével, Allan, et al., 99). Such defrmatin is absent, hwever, in many if nt mst f the segregatins, and it appears that nly minr slidstate flw and defrmatin ccurred after frmatin and emplacement f the gabbric segregatins. Sme fliatins may als reflect late melt flw and synmagmatic defrmatin (Fig. E), rather than subslidus defrmatin. This suggests that the lcus f dunite frmatin was near the end f emplacement f the peridtite at fairly lw pressures clse t the base f the crust. Mineralgy Mmineral analyses f a representative suite f dunites, trctlites, and ther gabbric segregatins are given in Tables thrugh. There is a significant range in livine cmpsitin, with the dunites ranging frm F 9 t F 89, trctlites frm F 89 t F 8, livine gabbrnrites frm F 88 8 t F 8, and a single livine-gabbr with F 8 9. Althugh there is cnsiderable verlap in cmpsitin, a general prgressin in livine cmpsitin tward mre irn-rich cmpsitins is seen with the successive appearance f Plagiclase and then clinpyrxene in the gabbric segregatins. This is cnsistent with an inferred sequence f the successive frmatin f dunite, fllwed by trctlite, livine-gabbr, and then livine-gabbrnrite and gabbr. The single analysis f livine in the dunites with a frsterite cntent (F 9 ) higher than that in the wall-rck harzburgite, is believed t be due t lcal subslidus reequilibratin f spinel and livine. Subslidus reequilibratin, where the lcal prprtin f spinel t livine is high, will shift the cmpsitin f the livine t mre magnesian cmpsitins (Dick and Bullen, 98; Irvine, 9, 9), and it is likely that we analyzed an livine grain sufficiently clse t a spinel grain that its frsterite cntent was raised by reequilibratin. Fr the remainder f the analyses, significant reequilibratin f spinel did ccur with livine, prducing a substantial shift tward mre irn-rich spinel cmpsitins (gethermmetry fr ur livine-spinel pairs shuld give temperatures f arund 800 C based n similar suites). The shift in spinel cmpsitin due t reequilibratin, hwever, is a functin f relative prprtins f spinel and livine (the lever-rule) and the relevant partitin cefficients fr the spinel end-members and livine. Because spinel is an accessry phase, this shift in cmpsitin is relatively unifrm and a direct functin f livine cmpsitin, preserving the riginal igneus trend. Olivine greatly exceeds spinel in vlume (rage mde: 8.% livine and % spinel [in the harzburgites]), s the effect f sub-slidus spinel-livine reequilibratin n livine cmpsitin is negligible. The gabbric segregatins lcally cntain abundant clinpyrxene and mre rarely rthpyrxene. Clinpyrxene is als fund as an intragranular accessry phase in plagiclase-bearing dunites and trctlites. In the pyrxene quadrilateral (Fig. ), the pyrxenes in the gabbric segregatins plt immediately t the right f the harzburgite array. Whereas clinpyrxene in the harzburgites has an rage Mg# [Mg/(Mg+Fe)] f 9. ±, it is significantly less magnesian in the dunites and gabbric segregatins, raging 9 ±.0 and extending t values as lw as Mg# 88. in ne gabbr. Althugh pyrxenes in the gabbric segregatins lie clse t the harzburgite array in the pyrxene quadrilateral, they are cmpsitinally very different with respect t incmpatible elements. Ntably they cntain t rders f magnitude mre titanium and sdium. Alumina cntents range upward frm that f pyrxene in the harzburgites t twice as much (. t. wt%). It is clear, therefre, that the melts with which these pyrxenes last equilibrated were t evlved t h been in equilibrium with the harzburgite. Chrmian spinel is a ubiquitus accessry in the dunites and gabbric segregatins. It is pltted with the spinels frm the harzburgites n the Cr-Al-Mg-Fe + face f the spinel cmpsitin prism in Figure Figure (facing page). epresentative trctlitic segregatins frm Site 89. A -cm scale bar is shwn at the lwer right f each pht. A, C, and F are riented fragments with tp tward the bttm f A, and t the left fr C and F. A. Sample -89C--, 9- cm, back sides f archive and wrking halves f the cre (t the left and right, respectively), shwing an irregular subvertical gabbric intrusin lcated between 08 and 0 cm. The segregatin grades tward the lwer left and upper right in the pht int trctlite, then Plagiclase dunite, and lcally dunite at its margins. Nte that the segregatin als grades internally frm a carse breccia f trctlite cnsisting f dunite clasts cemented by Plagiclase n the backside f the archive half, t a cm livine gabbr lens n the backside f the wrking half f the cre. B. Sample -89E--, Piece, 0- cm, (wrking half) shwing a small carse-grained granular gabbr vein with irregular sutured margins in serpentinized dunite. The gabbr is heavily altered, with Plagiclase cmpletely replaced by secndary hydrthermal minerals. It cntains carse granular clinpyrxene, ccasinally enclsing rund livine chadacrysts, intergrwn with carse-grained Plagiclase (pseudmrphs) and livine. Lcally Plagiclase enclses subhedral chrme spinel. The dunite is 98% serpentine after livine, cntains abut % hlly leaf t subhedral chrme spinel and % relict livine. N clinpyrxene relicts were bserved in the dunite. C. Sample ---, Piece 8, - cm (wrking half)- Disaggregated dunite with irregular ply-minerallic dunite clasts cemented by Plagiclase. Sample grades frm lwer right frm an autclastic breccia f dunite cemented by Plagiclase t the upper left where it is cmpsed largely f medium-grained trctlite with gabbric prprtins f Plagiclase that lcally enclses several very carse angular dunite clasts. D. Sample --8-, Piece, - cm (archive half). Hetergeneus trctlite with Plagiclase cntent ranging frm gabbric prprtins (apprximately 0 vl%) n the right, t areas with disaggregated dunite clasts at lwer left where dunite clasts make up clse t 8% f the sample. Clinpyrxene is present in several large intergranular grains, but mstly as crnas r selvages between Plagiclase and livine (Gillis, Mével, Allan, et al., 99). E. Sample -89E--, Piece 9, - cm (archive half). Trctlite with gabbric prprtins f Plagiclase and livine shwing a mderate fliatin f pssible late-magmatic rigin (Gillis, Mével, Allan, et al., 99). F. Sample --8-, Piece 0, - cm (archive half). Cmpund dike cnsisting f a thin selvage f medium-grained granular dunite n the left in sharp cntact with carse-grained trctlite cut in turn by medium-grained trctlite. The carse-grained trctlite cnsists f carse angular clasts f medium-grained granular dunite enclsed in a lesser amunt f carse-grained intergranular Plagiclase. The cntact between the medium and carse trctlites, while sutured n a thin sectin scale, macrscpically cuts acrss individual dunite clasts and is sharp in hand specimen. The medium-grained trctlite has medium- t carse-grained pikilitic t subphitic granular Plagiclase enclsing numerus medium- t fine-grained rund livine chadacrysts. The livine is frequently rimmed by a thin selvage r crna f clinpyrxene, suggesting a late reactin-relatin between livine and melt. The carse trctlite is interpreted as a disaggregated dunite impregnated by cumulus plagiclase, clinpyrxene, and livine.

