with Axial Depth and Crustal Thickness

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 92, NO. B8, PAGES , JULY 10, 1987 Gl bal Crrelatins f Ocean Ridge Basalt Chemistry with Axial Depth and Crustal Thickness EMILY M. KLEIN AND CHARLES H. LANGMUIR Lamnt-Dherty Gelgical Observatry and Department f Gelgical Sciences f Clumbia University, Palisades, New Yrk Reginal averages f the majr element chemistry f cean ridge basalts, crrected fr lw-pressure fractinatin, crrelate with reginal averages f axial depth fr the glbal system f cean ridges, including ht spts, cld spts, and back arc basins, as well as "nrmal" cean ridges. Quantitative cnsideratin f the variatins f each majr element during melting f the mantle suggests that the glbal majr element variatins can be accunted fr by -8-20% melting f the mantle at assciated mean pressures f 5-16 kbar. The lwest extents f melting ccur at shallwest depths in the mantle and are assciated with the deepest cean ridges. Calculated mean primary magmas shw a range in cmpsitin frm 10 t 15 wt % MgO, and the primary magma cmpsitins crrelate with depth. Data fr Sm, Yb, Sc, and Ni are cnsistent with the majr elements, but highly incmpatible elements shw mre cmplicated behavir. In additin, sme ht spts have anmalus chemistry, suggesting majr element hetergeneity. Thermal mdeling f mantle ascending adiabatically beneath the ridge is cnsistent with the chemical data and melting calculatins, prvided the melt is tapped frm thrughut the ascending mantle clumn. The thermal mdeling independently predicts the bserved relatinships amng basalt chemistry, ridge depth, and cmstal thickness resulting frm temperature variatins in the mantle. Beneath the shallwest and deepest ridge axes, temperature differences f apprximately 250'C in the subslidus mantle are required t accunt fr the glbal systematics. INTRODUCTION The temperature and flw regime f the mantle shuld, in part, cntrl the extent f partial melting that the mantle underges as it ascends beneath cean ridges. The extent f melting shuld, in turn, gvern bth the chemistry f cean ridge basalts and the thickness f the ceanic crust. Crustal thickness, t first rder, shuld be related thrugh isstatic cmpensatin t the zer-age depth f cean ridges. Thus, variatins in cean ridge basalt chemistry, axial depth, and crustal thickness shuld crrelate with each ther and with mantle temperature variatins. Despite the clear cnceptual relatinship amng these parameters, n glbal crrelatins between basalt chemistry and axial depth r crustal thickness have yet been established. Petrlgists and gechemists have examined certain aspects f this prblem, hwever, n a reginal basis. The basic cncept f axial "ht spts," which represent ne end-member in the glbal bathymetric spectrum, is related t increased mantle temperature, vluminus magmatism, shallw depths, and greater crustal thickness. In extensive studies f chemical variatins alng the nrthern Mid-Atlantic Ridge, Schilling et al. [1983] and Hamelin et al. [1984] bserved that maxima in La/Sm and radigenic strntium near Iceland and the Azres platfrm crrelate psitively with residual depth anmalies and crustal thickness. Furthermre, recent studies f ceanic ultramafic rcks, which are believed t represent the residues f partial melting, have shwn crrelatins between the degree f chemical depletin in basaltic cnstituents and residual depth [Michael and Bnatti, 1985] r residual geid height [Dick et al., 1984], particularly arund the Azres ht spt Cpyright 1987 by the American Gephysical Unin. Paper number 6B /87/006B great depth and negative gravity anmalies [Weissel ana Hayes, 1974; Hayes and Cnlly, 1972]. Cmparisn f the chemistry f basalts erupted in this "cld spt" with thse frm nrmal, as well as ht spt regins, led t a recgnitin that sme chemical parameters vary systematically with axial depth, even in the absence f ht spt anmalies. In the present study we have examined glbal systematics in basalt cmpsitin and fcused n the relatinship between chemistry and axial depth. Detailed studies f densely sampled ridge segments have shwn cnsiderable small-scale variability in basalt cmpsitin, suggesting lcally cmplex petrgenetic histries. In rder t identify glbal chemical systematics, these lcal variatins must be averaged ut. We have therefre used data averaged ver distances larger than presumed spreading cells. The identificatin f glbal chemical trends requires an extensive, high-quality, glbal chemical data set. It has nly been in the past few years, particularly thrugh the effrts f W. Melsn and J.-G. Schilling, that sufficient analyses have been reprted n fresh samples recvered frm the glbal system f spreading centers t permit a glbal synthesis such as that presented here. DATA TREATMENT AND COMPK.ATION Crrectin fr Lw-Pressure Fractinatin Prbably all cean ridge basalts have undergne sme shallw-level fractinatin in crustal magma chambers r cnduits, during which the abundances f all majr elements are mdified frm thse f their parental magmas. It is the majr element cntents f the parental magmas, hwever, that reflect the pressure, temperature, and cmpsitin f the in the Atlantic. On the ppsite bathymetric extreme, we have recently cmpleted a study f basalts erupted in the mantle frm which they are derived. A necessary first step, Australian-Antarctic Discrdance, a regin f anmalusly therefre, is t crrect fr the effects f shallw-level fractinatin. Tw appraches can be used t infer the majr element chemistry f parental magmas frm a suite f variably fractinated basalts. The first is t cnsider nly 8089

2 8090 KLEIN AND LANGMUIR; GLOBAL C ORREL ATIONS 3 ø90.80 <:1:.70 - i I I I.. 0 TAMAYO KOLBE INSEY e ø ø60 -.$0 5 b I! I $ ø O0 00( 0 0 O Od O I I I i I0 MgO Fig. 1. MgO (wt %) versus (a) Na20 (wt %) and (b) CaO/A120 3 fr basalts frm the Klbeinsey Rise (slid circles) [Schilling et al., 1983], the East Pacific Rise suth f the Tamay transfrm fault (pen circles) [Bender et al., 1984; C.H. Langmuir and J.F. Bender, unpublished data, 1987], and the Sutheast Indian Ridge in the vicinity f the Australian-Antarctic Discrdance (slid triangles) [Klein et al., 1984] Arrws in Figure la shw liquid lines f descent fr lw-pressure fractinatin fr each suite, calculated accrding t the methd f Weaver and Langmuir [1987]; als shwn are tw lines f cnstant Na8. 0 the calculated Na20 cntents at 8.0 wt % MgO, as discussed in the text; the frmula fr calculating Na8. 0 is given in the captin t Figure 2. cmpsitins with high MgO cntents, which are likely t have experienced fractinatin nly f ilvine. The number f ridge segments that can be studied thrugh this apprach, hwever, is limited; nly evlved basalts have been recvered frm many regins. The primary apprach used in this paper is t crrect individual xide abundances in suites f multiply saturated basalts t a cmparable level f MgO in rder t cmpare their chemistries. In additin, fr sme majr r trace element ratis that exhibit little variatin during fractinatin, arrays f data ver a specified range in MgO cntent have been used. We have als examined the mre limited data n lavas f high MgO cntent t ensure that ur cnclusins are nt an artifact f the crrectin methd. The apprach used t crrect fr fractinatin is illustrated in Figure l a, which shws data frm three areas encmpassing much f the range f axial depth displayed by glbal spreading centers. Althugh each f these areas is petrlgically cmplex in its wn right and prbably includes mre than ne distinct parental magma, data frm each area define an array which at any ne MgO cntent is distinct frm thse f the ther areas. Liquid lines f descent calculated fr lw-pressure (1 att) fractinatin f livine, plagiclase, and clinpyrxene frm a high MgO magma frm each area [Weaver and Langrnuir, 1987] pass thrugh the data array fr that area. Because the three liquid lines f descent are subparallel, differences in the Na20 abundances at the same MgO shuld, t first rder, be due t differences in the Na20 cntents f their parental magmas. By prjecting alng a line parallel t the liquid line f descent when MgO is less than 8.5 wt %, the Na20 cntent at sme reference value f MgO can be inferred. We have arbitrarily selected 8 wt % MgO as the reference value. The frmula used t calculate the Na20 cntent at 8 wt % MgO (Na8.0) is given in the captin t Figure 2. Nte that a value f Na8. 0 can be determined fr each and every sample with MgO between 5 and 8.5 wt % MgO. Fr tw f the suites, the values f Na8. 0 calculated fr all samples with MgO less than 8.5 wt % agree clsely with the actual abundance f Na20 in a sample with 8 wt % MgO frm each suite. Fr the lwest Na20 suite, the liquid line f descent has a flatter slpe, leading t errrs as high as 0.3 in Na8.0 fr individual pints with the lwest MgO; this represents, hwever, an extreme example f inaccuracy in the crrectin. Using this technique fr fractinatin crrectin, which is essentially the same as that f Langrnuir and Bender [1984], even a small number f analyses with MgO less than 8.5 wt % can be used t define a characteristic Na8. 0 value. Analgus reasning can be applied t FeO as well, leading t calculated values f Fe8. 0. The frmula used t calculate Fe8. 0 is als given in the captin t Figure 2. Figure lb shws the variatins in the CaO/A120 3 ratis fr the same samples. The CaO/A120 3 rati increases slightly during livine-plagiclase fractinatin but then des nt change substantially during fractinatin f the three-phase assemblage livine-plagiclase-clinpyrxene. The decreasing trend f CaO/A1203 fr the Klbeinsey Ridge data may reflect fractinatin at higher pressure where clinpyrxene may be a liquidus phase [e.g., Presnall and O'Dnnell, 1976; Bender et al., 1978; Dungan and Rhdes, 1978]. The CaO/A120 3 data frm the Australian-Antarctic Discrdance shw substantial variatin, which is als reflected in the trace element chemistry frm this regin, but samples frm a single dredge apprximately fllw the lwpressure fractinatin trend. Despite the cmplexities in the data and fractinatin histries, the three regins are distinguishable in their CaO/A1203 ratis, which crrelate inversely with Na8.0. Thus fr a variety f chemical parameters, it is pssible t "see thrugh" bth lw pressure fractinatin and lcal cmplexities t btain a reginal gechemical signature. Reginal averages f smthed axial depth, Na8.0, Fe8.0 and CaO/A120 3 in basaltic glasses and whle rcks are presented in Table 1. References fr the surces f the data, the number f analyses included in each average, and the standard deviatins fr each average are als reprted. The CaO/A120 3 ratis were calculated fr samples with greater than 5,vt % MgO, and the Na8. 0, and Fe8. 0 values were calculat d fr samples with MgO cntents between 5 and 8.5 wt %. Fr regins where nly whle rck data were availabl, and where infrmatin n the extent f whle rck alteraticn was given, analyses f highly altered samples were excluded frm ur cmputatins.

3 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS 8091 Because sampling density varies cnsiderably frm ridge t ridge, sme spreading centers, such as the nrthern Mid- Atlantic Ridge (MAR) r prtins f the East Pacific Rise (EPR), are better represented than ther, mre sparsely sampled spreading centers, such as the Suthwest r Sutheast Indian Ridges. The chemical data were averaged as Reginal averages f Na8. 0, CaO/A1203, and Fe8. 0 are fllws. Fr ridges where sampling density was high, pltted versus reginal averages f axial depth in Figures samples were gruped with respect t the tectnic 2a-2c. The data include nrmal mid-cean ridge basalts segmentatin f the ridge axis, and reginal averages were (MORB) frm the Atlantic, Pacific, and Indian cean ridges, calculated fr all samples within each ridge segment. Thus, enriched MORB frm axial ht spts (such as Iceland, the fr the densely sampled and tectnically cmplex Easter Azres, Easter micrplate, and the Galapags), basalts micrplate regin, the EPR was divided int seven segments erupted in back arc basins, and basalts erupted in such based largely upn transfrm-bunded segmentatin f the tectnically unique settings as the Cayman Trugh and ridge axes. Similarly, in the densely sampled regin just suth f the Kane Fracture Zne alng the MAR, a small Wdlark basin. There is a clear psitive crrelatin between Na8. 0 and depth (Figure 2a). The reginal averages ridge ffset was selected as a bundary between ridge f CaO/A1203 and Fe8.0 crrelate inversely with axial segments. Fr densely sampled spreading centers, analyses depth fr all basalts, with the exceptin f sme axial ht were thus generally averaged ver less than 100 km f ridge spts; sme ht spts, in particular the Azres, Jan Mayen, length. In cntrast, fr the numerus spreading centers and Galapags regins, shw lwer CaO/A1203 and Fe8. 0 where sampling density is sparse, averages were taken f all available data frm a particular regin, which may than ther basalts erupted at the same depth (Figures 2b and 2c). encmpass a few hundred kilmeters f ridge length. N average is reprted fr regins where nly ne analysis is available ver distances f a few hundred kilmeters. Within the verall crrelatins there are areas with Mst f the chemical data are frm fur labratries: W. Melsn's and H. Sigurdssn's micrprbe analyses f basaltic glasses, J.-G. Schilling's analyses f basaltic whle rcks, and ur wn analyses f hand-picked glass pwders by plasma spectrmetry. Our cmparisn f analyses by the fur labratries suggests that there are systematic interlabratry biases, particularly fr MgO, Na20, and T io 2. These biases, hwever, are small cmpared t the range f reginal chemical variatins presented herein. The analyses frm Melsn's micrprbe are frm areas that span the entire bathymetric range, and therefre chemical crrelatins with depth cannt be attributed t interlabratry biases. Standard deviatins fr all reginal averages f NaB. 0, Fe8. 0 and CaO/A120 3 are reprted in Table 1. Fr the vast majrity f averages, the standard deviatins are less than 5-10% f the ttal range f the data which shws that the glbal systematics are nt an artifact f the averaging prcess. A small number f reginal averages, particularly in the vicinity f recgnized ht spts, exhibit larger standard deviatins. Reginal axial depth values reprted in Table 1 represent ur best estimate f the average ver the range f bathymetric data published fr a particular regin. References fr the bathymetric data frm which ur estimates were made are included in the ftntes f Table 1. In determining the reginal depth values fr regins f gd bathymetric cverage, we have been guided by the wrk f Le Duaran and Francheteau [1981], wh determined a smthed axial depth prfile fr the nrthern MAR by taking mving averages with a 100 km step. Qualitatively, ur value fr reginal axial depth wuld represent the mean depth f such a smthed prfile ver the ridge length in questin. Fr a few regins, hwever, the reprted bathymetric data are limited, and in such cases, we have simply used the mean f the axial depth infrmatin that is available in published papers. It shuld be nted that fr the purpses f examining glbal crrelatins between axial depth and chemistry, an errr f as much as a few hundred meters in axial depth is small cmpared t the ttal glbal spreading center bathymetric range f 5600 m. RESULTS Crrelatins Between Chemistry and Axial Depth different lcal systematics. Fr example, averages taken ver the tpgraphically highest ridge segments assciated with Atlantic and Pacific ht spt centers (Figure 2a) tend t have higher Na8.0, fr the same depth (r are shallwer fr the same Na8.0), and therefre the data fr mst ht spt centers skirt the upper bundary f the brad band defining the glbal crrelatin. Much f the lwer bundary f the crrelatin is frmed f samples frm the margins f ht spts, that is, the bathymetrically deeper regins n the peripheries f ht spts. This can lead t the ppsite crrelatin between Na8.0 and depth fr these ht spt areas when any ne f them is cnsidered in islatin. Arund the Galapags ht spt, fr example, all data fall within the brad, psitive, glbal crrelatin band (Figure 2a), yet five f the seven ridge segments define an inverse trend within this band. The trends f ppsite slpe bserved arund ht spts apparently reflect prcesses ccurring at shrter wavelengths than the verall crrelatin. Figure 2d shws the data frm Table 1 averaged ver >500 km f ridge length, using majr tectnic bundaries as delimiting pints. Over these lnger ridge lengths, the variatins within each ht spt regin are averaged, the crrelatin becmes mre clearly defined and distinctly curvilinear, and different cean basins frm distinct fields. This suggests that the differences in Na8. 0 between ht spt centers and margins is a mediumwavelength perturbatin n the prcesses that gvern the glbal crrelatin between depth and Na8.0. The width f the glbal Na8.0-depth crrelatin (up t ~1500 m in depth r 0.6 in Na8.0) is substantial and clearly indicates the existence f multiple causes f variatin in depth and chemistry, even when shrt-wavelength variatins are averaged ut. In this paper, hwever, we cncentrate n the cause nly f the verall glbal trends f the data because these trends shuld reflect first-rder causes f MORB variatins n a glbal scale. Cmparisn With Previus Chemistry/Depth Crrelatins There have been several previus papers which have shwn relatinships between chemistry and depth fr ceanic basalts. All f these previus effrts have been

4 8092 KLI IN AND LANGMUIR: GLOBAL CORRI LATION8 I I I I I øøe e x ATLANTIC x PACIFIC O INDIAN f SOUTHERN RIDGES BACK ARC BASINS CAYMAN TROUGH 8 WOODLARK BASINS I HOT SPOT CENTERS ARE CIRCLED I { I i % , ,60 - I I I I I 8- Od j I I I I I 0 I DEPTH ( rn ) Fig. 2. Reginal averages f axial depth versus (a) Na8. 0 (b) CaO/A120 3, and (c) Fe8. 0 fr samples frm the Mid- Ariantie Ridge (slid circles), Padtic ridges (crsses), Indian and suthern ridges (pen diamnds), back are basins (slid squares), and the Mid-Cayman Rise and Wdlark basin (slid diamnds); recgnized axial ht spt centers are circled. Data and references are given in Table 1. Data pints frm the Azres, Jan Mayen and Galapags ht spt centers which are anmalus n Figures 2b and 2c are enclsed in a dashed Held. (d) Lng-wavelength NAB.0 versus axial depth; data frm Table I have been cmbined and, where sampling density permitted, averaged ver >500 km f ridge length; in particular, ht spt centers and margins have been averaged tgether. NAB.0 = Na '(MGO) ; FEB.0 = FeO '(MGO) Na8.0 and FEB.0 are calculated fr samples with wt % MgO; CaO/A1203 are calculated fr samples with >5.0 wt % MgO. restricted in cverage, hwever, and the results presented herein differ in several fundamental ways. The mst well-knwn relatinship between chemistry and depth is the decrease in incmpatible element abundance and La/Sm with increasing distance frm Iceland (and hence increasing depth) alng the Reykjanes Ridge, first established by Schilling [1972]. It has since been shwn that fr many ht spts near ridges there is a fairly regular change in La/Sm and radigenic istpe ratis with increasing distance frm the ht spt, and therefre with increasing depth [e.g., Schilling et al., 1982, 1983, 1985; Sun et al., 1975; Hart et al., 1973; Verrna and Schilling, 1982]. Hamelin et al. [1984] prpsed an verall crrespndence between depth and Pb and Sr istpic

5 KLEIN AND LANOMUIR: GLOBAL CORRELATIONS P.O 1.5, I I I I i DEPTH ( m ) I Fig. 2. (cntinued) cmpsitin fr the Nrth Atlantic, althugh the slpe f the crrelatin is different fr each ht spt. There are tw imprtant distinctins between the glbal crrelatins f Figure 2 and the previus results frm arund ht spts. First, the previus crrelatins were based primarily n incmpatible trace elements and radigenic istpes and nt n majr elements. Secnd, all f the previus crrelatins between depth and incmpatible element r istpic cmpsitin apply nly t individual ht spts and d nt apply glbally. Fr example, the MAR near the Azres platfrm is much deeper than is The fact that reginal averages frm all spreading centers, with the exceptin f sme ht spt centers, define Iceland, yet the La/Sm and 87Sr/86Sratis near the Azres crrelatins between Na8. 0, CaO/A1203, Fe8. 0, and depth are much higher. Indeed, ne f the majr pints made by (Figure 2) suggests that there are prcesses that gvern bth Schilling et al. [1983] was the cntrast in chemistry between the Azres and Iceland regins. But such cntrasts are nt restricted t htspts. Sr istpic data fr sme basalts frm the Australian-Antarctic Discrdance are similar t thse reprted frm Iceland, yet the tw regins represent fractinatin, majr element surce hetergeneity, and extremes in bth NaB. 0 and bathymetry [Klein et al., 1984; varying extents f partial melting. Althugh there is Schilling et al., 1983]. Furthermre, we nted abve the cntrast between lcal Na8.0-depth variatins arund sme individual ht spts and the verall glbal crrelatin. If the ht spt signature dminated the glbal crrelatin between depth and Na8. 0 then ne might expect a negative glbal crreladn f Na8.0 and depth, since shallw ht spt centers tend t be higher in Na8.0 than the deeper margins f ht spts. Thus there are fundamental distinctins between the previusly nted crrelatins arund ht spts and the glbal crrelatins presented in Figure 2. Michael and Bnatti [1985] have recently shwn a crrelatin between the cmpsitin f minerals frm ultramafic rcks recvered frm the Nrth Ariantic and depth f the ridge axis. In additin, Dick et al. [1984] shwed crrelatins between the chemistry f abyssal ultramarie rcks and bth residual geid and the chemistry f spatially assciated basalts fr several lcalities in the Atlantic and Indian ceans. The cmbinatin f these bservatins leads t a result that is cmpletely cnsistent with the glbal regularities f Figure 2. In view f the paucity f the abyssal ultramafic data base and the inherent ambiguities in the representativehess f small ultramafic samples, it is remarkable that these studies captured the essence f the glbal regularities illustrated in Figure 2. We nte, hwever, that the results f Michael and Bnatti [1985] and Dick et al. [1984] were dminated by data frm the central Nrth Atlantic. In the glbal crrelatins f Figure 2 it appears that the central Nrth Atlantic regin may be in fact chemically anmalus in a glbal cntext. POSSmLE CAUSES OF THE GLOBAL CORRELATIONS axial depth and aspects f basalt chemistry n a glbal scale, despite the cmplicated petrgenetic histries f lcal areas. In the fllwing, we cnsider three pssible explanatins fr the bserved crrelatins: clinpyrxene evidence fr the imprtance f all f these t sme extent in varius areas, the partial melting mdel alne prvides a straightfrward explanatin f the first-rder crrelatin between reginal basalt chemistry and average depth. Much f the discussin that fllws in this sectin invlves detailed gechemical mdeling and cmparisn f the glbal data t the results f experimental studies n the melting behavir f peridires. The key pint is that the verall glbal systematics are cnsistent with varying extents f melting at varying pressures, with basalts frm the shallwest ridge segments prduced by larger extents f melting deeper in the mantle. The implicatins f these cnclusins are explred in the discussin sectin. C linpyrxene Fractinatin Sdium behaves as a mderately incmpatible element with respect t clinpyrxene and the CaO/A120 3 rati in clinpyrxene is high [e.g., Dungan and Rhdes, 1978]. Thus substantial clinpyrxene fractinatin wuld be expected t prduce an inverse crrelatin between CaO/A1203 and Na8. 0 similar t that bserved. The ttal variatin in reginal CaO/A1203 is Typical lw-pressure clinpyrxenes cntain abut 20 wt %

6 8094 KLEIN AND LANOMUIR: GLOBAL CORRELATIONS TABLE 1. Reginal Averages f Smthed Axial Depth and Parameters f Basalt Chemistry fr Oceanic Spreading Centers Latitude Smthed r Axial CaO! Regin Lngitude, Depth, Na8.0 s.d. Fe8.0 s.d. N A1203 s.d. N Reference deg km Ocean Ridges Nrth Atlantic N , N , N , N , N , N , N , N , N N , N , N ,3, N ,3, N , N , N , N , N , N , N , N , N , N ,6' N ,6' N ,6' N ,6' N ,8* N ,8* N ,6* N ,10,6' N ,10,6' N ,6* N ,6* N ,6* Suth Atlantic S ,6' 34.55S S S S American-Antarctic W ,6' W ,6' W ,6' W ,6' W ,6' Suthwest Indian S ,6' S ,6,16' Central Indian S ,6' S ,6' S ,6* Carlsberg N ,6* Red Sea N ,6' Sutheast Indian E ,21' E ,21' E ,21' Juan de Fuca 44.75N ,23* Grda N * East Pacific Rise N ,25* N ,25* N , N , N , N ,25 Galapags Spreading Center W , W , W , W ,28