22 H.J.B. DICK, J.H. NATLAND 8, and presented in an expanded prjectin in Figure. The spinels in the dunites and gabbric segregatins define a trend extending upward frm the harzburgite array tward mre irn-rich and smewhat mre chrmian cmpsitins. This trend is expected fr fractinatin f livine and Plagiclase as crystallizatin f Plagiclase increases melt Cr# even as cprecipitatin f livine rapidly reduces melt Mg/ (Mg+Fe) (Dick and Bullen, 98). By cntrast, early precipitatin f clinpyrxene, which cntains substantial Cr, wuld cause the trend t drp tward less chrmian cmpsitins with increasing irn cntent. We nte that while subslidus reequilibratin shifts the cmpsitin f accessry spinel t mre irn-rich cmpsitins, this cannt accunt fr the variatins in Cr and Al, which must relate t igneus petrgenesis. eequilibratin with clinpyrxene will affect slightly the partitining f Cr and Al, shifting the cmpsitin f the c-existing spinel t higher chrme cntents (Kmr et al., 98a). Where there are large mdal variatins in the prprtin f pyrxene, this can prduce a cnsiderable spread in spinel Cr#. In ur samples, hwever, reequilibratin with clinpyrxene bviusly cannt accunt fr the systematic trend tward higher chrme cntents in the dunites and trctlites that cntain little r n dipside. Interestingly, the livine-gabbrs, which d cntain significant dipside, and therefre ptentially culd h higher spinel Cr# due t reequilibratin, define a separate field shifted tward lwer Cr and Mg. This drp in spinel Cr#, then, likely reflects three-phase saturatin and precipitatin f chrmian dipside. Similarly, the large spread in spinel Mg# requires large variatins in the relative prprtins f silicates and xides in the varius samples t accunt fr by reequilibratin (Dick and Bullen, 98; Irvine, 9, 9). Tgether with the systematically lwer frsterite cntent f the cexisting livine, hwever, this variatin is easily explained by crystal-fractinatin. Shwn in Figure is Cr number (Cr/Cr+Al) pltted against the weight percent TiO in spinel. It is nce again clear frm this plt that the melt frm which the spinels precipitated was nt in equilibrium with spinel in the harzburgite which cnsistently has far lwer TiO. High TiO in spinel is assciated with the crystallizatin f trapped melt in abyssal peridtites, and is a characteristic feature f Cr spinel in dunites and trctlites in bth philitic and ceanic peridtites and dunites (Dick, 9b; Hebert et al, 98; Dick and Bullen, 98; Girardeau and Francheteau, 99; Cannat et al, 990). The enrichment and extreme variatin f TiO in the gabbric segregatins and dunites is quite ntable, particularly when it is examined in a plt against livine cmpsitin (Fig. ), where there is a six-fld increase in spinel TiO ver nly a % t % change in livine frsterite cntent. This variatin cannt be accunted fr by reequilibratin, as the dunites cntain n titaniferrus phase ther than spinel. It must, therefre, reflect large variatins in the cmpsitin f the liquids migrating thrugh the dunites. Plagiclase in the gabbric segregatins has a bimdal cmpsitin distributin (Fig. ) with a peak in abundance, cnfined t livine gabbrs and gabbrs, at arund An 9, and a secnd higher peak, represented by all rck types at An 8. Individual spt analyses in the first grup, which we term ultra-calcic, extends frm An 99 up t An 98, and in the secnd grup, which is merely calcic, frm An 88 dwn t An. Shwn in Figure are Plagiclase cre and rim cmpsitins. Substantial zning exists, with tw f the ultra-calcic anrthites exhibiting slight reverse zning tward mre calcic rims, while three are weakly and tw strngly nrmally zned with mre sdic rims. The mre "sdic" calcic ppulatin als shws bth nrmal and reverse zning. Overall, reverse zning is smewhat mre cmmn than nrmal zning. The Plagiclase in the Site 89 gabbric segregatins is significantly mre calcic verall than the Plagiclase fund in the Hle 89G gabbrs, thugh there are sme very calcic Plagiclase xencrysts in the Hle 89G gabbrs. This is clearly illustrated by gabbrnrite Sample -89G-9-, 8-88 cm, which cntains bth a high calcium Plagiclase suite, and a mre sdic suite lying clse t CJ U öj 0 - O Dunite + Gabbr ffi Olivine gabbr Trctlite Φ Trctlitic dunite A Plagiclase dunite '+ m e fi fi ij i O P i,i 0 0 Spinel Mg*00/(Mg+Fe) Plag-OI fractinatin mm O Ol-Plag.-Cpx fractinatin Figure. Expanded plt f spinel cmpsitins in the Cr-Al-Mg-Fe face f the spinel cmpsitin prism. The Mg-Fe axis is distrted (lengthened) t allw better discriminatin f the cmpsitin fields fr different types f gabbric segregatin. General trends fr spinel cmpsitins fr prgressive partial melting (dark shaded arrw) and fr plagiclase-livine and livineplagiclase-clinpyrxene fractinatin are shwn frm Dick and Bullen (98). 0.ε U - 0 _ OQ β O O Q 0 ffl rp ffl Δ Δ P Dunite.. TiO in Spinel (wt.%) Trct. dunite Trctlite Olivine gabbr Plagiclase dunite Figure. Weight percent TiO pltted against the mlecular Cr/(Cr+Al) (Cr#) fr spinel in Site 89 harzburgites, dunites, and gabbric segregatins. the maxima in Plagiclase abundance fr Hle 89G (Fig. ). Natland and Dick (this vlume) pstulate that the high-calcium plagiclase suite frm Hle 89G represents xencrysts crystallized in the shallw mantle r lwer crust, arguing that this represents intrusin f relatively unmdified mantle melts t high levels in the plutnic sequence directly beneath the melt lens. The mre sdic main plagiclase suite at Hle 89G, they believe, derives by crystallizatin frm liquids representing pst-cumulus migratin f highly fractinated (ferrbasaltic t ferrandesitic) melts thrugh an pen crystal netwrk. The mre calcic Plagiclase xencrysts were preserved due t incmplete reequilibratin prir t sealing f the cumulate by adcumulate crystal grwth. In Figure, we shw that the anrthite cntent f the calcic plagiclase suite frm Site 89 plts alng a reasnable extensin f the field fr the Hle 89G high-level gabbrs. The Site 89 calcic suite als lies within the cmpsitin range fr depleted MOBs similar t thse frm Hle 0B (Kemptn et al., 98; Natland et al., 98). The majrity f MOB glasses are characterized by Ca/(Ca+Na) values frm t 8 crrespnding t equilibrium feldspar cmpsitins ranging frm An 0 t An 8, thugh MOBs as calcic as Ca/ (Ca+Na) = 8 d ccur (Panjasawatwng et al., 99). Since the mre calcic Hess Deep basalt glasses pltted in Figure prject back t Ca/(Ca+Na) greater than 80 (Fig. IB), we cnclude that the i i _

23 LATE-STAGE MELT EVOLUTION AND TANSPOT. - Site 89 s, Dunites and Gabbric Segregatins Nrth Slpe D Tnalite Oxide gabbrnrite Gabbrnrite L Q I 0 S ID I $ * Frsterite O Dunite O Trctlite * Olivine gabbrnrite + Olivine gabbr cntent 8 8 Figure. Weight percent TiO pltted against the frsterite cntent f cexisting livine. Shwn fr cmparisn are the apprximate trends fr meltrck reactin where Mg/(Mg+Fe) is buffered by the mantle, and fr clsedsystem fractinal crystallizatin withut interactin with the mantle. liquids that crystallized the Site 89 calcic Plagiclase suite may als be reasnable parent magmas fr the Hle 89G gabbrs. The cmpsitin range f the ultra-calcic Plagiclase suite, hwever, lies entirely utside the knwn range f equilibrium cmpsitins fr erupted MOB and wuld seem t calcic t h crystallized frm a reasnable parent magma fr mst f the Hle 89G gabbrs. Based largely n textural evidence, and the presence f ultra-calcic Plagiclase xencrysts in sme Hle 89G gabbrs (Natland and Dick, this vlume), we prefer the hypthesis that the ultracalcic Plagiclase is igneus, nt metamrphic. It shuld be nted, hwever, that given the extensive alteratin it is pssible that the ultra-calcic Plagiclase culd represent cryptic metasmatic effects related t rdingitizatin f the gabbric segregatins during serpentinizatin. Igneus Plagiclase this calcic has previusly been reprted nly in high-calcium bninite lavas frm island arcs (Falln and Crawfrd, 99). The petrgenesis f high-calcium bninite lavas dredged frm the nrth Tnga ridge (Falln and Crawfrd, 99) and frm sme philitic bninites and assciated gabbrs frm the Trds philite (Duncan and Green, 98; Malpas et al., 989; Thy et al., 989). The bimdal ppulatin, and the presence f bth nrmal and reverse zning argues fr a cmplex crystallizatin histry invlving mixing f a highly refractry calcium- and alumina-rich melt with less calcic and aluminus primitive MOB. Gechemistry Clinpyrxenes in the gabbric segregatins frm Site 89 h incmpatible trace element cncentratins that verlap thse frm the Hle 89G gabbrs, despite the mre evlved majr element cmpsitin f the latter, and exhibit bth higher and lwer cncentratins (Fig. 9). Als shwn fr cmparisn are the fields fr phencryst and grundmass clinpyrxene in Hle 0B diabase. The grundmass 0B clinpyrxenes prvide values apprpriate fr in situ crystallized melt at lw pressures where mst f the titanium is cntained in ilmenite and ther late magmatic phases nt fund in the Site 89 gabbric segregatins. Ntably, several Site 89 trctlites and trctlitic gabbrs extend t values eight times that f the 0B grundmass. Such high values indicate that, despite the lack f late magmatic phases, these segregatins mst likely represent hybrid rcks cmpsed f disaggregated dunite impregnated by crystallized trapped melt. CD E " 0 - " 0-0 Hle 89G D Gabbr Gabbrnrite n Olivine gabbrnrite Z Olivine gabbr Gabbrnrite sample 89G- 9-, 8-88 cm D Gabbr Gabbrnrite Olivine Gabbrnrite E D Olivine Gabbr Trctlite Anrthite (%) Site Figure. Histgram f Plagiclase cmpsitins frm the Nrth Wall (slpe) f Hess Deep and Site 89 and 89 gabbrs and gabbric segregatins. Data fr the Nrth Wall (slpe) and fr Hle 89G are frm Natland and Dick (this vlume). The plt represents the unraged cmpsitins frm the data used t cmpute the rage cmpsitins reprted here and in Natland and Dick (this vlume). The large range in Zr/Ti ratis drps tward values within the cmpsitin field f the abyssal harzburgite array f Jhnsn et al. (990). This spread in cmpsitin argues fr a range in melt cmpsitins. This range culd be explained by melts derived by varying degrees f melting in the underlying mantle clumn and varying degrees f aggregatin. Mre likely, the large variatin in Figure 9 represents reactin with mantle peridtite. This is strngly supprted by the EE patterns pltted fr the same rcks in Figure 8. Prgressive fractinal melting prduces sequential melts with a range f slpes in such EE-plts (Jhnsn et al., 990), yet the Site 89 gabbric segregatins h nearly unifrm patterns, indicating derivatin frm a single primary melt. Based n rare earth element cncentratins, there are tw ppulatins f gabbric segregatins, labeled C- and L-type in Figure 8. The calculated liquid cmpsitins in equilibrium with the C-type gabbrs are similar t nrmal MOB and verlap the mre depleted end f the field fr EP basalts dredged t the west f Hess Deep (Fig. 9A). The secnd ppulatin has higher EE cncentratins relative t the first, but with the same slpe and shape except fr a prnunced Eu-anmaly indicating significant Plagiclase fractinatin. It is entirely verlapped by the field fr the Hle 89G gabbrs (Fig. 9B), which is cnsistent with their derivatin frm the same magma by similar degrees f crystallizatin. In the case f the Site

24 H.J.B. DICK, J.H. NATLAND Gabbr» v Gabbrnrite Olivine Gabbr Plagiclase Dunite * Trctlite Hle 89C β Δ Δ i i i a - a D D A Δ A A D A Hle i Δ 0 Number A Δ a W 9 " D Δ Δ A A Hle 89E 0 O ü 00 <& 0 B 0. i r Site 89 C-type clinpyrxenes Ce Nd * Gabbr Olivine Gabbr Olivine Gabbrnrite EP - N basalts Figure. Anrthite cntents f rims (pen symbls) and cres (slid symbls) f individual Plagiclase grains fr Site 89 gabbrs and trctlites pltted sequentially by analysis number t highlight the zning patterns in the tw ppulatins represented in Figure. ü O ü - i A L-type i -y B ^ * * C - type i i i i i \ \ Site 89 gabbric intrusins O Hle 89C D Hle Hle 89E i i Ce Nd Sm Gd Dy Er Yb Plagiclase dunite & trctlites (L-type) Ce Nd Sm Gd Dy Er Yb Figure 8. A. EE patterns fr all Site 89 gabbric segregatins analyzed nrmalized t chndrites using the values f Anders and Grevesse (989). B. EE patterns fr dipsides in the gabbrnrite and trctlite segregatin and the wall-rck harzburgite frm Hle 89C anntated with the cmpsitin f cexisting Plagiclase and livine. L-type refers t gabbric segregatins that h EE patterns and mineralgy suggestive f the crystallizatin f trapped melt. C-type refers t gabbric segregatins believed t reflect sme frm f cumulus crystallizatin frm a liquid. 89 gabbrs, hwever, the very frsteritic livine, and very calcic Plagiclase indicates that the cmpsitin f the melt was buffered during crystallizatin by melt-rck reactin. Clinpyrxene in tw livine gabbrs and ne gabbr cntaining ultra-calcic Plagiclase were analyzed fr EE, and all cntained C- type EE patterns. In cntrast, clinpyrxene in segregatins cntaining mre sdic Plagiclase included bth L- and C-type patterns. Thus, the bimdal distributin f Plagiclase cmpsitins appears ò 0 Site 89 L-Type Clinpyrxenes TV Trctlitic gabbrnrite A Trctlite [~l 89G gabbrs Ce Nd Sm Gd Dy Er Yb Figure 9. A. EE patterns fr dipsides in C-type gabbric segregatins (slid symbls) and the cmpsitin f calculated c-existing liquids (pen symbls). Field fr East Pacific ise basalts is frm Lnale et al. (99). B. EE patterns f dipsides in L-type gabbric segregatins (slid symbls) with the calculated equilibrium liquid cmpsitins (pen symbls). Shwn fr cmparisn is the field fr Hle 89G gabbrs (Dick, unpubl. data). unrelated t the phenmena giving rise t the L- and C-type EE patterns. Shwn in Figure A is a small planar zned gabbric intrusive vein in harzburgite tectnite that displays the entire range f EE cncentratins seen in the gabbric segregatins at Site 89. It cnsists f patchy livine gabbrnrite segregatins at the center, enclsed in clinpyrxene-bearing trctlite, which in turn grades utward int Plagiclase dunite. The feature is riented near-vertically, is t cm thick, and extends several meters up a near cntinuus sectin f cre frm Hle 89C, thugh it lcally tapers ut. This is als the trctlite cntaining the cmplex carse skeletal spinels intergrwn with Plagiclase. The preservatin f such textures argues fr relatively rapid frmatin f this small intrusin. Mst f the feature cnsists f disaggregated dunite impregnated with Plagiclase and minr clinpyrxene. The intrusin is reasnably interpreted as being clse t the tip f a fracture-cntrlled intrusive dike in the harzburgite. The perfectly preserved primary igneus textures demnstrate that intrusin fllwed emplacement f the peridtite t near the base f the crust, as plastic defrmatin accmpanying further mantle flw wuld h destryed the delicate spinel textures. Shwn in Figure 8B are the EE patterns f dipside in the livine gabbrnrite at the center f the intrusin, several patterns frm the trctlites and Plagiclase dunite and an extremely depleted dipside pattern frm the verlying wall-rck harzburgite. The livine gabbrnrite has the lwest EE cncentratins in the intrusin and n eurpium anmaly (C-type segregatin). The trctlites and ne Plagiclase dunite shw the same pattern, but with a prnunced eurpium anmaly and a five-fld enrichment in EE (L-type). At the same time the C-type livine gabbrnrite has mre irn-rich livine. The intrusin des nt cntain any f the ultra-high-calcium plagi-

25 LATE-STAGE MELT EVOLUTION AND TANSPOT clase, thugh, despite the enrichment in incmpatible trace elements, Plagiclase is nly very slightly less calcic in the Plagiclase dunite than that in the gabbrnrite. Ordinarily, a five-fld enrichment in highly incmpatible elements is assciated with abut 80% crystallizatin f a MOB melt. The behavir f the majr elements in the gabbric vein cntrasts sharply t that f the incmpatible elements, and is cntrary t that expected fr nrmal fractinal crystallizatin where Mg/(Mg+Fe) and Ca/(Ca+Na) shuld drp significantly. Where a melt crystallizes thrugh reactin with the mantle, hwever, as its mass shrinks, the melt becmes mre magnesian due t bth assimilatin f enstatite frm the harzburgite, and reequilibratin with residual livine and pyrxene in the matrix. At the same time, as it fractinates, nly livine, Plagiclase, and minr clinpyrxene, the incmpatible elements are strngly enriched in the melt. Calcium additin frm assimilated enstatite and dipside, as well as melt reequilibratin with the remaining residual mantle mineralgy ffsets the effects f plagiclase fractinatin n melt Ca/Na, causing it t remain nearly cnstant in the case f the gabbric vein. DISCUSSION OF THE GABBOIC SEGEGATIONS The Site 89 sectin bears a clse resemblance t the stratigraphy in philite cmplexes immediately beneath the "petrlgic" Mh (e.g., Budier and Cleman, 98; Ceuleneer and Niclas, 98; Niclas, 989; Niclas et al., 988; Niclas and Vilette, 98; Quick, 98a; amp, 9; SFyeva et al, 980). These latter rcks crrespnd t znes in the philite mantle sectins where harzburgites are crsscut by dunite pds, which ften cntain melt segregatins and impregnatins at their cres. Dunite and gabbric impregnatins in dunite ccurring with harzburgite tectnites appear t be widely distributed acrss the intra-rift high based n the dive and dredge results f Francheteau et al. (990), and h been previusly interpreted as representing the mantle transitin zne immediately underlying the crust (Hekinian et al., 99). The lack f in situ stratigraphy, hwever, has limited detailed interpretatin f the dredge and submersible grab samples as the exact relatinship f ne rck t anther can nly be dne by inference based n the stratigraphy f philites. Hekinian et al. (99) als reprt an extensive suite f primitive gabbrs in their dredge and dive cllectins that they interpret as crrespnding t the layered gabbrs seen in the lwer prtins f the Oman philite plutnic sequence. Many f these mineralgically fall in a cmpsitinal gap between the Site 89 gabbric segregatins and the Hle 89G high-level gabbrs. Sme f the samples they include in this suite, hwever, resemble the gabbric segregatins drilled at Site 89, and may h been inadvertently placed in the wrng part f the crust mantle sequence. Origin f the Dunites The rigin f dunites crsscutting mantle harzburgite tectnites is nw fairly well understd. Thermdynamically, a melt frmed at high pressure must shift its cmpsitin t remain in equilibrium with the mantle as it mves upward (O'Hara, 98; Kelemen et al., 990; Kushir, 9; Kushir, 9). It can d this by fractinating livine, and by disslving pyrxene frm the mantle as it ascends. If melt flw thrugh the mantle is sufficiently rapid, r sufficiently fcused, then the melt may fall ut f equilibrium with the fur-phase mantle assemblage. In this case it will incngruently disslve enstatite and dipside, leaving nly an livine residue behind. The "pdifrm dunites" fund in the mantle sectins f many philites are widely believed t be the prducts f such melt-rck reactin between mantle and ascending magmas (Dick, 9a; Kelemen, 98; Kelemen et al., 990; Quick, 98a). This disequilibrium prcess is ptentially a majr mdifier f melt chemistry and may cntradict many cmmn assumptins abut MOB petrgenesis if it is hw the Hess Deep dunites frmed. Sme f the dunite, hwever, may represent simple fractinal crystallizatin f livine frm an ascending melt (e.g., O'Hara, 98) in a permeable flw cnduit where the mantle is already stripped f pyrxene. The inferred lithlgic sequence at Site 89, with harzburgite enclsing dunite, which in turn enclses trctlite, which then may enclse livine gabbr and gabbrnrite (Fig. ) prvides direct chemical and textural evidence that the melts frm which the gabbrs crystallized were nt in equilibrium with the wall rck. This supprts frmatin f the dunites by melt-rck reactin during migratin f melts frm higher pressures thrugh the shallw mantle. In nly ne case did we find a