7 KLEIN AND LANOMUIR: GLO! AL CORRELATIONS 8095 TABLE 1. (cntinued) Latitude Smthed r Axial CaO/ Regin Lngitude, Depth, Na8.0 s.d. Fe8. 0 s.d. N A1203 s.d. N Reference deg km Easter Micrplate Eastern Ridge Western Ridge W , W , W , S , S , S , S , S , S , S ,30 Back Arc Basins Marianas Trugh 17.39N ,6,32' 18.01N ,6,32' Lau Basin S ,6,32* Sctia Sea S ,6,32* Other Spreading Centers Cayman Trugh N ,6,36* N ,6,36* N ,6,36* Wdlark Basin N Na8. 0 and Fe8. 0 are calculated values f Na20 and FeO at 8 wt % MgO, as discussed in the text and captin t Figure 2; s.d. refers t ne standard deviatin. N is the number f samples used t cmpute each mean. Reference clumn indicates reference numbers fr the surces f the bathymetric and chemical data, as indicated belw; where nly ne number is given, chemical and bathymetric data are frm the same reference. Smthed axial depth refers t average depth t nevlcanic zne. * Data frm these references are included in a 1981 versin f the Smithsnian Institutin catalg f basalt glasses; see reference 6. The majrity f the chemical data frm the Nrth Atlantic are either frm Schilling et al. [1983] r W. Melsn (see reference 6); bth sets f data were used in rder t achieve the fullest pssible cverage; fr a few Nrth Atlantic ridge segments, there is verlap between the tw data sets. References: 1, Vgt [1986]; 2, Neumann and Schilling [1984]; 3, Schilling et al. [1983]; 4, Lamnt-Dherty Gelgical Observatry (L-DGO) unpublished plasma analyses; 5, Le Duaran and Francheteau, [1981]; 6, 1981 versin f the Smithsnian Institutin catalg f basalt glass analyses, which includes analyses reprted by Melsn et al. [1976, 1977] and Melsn and O'Hearn [1979, 1986]; 7, Phillips and Fleming [1978]; 8, Stakes et al. [1984]; 9, Pckalny et al. [1985]; 10, Bryan et al. [1981]; 11, N. Cherkis (persnal cmmunicatin, 1987); 12, Humphris et al. [1985]; 13, Lawyer and Dick [1983], samples f lerex et al. [1985]; 14, Sclater et al. [1976]; 15, Sclater et al. [1981]; 16, Price et al. [1986]; 17, Fisher et al. [1971]; 18, McKenzie and Sclater [1971]; 19, Bnatti et al. [1984]; 20, Weissel and Hayes [1974]; 21, E. Klein et al. (manuscript in preparatin, 1987); 22, Kappel and Ryan [1986]; 23, E. Kappel (unpublishedata, 1986); 24, Tamay Scientific Team [1984]; 25, L-DGO, unpublished analyses: samples frm 5-14øN f Langmuir et al. [1986]; 26, Macdnald et al. [1984]; 27, Christie and Sintn [1981]; 28, Schilling et al. [1982]; 29, Hey et al. [1985]; 30, Schilling et al. [1985]; 31, Fryer and Hussng [1981]; 32, Fryer et al. [1981]; 33, Hawkins [1976]; 34, Barker [1972]; 35, Strup and Fx [1981]; 36, Thrnit ',, et al. [1980]; 37, Perfit et al. [1987]; 38, Klitgrd and Mammerick [1982]. CaO and 4 wt % A120 3 [Shibata et al., 1979; Grve and ne wuld therefre expect the cmpsitins frm different Bryan, 1983]. Thus, t prduce the bserved range in areas t cnverge at high MgO cntents, and the bserved CaO/A1203 wuld require apprximately 25-30% equilibrium variatins in chemistry t ccur nly at lw values f MgO. crystallizatin f lw-pressure clinpyrxene. This amunt In fact, the chemical variatins between different regins f crystallizatin, hwever, wuld increase the Na20 abundances in the residual liquids by nly 30-50%, much less than the factr f 2 variatins bserved. If highpressure, aluminus clinpyrxene were the crystallizing phase, mre crystallizatin wuld be required t prduce the bserved range in CaO/A120 3 ratis, which might lead t the requisite increase in Na20. remain distinct fr all extents f fractinatin (e.g., Figure 1) and appear t be fundamental features f the parental magmas. Secnd, because clinpyrxenes have less FeO than the magmas frm which they crystallize, clinpyrxene crystallizatin wuld lead t prgressively higher FeO cntents with decreasing CaO/A120 3 and increasing NaO. This is ppsite t the bserved glbal trends; areas f high Hwever, there are tw aspects f the data which are nt CaO/A1203 and lw Na8.0 have the greatest FeO cnsistent with high-pressure clinpyrxene fractinatin. abundances, nt the least. Thus, althugh high-pressure First, such crystallizatin wuld prgressively lwer MgO; clinpyrxene fractinatin may ccur, it is nt an adequate

8 8096 KLEIN AND LANOMUIR: GLO! AL C ORREL ATIONS explanatin fr the glbal crrelatins. cause sme f the scatter within particularly fr FEB.0. It may, hwever, the crrelatins, Varying Surce Cmpsitins A secnd pssible mdel is that the depth/chemistry systematics result frm surce hetergeneity in majr element cmpsitin and hence mineralgy. While it is clear that the mantle is hetergeneus with respect t highly incmpatible trace element abundances and istpic cmpsitin [e.g., Schilling et al., 1983], the evidence fr majr element surce hetergeneity n the scale sampled by melting has been mre ambiguus [Langmuir and Hansn, 1980; Sigurdssn, 1981; Bryan and Dick, 1982; Dick et al., 1984]. Nevertheless, it is imprtant t explre the pssibility that the bserved chemical systematics result frm majr element surce hetergeneity. In the simplest mdel, all samples wuld be prduced by apprximately the same extents f melting and the bserved chemical variatins wuld reflect the extent f fertility r depletin f the surce. Samples prduced by melting f a mre fertile surce, fr example, wuld be expected t have higher Na20, A1203, and FeO than samples prduced by the same extent f melting f a mre depleted surce. This wuld lead t a psitive rather than the bserved inverse crrelatin between NAB.0 and FEB.0. This simple mdel will nt strictly apply, hwever, because the mre depleted mantle wuld have a higher slidus temperature and hence wuld melt less, leading t a relative enrichment in Na20. Surce depletin is thus clinpyrxene. Fr the extents f melting relevant t cnpensated fr, t sme extent, by the decrease in melting. MORB (see quantitative mdeling belw), plagiclase Hwever, fr elements with small partitin cefficients, stability is expected t play nly a minr rle in the such as Na20, the effect f surce depletin far utweighs melting systematics f Na20. T a first rder, it appears, that f decreased melting. Hence a simple mdel f majr element hetergeneity predicts a psitive rather than the bserved inverse crrelatin between NaB. 0 and FeB. 0. therefre, that Na20 behaves as a mderately incmpatible trace element, shwing highest cncentratins at the smallest extents f melting and decreasing in cncentratin Majr element surce hetergeneity may, hwever, by dilutin as the extent f melting increases [Jaques and cntribute t sme f the scatter f Figures 2a-2c. Green, 1980; Fujii and $carfe, 1985], Therefre, if NAB.0 In particular, tw aspects f the data, may require surce apprximates the sdium abundances in relatively primitive hetergeneity. First, the >500 km-wavelength chemical magmas and the Na20 cntent f the surce is cnstant, averages (Figure 2d) shw that the Indian and Nrth Atlantic variatins in the abundances f Na20 frm regin t regin ridges frm distinct curvilinear trends within the verall may indicate variatins in the extents f melting. Dick et glbal crrelatin. This fine-structure within the verall al. [1984] and Michael and Bnatti [1985] reached similar crrelatin may result frm majr element and mineralgical cnclusins regarding the behavir f Na20 during melting hetergeneities. Secnd, samples frm ridges near the f the manfie, based n their studies f ceanic peridtires. Azres, Jan Mayen, Galapags and, t a lesser extent, Easter CaO and A120 3 als appear t shw systematic behavir micrplate ht spt regins d nt plt n the glbal trends during melting f spinel lherzlite (Figure 3a). Fujii and f CaO/A1203 and FeB. 0 versus depth, but shw markedly Scarfe [1985] and Jaques and Green [1980] shwed that lwer values fr the same depth cmpared t ther regins A1203 abundances are highest in the first increments f melt (Figures 2b and 2c). Schilling et al. [1983] attributed the lw CaO/A120 3 ratis and lw Sc abundances in the frmer tw areas t enhanced clinpyrxene crystallizatin. Langmuir and Hansn [1980] nted the unusually lw FeO cntent f basalts frm the MAR near the Azres and suggested that the lw FeO resulted frm majr element hetergeneity. Sigurdssn [1981] bserved that bth the FeO and SiO2 cntents f these samples were t lw t be explained exclusively by clinpyrxene fractinatin. Thus the Azres, Jan Mayen, Galapags, and, t a lesser extent, Easter micrplate plumes appear t be anmalus in a glbal cntext and may represent evidence f majr element surce hetergeneity. Varying Extents f Partial Melting Melting beneath ridges is likely t ccur by adiabatic upwelling f the mantle. In this case, fr a given mantle cmpsitin, the amunt f melt prduced shuld be cntrlled largely by the temperature f intersectin f the mantle slidus. An bvius third mdel t explre, then, is that the glbal crrelatins f Figure 2 reflect different extents f melting in respnse t variatins in the temperature f the mantle. T explre this mdel, it is necessary t understand hw majr elements vary in respnse t differences in the extent and pressure f melting f the mantle. EVALUATION OF THE PARTtA MELTING HYPOTHF3IS Majr Element Systematics During Partial Melting A number f experimental studies in recent years [e.g., Jaques and Green, 1980; Fujii and Scarfe, 1985; Takahashi, 1985; Takahashi and Kushir, 1983; Mysen and Kushir, 1977] have prvided an adequate data base t infer the respnse f mst majr elements t increasing extents f melting. In this sectin we attempt t synthesize these data t cnstrain the extents and pressures f melting fr the basalt data. Experimental data shw that Na20 is perfectly incmpatible with respect t livine, strngly incmpatible with respect t rthpyrxene, and that at pressures abve plagiclase stability (apprximately 8-9 kbar [Presnall et al., 1979]) the manfie budget f Na20 resides primarily in and decrease with further extents f melting f spinel lherzlite. CaO appears t shw smewhat mre cmplicated melting behavir. CaO increases as clinpyrxene melts, but when clinpyrxene is n lnger present as a residual phase, CaO decreases with further melting. Thus the CaO/A120 3 rati shws the fllwing systematics: CaO/A120 3 is at a m'mimum as melting begins; it increases thereafter until clinpyrxene has melted ut, at which pint it has a higher value than the initial surce because sme A120 3 remains in rthpyrxene; as melting prceeds and A1203 frm rthpyrxene is added t the melt, the CaO/A120 3 rati decreases slighfiy. It is imprtant t nte that as lng as clinpyrxene is present