gabbric segregatin directly in cntact with harzburgite. In all ther cases, where the relatinships were preserved, gabbr was in cntact with dunite. This is als lcally the case in the Oman philite where melt extractin and hydrfracture are pstulated t h ccurred in the shallw mantle sectin (Ceuleneer and abinwicz, 99; Niclas, 990). In such areas, trctlites with virtually identical textures t thse shwn in Figure are abundant in dunite pds and dikes cutting the harzburgite. Elsewhere in the Oman mantle sectin, gabbric dikes are fund directly in cntact with the harzburgite. These gabbrs are believed t represent higher level melt transprt thrugh the mantle near the base f the crust when the sectin was cler than in the zne f melt aggregatin and segregatin (Ceuleneer and abinwicz, 99; Niclas, 990). This is als the likely rigin fr the ne Hess Deep gabbric segregatin fund in cntact with harzburgite. Sequence and Depth f Crystallizatin f the Gabbric Segregatins Frm the textural and chemical prgressin crrespnding t the Site 89 lithstratigraphy we cnclude that the sequence f crystallizatin in these melts is livine, livine-spinel, livine-spinel-plagiclase, livine-spinel-plagiclase-clinpyrxene, livine spinel-plagiclase-clinpyrxene-rthpyrxene, and finally in the extreme Plagiclase clinpyrxene. A similar sequence, with Plagiclase preceding livine, was previusly reprted fr gabbrs dredged with peridtites and dunites at the Garret Fracture Zne (Hebert et al., 98): a sequence that has lately als been attributed t the crustmantle transitin zne beneath the EP (Cannat et al., 990). This is the same sequence as seen in abyssal thleiites (Bryan and Grve, 98; Miyashir et al., 99; Miyashir et al., 90), and wuld be expected fr the crystallizatin f MOB in the shallw mantle. At high pressure, hwever, r fr extremely depleted melt cmpsitins, such as high magnesian andesite r bninite, clinpyrxene wuld be expected t precede Plagiclase n the liquidus (Elthn, 99). Petrgenetically, then, the rder f appearance f livine, plagiclase and clinpyrxene is imprtant t the debate ver the depth f segregatin f MOB. Thus, particular significance has been attached t the appearance f wehrlites in many philite cmplexes as an indicatr f high pressure crystallizatin fr MOB (Casey et al., 98; Elthn, 989; Elthn and Scarfe, 98). Alternatively, the presence f such primitive livine and clinpyrxene cumulates has been taken t indicate the melt frm which they crystallized was nt a MOB, but an arc-related magma. We h fund n such wehrlites in any f the gabbric segregatins we h examined: a rck where clinpyrxene is an early crystallizing (cumulus) phase with livine preceding the appearance f Plagiclase. Hekinian et al. (99) refer t wehrlitic segregatins and impregnatins in their Hess Deep dredge and dive samples. The term wehrlite, hwever, used in the cntext f the Hess Deep samples is smewhat misleading in light f the preceding debate. Nne f the Hess Deep rcks appear t equate t the wehrlitic sequences seen in many philites such as Oman and Trds where clinpyrxene ccurs as an early cumulus phase with livine in massive wehrlite utcrps. Althugh clinpyrxene ccurs lcally in the Hess Deep dunites in the absence f Plagiclase, it is vlumetrically minr, and can be ex-

26 H.J.B. DICK, J.H. NATLAND plained as the result f late-stage crystallizatin f an intergranular highly reacted MOB magma (Girardeau and Francheteau, 99; Hekinian et al., 99). If these clinpyrxene-bearing "wehrlitic" rcks h indeed crystallized at high pressures, it is then hard t explain why they are nt extensively recrystallized due t plastic defrmatin accmpanying extensive mantle flw t the base f the crust. The Origin f the Bimdal EE Patterns: Evidence fr Trapped Melt A striking feature f the EE patterns f the Site 89 gabbric segregatins is their bimdal distributin. We cnsider tw scenaris t explain this feature: (A) The intrusin represents the fllwing sequence: () initial intrusin f a MOB-like liquid near the tip f a fracture system int harzburgite immediately belw the Mh at a temperature significantly belw the liquidus, but abve the slidus f the basalt; () reactin with the harzburgite, disslving enstatite and dipside t frm dunite, lcally disaggregating the peridtite at the center f the fracture; () initial crystallizatin f livine and skeletal spinel ccurred due t slight undercling f the melt; () a rapid, near simultaneus appearance f Plagiclase and clinpyrxene n the liquidus with spinel and livine as the liquid cled, prducing a C-type livine gabbr cumulate at the center f the vein; () migratin f the crystallizing liquid ut int the dunite as it fractinated, precipitating clinpyrxene reflecting clse t 80% reductin f the liquid mass, and therefre a five-fld enrichment in incmpatible elements, but with liquidus cmpsitins f livine, Plagiclase, and pyrxene buffered by melt-mantle reactin t high-mg and high-ca cmpsitins. (B) The abve scenari, hwever, des nt explain the striking bimdal cmpsitin f the EEs in the gabbric segregatins. As the bimdality is fund fr all the gabbric segregatins, including segregatins frm hles 0 m apart, we suggest that the type-l patterns represent in situ crystallizatin f trapped melt, and the type-c represent cumulus crystallizatin frm primitive MOB passing up thrugh the pen fracture. The sequence f events we envisin fr this scenari is: () At the initiatin f the fracture, dilatin at the fracture tip pens up the harzburgite matrix, allwing melt t invade it and disslve pyrxene by reactin, prducing dunite. () As dike prpagatin prceeds, melt in the matrix is gradually expelled back int the dike, mixing with fresh MOB mving thrugh the dike. Lcally, the melt may nt be fully expelled frm the matrix, and the trapped melt may underg extensive in situ crystallizatin t frm L- type gabbric segregatins. Such in situ crystallizatin f trapped melt prduces a gabbric assemblage f livine, Plagiclase, and clinpyrxene that impregnates the residual dunite. A characteristic feature f this assemblage is that it has the same EE pattern as the Hle 89G high-level gabbrs, because bth represent extensive in situ ctectic crystallizatin f MOB. The five-fld enrichment f EE in the clinpyrxene reflects the percentage f clinpyrxene that crystallized frm the melt relative t Plagiclase and livine, which much mre efficiently exclude these elements (rughly 0% clinpyrxene, 0% livine, and 0% Plagiclase assuming ctectic crystallizatin). The large Euanmaly reflects uptake f Eu by the Plagiclase in the impregnating assemblage. The Mg-rich livine and calcic Plagiclase, relative t the Hle 89G assemblage, n the ther hand, reflects the extensive reactin f the melt with the mantle assemblage and buffering f melt Mg and Ca during crystallizatin. It culd be, as fr the Hle 89G gabbrs (Natland and Dick, this vlume), that the impregnating assemblage may nt represent cmplete liquid crystallizatin, and that sme late liquid may h escaped by infiltratin ut thrugh the peridtite r t the center f the dike. Determining the latter, hwever, requires a full mass balance, with cmplete majr and trace element cmpsitins fr all the phases, and accurate mdal prprtins fr the trctlite, which may nt be pssible t btain given the degree f alteratin f the gabbric segregatins. The high Zr and Ti cntents f the L-type clinpyrxenes, n the ther hand, d make it reasnable that sme f these gabbric segregatins d represent trapped liquid (Fig. 9; Table 8). The slidificatin f trapped melts by reactin in the shallw mantle, as prpsed fr the L-type gabbric segregatins, can simply be thught f as equating t the bulk cmpsitin f trapped melt + peridtite lying belw the mantle slidus. Thus, by reactin, a liquid initially at 0 C that is nt in equilibrium with the mantle, will react, bringing its cmpsitin tward equilibrium with the hst peridtite. As this reactin prceeds, if the bulk cmpsitin f mantle plus melt is belw the slidus, then the liquid will cmpletely crystallize befre it reaches equilibrium with the mantle assemblage and becmes a stable phase. An interesting sidelight t this prcess is that it prceeds essentially isthermally, buffered by the mantle temperature. This likely explains the absence f late-magmatic phases such as apatite, ilmenite, r zircn in the L-type gabbric segregatins, as these generally becme stable n the basalt liquidus at relatively lw temperatures (<l 00 C). Instead, clinpyrxene (and spinel fr titanium) prxies as a hst fr the highly incmpatible elements, and has cncentratins reaching thse fund in sme primitive MOBs (e.g., Fig. 9; Table 8). The Bimdal Cmpsitin f Plagiclase and the Origin f Ultra-calcic Plagiclase Anther striking feature f the Site 89 gabbric segregatins is the bimdal Plagiclase cmpsitins that argue fr the presence f at least tw different liquids. The ultra-calcic Plagiclase fund in a number f the livine gabbrs are at present unique frm a mid-cean ridge envirnment. Similar ultra-calcic Plagiclase, thugh nt extending t quite as high anrthite-cntents (An 9 ), has been widely reprted in MOB (Duncan and Green, 980; Elthn and Scarfe, 98; Panjasawatwng et al., 99; Sintn et al., 99). The anrthite cntent f Plagiclase has been fund t be dependent n three majr factrs, increasing with melt Ca/(Ca+Na), alumina cntent Al/(A+Si) and decreasing pressure ver the range -0 kb (Panjasawatwng et al., 99). Several mdels h been used t explain these xencrysts. One suggests that they are the prducts f high-pressure fractinatin in assciatin with high-mg pyrxene f MOB (Elthn, 98). Anther mdel suggests that they are the prduct f crystallizatin frm refractry melts frmed at the end f the melting prcess at relatively shallw depths (Duncan and Green, 980; Panjasawatwng et al., 99; Sblev and Shimizu, 99). The presence f high-calcium Plagiclase in the Hess Deep Site 89 gabbric segregatins assciated with high-mg dipside, hwever, suggests a third pssibility. They may be the prducts f melt-rck reactin where melts stagnate in melt-transprt cnduit in the shallw mantle. Wall-rck reactin, with disslutin f enstatite frm a highly depleted mantle sectin and precipitatin f livine, essentially adds nly silica, calcium, alumina, and minr titanium t the melt, while at the same time buffering melt Mg/Fe. This reactin will bring Plagiclase, and eventually clinpyrxene, nt the liquidus, as bserved in the Hess Deep gabbric segregatins. Thus, a mnitr f the reactin prgress, in additin t spinel TiO, will be melt alumina and CaO/Na O, which will change dependent n the relative prprtins f assimilant and precipitant during reactin and the cmpsitin f the assimilant and precipitate. Melt-rck reactin can increase melt Ca/Na and alumina where nly livine is fractinated whether melt mass increases r decreases. Simple mass balance (Table 9) shws that melt alumina and Ca/ Na rati increases simply thrugh assimilatin f pyrxene and fractinatin f livine and Plagiclase where the mass prprtin f pyrxene disslved rughly exceeds the mass fractin f Plagiclase precipitated. If the mass fractin f Plagiclase precipitated exceeds that f pyrxene assimilated, Ca/Na shuld drp. By simple disslutin f pyrxene and precipitatin f livine during isbaric crystallizatin f trapped melt in the Hess Deep sectin, hwever, it is very 8

27 LATE-STAGE MELT EVOLUTION AND TANSPOT Table 9. Simple mass balances fr melt-mantle reactin. Melt ALV8,- FAMOUS Enstatite Assimilant Dipside Olivine Precipitant Plagiclase Add 0% Px ppt % 0 Add 0% Px ppt 0% 0 Add 0% Px ppt 0% Ol Add 0% Px ppt % Ol ppt % PI Add 0% Px ppt 8% Ol ppt % PI 0 C High alumina melt inclusin SiO TiO A O FeO MnO MgO CaO Na O K O P O Cr O, Ttal Mg# CaO/Na O Ca/(Ca+Na) Al/(A+Si) na na na na na na na na na na na na na na Ntes: FAMOUS basalt glass is frm Bryan and Mre (9); all ther data cme frm this paper. The relative prprtin f enstatite t dipside in the rage Hess Deep harzburgite is :, which is the rati used fr the assimilant (Px) in the calculatins. The prprtin f Plagiclase t livine (:) used in sme f the calculatins is equal t the plagiclase-livine ctectic prprtins fund fr natural MOB (e.g., Dick and Bryan, 98). Mg# = Mg/(Mg+Fe) using atmic prprtins. Px = pyrxene, Ol = livine, PI = Plagiclase, na = nt analyzed. High alumina melt inclusins in An 9 Plagiclase quenched at 0 C frm a high alumina thleiitic basalt frm the Galapags Platfrm (Sintn et al., 99). unlikely that the liquid wuld reach Ca/Na ratis in equilibrium with the mst calcic igneus Plagiclase we h fund. This reflects the lw prprtin f clinpyrxene in the Hess Deep harzburgites. The simple mass balance in Table 9, hwever, des shw that melt-rck reactin can accunt fr the near cnstant anrthite cntent f plagiclase in the Hle 89C micr-intrusin described earlier (Fig. 8B). We als nte that simple mass balance accunts nly fr the effects f disslutin and precipitatin f phases and excludes reequilibratin with the residual matrix mineralgy. Abut % f the ttal calcium in Site 89 harzburgites is included in livine, and reequilibratin between trapped melt and residual livine alne may h a dramatic effect n melt Ca/Na. Prgressive equilibratin f melt with the residual mantle matrix seems likely t be part f the explanatin fr the highly calcic Plagiclase fund in many f the gabbric segregatins frm Site 89. Sintn et al. (99) h suggested a mdel similar t what we envisage based n high alumina melt inclusins in high calcium plagiclase (An 9 t An 9 ) in Galapags Platfrm thleiites. These authrs nte the pseudinvariant crystal-liquid reactin presented by Kinzler and Grve (99), in which spinel + clinpyrxene + rthpyrxene react t liquid + livine + Plagiclase, and that during such a reactin, bth crystal and liquid cmpsitins are effectively buffered in terms f majr elements by the bulk mineralgy f the mantle while majr and trace elements are free t vary. They envisaged, therefre, the magma bdies underging such a reactin as ccurring as relatively small dikes within a crystal mush regin in the lwer crust r upper mantle. We believe that we h dcumented exactly this scenari in the Site 89 gabbric segregatins. The alternative mdel that the ultra-calcic Plagiclase represents crystallizatin f ultra-depleted liquids prduced at the end f a fractinal melting sequence is undubtedly viable fr such Plagiclase in sme MOBS, given the presence f such liquids in MOB xencrysts (Sblev and Shimizu, 99). In this light it is interesting t nte that the ultracalcic Plagiclase is limited t C-type livine gabbrs and ne gabbr calculated t be in equilibrium with a primitive East Pacific ise MOB frm the regin, nt an ultra-depleted melt. While an ultra-depleted melt has a great depletin in LEE and a very steep EE pattern, melt-rck reactin in the shallw mantle will h little effect n the trace element cmpsitin f MOB as the mantle at this level is essentially stripped f trace elements. In the same light, simple in situ crystallizatin f trapped melt reacting with a relatively small vlume f mantle matrix als des nt appear t prduce apprpriate liquids as evidences by the Hle 89C micrintrusin. We nte that the effect f melt-rck reactin in the mantle is very sensitive t the rati f dipside t enstatite, with melts becming substantially mre calcic by reactin with increasing mantle dipside cntent. The lcal harzburgite at Site 89 is extremely depleted in dipside, which may accunt fr why crystallizatin f the Hle 89C micrintrusin did nt prduce very calcic Plagiclase. Aggregate MOB melts reacting at depth with a less depleted mantle, hwever, are likely t be much mre calcic, and this may accunt fr the magmas prducing the ultra-calcic Plagiclase in the Site 89 gabbric segregatins. It is reasnable that such melts may represent the migratin f late stagnating melt up a dying cnduit at the end f a cycle f hydrfracture-driven melt extractin frm the mantle. eintrusin f new batches f melt up thrugh a cnduit cntaining stagnated melt wuld mix with the remaining basalts. As the vlume f stagnated melt is presumable small, it wuld h little effect n the liquid cmpsitin f the erupted basalt, thugh it may accunt fr much f the vertical scatter f the Hle 89G Plagiclase abut any likely crystallizatin trend fr a single primary melt in Figure. Fractinal Crystallizatin vs. Melt-rck eactin We h fund cnsiderable evidence in the Site 89 gabbric segregatins, and in the petrgraphy f the residual mantle harzburgites fr the effects f cnductive cling beneath the EP n the evlutin f shallw magmas. This ptentially argues fr significant chemical fractinatin f the magmas by simple crystallizatin, as ppsed t the putative effects f melt-rck assimilatin. Cnsiderable insight int this prcess can be gained by cmparing the titanium cntent f spinel t the cmpsitin f the cexisting livine. Fr melt-rck reactin, the Mg# f the melt is effectively buffered by the mantle, while there is a large systematic decrease in liquid Mg# fr fractinal crystallizatin. Titanium, n the ther hand is present in relatively small quantities in the residual mantle and behs as an incmpatible element with nearly linear enrichment thrughut much f the early stages f fractinal crystallizatin. Given the very small prprtin f spinel in the gabbrs and dunites (typically % r less), precipitatin f spinel cannt h had a significant effect n titanium in the melt. Mrever, neither livine r plagiclase takes up titanium. Thus, titanium shuld beh as a nearly incmpatible element in the melt during frmatin f the dunites and trctlites. The large enrichments in titanium seen in Figure, therefre, wuld require 80%-90% clsed system fractinal crystallizatin accmpanied by a very large change in Mg/(Mg+Fe) in livine precipitating with the spinel. Such extensive fractinal crystalli- 9

28 H.J.B. DICK, J.H. NATLAND zatin wuld als nrmally be accmpanied by the appearance f late magmatic phases such as apatite and ilmenite, nne f which is present in the Site 89 gabbric segregatins and dunites. Thus, simple clsed-system fractinal crystallizatin alne cannt accunt fr the trend in spinel and livine cmpsitin bserved in the Site 89 dunites and gabbric segregatins. Large increases in titanium in spinel with little r n change in the cmpsitin f the cexisting livine, hwever, h been identified in dunites dredged with abyssal peridtites and in pdifrm dunite cmplexes crsscutting mantle harzburgites in philites (Dick and Bullen, 98). These enrichments are believed t represent melt-rck reactin between migrating melt and the mantle (Dick and Kelemen, 99). Assimilatin f mantle peridtite by decmpressing melts migrating thrugh the peridtite is driven by the shift in the liquidus away frm livine, leading t precipitatin f livine and disslutin f pyrxene, while buffering the liquid cmpsitin clse t equilibrium with mantle livine. If this takes place at cnstant melt mass, then there is little r n increase in melt titanium. If, n the ther hand, mre livine is precipitated than pyrxene is disslved, then the cncentratin f incmpatible elements may increase rapidly as shwn by the melt-rck reactin trend in Figure with little r n change in the cmpsitin f the liquidus livine. Thus, the titanium cntent f spinel precipitated frm a migrating melt can be cntrlled by tw different prcesses: () the degree f melt-rck interactin and the extent t which melt mass is increasing r decreasing during melt transprt, and () the amunt f fractinal crystallizatin f livine and Plagiclase due t simple clsed-system fractinal crystallizatin where interactin between mantle and melt is negligible. Where melt mass decreases, with limited interactin with the mantle, the cmpsitin f spinel and livine precipitating frm the melt will be affected by bth prcesses and will lie between the tw trends as shwn in Figure. This is