9 - KLEIN AND LANOMUIR: GLOBAL CORRELATIONS _ I I I I I FUJII 8 SCARFE 10kb I0 kb MELTI c 25 CALCULATED MAXIMUM MELTING CURVE /' I0 kb 1310øC FUJII I SCARFE IOkb MELTING I0-13 IOkb - ( ) ( ) ' _ _ 1250øc I I I I I ,0 3,5 N PATHS OF ADIABATIC MELTING FRACTIONAL CRYSTALLIZATION OF OLIVINE Fig. 3. Nag.0 versus A120 3 (wt %) and CaO (wt %) fr representative sarnples with wt % MgO. Symbls are as in Figure I0 12 Als shwn are fields fr prgressive melting f tw synthetic FeO (ctin mle %) spine1 lherzlites at 10 kbar, and the tw melt cmpsitins fr the reprted temperatur extremes f 1250'C and 1310'C [frm Fujii and Scarfe, 1985]. Arrws indicate directin f cmpsitinal change Fig. 5. FeO versus MgO (catin mle percent) [after Langmuir and with increasing extents f melting. NaB. 0 was calculated frm the Hansn, 1980]. The 0% melting line and the maximum melting reprted analyses after crrectin t 8.5 wt % MgO by remval f curve are calculated fr an initial bulk cmpsitin fr pyrlite F90 livine. Nte that the trends f the basalt data are sub-parallel reprted by Jaques and Green [1980]. Als shwn are melt t the trends f the experimental melt fields. cmpsitins prduced by prgressive melting f pyrlite (pen triangles) and Tinaquill lherzlite (slid triangles) at pressures f 5, 10, and 15 kbar, reprted by Jaques and Green [1980]. The star as a residual phase, increasing extents f melting cause an indicates a melt cmpsitin prduced by melting f natural spinel increase in the CaO/A1203 rati. The data f Jaques and lherzlite at 10 kbar, reprted by Takahashi [1985]. Als shwn Green suggest further that there is a slight pressure are paths f adiabatic melting (dashed arrws labelled X, Y, Z). The slid arrw shws a liquid line f descent fr fractinatin f dependence t the partitining f A120 3 during melting; at higher pressure, the slubility f A120 3 in pyrxenes increases, leading t lwer A120 3 in cexisting magmas (see Figure 4). The distributin f FeO and MgO during melting f the mantle has been calculated and discussed in detail by 18 I I I IO livine frm ne, arbitrary experimental melt cmpsitin, calculated accrding t the melthd f Weaver and Langmuir [1987]. Nte that crystallizatin f livine decreases MgO cntents f residual liquids but des nt appreciably change their FeO cntents. Representative data are shwn fr high MgO basalts frm the Klbeinsey Ridge (K), East Pacific Rise near the Tamay transfrm fault (T), and the Australian-Antarctic Discrdance (A), which span mst f the range in chemistry and bathymetry presented in Table 1. Langmuir and Hansn [1980]. Experimental melt - cmpsitins f Jaques and Green [1980] are shwn n the FeO/MgO diagram in Figure 5, with a melt field fr pyrlite calculated by the methd f Langmuir and Hansn [1980]. The new experimental data are cnsistent with the calculatins in the sign and magnitude f the pressure dependence, as well as the inferred path f adiabatic melting. F:.25 The data f Jaques and Green d nt agree with the single I0 10-kbar pint f Takahashi [1985], hwever, and there are I I I sme discrepancies in terms f the variatins f temperature 0 5 I0 15 with MgO cntent. P(kb) Like FeO, the systematics f SiO 2 variatins during Fig. 4. A1203 in melts prduced by prgressive melting f pyrlite melting depend n the pressure as well as the extent f (pen triangles) r Tinaquill lherzlite (slid triangles) versus the melting. Experimental studies shw that at any ne pressure f melting; data are frm Jaques and Green [1980]. F pressure, S io 2 increases with increasing extents f melting, indicates extent f melting fr each field; where necessary, melt cmpsitins at the particular extent f melting indicated have been t apprximately 40% melting [Jaques and Green, 1980; extraplated frm the reprted melt intervals. K, T, and A refer t Mysen and Kushir, 1977]. MgO cntents als increase averages fr samples frm the Klbeinsey Ridge, EPR suth f the with increased melting at any ne pressure, which leads t Tamay transfrm, and the Australian-Antarctic Discrdance, greater extents f livine fractinatin t reach a reference respectively, and were pltted accrding t their A120 3 abundances value f MgO. Since livine has a lw SiO2 cntent, such and inferred extents f melting, back-crrected t primary MgO cntents f 15%, 12%, and 10%, respectively, t crrespnd t fractinatin f livine t lwer MgO cntents leads t still Figure 5. higher S io2 cntents in the fractinated magma. These tw

10 8098 KLEIN AND LANOMUIR: GLOBAL CORRELATIONS a element cmpsitins alne. The systematics can be applied t small-scale petrgenetic studies as well as t the glbal scale prblems cnsidered in this paper kb O Si02 at 9wt % MgO Klbeinsey Ridge, have undergne larger extents f melting Fig. 6. (a) SiO 2 cntents (wt %) calculated t 9 wt % MgOby remval f F90 livine in melts frm the experiments f Jaques than areas where basalts have high Na8. 0 and lw and Green [1980], and Takahashi [1985] pltted versus the extent CaO/A Figure 3 shws sme representative reginal f melting (extents f melting fr the data f Takahashi were averages f CaO and A120 3 fr samples with wt % cnstrained by Na20 and TiO2 abundances. The 10- and 5-kbar MgO (t minimize the effects f fractinatin) pltted fields verlap slightly. Nte that SiO 2 at 9 wt % MgO increases with increasing extents f melting; fr the same extent f melting, against reginal Na8.0 values frm Table 1; als pltted are melts prduced at higher pressure have lwer SiO 2 cntents. Als experimentally determined melt variatins in CaO and A1203 shwn are mean SiO 2 cntents fr basalts frm the Klbeinsey Ridge (K), Tamay regin f the EPR (T), the Australian-Antarctic Discrdance (A), and the Mid-Cayman Rise (C). Extents f melting 3.5 i fr these areas were estimated frm their Na8. 0 cntents as discussed in the text. (b) Reginal averages f SiO2 determined by plasma emissin spectrmetry versus Na8.0 (frm Table 1), with the exceptin f the SiO2 data fr Cayman and Klbeinsey, which are frm the Smithsnian micrprbe (see reference 6, Table 1) and Schilling et al. [1983], respectively, and were crrected t a cmparable basis (nrmalized t 100%, ttal FeO as Fe203, and an 3.0 interlabratry crrectin factr f applied t the Smithsnian data). Symbls as in Figure 2. Nte that fr the same Na8. 0 samples frm the Pacific are ffset t lwer SiO 2 abundances, cmpared t samples frm Atlantic and Indian cean ridges. The bars n the ne pint shw the standard deviatin in SiO 2 fr 42 analyzed samples frm the Tamay regin. additive effects prduce higher SiO 2 cntents in basalts evlved frm parental magmas derived by greater extents f melting at a given pressure. This smewhat surprising result is illustrated in Figure 6a, and is cnsistent with experimental data frm fur labratries [Jaques and Green, 1980; Fujii and Scarfe, 1985; Takahashi, 1985; Mysen and Kushir, 1977]. Hwever, the abundance f SiO 2 in the melt als depends strngly n the pressure f melting. Fr a given extent f melting, increased pressure causes melts t have lwer SiO 2 cntents (see Figure 6a). A summary f the behavir f the majr elements during melting f the mantle is presented in Table 2. When cmbined with planetary cnstraints n the mantle cmpsitin, Table 2 and assciated figures allw estimates f extents and pressures f melting t be made frm majr Applicatin f Melting Systematics t the Glbal Chemistry/Depth Crrelatins The glbal MORB data set shws the fllwing general characteristics, as exemplified by data frm shallw, "nrmal," and deep regins, that is, the Klbeinsey Ridge, the East Pacific Rise near the Tamay transfrm, and the Australi. an-antarctic Discrdance, respectively. Regins with lw Na8. 0 such as the Klbeinsey Ridge, have relatively high FeO and CaO/A120 3 and lw SiO 2 Cnversely, regins with high Na8.0 such as the Australian- Antarctic Discrdance and the Cayman Trugh, have relatively lw FeO and CaO/A120 3 and higher SiO2. The relative variatins in Na8. 0 immediately suggesthat lwer Na8. 0 regins are derived by greater extents f melting. The glbal inverse crrelatin between CaO/A120 3 and Na8.0 is als cnsistent with the effects f different extents f melting, with clinpyrxene as a residual phase fr mst regins; hwever, fr the area immediately arund Iceland, where CaO/A120 3 may begin t decrease slightly as Na8. 0 decreases (see Figure 2b), clinpyrxene may n lnger be a residual phase. This suggests that areas erupting basalts with lw Na8.0 and high CaO/A1203, such as the z 2.5?.0 I 49ø SiOz (wt %) Fig. 6 (cntinued)

11 KLEIN AND LANGMUIR: GLOnAL CORRELATIONS 8099 Table 2. Systematics f Melting f Spinel Lherzlite Frm F=0 t 0.30 Element Instanteus Melts Pled Melts General in an Adiabatically frm an Adiabatically Systematics Upwelling Clumn Upwelling Clumn Figure SiO2 A1203 MgO FeO Na20 CaO fr cnstant MgO, increases with F, decreases with increasing P decreases with F, decreases with increasing P increases with F at cnstant P, increases with increasing P cnstant with F at cnstant P, increases with increasing P decreases with F; behaves as a mderately incmpatible element increases with F decreases with F apprximately cn stant decreases with increasing P decreases with increasing P increases with increasing P decreases with F increases with increasing P decreases with F decreases with increasing P increases with F while cpx is increases with F increases with inresidual, then decreases with while cpx is creasing P while increasing F residual cpx is residual, then decreases 6aand 13 3aand b versus Na8.0 with prgressive melting f a spinel lherzlite [Fujii and Scarfe, 1985]. Each element varies in the sense expected by different extents f melting. Understanding the variatins in FeO and SiO 2 requires cnsideratin f the pressure f melting as well as the extent f melting. Discussin f the effects f pressure n melting systematics must, in turn, distinguish between segregatin pressure and the pressure at the intersectin f the slidus. In Figure 5, the lwer dashed arrw (X) shws the adiabatic path f melting that wuld result frm an upwelling diapir that intersects the manfie slidus at abut 7 kbar. Magmas derived by lwer extents f melting in such a diapir wuld have bth higher Na8.0 and Fe8. 0 relative t magmas frmed by greater extents f melting in the same diapir. Thus, if FeO were cntrlled simply by the extent f melting in diapirs that intersected the slidus at a single pressure, ne wuld expect t see a psitive crrelatin between Fe8. 0 and Na8.0, which is nt bserved. A secnd pssibility is the case where tw diapirs intersect the slidus at different pressures but segregate at the same pressure. In Figure $, diapirs X and Y intersect the slidus at >20 kbar and 15 kbar, respectively. If bth diapirs segregated within the 10-kbar melt field, then magmas derived frm the tw diapirs wuld have apprximately the same Fe8. 0 cntents, but X, having undergne greater extents f melting wuld have lwer Na8. 0 than Y wuld have. This scenari wuld therefre lead t n crrelatin bserved inverse crrelatin between Na8. 0 and Fe8. 0 (cmpare Figures 2a and 2c). Using the Jaques and Green data t calibrate the pressure effects, the high Na8.0 (lw Fe8.0) data (e.g., the Australian-Antarctic Discrdance) are cnsistent with mean segregatin pressures f abut 5 kbar, while the medium Na8. 0 (and Fe8.0) data (e.g., EPR, Tamay regin) require mean pressures f 10 kbar. The Klbeinsey Ridge data (lw Na8.0, high Fe8.0) suggest mean segregatin pressures f abut 15 kbar. Figure 5 als implies that the samples derived by higher extents f melting at higher mean pressures have undergne mre fractinatin f livine prir t eruptin. If the Jaques and Green [1980] data are pertinent, MgO cntents f primary magmas range frm abut 10 wt % ( 14 catin mle %) fr the deep cean regins prduced by the smallest extents f melting, and up t 15 wt % ( 20 catin mle %) fr the shallwest regins arund Iceland, prduced by larger extents f melting. The SiO 2 variatins with pressure and extent f melting are subtle, and t discern their systematics in the data requires gd analytical precisin. Lamnt plasma data n glasses are precise fr SiO 2 t wt %, substantially better than micrprbe data. Figure 6 shws reginal averages f SiO 2 fr samples with wt % MgO versus NAB.0 (frm Table 1) fr all f the areas fr which there are plasma data, with the additin f data frm the literature fr the critical areas at the extreme chemical and depth ranges (Mid-Cayman Rise and Klbeinsey Ridge), fr which we have used the Smithsnian micrprbe data [Thmpsn et al., 1980] and the whle rck data f Schilling et al. [1983]. There is a psitive crrelatin between NAB.0 and SiO 2 (Figure 6b), and apparently the data frm the Pacific are between Na8. 0 and Fe8. 0. Cnsider, last, a scenari in which a diapir that intersects the slidus at a higher pressure als segregates at a higher pressure. In Figure $, diapirs Z, slightly ffset t lwer SiO2. Y, and X intersect the slidus at successively higher pressures; the mean pressures f melting within each diapir will increase prgressively frm Z t X and lead t increasing Fe8. 0, and the mean extent f melting will increase frm Z t X. This scenari is cnsistent with the If the glbal chemical variatins were prduced by different extents f melting at a cnstant equilibratin pressure (t.e., cnstant pressure f melt segregatin), ne wuld expect an inverse crrelatin between SiO 2 and Na2, cntrary t what is bserved. Thus the S io2 data, like the FeO data, suggest that the areas where lwer extents f melting ccurred als melted at lwer mean pressures. The SiO 2 data can be used t quantitatively cnstrain mean pressures f melting and suggest a pressure range f 5-20 kbar (cmpare Figures 5a and 5b), which is apprximately the same as that inferred frm FeO.