an assimilatin-fractinal-crystallizatin (AFC) cmpsitin field fr melt-mantle interactin. It is imprtant t nte that the degree f melt-rck interactin will be strngly affected by the velcity and flux f melt thrugh the mantle cnduit. Thus, where large vlumes f melt pass thrugh, there will be a negligible effect n melt cmpsitin. If melts pass slwly up thugh the cnduit r stagnate there fr any perid, then the effects can be extreme. It is clear that mst f the melts frm which the spinels in the Site 89 gabbric segregatins precipitated were strngly influenced by interactin with the mantle, but were als affected t a significant degree by fractinal crystallizatin. This wuld again appear t demnstrate that the gabbric segregatins frmed at shallw depths where there was significant cnductive heat lss in excess f the mantle adiabat. Fracture Cntrl f Late-stage Melt Transprt The mantle sectin at Hess Deep prvides direct evidence f melt transprt and melt-rck interactin in the shallw mantle. The ccurrence f numerus dunite dikes argues fr an episdic tapping and transprt f melt ut f the shallw mantle by repeated fracture events. The existence f brittle fracture in the mantle beneath the EP suggests that there is an effective "lithsphere" there capable f supprting significant shear stress n the time scale needed fr brittle fracture. Hw this might wrk is illustrated in Figure 0. As melt rises thrugh the shallw mantle tward the crust, the melt velcity slws and melt fractin increases. The increased melt fractin weakens the mantle immediately beneath the crust, reducing the effective strength and thickness f the "lithspheric" zne. This, in turn, results in lcal failure and fracture in the zne f "lithspheric" necking beneath the ridge. These fractures then prvide a high permeability pathway fr melt, causing it t migrate ut f the surrunding mantle tward, int, and up alng the fractures. As melt drains frm the mantle, hwever, the lithspheric zne strengthens and becmes thicker, and the melt cnduits clse as smaller and smaller vlumes f late, presumably mre reacted, melt migrate up the cnduit frm depth. C- type gabbric segregatins crystallize at this stage frm variably reacted melts ranging frm primitive MOB t high-alumina basalt mving up the cnduits frm depth. At sme pint pckets f melt may be trapped, either by sealing f the surrunding rck by adcumulate grwth, r by simple clsing f the fissure. This melt then slwly crystallizes in situ by reactin with the mantle t prduce L-type gabbric segregatins. The prcess then repeats itself as melt cntinues t rise up int the base f the lithsphere frm the mantle melting clumn. We nte that ur mdel fr melt migratin beneath the EP is similar t thse presented by wrkers studying gabbric segregatins and dunites in mantle philites (Ceuleneer and abinwicz, 99; Ceuleneer et al., 988; Niclas et al., 988; abinwicz et al., 98; abinwicz et al., 98). An interesting cnsequence f such a mdel, hwever, is that the mantle in the zne f melt aggregatin and hydrfracture shuld be clse t equilibrium with the melt. This is clearly nt the case fr the Hess Deep sectin, yet it is als clear frm the extensive evidence fr melt-rck interactin that the gabbric segregatins and the dunites frmed at high temperatures near the liquidus f primitive MOB, cnditins expected in the mantle immediately beneath the ridge. This bservatin, hwever, is cnsistent with a highly fcused flw f melt in the mantle beneath the EP. In such a situatin, where melt may be aggregating and flwing up thrugh the mantle by prus flw in a narrw finger extending up tward the base f the lithsphere perhaps nly hundreds f meters wide, fracture and the frmatin f a melt-transprt cnduit wuld be initiated where the cnduit extends int and lcally weakens the verlying "lithspheric" cap. Once initiated, hwever, such fractures wuld likely rapidly prpagate laterally fr cnsiderable distance int the mantle n either side as well as up tward the crust in a manner analgus t the frmatin f sheeted dikes. This wuld bring fully aggregated MOB int cntact with the highly depleted mantle utside r abve the zne f fcused melt-flw, where it wuld react with and be mdified by that mantle. Thus, the gechemistry f the Site 89 sectin strngly supprts a mdel f highly fcused lcal melt flw in the mantle beneath the EP. A simple unifrm tw-dimensinal flw f melt thrugh the mantle gemetrically requires the shallwest mantle immediately belw the crust t be nearly everywhere in equilibrium with MOB. This is clearly nt the case. The fcusing might be n quite a different scale than at slw-spreading ridges, hwever, with many lcal znes beneath a single ridge segment rather than a single majr zne, reflecting a mre tw-dimensinal structure f mantle (as ppsed t melt) flw beneath fast-spreading ridges (Lin and Phipps Mrgan, 99). The gabbric segregatins and the enclsing dunites are best explained as the prducts f shallw-level melt transprt thrugh the mantle accmpanied by crystallizatin f livine, Plagiclase, clinpyrxene, and spinel. The crystallizatin f the type-c gabbric assemblage demnstrates that fully aggregated MOB passed thrugh the dunites, as fractinal melts wuld h crystallized melts with EE patterns with varying slpes reflecting melt extractin at different degrees f melting (Jhnsn et al., 990). The crystallizatin f the three-phase silicate assemblage, hwever, wuld nt be expected if melting f the mantle cntinued t the base f the crust. Experimental petrlgy shws that liquids ascending thrugh the mantle with increasing magma mass will precipitate nly livine and spinel, as melt cmpsitin shifts t maintain equilibrium with the wall rcks (Kushir, 9; Kushir and Schairer, 9; O'Hara, 98). Crystallizatin f Plagiclase and clinpyrxene in additin t livine requires magma mass t decrease, and therefre t underg cnductive heat lss t the mantle wall rck. The temperature f the mantle at the time f crystallizatin f the melt segregatins can be estimated using the cmpsitin f the livine, Plagiclase, and pyrxene. The extent f crystallizatin f melts passing thrugh the cnduit will be limited by the wall-rck temperature, and hence the 0

29 LATE-STAGE MELT EVOLUTION AND TANSPOT Figure 0. Mdel fr dunite dike frmatin and melt migratin in the shallw mantle beneath the East Pacific ise. Light stippled pattern is harzburgite; lightweight arrws shw directin and lcus f perclating prus melt flw. Heavy black line in (B) indicates a dwnward prpagating crack in the mantle beneath the ridge. Heavy stippled pattern indicates dunite frmatin due t accelerated upward melt flw, which causes the melt t lse saturatin in pyrxene. Heavyweight arrw shws flw f segregated melt in pen fracture. Hashed pattern at center f dunite in (E) is a C-type gabbric segregatin crystallizing n the walls f the cnduit frm melt migrating frm depth up thrugh the pen cnduit. Final panel shws the dike after cnduit cllapse, with a renewed upward perclatin f melt thrugh the mantle matrix. Inset shws late intergranular clinpyrxene (heavy diagnal lines) and plagiclase (light diagnal lines) crystallized frm trapped melt in the dunite, frming an L-type gabbric segregatin. Scale is intentinally left ambiguus, but the rage thickness measured fr dunite at Site 89, crrected fr the steep inclinatin f mst harzburgite-dunite cntacts, is abut m. minimum frsterite and anrthite cntents f the livine and clinpyrxene. Allan and Dick (this vlume) use this principle t estimate a mantle temperature f rughly 0 C fr the Site 89 mantle sectin at the time f crystallizatin f the gabbric segregatins. CONCLUSIONS As a result f ur investigatins and analysis f the Site 89 mantle sectin, we draw a number f majr cnclusins relevant t the generatin f melts, shallw mantle melt transprt and the nature f the mantle beneath the equatrial EP.. The highly depleted cmpsitin f the Site 89 harzburgites and their unusual EE patterns, when cnsidered in the light f the cmpsitin f the spatially assciated basalts and gabbrs, shw that the Hess Deep sectin f EP crust was generated by melting f an initially majr- and trace-element-depleted mantle surce cmpared t the mantle surce in the Atlantic and Indian Oceans. Given the smewhat thinner than nrmal EP crust in the regin (-. km; Znenshain et al., 980), n unusual gethermal gradient r initial mantle temperature seems called fr. We nte that an unusually depleted mantle surce has als been invked t explain the highly depleted nature f Hle 0B MOB at 8 W- N (Auti et al., 989; Auti and hdes, 98) sme 00 km t the east f Hess Deep suth f the Ccs-Nazca idge. Cllins et al. (989) reprted anmalusly thin seismic crust at Site 0B, and a discrepancy with the crrelatin f Na 8 and crustal thickness pstulated by Klein and Langmuir (98). The nature f the depletin at these lcatins may be smewhat different in nature and rigin; nnetheless, this pints up the dangers in assuming that the Pacific mantle can be even rughly equated in cmpsitin t that f the Atlantic and Indian Oceans (e.g., Klein and Langmuir, 98), and certainly challenges simple crrelatins f the majr element depletin f basalts and peridtites t crustal thickness.. Like mantle peridtites dredged frm the Atlantic and Indian Oceans (Jhnsn et al., 990), the Hess Deep mantle harzburgites are in equilibrium with nly extremely refractry late near-fractinal melts f any reasnable mantle surce cmpsitin. This demnstrates that East Pacific ise MOB is als generated by pen-system near-fractinal melting and nt by batch equilibrium melting.. The gabbric segregatins frm Site 89 shw that late-stage mantle-melt transprt immediately beneath the Mh is, at least in part, fracture cntrlled. Parental melts transprted thrugh these cnduits were fully aggregated MOB, demnstrating that fractinal melts are aggregated at sme pint prir t intrusin t the crust, at least at this lcatin.. Melt-rck (mantle) interactin in the shallw mantle has prduced a significant vlume f dunite in the Site 89 sectin. The verall effect n MOB melt cmpsitin may be small, but this des h significant ramificatins fr interpreting the genesis f MOB based n experiments n natural basalts: (A) the assimilatin f arbitrary amunts f dipside and enstatite frm the wall-rck harzburgite implies that the depth f melt segregatin cannt be interpreted simply frm these experiments; (B) MOB ascending thrugh the shallw mantle is generally in equilibrium with mantle livine at lw pressures, but nt much else; (C) high-magnesium pyrxene and high-calcium Plagiclase, fund as ccasinal xencrysts in MOB at many lcalities in the ceans, may h riginated as prducts f melt-rck reactin in the mantle, and shuld nt, by themselves, be interpreted as evidence fr fractinal crystallizatin f these phases frm MOB at high pressures. Taken tgether, these results shw that high-magnesian primary MOB magmas segregated at high pressures are nt as strng as has been previusly suggested.. The best estimate f the cmpsitin f primary MOBS, and therefre f the verall cean crust, is prbably made by taking the cmpsitin f typical mid-cean-ridge basalts saturated in livine and Plagiclase and extraplating back alng liquid lines f descent t simple equilibrium with livine in the shallw mantle. Tw additinal cmpnents may be imprtant in accunting fr MOB cmpsitin and variability: () a mre aluminus calcic liquid representing reacted melts prduced during shallw mantle melt transprt, and () ultra-depleted partial melts f mantle peridtite generated high in the melting clumn that did nt share the mantle "plumbing system" trrsed by mre typical basalts. ACKNOWLEDGMENTS This research was supprted in part by grants frm the JOIDES US Science Supprt Advisry Cmmittee, by funds prvided by The Van Allen Clark Chair at Wds Hle Oceangraphic Institutin, and by funds prvided by NSF Grant OCE Nilanjan Chatterjee prvided technical assistance and cnducted mst f the

30 H.J.B. DICK, J.H. NATLAND actual electrn micrprbe analysis. Dr. Nbu Shimizu prvided enrmus help in running the in micrprbe. This manuscript benefited greatly frm reviews by Fred Frey and Jean Luis Bdinier, and frm discussins with Peter Kelemen, Stan Hart, Nbu Shimizu, Peter Mayer, and Greg Hirth. We wuld als like t acknwledge the able crew f the JOIDES eslutin, withut whm nne f this wuld h been pssible. EFEENCES Albee, A.L., and ay, L., 90. Crrectin factrs fr electrn micrprbe analysis f silicates, xides, carbnates, phsphates and sulfates. Anal. Chem., :08-. Anders, E., and Grevesse, N., 989. Abundances f the elements: meteritic and slar. Gechim. Csmchim. Acta, :9-. Auti, L.K., and hdes, J.M., 98. Csta ica ift Zne basalts: gechemical and experimental data frm a pssible example f multistage melting. In Cann, J.., Langseth, M.G., Hnnrez, J., Vn Herzen,.P., White, S.M., et al., Init. epts. DSDP, 9: Washingtn (U.S. Gvt. Printing Office), 9-. Auti, L.K., Sparks, J.W., and hdes, J.M., 989. Gechemistry f Leg basalts: intrusive feeders fr highly depleted pillws and flws. In Becker, K., Sakai, H., et al., Prc. ODP, Sci. esults, : Cllege Statin, TX (Ocean Drilling Prgram), -. Bence, A.E., and Albee, A.L., 98. Empirical crrectin factrs fr the electrn micranalysis f silicates and xides. J. Gel., :8-0. Blmer, S.H., Meyer, P.S., Dick, H.J.B., Ozawa, K., and Natland, J.H., 99. Textural and mineralgic variatins in gabbric rcks frm Hle B. In Vn Herzen,.P., binsn, P.T., et al., Prc. ODP, Sci. esults, 8: Cllege Statin, TX (Ocean Drilling Prgram), -9. Bnatti, E., Peyve, A., Kepezhinskas, P., Kurentsva, N., Seyler, M., Skltnev, S., and Udintsev, G., 99. Upper mantle hetergeneity belw the Mid-Atlantic idge, 0 - N. J. Gephys. es., 9:-. Budier, F., and Cleman,.G., 98. Crss sectin thrugh the peridtite in the Samail philite, sutheastern Oman Muntains. J. Gephys. es., 8:-9. Budier, F., and Niclas, A., 9. Fusin partielle gabbrique dans la Iherzlite de Lanz. Bull. Suisse Mineral. Petrgr., :9-., 9. Structural cntrls n partial melting in the Lanz peridtites. Bull. Oreg. Dep. Gel. Miner. Ind., 9:-8. Bryan, W.B., and Grve, T.L., 98. Cntrasting Atlantic and Pacific MOB phencryst assemblages. Es, :. Bryan, W.B., and Mre, J.G., 9. Cmpsitinal variatins f yung basalts in the Mid-Atlantic idge rift valley near lat 9'N. Gel. Sc. Am. Bull., 88:-0. Cannat, M., 99. Emplacement f mantle rcks in the seaflr at mid-cean ridges. J. Gephys. es., 98:-. Cannat, M., Bideau, D., and Hebert,., 990. Plastic defrmatin and magmatic impregnatin in serpentinized ultramafic rcks frm the Garrett transfrm fault (East Pacific ise). Earth Planet. Sci. Lett., 0:-. Casey, J.F., Karsn, J.A., Elthn, D., sencrantz, E., and Titus, M., 98. ecnstructin f the gemetry f accretin during frmatin f the Bay f Islands Ophilite Cmplex. Tectnics, :09-8. Cassard, D., Niclas, A., abinvitch, M., Mutte, J., Leblanc, M., and Prinzhfer, A., 98. Structural classificatin f chrmite pds in Suthern New Calednia. Ecn. Gel., :80-8. Ceuleneer, G., and Niclas, A., 98. Structures in pdifrm chrmite frm the Maqsad district (Sumail philite, Oman). Miner. Depsita, 0:- 8. Ceuleneer, G., Niclas, A., and Budier, F., 988. Mantle flw patterns at an ceanic spreading centre: the Oman peridtite recrd. Tectnphysics, :-. Ceuleneer, G., and abinwicz, M., 99. Mantle flw and melt migratin beneath cean ridges: mdels derived frm bservatins in philites. In Phipps Mrgan, J.B., Blackman, D.K., and Sintn, J.M. (Eds.), Mantle Flw and Melt Generatin at Mid-Ocean idges. Gephys. Mngr., Am. Gephys. Unin, :-. Chayes, F., 9. Petrgraphic mdal analysis: an elementary statistical apprach: Lndn (Jhn Wiley & Sns). Cllins, J.A., Purdy, M.G., and Brcher, T.M., 989. Seismic velcity structure at Deep Sea Drilling Prject Site 0B, Panama Basin: evidence fr thin ceanic crust. J. Gephys. es., 9: Dick, H.J.B., 9a. Evidence f partial melting in the Jsephine Peridtite. Bull. Oreg. Dep. Gel. Miner. Ind., 9:9-., 9b. Partial melting in the Jsephine peridtite. I. The effect f mineral cmpsitin and its cnsequence fr gebarmetry and gethermmetry. Am. J. Sci., : , 989. Abyssal peridtites, very slw spreading ridges and cean ridge magmatism. In Saunders, A.D., and Nrry, MJ. (Eds.), Magmatism in the Ocean Basins. Gel. Sc. Spec. Publ. Lndn, :-0. Dick, H.J.B., and Bryan, W.B., 98. Variatin f basalt phencryst mineralgy and rck cmpsitins in DSDP Hle 9B. In Dmitriev, L., Heirtzler, J., et al., Init. epts. DSDP, : Washingtn (U.S. Gvt. Printing Office), -. Dick, H.J.B., and Bullen, T., 98. Chrmian spinel as a petrgenetic indicatr in abyssal and alpine-type peridtites and spatially assciated lavas. Cntrib. Mineral. Petrl, 8:-. Dick, H.J.B., and Fisher,.L., 98. Mineralgical studies f the residues f mantle melting: abyssal and alpine-type peridtites. In Krnprbst, J. (Ed.), Kimberlites II: The Mantle and Crust-Mantle elatinships: Amsterdam (Elsevier), Dick, H.J.B., Fisher,.L., and Bryan, W.B., 98. Mineralgic variability f the uppermst mantle alng mid-cean ridges. Earth Planet. Sci. Lett., 9:88-0. Dick, H.J.B., and Kelemen, P.B., 99. Chrmian spinel as a petrgenetic indicatr f magmagenesis in shallw mantle rcks. Es, :. Dick, H.J.B., Meyer, P.S., Blmer, S., Kirby, S., Stakes, D., and Mawer, C, 99a. Lithstratigraphic evlutin f an in-situ sectin f ceanic Layer. In Vn Herzen,.P., binsn, P.T., et al., Prc. ODP, Sci. esults, 8: Cllege Statin, TX (Ocean Drilling Prgram), 9-8. Dick, H.J.B., binsn, P.T., and Meyer, P.S., 99. The plutnic fundatin f a slw-spreading ridge. In Duncan,.A., ea, D.K., Weissel, J.K., vn ad, U., and Kidd,.B. (Eds.), The Indian Ocean: A Synthesis f esults frm the Ocean Drilling Prgram. Gephys. Mngr., Am. Gephys. Unin, 0:-9. Dick, H.J.B., Schuten, H., Meyer, P.S., Gall, D.G., Bergh, H., Tyce,., Patriat, P., Jhnsn, K.T.M., Snw, J., and Fisher, A., 99b. Tectnic evlutin f the Atlantis II Fracture Zne. In Vn Herzen,.P., binsn, P.T., et al., Prc. ODP, Sci. esults, 8: Cllege Statin, TX (Ocean Drilling Prgram), Dick, H.J.B., and Sintn, J., 99. Cmpsitinal layering in alpine peridtites: evidence fr pressure slutin creep in the mantle. J. Gel., 8:0-. Dickey, J.S., Jr., Yder, H.S., Jr., and Schairer, J.F., 9. Incngruent melting f chrmian dipside and the rigin f pdifrm chrmite depsits. Gel. Sc. Am., Abstr., :-. Duncan,.A., and Green, D.H., 980. le f multistage melting in the frmatin f ceanic crust. Gelgy, 8:-., 98. The genesis f refractry melts in the frmatin f ceanic crust. Cntrib. Mineral. Petrl, 9:-. Elthn, D., 99. High magnesia liquids as the parental magma fr cean flr basalts. Nature, 8:-8., 98. Petrlgy f gabbric rcks frm the Mid-Cayman ise spreading center. J. Gephys. es., 9:8 8. -, 989. Pressure f rigin f primary mid-ceanic ridge basalts. In Saunders, A.D., and Nrry, MJ. (Eds.), Magmatism in the Ocean Basins. Gel. Sc. Spec. Publ. Lndn, :-. Elthn, D., and Scarfe, CM., 98. High-pressure phase equilibria f a highmagnesia basalt and the genesis f primary ceanic basalts. Am. Mineral, 9:-. England,.N., and Davies, H.L., 9. Mineralgy f ultramafic cumulates and tectnites frm eastern Papua. Earth Planet. Sci. Lett, :^. Falln, T.J., and Crawfrd, A.J., 99. The petrgenesis f high-calcium bninite lavas dredged frm the nrthern Tnga idge. Earth Planet. Sci. Lett., 0:-9. Francheteau, J., Armij,., Cheminée, J.L., Hekinian,., Lnale, P.F., and Blum, N., 990. Ma East Pacific ise ceanic crust and uppermst mantle expsed by rifting in Hess Deep (equatrial Pacific Ocean). Earth Planet. Sci. Lett., 0:8-9. Gillis, K., Mével, C, Allan, J., et al., 99. Prc. ODP, Init. epts., : Cllege Statin, TX (Ocean Drilling Prgram). Girardeau, J., and Francheteau, J., 99. Plagiclase-wehrlites and peridtites n the East Pacific ise (Hess Deep) and the Mid-Atlantic idge (DSDP Site 9) evidence fr magma perclatin in the ceanic upper mantle. Earth Planet. Sci. Lett, :-9.