12 8100 KLEIN AND LANOMUIR: GLOBAL CORRELATIONS melting mve parental magma cmpsitins tward silica, while increased pressures f melting shift melts away frm silica and tward the livine apex. The data suggesthat the dminant cntrl n psitin f livine + plagiclase- Fig. 7. Pseud-ternavdiagram f nnnative dipside-livine-silica saturated magmas n this diagram relative t silica is the (Di, O1, Si) prjected frm plagiclase; calculated accrding t the extent f melting rather than the pressure f melting. Thus methd f Walker et al. [1979]. K, T, and A are as in Figure 4. Als shwn are the results f prgressive melting f tw synthetic the use f this diagram t cnstrain pressures f melting is spinel lherzlites at 10 kbar (pen and slid triangles) reprted by nt straightfrward. Fr cnstraints n pressure, it appears In the discussin abve, majr element systematics during melting, as a functin f bth pressure and extent f melting, have been examined element. by element. In rder t investigate hw the majr elements vary in cncert, a cmmn apprach has invlved the use f the pseudternary f Walker et al. [1979] and Stlper [1980]. Figure 7 shws the plagiclase prjectin f the pseud-ternary with the data f Fujii and Scarfe [1985] fr cmpsitins prduced by equilibratin f basalt with spinel lherzlite ver a range f temperatures at 10 kbar. Melt cmpsitins at the lwest temperatures, analgus t smaller extents f melting, plt n the silica-undersamrated side f the psuedternary. With prgressively larger extents f melting at a single pressure, the cmpsitins becme mre enriched in the silica cmpnent. This finding was als nted by Takahashi and Kushir [1983] and Presnall and Hver [ The data clearly shw that there are n five-phase invariant pints n this prjected phase diagram (see, e.g., Fujii and Scarfe [1985]). The psitins f the s-called isbaric invariant pints were inferred frm the disappearance f clinpyrxene as a liquidus phase in experimental charges [Stlper, 1980]. Since the pints are nt invariant, they simply shw the pint alng melting curves where clinpyrxene melts ut f the residue. With increased pressure the melting curves shift tward the livine apex [Stlper, 1980; Takahashi and Kushir, 1983; Presnall and Hver, 1984]. Als shwn in Figure 7 are basalts frm three regins encmpassing the range in chemistry and bathymetry presented in Table 1. The relative psitins f the three type regins n this diagram are cnsistent with the experimental data suggesting varying extents f melting. The Klbeinsey data, which the chemical systematics suggest are derived by greater extents f melting at higher mean pressures, are highest in the silica cmpnent, (althugh lwest in SiO2). Thus the psitin f a parental magma n this diagram is determined by tw cmpeting effects: increasing extents f Fujii and Scarfe [1985]. Arrws indicate directin f cmpsitinal t be simplest t examine variatins in the abundance f change in the melts with increasing extents f melting. individual xides that are pressure-sensitive, such as FeO and SiO2. The A1203 abundances in MORB als supprt these estimates f the range in pressures f melting. In additin Quantitative Cnstraints n the Extents f Melting t the inverse variatin between A120 3 and extent f melting (Figure 3), A120 3 abundanceshw a subtle inverse The majr element data allw upper limits t be placed n crrelatin with pressure (Figure 4). The high A1203 the extents f melting fr the different ceanic regins. cntents f the MORB data suggest that the mean pressure Data frm primitive mantle ndules [Jagutz et al., 1979], f melting ranges frm >5 t <15 kbar, if the Jaques and peridtite suites [Frey et al., 1985], and chndritic Green data are pertinent (Figure 4). Thus all the majr cnsideratins [Hart and Zindler, 1987] suggesthat Na20 in element data are remarkably cnsistent with an rigin by the undepleted mantle is 0.30 _+ ~0.03, CaO abut 3.2 wt %, different extents f melting ver different pressure intervals; and A120 3 abut 4.1 wt %. Since the range in Na20 fr the basalts after crrectin fr livine fractinatin is 1.5 wt % melts prduced by greater extents f melting appear t be generated at higher pressures f melting. Na20 (fr the Klbeinsey Ridge) t 3.4 wt % Na20 (fr the Cayman Trugh), this wuld imply a range in extent f Implicatins fr Use f the Walker/Stlper melting (F) frm 0.20 t 0.09, if sdium were perfectly Prjected Phase Diagram incmpatible. Because Na20 is nt perfectly incmpatible, hwever, these values represent upper limits n F fr their respective regins. Upper limits can als be btained fr CaO and A Hw clse these limits apprach actual values depends n hw cmpatible the element is during melting. Fr the Klbeinsey Ridge, the upper limits n F are 0.32 fr A1 and 0.29 fr Ca. Fr the Mid-Cayman Rise the upper limits are 0.25 fr A1 and 0.33 fr Ca. Thus, fr the lwest extents f melting beneath Cayman, Ca and A1 clearly behave mre cmpatibly and d nt give useful upper limits, while fr greater extents f melting beneath Klbeinsey Ridge the upper limits are much clser t that prvided by Na. If the MORB surce is mre depleted than primitive mantle, these upper limits wuld be decreased in prprtin t the depletin f the surce. Thus the bunds f <9% t <20% melting are difficult t exceed. The upper limit f 20% melting is prbably a gd estimate f the extent f melting required t prduce the Klbeinsey data, since Na is likely t be very incmpatible nce clinpyrxene disappears frm the residue. Fr the lwer extents f melting, if ne assumes <15% clinpyrxene in the residue and a distributin cefficient fr Na20 in clinpyrxene f 0.15, then ne btains an estimate f 7-8% melting fr the highest Na20 melts. Thus, the likely range f F t accunt

13 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS a b x x xx IP I I ' NAB.0 I i I I I 500- C..,,.. //' 40( - / - ///,,,?...,,/ - // UILIBRIUM / /- / CRYSTALLIZATION -. / / Of OLIVINE / _ h YO./ /. FRACTIONAL I0 ' /' CRYSTALLIZATION I i i i I i 8 i0 iz MgO(wt%) Fig. 8. Na8.0 versus (a) (Smffb) N and (b) Sc (ppm). Symbls are as in Figure 2. (Smffb) N was calculated nly fr samples with (La/Ce)N _<1. Se abundances are fr samples with >7.5 wt % MgO. Stars are fr samples frm 140 Ma crust, as discussed in the text. Data are frm Schilling et al. [1983], E. Klein et al. (manuscript in preparatin, 1987), Rice et al. [1980], Bryan et al. [1981], Humphris et al. [1985], Price et al. [1986], Schilling et al. [1982], Fryer et al. [1981], Perfit et al. [1987]; lerex et al. [1983], Bender et al. [1984], Kay et al. [1970], Schilling [1969], Dickey et al. [1977], Saunders and Tarney [1979], and M. Perfit and G. Waggner (unpublished data, 1986). (c) Ni (ppm) versus MgO (wt %) fr samples frm Klbeinsey Ridge (slid circles), EPR suth f the Tamaye transfrm (crsses), and the Australian-Antarctic Discrdance (pen diamnds). Data frm Schilling et al. [1983], Bender et al. [1984], C.H. Langmuir and J.F. Bender (unpublished data, 1987), and E. Klein et al. (manuscript in preparatin, 1987). Als shwn is a field fr melting 0-30% f the manfie, after Hart and Davis [1978] and Nabelek and Langmuir [1986]. Arrws indicate changes in residual liquid cmpsitins fr equilibrium (dashed) and fractinal (slid) crystallizatin f elivine frm varius primary magma cmpsitins. Nte that the three MORB suites, AAD, Tamaye, and Klbeinsey, must be derived frm parental magmas with prgressively higher MgO and Ni cntents, in agreement with Figure 5. lb fr the glbal range in data is apprximately 8-20% melting. Trace Element Cnsistency With the Partial Melting ttypthesis Glbal systematics f trace element variatins fr elements that are nt highly incmpatible supprt the inferences frm majr elements presented abve. Figure 8a shws reginal averages f available data fr Sc abundances and (Sm/Yb)N ratis versus NaB. 0. In rder t minimize the effects f surce hetergeneity r surce vlume differences fr strngly incmpatible elements [e.g., McKenzie, 1985; O'Hara, 1985; Zindler and J agutz, 1987], (Sm/Yb) N ratis are pltted nly fr regins that exhibit (La/Ce)N<i. Because Sm, Yb, and Sc are nly mderately incmpatible during melting f mantle assemblages, these element can be mdeled in a simpler way than highly incmpatible elements because their abundances in the surce shuld apprximate thse f the bulk mantle and are unlikely t be substantially mdified by prcesses such as mantle metasmatism, remval f small amunts f melt, r frmatin f the cntinents. The equatin fr batch melting f the mantle where the bulk distributin cefficient fr the mineralgy f the slid that is melting differs frm that f the bulk mantle is Cz,/C=I/[D O+F(1- p)] (1) where C L is the abundance f the element in the magma, C is its abundance in the surce, F is the fractin f melt, D is the bulk distributin cefficient at the nset f melting, and g is the distributin cefficient f the actual prtin f the slid that is being cnverted int liquid [Shaw, 1970]. Assuming that g is cnstant, then fr any tw extents f melting, F 1 and F 2, manipulatin f equatin (1) leads t: D = C * (F2CL2 - FICL 1) I [CL2*CLI(F2- F1)] (2) Using the tw extreme values f F estimated abve frm the majr elements, C 'S estimated frm primitive mantle abundances, and the tw C L'S frm the measured basalt abundances, D can be determined frm equatin (2), and g frm equatin (1). Using C abundances f 17, 0.38, and 0.42 ppm fr Sc, Sin, and Yb, respectively [J agutz et al., 1979], and values f F f 0.08 and 0.20 fr the tw endmembers in the MORB array, we btain values fr D f 0.69, 0.035, and fr these three elements, and values fr g f 2.75, 0.18, and These values fr bth D and g are apprpriate prvided clinpyrxene is the dminant Sc-, Sm-, and Yb-bearing mineral entering the melt. Depending upn the mineral/melt distributin cefficients used [e.g., Irving, 1978; Hendersn, 1982], the values fr g suggesthat apprximately 70% f the melt is cmpsed f clinpyrxene, and 30% is cmpsed f elivine and rthpyrxene. Furthermre, because the calculated D 'S are threefld t fivefld smaller than the values fr g, these calculatins suggest that the riginal bulk surce cntained ~15% clinpyrxene, which is cnsistent with primitive upper mantle estimates [J agutz et al., 1979]. Hart and Davis [1978] presented data which shwed that varius basalt suites had different MgO-Ni characteristics, and Nabelelc and Langmuir [1986] emphasized the substantial differences in Ni abundances amng MORB suites. Bth papers suggested the MgO-Ni systematics might be related t melting f the mantle under varying

14 8102 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS õ % ]- P: 20 kb ] ' *P: 40kb - I I i I I I I I i i i I 6 8 I MgO (wt%) Fig. 9. MgO versus (a) Na20, and (b) and (c) TiO 2. In Figures 9a and 9b, asteriks indicate parental magma cmpsitins calculated fr P = 40, 30, 20, and 14 kbar, as discussed in the text; slid curves are liquid lines f descent calculated fr each parental magma cmpsitin, accrding t the methd f Weaver and œangmuir [1987]; lcatins f l, pl, and cpx alng each liquid line f descent indicate calculated appearances f livine, plagiclase, and clinpyrxene; dashed curves, labeled X and Y, graphically shw the effects f an apprximate 1 wt % decrease r increase in the MgO cntents where plagiclase jins livine n the 20- and 30-kbar liquid lines f descent; the dtted line at 8 wt % MgO is shwn fr reference and indicates the apprximate lcatin f a calculated value f Na8.0 r Ti8. 0 fr each liquid line f descent. Nte that fr Na20 (Figure 9a), a change f ñ lwt % MgO where plagiclase jins livine n the liquidus OC and Y) has little effect n the apparent, relative Na20 abundances in the 20- and 30-kbar parental magmas; in cntrast, due t the abrupt bends in the liquid lines f descent fr TiO 2 ( Figure 9b), a change f ñ 1 wt % MgO where plagiclase jins livine n the liquidus reverses the apparent, relative TiO 2 cntents in their respective parental magmas. In Figure 9c are shwn fields fr data n basalts frm the Mid-Cayman Rise, EPR suth f the Tamay transfrm, Australian-Antarctic Discrdance, and Klbeinsey Ridge; arrws indicate prtins f the liquid lines f descent shwn in Figure 9b. cnditins. Figure 8c shws data fr MgO versus Ni fr three nrmal MORB suites representing the range in axial depth shwn in Table 1. Als shwn is the range f mantle melt cmpsitins, calculated as by Nabelek and œangmuir [1986], and three trajectries shwing fractinatin f livine frm different parental magmas. Fr a given MgO cntent the suite with the lwest Ni cntent is derived frm parental magmas with the highest MgO cntents. Higher MgO wuld be assciated with parental magmas generated by greater extents f melting at higher pressure. The rder f the trends in Figure 8c is thus cnsistent with the majr element cnstraints presented abve. We turn last t the systematics f TiO 2. Because TiO 2, like Na20, behaves as a mderately incmpatible element during melting, ne wuld expect a crrespndence between the abundances f these tw xides frm regin t regin. This is partially true: high TiO2 and Na20 abundances ccur in basalts frm the Mid-Cayman Rise, and lw TiO 2 and Na20 abundances ccur in samples frm the Klbeinsey Ridge (Figure 9c). Hwever, in rder t cmpare fully TiO 2 abundances in suites f variably fractinated basalts frm different regins, we must crrect TiO 2 cntents fr shallwlevel fractinatin t a cmparable value, as we did fr Na20. The inaccuracies in crrectin fr lw pressure fractinatin depend n the trajectry f the liquid line f descent as fractinatin prceeds. On a plt f MgO versus Na20, livine fractinatin prduces a smth, gradual increase in Na20, with decreasing MgO. When plagiclase and then clinpyrxene jin livine n the liquidus, the slpe f this liquid line f descent increases nly slightly because clinpyrxene and particularly plagiclase remve Na20 frm the liquid (Figure 9a). Because the slpe f the liquid line f descent des nt change appreciably as new phases appear, it is pssible t crrect the Na20 abundances in v ariably fractinated basalts t a cmparable value f MgO with minimal errr (Figure 9a). The liquid line f descent fr TiO 2, hwever, differs marked!y frm that f Na20. During livine fractinatin, n a plt f MgO versus TiO 2, TiO 2 increases slightly with decreasing MgO; when plagiclase jins livine n the liquidus, hwever, the slpe f the liquid line f descent changes abruptly, and TiO 2 increases substantially with decreasing MgO (Figure 9b). Thus, in cntrast NaB. 0, the value f Tis. 0 depends critically n the precise MgO cntent where plagiclase jins livine n the liquidus (see Figure 9b). The lcatin f this abrupt bend in the liquid line f descent, hwever, depends n pressure and water cntent as well as cmpsitin. Fr example, a higher water cntent suppresses the appearance f plagiclase t lwer values f MgO. Indeed, J.M. Sintn and P. Fryer (Mariana trugh lavas frm 18øN and the rigin f back arc basin basalts, submitted t Jurnal f Gephysical Research, 1987) and Fryer et al. [1981] have argued that the lw abundances f T io 2 in Marianas back arc basin lavas result frm the suppressin f plagiclase crystallizatin t lwer MgO cntents. This sensitivity f TiO 2 abundances t the precise MgO where plagiclase appears n the liquidus may in part explain the imperfect relatinship between TiO 2 cntent and the extent f melting determined by ther majr elements. Samples frm the Klbeinsey Ridge, Tamay regin, and the

15 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS 8103 Mid-Cayman Rise shw the expected relative TiO 2 abundances, but samples frm the Australian-Antarctic Discrdance appear t be anmalusly lw in TiO 2. This prblem als exists fr FeO and may in part explain the greater scatter f the Fe8. 0 diagram (Figure 2c). The TiO 2 discrepancy may als arise in part frm the discrepant behavir f the "incmpatible elements" as a grup. These elements (including, e.g., K and Ba) are very sensitive t extractin r additin f small melt fractins, and d nt shw clear glbal crrelatins with depth. Sme very shallw regins, such as the Klbeinsey Ridge, are highly depleted in incmpatible elements [Schilling et al., 1983], while shallw ht spts are ften enriched. In additin, axial ht spts, such as Iceland, are als ften high in TiO 2 and have TiO2/Na20 ratis in excess f fertile mantle surces. Since the partitin cefficient fr Na2 appears t be less than that f TiO 2 during mantle melting, this high rati is ften difficult t recncile with melting systematics, and the TiO 2 data are nt cmpletely cnsistent with the inferences frm the majr elements. Surrvnw-y In summary, majr element and much trace element data are cnsistent with the derivatin f the brad glbal variatins in MORB chemistry by melting between 8 and 20% f the mantle with clinpyrxene present as a residual phase fr all but the very greatest extents f melting. Melts derived by lwer extents f melting equilibrate at lwer pressures in the mantle. The inferred extents f melting crrelate with the depth f the spreading center axes. Sme ht spt centers are cnsistent with these cnclusins, but thers exhibit anmalus behavir in sme elements. It is cnceivable that these latter anmalies are caused by mdificatins f the phase equilibria by vlatiles [Schilling et al., 1983], but they may als reflect majr element hetergeneity in the mantle surce. Incmpatible element abundances are strngly affected by prcesses independent f thse causing the glbal majr element variatins. DISCUSSION Cnstraints n Mantle Temperature and Melting Prcesses It has lng been suggested that melting beneath cean ridges ccurs in respnse t adiabatic upwelling f the mantle beneath the ridge. The extents f melting resulting frm adiabatic decmpressin f mantle matedhal have been calculated frequently, using cnstraints frm experimental data, thermdynamic prperties f slid and liquid, and mantle temperature estimates [e.g., Verhgen, 1954; Cawthrn, 1975; Ahem and Turctte, 1979; McKenzie, 1984]. The bttm line in these calculatins is the percentage f melt prduced per kilbar f pressure release after intersectin f the mantle slidus. The percentage f melt will vary slightly as a functin f pressure, but in view f the uncertainties in heat capacities, heats f fusin, and the extent f melting per degree abve the slidus, a cnstant value may be warranted. Ahem and Turctte [1979] calculate this percentage t be 1.2% melting per kilbar f pressure release, and we use their value. The calculatins f McKenzie [1984] range frm 0.72 t 1.5% melt/kbar, with a best estimate f,0.95% melt/kbar. These authrs and, in additin, Reid and Jacksn [1981] and Sleep and Windley [1982] made the additinal pint that the thickness f the cean crust shuld relate t the amunt f melt prduced during upwelling beneath the ridge, althugh this depends n the mantle flw regime. Since we have independent estimates frm the basalt chemistry f extents and pressures f melting, it is f interest t cmpare the thermal cntraints frm the majr and trace element chemistry with the adiabatic upwelling mdel and t determine what srt f melting mdel is cnsistent with the petrlgical data. In thery, there are several pssible mechanisms by which varying extents f melting culd ccur beneath ridge axes and thus generate the bserved spectrum f majr element variatins. Three pssible mechanisms are illustrated in Figure 10. First, all melts might be generated alng a single mantle thermal gradient but segregate at different depths and pressures (Figure 10a). This mdel is incnsistent with the data because it requires that the largest extents f melting segregate the shallwest, while the data suggest the ppsite. In the secnd mdel, a single mantle temperature gradient acts upn different mantle cmpsitins (Figure 10b). In this case, the mre depleted mantle, with lwer Na20, wuld intersect its slidus shallwer and wuld melt less, leading t higher Na20. The effects f melting and surce cmpsitin wuld therefre have ppsite effects n Na20, and the net result is nt bvius. In the third mdel, different regins f the mantle are at different temperatures at the same depth and therefre intersect the mantle slidus at different depths upn upwelling (Figure 10c). This is the mdel cnsidered by Cawthrn [1975], Ahem and Turctte [1979], Langmuir and Hansn [1980], Sleep and Windley [1982], and McKenzie [1984] and is the ne we cnsider in mre detail in light f ur majr element chemical cnstraints. In the simplest versin f this mdel, a diapir rises and underges partial melting and the melt segregates at sme cnstant pressure beneath the ridge. This mdel is implicit in many discussins f mantle melting where the "segregatin pressure" f MORBs is discussed [e.g., Presnall et al. 1979; Elthn and Scarfe, 1984]. If this were the case, hwever, ne wuld expect all the majr element data t be related by different extents f melting at a single pressure, the final pressure f equilibratin. The majr element data, hwever, are nt cnsistent with a cnstant pressure f melt segregatin. On the cntrary, segregatin pressures appear t vary, and there is a fundamental relatinship between pressure and extent f melting. Thse magmas derived by greater extents f melting recrd deeper segregatin frm their mantle surces. Integrated Melting Clumn A way t recncile the data with the simple adiabatic melting mdel is t envisin a prcess in which the melt is sampled frm thrughut the ascending mantle clumn and the erupted magma represents a mean f the melts frm the entire clumn. Fr this mdel t be physically realistic, it requires that melts can ascend rapidly relative t the mantle matrix. The mdel is essentially hw Ahern and Turctte [1979] and McKenzie [1984] envisined melting t ccur beneath cean ridges. In regins where the mantle intersects the slidus deeper, the melting clumn extends t greater depths, s the magmas will be generated ver a higher mean range f pressures. In such a mdel, the mean extent f melting is substantially less than the maximum