31 LATE-STAGE MELT EVOLUTION AND TANSPOT Hart, S.., and Dunn, T., 99. Experimental CPX/melt partitining f trace elements. Cntrib. Mineral. Petrl, :-8. Hebert,., Bideau, D., and Hekinian,., 98. Ultramafic and mafic rcks frm the Garret transfrm fault near θ's. n the East Pacific ise: igneus petrlgy. Earth Planet. Sci. Lett., :0-. Hekinian,., Bideau, D., Francheteau, J., Cheminée, J.L., Armij,., Lnale, P., and Blum, N., 99. Petrlgy f the East Pacific ise crust and upper mantle expsed in the Hess Deep (eastern equatrial Pacific). J. Gephys. es., 98: Hess, P.C., 99. Phase equilibria cnstraints n the rigin f cean flr basalts. In Phipps Mrgan, J., Blackman, D.K., and Sintn, J.M. (Eds.), Mantle Flw and Melt Generatin at Mid-Ocean idges. Gephys. Mngr., Am. Gephys. Unin, :-0. Hdges, F.N., and Papike, J.J., 9. DSDP Site : magmatic cumulates frm ceanic layer. J. Gephys. es., 8:-. Irvine, T.N., 9. Chrmium spinel as a petrgenetic indicatr, Part : thery. Can. J. Earth Sci., :8-., 9. Chrmium spinel as a petrgenetic indicatr, Part : petrlgical applicatins. Can. J. Earth Sci., :-0. Jaques, A.L., and Green, D.H., 980. Anhydrus melting f peridtite at 0- kb pressure and the genesis f the thleiitic basalts. Cntrib. Mineral. Petrl., :8-0. Jhnsn, K.T.M., and Dick, H.J.B., 99. Open system melting and tempral and spatial variatin f peridtite and basalt at the Atlantis II fracture zne. J. Gephys. es., 9:99-9. Jhnsn, K.T.M., Dick, H.J.B., and Shimizu, N., 990. Melting in the ceanic upper mantle: an in micrprbe study f dipsides in abyssal peridtites. J. Gephys. es., 9:-8. Karsn, J.A., Hurst, S.D., and Lnale, P.F., 99. Tectnic rtatins f dikes in fast-spread ceanic crust expsed near Hess Deep. Gelgy, 0:8-88. Kelemen, P.B., 98. Assimilatin f ultramafic rcks in subductin-related magmatic arcs. J. Gel, 9:89-8. Kelemen, P.B., Dick, H.J.B., and Quick, J.E., 99. Frmatin f harzburgite by pervasive melt/rck reactin in the upper mantl.: Nature, 8:-. Kelemen, P.B., Jyce, D.B., Webster, J.D., and Hllway, J.., 990. eactin between ultramafic rck and fractinating basaltic magma II. Experimental investigatin f reactin between livine thleiite and harzburgite at II 0-00 C and kb. J. Petrl, :99-. Kemptn, P.D., Auti, L.K., hdes, J.M., Hldaway, MJ., Dungan, M.A., and Jhnsn, P., 98. Petrlgy f basalts frm Hle 0B, Deep Sea Drilling Prject, Leg 8. In Andersn,.N., Hnnrez, J., Becker, K., et al., Init. epts. DSDP, 8: Washingtn (U.S. Gvt. Printing Office), 9-. Kinzler,.J., and Grve, T.L., 99. Primary magmas f mid-cean ridge basalts,. Experiments and methds. J. Gephys. es., 9: Klein, E.M., and Langmuir, CH., 98. Glbal crrelatins f cean ridge basalt chemistry with axial depth and crustal thickness. J. Gephys. es., 9: Kmr, S.C., Elthn, D., and Casey, J.F., 98a. Mineralgic variatin in a layered ultramafic cumulate sequence at the Nrth Arm Muntain Massif, Bay f Islands Ophilite, Newfundland. J. Gephys. es., 90:0-., 98b. Serpentinizatin f cumulate ultramafic rcks frm the Nrth Arm Muntain massif f the Bay f Islands philite. Gechim. Csmchim. Acta, 9:-8. Kushir, I., 9. The system dipside-frsterite-enstatite at 0 kilbars. Yearbk Carnegie Inst. Washingtn, :0-08., 9. The effect f water n the cmpsitin f magmas frmed at high pressures. J. Petrl, :-. Kushir, J., and Schairer, J.F., 9. New data n the system dipside-frsterite-silica. Yearbk Carnegie Inst. Washingtn, :9-0. Lin, J., and Phipps Mrgan, J., 99. The spreading rate dependence f three-dimensinal midcean ridge gravity structure. Gephys. es. Lett., 9:-. Lindsley, D.H., and Andersen, D.J., 98. A tw-pyrxene thermmeter. J. Gephys. es., 88:A88-A90. Lnale, P., 988. Structural pattern f the Galapags micrplate and evlutin f the Galapags triple junctins. J. Gephys. es., 9:. Lnale, P., Blum, N., and Puchelt, H., 99. The triple junctin at the suthern end f the Pacific-Ccs East Pacific ise. Earth Planet. Sci. Lett., 09:-9. Malpas, J., Brace, T., and Dunswrth, S.M., 989. Structural and petrlgic relatinships f the CY- drill hle f the Cyprus Crustal Study Prject. Pap. Gel. Surv. Can., 88-9:9-8. Medaris, L.G., Jr., 9. High-pressure peridtites in suthwestern Oregn. Gel. Sc. Am. Bull., 8:-8. Medaris, L.G., Jr., and Dtt,.H., Jr., 90. Mantle-derived peridtites in suthwestern Oregn: relatin t plate tectnics. Science, 9:9-9. Meyer, P.S., Dick, H.J.B., and Thmpsn, G., 989. Cumulate gabbrs frm the Suthwest Indian idge, S- 'E: implicatins fr magmatic prcesses at a slw spreading ridge. Cntrib. Mineral. Petrl, 0:-., Michael, PJ., and Bnatti, E., 98a. Peridtite cmpsitin frm the Nrth Atlantic: reginal and tectnic variatins and implicatins fr partial melting. Earth Planet. Sci. Lett., :9-0., 98b. Petrlgy f ultramafic rcks frm Sites, 8, and 0 in the Nrth Atlantic. In Bugault, H., Cande, S.C., et al., Init. epts. DSDP, 8: Washingtn (U.S. Gvt. Printing Office), -8. Miyashir, A., Shid, F., and Ewing, M., 99. Diversity and rigin f abyssal thleiite frm the Mid-Atlantic ridge, and 0 N latitude. Cntrib. Mineral. Petrl, :8-., 90. Crystallizatin and differentiatin in abyssal thleiites and gabbrs frm mid-ceanic ridges. Earth Planet. Sci. Lett., :-. Mutte, J., 99. Le Massif de Tiebaghi, Nuvelle Calednia et ses gites de chrmite [Ph.D. thesis]. L'Ecle Natinale Superieure des Mines de Paris. Mysen, B.O., and Bettcher, A.L., 9. Melting f a hydrus mantle, I. Phase relatins f natural peridtite at high pressures and temperatures with cntrlled activities f water, carbn dixide, and hydrgen. J. Petrl, :0-8. Natland, J.H., Adamsn, A.C., Lrne, C, Melsn, W.G., and O'Hearn, T., 98. A cmpsitinally nearly steady-state magma chamber at the Csta ica ift: evidence frm basalt glass and mineral data, Deep Sea Drilling Prject Sites 0, 0, and 0. In Cann, J.., Langseth, M.G., Hnnrez, J., Vn Herzen,.P., White, S.M., et al., Init. epts. DSDP, 9: Washingtn (U.S. Gvt. Printing Office), Niclas, A., 989. Structure f Ophilites and Dynamics f the Oceanic Lithsphere: Drdrecht (Kluwer)., 990. Melt extractin frm mantle peridtites: hydrfracturing and prus flw, with cnsequences fr ceanic ridge activity. In yan, M.P. (Ed.), Magma Transprt and Strage: Chichester (Wiley), 9. Niclas, A., Budier, F., and Bullier, A.M., 9. Mechanisms f flw in naturally and experimentally defrmed peridtites. Am. J. Sci., :8-8. Niclas, A., Budier, F., and Ceuleneer, G., 988. Mantle flw patterns and magma chambers at cean ridges: evidence frm the Oman Ophilite. Mar. Gephys. es., 9:9-0. Niclas, A., and Vilette, J.F., 98. Mantle flw at ceanic spreading centers: mdels derived frm philites. Tectnphysics, 8:9-9. Nilssn, K., 99. Oxidatin state, sulfur speciatin, and sulfur cncentratin in basaltic magmas: examples frm Hess Deep and the Lau Basin [Ph.D. dissert.]. Univ. Califrnia-San Dieg. O'Hanley, D.S., 99. Slutin t the vlume prblem in serpentinizatin. Gelgy, 0:0-08. O'Hara, MJ., 98. Are cean flr basalts primary magma? Nature, 0:8-8. Ozawa, K., Meyer, P.S., and Blmer, S.H., 99. Mineralgy and textures f irn-titanium xide gabbrs and assciated livine gabbrs frm Hle B. In Vn Herzen,.P., binsn, P.T., et al., Prc. ODP, Sci. esults, 8: Cllege Statin, TX (Ocean Drilling Prgram), -. Panjasawatwng, Y., Danyushevsky, L.V., Crawfrd, A.J., and Harris, K.L., 99. An experimental study f the effects f melt cmpsitin n plagiclase-melt equilibria at and 0 kb: implicatins fr the rigin f magmatic high-an Plagiclase. Cntrib. Mineral. Petrl, 8:0-. Presnall, D.C., Dixn, J.., O'Dnnell, T.H., and Dixn, S.A., 99. Generatin f mid-cean thleiites. J. Petrl, 0:-. Presnall, D.C., Dixn, S.A., Dixn, J.., O'Dnnell, T.H., Brenner, N.L., Schrck,.L., and Dycus, D.W., 98. Liquidus phase relatins n the jin dipside-frsterite-anrthite frm atm t 0 kbar: their bearing n the generatin and crystallizatin f basaltic magma. Cntrib. Mineral. Petrl, :0-0. Quick, J.E., 98a. The rigin and significance f large, tabular dunite bdies in the Trinity peridtite, nrthern Califrnia. Cntrib. Mineral. Petrl, 8:-.