16 8104 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS ) b) c) B extent f melting reached at the tp f the clumn. Thus clinpyrxene culd be absent frm the residual mantle immediately beneath the ridge, althugh the erupted magma (the integrated melt frm the entire clumn) may still retain the chemical signature f residual clinpyrxene (r garnet) frm deeper in the melting clumn. Qualitatively, this mdel is clearly cnsistent with the data, but it wuld be useful t btain quantitative cnstraints. In realistic terms, a quantitative mdel is difficult because it entails melt and slid ascending at different rates, with the tw interacting as further melting prceeds. In additin, the rates f ascent f melt and slid may change as a functin f melt fractin, and sme cmpnent f lateral flw f melt beneath the ridge may ccur. It is useful, hwever, t see hw well a cnceptually simple mdel accunts fr the data. This simplified mdel can be envisined in the fllwing way. An ascending mantle diapir begins melting upn intersectin f the slidus and cntinues t melt with decreasing pressure until a final pressure f melting is reached. Thus prir t melt segregatin the melting clumn extends frm the slidus t the final pressure f melting. Within this clumn, the amunt f melt present at a particular depth (the instantaneus melt) is gverned by the difference in pressure relative t the slidus. The chemistry f each instantaneus melt is gverned by the extent f melting by which it was prduced and the pressure f its lcatin in the clumn. If instantaneus melts segregate frm thrughut the clumn and pl at shallw depth, the pled melt will represent a weighted mean f the cmpsitins f all instantaneus melts. It may be necessary t emphasize that the chemical data require substantial mean pressures f mantle equilibratin, far belw the crust, which implies that melts can ascend withut further chemical interactin with the mantle matrix. This has implicatins fr the mechanisms f ascent, but may als result because the verlying mantle is always mre depleted and cler than the immediate surce f the melt. P Quantitative Evaluatin f the Integrated Melting Clumn Several authrs in the last 15 years have calculated the extent f melting resulting frm adiabatic ascent in the mantle [e.g., Cawthrn, 1975; Ahern and Turctte, 1979; Sleep and Windley, 1982; McKenzie, 1984]. In rder t relate these calculatins t petrlgy, it is useful t define mean pressures and extents f melting that ccur within a melting clumn. If the pressure f intersectin f the slidus is P, and the final pressure f melting is Pf, then there exists a functin F(P) that gives the amunt f melt present at any pressure Fig. 10. Schematic mantle pressure versus temperature prfiles. In Figures 10a-10c, "A" and "B" represent the paths fllwed t between P and Pf. The ttal amunt f melt (F?) present within a unit clumn is then generate magmas erupted alng shallw r deep ridges, respectively. (a) The mantle is characterized by a single thermal gradient, the slidus is intersected at the same pressure everywhere, but magmas Pf segregate at different depths and therefre exhibit different extents f melting. (b) The mantle is characterized by a single thermal FT = I F(P) dp (3) gradient but there are markedly different manfie cmpsitins P beneath different ridges; "A" intersects the slidus deeper, due t its mre fertile nature, and melts mre; "B" represents mre depleted mantle that intersects its slidus shallwer and melts less. (c) The mean fractin f melting, F, is the ttal amunt f melt Different regins f the mantle are at different temperatures at the in the clumn divided by the clumn height: same depth and therefre intersect the slidus at different pressures, depths, and temperatures. "A" represents a htter regin f mantle which therefre intersects the slidus deeper and melts mre; "B" Pf represents a cler mantle regin which intersects the slidus shallwer and melts less. As discussed in the text, Figure 10c (4) depicts ur preferred mdel. P

17 KLEIN AND LANOMUIR: GLOBAL CORRELATIONS 8105 F(P) I Figure 12 plts the results f these calculatins. Fr an ascending mantle clumn the mean amunt f melting, the mean pressure, and the crustal thickness axe pltted as a functin f pressure f intersectin f the slidus (P)- Fr mean melting f 8%, the pressure f intersectin f the slidus is 14 kbar, the mean pressure is 4.8 kbax and the crustal thickness 3.6 km. Fr mean melting f 20%, the pressure f intersectin f the slidus is 40 kbar, the mean pressure is 16.5 kbar and the crustal thickness is 22.5 km. In cntrast t the simple adiabatic mdel that assumed a cnstant pressure f melt segregatin, these mean pressures are similax t thse inferred frm the FeO and S io 2 Fig. 11. Schematic representatin shwing F(P), the amunt f systematics presented abve. Thus independent appraches melt present at a particular pressure P fr ascending mantle which intersects the slidus at pressure P and melts n further at pressures less than Pf. In calculatins presented in the text, F(P) is assumed t be 1.2 ' meltjkbar pressure release. The ttal amunt f melt present abve the slidus, FT, as discussed in the text, is equal t the area unde._r the diagnal line. Als shwn is the mean pressure f melting, P, defined as the pressure. at which half f the melting ccurs shallwer and half deeper than P. F(P) is a cmplex functin but can be apprximated by a cnstant value f 1.2% melt prduced per kilbar f pressure release at every depth within the ascending clumn [Ahern and Turctte, 1979]. Using this apprximatin, the munt f melt present at any pressure P, F(P), is [0.012*(P -P)]. Then: the cntributin f water pressure is ignred, then Pfcan be determined simply by setting it equal t crustal thickness. Pi= * (P- pf)2 (8) Thus equatins (5)-(8) give the crustal thickness, the mean extent f melting, the mean pressure f melting, and the final pressure f melting in terms f the initial pressure f intersectin f the slidus..pf Pf FT: I F(P)dP = I (0.012P P) dp P P 15 and the mean fractin f melting is' = *(P - Pf)2 (5) F = * (P-P f) (6) Nte that in equatins (5) and (6) the cnstant has units f kbar-1. Thus, in (5) the ttal amunt f melt has units f kbax, which can be cnverted t crustal thickness nce the mean clumn density is knwn (see appendix). One can define the mean pressure f melting, P, as that pressure where half f the melt is prduced shallwer and half deeper than P (Figure 11). Then it can be shwn that øC / 1400ø 1500øC P = P * (P - Pf) (7) I I I I I I õ I I I 35 4O If mantle ascends all the way t the base f the crust, then P (kb) the final pressure f melting crrespnds t the pressure at the base f the crust. Because we assume that the ttal Fig. 12. Results f calculatins discussed in the text, shwin_r P, amunt f melt prduced, F T, segregates t frm the crust, the initial pressure f intersectin f the slidus, versus F (the then equatin (5) gives crustal thickness D c (in units f mean percentage f melt prduced in the ascending diapir), P (the mean pressure f melting), and the ttal crustal thickness prduced, kbar). (Nte that Ahern and Turctte [1979] and McKenzie assuming cmplete segregatin. Als shwn are mantle temper- [1984] used Pi = 0 and thus substantially verestimate atures at the intersectin f the slidus, assuming a mantle slidus crustal thickness, particularly fr higher values f P.) If slpe f 12øC/kbar.

18 8106 KLEIN AND LANOMUIR: GLOBAL CORRELATIONS f simple thermal mdeling f the melting prcess and evaluatin f MORB chemistry lead t cnsistent results. The mdel presented abve has specific implicatins fr mantle temperatures. Assuming a slidus temperature f estimated plybaric melting curves (Ci (F)) fr intersectin f the slidus at 40, 30, 20, and 14 kbar are shwn ir Figure 13b. The experimental data n the less abundant xides Na øC at 1 atm and a sliduslpe f 12øC/kbar [e.g., and TiO 2 are less reliable and the pyrlite abundances differ Takahashi and Kushir, 1983], then between areas frm mantle abundances by a factr f tw. Fr these tw underging minimum and maximum extents f melting, elements, therefre, the plybaric melting paths fr Na20 thermal differences f 300øC at their slidi (Figure 12) and and TiO2 are given by equatin (1); D's and 's have been -250øC at equivalent depths belw the slidus (see Figure estimated frm the experimental data f Takahashi and 10c) are implied, prvided the ascending mantle is thermally islated frm its surrundings. BASALT CHEMISTRY, CRUSTAL THINS, AND DEPm The calculatins presented in Figure 12 cmbined with the systematics f majr element variatins during partial melting can be used t calculate the relatinships amng basalt chemistry, crustal thickness, and axial depth. Calculatin f basalt chemistry requires integrating melt cmpsitins ver the entire melting clumn. These cmpsitins can then be used t calculate crustal density, frm which crustal thickness can be calculated frm equatin (5). Calculatin f axial depth requires knwledge f the thickness and density f the mantle residue as well as the crust, and in additin must take int accunt the cntributin frm mantle temperature, since ridges underlain by htter mantle wuld be shallwer simply due t thermal effects alne. These calculatins are dealt with in turn in the fllwing sectins. Calculatin f Integrated Melt Cmpsitins In rder t estimate the cmpsitin f the integrated (pled) melts, it is necessary t determine the abundance f each element in all instantaneus melts thrughut the (plybaric) melting clumn. Results f mantle melting experiments cannt be applied directly t this prblem, hwever, because the experiments reprt changes in melt cmpsitin with increasing extents f melting at a fixed pressure. The results f these isbaric melting experiments, hwever, can be used t estimate plybaric melting systematics by using the value f 1.2% melt/kbar t determine the amunt f melt present at a particular pressure; the isbaric melting experiments can then be used t determine the cmpsitins f the instantaneus melt at that particular pressure and extent f melting. Fr this, results n the prgressive isbaric melting f peridtitic cmpsitins have been pltted as the abundance f each f the xides SiO2, A120 3, FeO, and CaO versus extent f melting fr each pressure. A similar apprach t calculating mantle melting has been taken by D. McKenzie (persnal cmmunicatin, 1986). We have used data frm varius experimental studies [e.g., Takahashi, 1985; Mysen and Kushir, 1977] where changes in melt cmpsitin versus extent f melting have been (r can be) determined, but we have placed particular emphasis n the pyrlite melting studies f Jaques and Green [1980]. Abslute cncentratin values fr CaO, A120 3, and FeO have been adjusted t mre recent estimates f bulk silicate mantle cmpsitin [e.g., Hart and Zindler, 1987; Jagutz et al., 1979], and the melt cmpsitins changed prprtinately. MgO was calculated fllwing the methd f Langrnuir and Hansn [1980] using the calculated FeO cntents. Fr illustrative purpses, isbaric melting data fr SiO 2 are shwn in Figure 13a and Kushir [1983]. The Kd's fr Na20 in clinpyrxene and rthpyrxene have been taken as 0.15 and 0.04; and Kd's fr TiO2 in clinpyrxene and rthpyrxene were taken as 0.15 and 0.1. D's and 's have been calculated assuming that Na20 and TiO2 are partitined nly int clinpyrxene and rthpyrxene and that clinpyrxene and rthpyrxene cntribute 15% and 20%, respectively, t D, and 70% and 10%, respectively, t. In additin, we have assumed that clinpyrxene melts ut f the residue at 30% melting, and therefre the abundances f Na20 and TiO2 in instantaneus melts prduced by > 30% are given by ~I/F fr Na20 and 1/(0.02+F(0.98)) fr TiO2. The plybaric melting path fr each xide gives the abundances f the xide in instantaneus melts thrughut the melting clumn (Ci (F)). If these instantaneus melts pl frm thrughut the clumn, the mean abundance f an xide(c/) in the pled melt is given by c%:[ / 0 0 where Ff is * (P- Pf). ] (9) Majr element cmpsitins f pled melts calculated as described abve are shwn in Table 3. Because the estimates f the plybaric melting paths require extraplatin back t 0% melting, the abundances f thse xides that shw large changes in cncentratin at small extents f melting and shw strng pressure dependencies are nt well cnstrained; this is the case fr S io2 (Figure 13) and fr A1203. Given the scant experimental data and the resultant uncertainties invlved in determining the plybaric melting paths fr each xide, we stress that the values presented in Table 3 shuld be viewed as estimates. We expect that the general trends fr each xide with changing P are crrect but that further refinements n these calculatins must await mre detailed experiments n the melting behavir f peridtires. The calculatins abve are based n the results f experimental studies perfrmed primarily n spinel peridtites. Mre detailed experiments within the garnet peridtitc facies are needed fr evaluatin f the melting systematics within the deepest parts f the melting clumn. Presnall and Hver [1987] have pinted ut that melting systematics in the plagiclase peridtitc facies may differ significantly frm thse in the spinel peridtitc facies. Furthermre, the melting behavir f Ca and, particularly, A1 and Na may differ between plagiclase and spinel peridtitc facies because these elements are the dminant cnstituents f plagiclase. The value f the percent melting per kilbar is unlikely t change substantially, hwever, because it is cntrlled primarily by the heat f fusin and heat capacity f the mantle minerals. Thus, fr

19 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS 8107 õ õ i 'i i i,.okb apprpriate mantle cmpsitins (5-10% aluminus phase), plagiclase is likely t be melted ut f the residue by abut 15% melting. If plagiclase becmes stable at 8-9 kbar, then fr any value f P greater than 20 kbar, the extent f melting achieved by the pressure f plagiclase stability is likely t equal r exceed the amunt f melting where plagiclase may be a residual phase. Thus it is nly fr the very clest regins (smallest extents f melting) that plagiclase may remain in the residue. A full evaluatin f the imprtance f plagiclase must await experiments n apprpriate mantle cmpsitins in the plagiclase stability field. Calculatin f Crustal Thickness Given the cmpsitins in Table 3, it is pssible t calculate crustal density, and hence cnvert crustal thickness in units f kilbars in equatin (5) t a thickness in kilmeters. T d this, the calculated crustal cmpsitins in Table 3 were cnverted t nrmafive minerals, and the knwn mlar vlumes and masses f these minerals were used t calculate crustal density. Crustal densities were pltted versus P, and an empirical curve was fitted t the data. The mean density fr the crust (Pc) prduced by intersectin f the slidus at a given P is thus determined by the fllwing empirical equatin: Pc = * P (ø'ø 8) (10) Crustal thickness is then btained frm equatin (A5) in the appendix. Nte that these calculatins predict a relatinship between crustal chemistry and crustal thickness independent f the bservatinal data. The calculatins are cmpared t available data in Figure 14. Figure 14 shws that the predicted inverse crrelatin exists between seismically determined estimates f crustal thickness and Na8. 0 fr varius regins encmpassing the range f chemistry and bathymetry shwn in Table 1. TABLE 3. Calculated Pled Melt Cmpsitins and Other Physical Parameters fr Varying Pressure f Intersectin f the Shdus PO 40 kbar 30 kbar 20 kbar 14 kbar 4 Fig. 13. SiO 2 (wt %) versus F (extent f melting). (a) Results f several experimental studies n the isbaric melting f varius peridtitic cmpsitins at the pressures indicated. Surces and starting cmpsitins are: pyrlite (circled crsses), Tinaquill lherzlite (crsses, nly 10-kbar melts shwn) [ffaques and Green, 1980]; garnet lherzlite + water (slid circles) [Mysen and Kushir, 1977]; natural spinel lherzlite (asteriks; extents f melting estimated frm reprted Na20 and TiO 2 abundances) [Takahashi, 1985]. (b) Frm the trends f the experimental data shwn in Figure 13a, with particular emphasis n the pyrlite melting studies, isbaric melting curves (dashed) were estimated fr 2, 5, 10, and 15 kbar. Assuming 1.2% melt/kbar pressure release, plybaric melting curves (slid) were estimated fr P f 40, 30, 20, and 14 kbar, by fitting t the isbaric melting curves (e.g., fr P=40, 30% melting will be achieved by 15 kbar). The plybaric melting curves estimate the SiO2 abundances in instantaneus melts present in the melting clumn at each pressure abve the slidus (Ci(F)). Pled Melt Cmpsitins SiO A FeO MgO CaO Na TiO Sum Mg# CaO/A Physical Parameters F % P kbar Dc, km Dw, kin* * Depth f cmpensatin = 200 km.

20 8108 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS i i i I I crrelate with axial depth, ur findings have imprtant implicatins fr mdels f ridge axis bathymetry. Cntributins t average ridge axis bathymetry frequently 2O discussed include variatins in crustal thickness, mantle temperature, and mantle flw. The fact that Na8. 0 (and by inference, extent f melting) generally crrelates with crustal thickness and with axial depth (Figure 2) supprts IO 8 gephysical mdels f ridge axis bathymetry which, t first rder, call upn isstatic cmpensatin f varying crustal thicknesses t accunt fr variatins in residual depth (see, 6 e.g., review by Watts and Daly [1981]). The mdel develped in the discussin, hwever, has several aspects in additin t crustal thickness which wuld cntribute t axial depth. The bserved variatins in crustal -% thickness may result frm temperature differences f as much as 250øC at equivalent depths in the subslidus mantle. 2.0 Since a clumn f mantle f higher mean temperature will be Ns. less dense, ridges underlain by higher mantle temperatures will be shallw simply due t thermal effects alne. In Fig. 14. Seismically determined estimates f crustal thickness additin, htter mantle starts melting deeper and creates a versus Na8. 0 with the exceptin f the crustal thickness value fr thicker mantle residue, and the mantle residue after melting the Cayman Trugh (C) which was estimated frm gelgic is lighter than the fertile mantle prir t melting [e.g., evidence. Data surces fr crustal thickness are as fllws: A (Azres regin), Searle [1976]; C (Mid-Cayman Rise), Strup and Oxburgh and Parrnentier, 1977]. It is therefre necessary t Fx [1981]; D (MAR at apprximately 45'N), Lewis and Snydsman try t cnstrain the separate effects f variatins in mantle [1979]; F (MAR at apprximately 37'N), writmarsh [1973] and temperature, residue thickness, and crustal thickness. Fwler [1976]; I (Iceland regin), Palmasn and Saernundssn In the appendix, we have calculated the expected range in [1974] and Bjrnssn [1984]; J (Juan de Fuca), McClain and Lewis axial depth f isstatically balanced clumns resulting frm [1982]; K (MAR, Kane transfrm regin), Detrick and Purdy [1980]; M (Mariana Trugh), LaTraille and Hussng [1980]; R (Reykjanes the cupled effects f variatins in the extent f melting, Ridge), Bunch and Kennett [1980]; S (EPR near Siqueirs transfrm), crustal thickness, and mantle temperature. The results f the Orcutt et al. [1976]. Values fr Na8.0 fr each regin are frm calculatins are presented in Table 3 and Figure 15, where Table 1. Where infrmatin n the range f crustal thickness the differences in water depth fr specified P's are expressed estimates fr a particula regin were given, the range is included in the figure. The star is fr samples frm 140 m.y. crust frm the relative t an assumed water depth f zer fr P=40 kbar. western Nrth Atlantic [Purdy, 1983; Byerly and Sintn, 1980]. The relative changes in water depth als depend n the Als shwn are calculated curves fr crustal thickness versus Na8.0; assumed depth f cmpensatin (Figure 15a). Fr a crustal thickness is determined by equatin (A5), and Na8. 0 values cmpensatin depth f 200 km and mean extents f melting are derived by fractinatin t 8 wt % MgO frm the parental between 8% and 20%, the ttal range in water depth is 4.1 magma cmpsitins in Table 3. The tw curves crrespnd t different values f mantle Na20 (C r 0.30 wt %). km (Figures 15a and 15b). The effects f temperature variatins alne n the calculated water depths can be determined by setting the cefficients f thermal expansin Althugh the slpes f the calculated curves are clearly t zer in equatins (A15) and (A17); the ttal range in cnsistent with the available data, the relative psitin f water depth fr 8-20% mean melting then becmes 3.2 km the curves is quite sensitive t the surce abundance f (Figure 15b). Thus temperature variatins alne cntribute N a20, as is shwn. The best fit is fr a surce apprximately 1 km (<25%) t the ttal depth range. cncentratin f 0.26 wt % Na20, which is slightly less Further, we can examine the effect f variatins in crustal than primitive mantle abundances (0.30 wt %+0.03), thickness alne by assuming a mantle f cnstant suggesting that the MORB mantle may be slightly depleted temperature and density. The ttal range in water depth fr relative t bulk silicate mantle. It shuld als be nted that 8-20% mean melting then is nly 2.5 km. Thus variatins the calculated curves assume cmplete segregatin f melt in crustal thickness accunt fr apprximately half f the frm thrughut the melting clumn, and hence maximum ttal depth range. crustal thickness. If sme percentage f melt is retained T cmpare calculated depths t bserved depths, it is within the residual matrix, as is likely, crustal thickness necessary t distinguish between axial depth, which includes will decrease prprtinately. T first rder, hwever, the the effects f rift valleys r axial highs, and reginal depth. general crrespndence f the data t the calculated curves f Figure 15a shws NaB. 0 pltted against ridge depth that has Figure 14 suggesthat variatins in the extent f melting been averaged frm zer-age t a 5 m.y. ischrn, [Andrews and hence chemistry crrespnd with changes in the et al., 1985; W. Haxby et al., manuscript in preparatin, thickness f the ceanic crust. 1987]. Als pltted are calculated curves f relative Calculatin f Relative Changes in Axial Depth variatins in axial depth fr cmpensatin depths f 150, 200, and 250 km, as described abve. The verall trend f In the discussin abve, variatins in basalt chemistry the data agrees we1! with the calculated curves, but there are were used t infer bth variatins in the thermal structure f sme discrepancies. In particular, mst f the ht spt the mantle underlying different spreading centers and in centers appear t be anmalusly shallw, r anmalusly crustal thickness. Because the chemical variatins als high in Na20, cmpared t the calculated curves. Thus the

21 KLEIN' AND LANOMUIR: GLOBAL CORRELATIONS z i i I I i Dmax:150km 200km 250kin CRUST AND RESIDUE Dmax = 200 km CONTR I BUT IONS (N Temperature Variatins)_ CRUSTAL THICKNESS CONTRIBUTION CRUST, RESIDUE AND TEMPERATURE CONTRIBUTIONS I I I I I i 2000 : DEPTH (rn) Fig. 15. (a) Data pints shw Na8.0 frm Table 1 versus ridge depth averaged t a 5 m.y. ischrn; 5 m.y. average depths are predminantly frm Andrews et al. [1985] and W. Haxby et al. (manuscript in preparatin, 1987), supplemented with estimates frm the data f Fryer and Hussng [ 1980], Strup and Fx [ 1981], and the ROSE expeditin (E. Vera, persnal cnununicatin, 1987). Symbls as in Figure 2. Curves shw Na8. 0 and depth variatins calculated as described in the appendix fr different depths f cmpensatin (150, 200, r 250 kin); Na8.0 fr the calculated curves are derived by fractinatin frm the cmpsitins presented in Table 3. (b) Curves shwing relative cntributins t depth f the varius parameters discussed in the appendix, fr a cmpensatin depth f 200 kin. Curve labeled "crust, residue, and temperature cntributins" is the same as the D max = 200 km curve in Figure 15a; curve labeled "n temperature variatins" is calculated by seuing a= 0 in equatins (A15) and (A17). Curve labeled "crustal thickness cntributin" is calculated assuming a cnstant mantle density f 3.34 gm/cm 3. surces fr ht spt centers may either be slightly higher in N a20, r their anmalus depths may result frm a cmpnent f mande flw [e.g., Mrgan, 1972; McKenzie et al., 1974; Sclater et al., 1975]. Cmparisn With Studies f Oceanic Ultramafic Rcks As nted abve, studies f ceanic peridtites [Dick et al., 1984; Michael and Bnatti, 1985] are remarkably cnsistent b _ with the glbal crrelatins and inferences derived frm them. Thus the results f this study strngly reinfrce the methdlgy and cnclusins f these previus studies. There are tw aspects f the ultramafic data, hwever, which need further discussin fr cmparisn between the peridtitc and basalt data. Bth ultramafic studies were heavily influenced by samples recvered near the Azres platfrm. Langmuir and Hansn [1980] pinted ut the anmalusly lw FeO f basalts frm this regin and suggested that this may result frm a higher Mg/(Mg+Fe) rati in the mantle surce. MORB frm the Azres regin are als anmalus in the cntext f the glbal crrelatins (Figure 2). If the Azres regin surce had higher Mg/(Mg+Fe) t begin with, then the depletin in the ultramafics caused by melting wuld have been in additin t the preexisting depletin. This may in part explain the highly depleted ultramafics frm this regin. The ultramafic samples, eraplaced at crustal levels, als necessarily represent the residues f the very last increment f melt remved frm the mantle. If the erupted melt is a mixture f melts frm the entire melting zne, then parental cean ridge basalts shuld nt be in equilibrium with the sampled ultramafics. Instead, ne might expect the ultramafics t be far mre depleted than expected frm nrmal MORB chemistry because the ultramafics may represent the end result f a cntinuus melting prcess [Bender et al., 1984]. This cmplicates a direct cnnectin between the peridtitc and basalt data sets. The rarely bserved, but ften inferred, very depleted MORB endmember capable f crystallizing An92 plagiclase may be a remnant f melts in equilibrium with the depleted peridtites at the tp f the melting clumn. Implicatins fr the Cmpsitins f Primary Magmas Our calculatins f the pled melt cmpsitins as a functin f P have imprtant implicatins fr the nging debate ver the cmpsitin and equilibratin pressure f primary MORB magmas. One grup f experimentalists has argued fr primary magmas equilibrated at pressures f greater than 15 kbar, and with MgO cntents in excess f 14 wt %, and even in excess f 20 wt % [e.g., O'Hara, 1968; Elthn and Scarfe, 1984; Stlper, 1980]. Others have argued fr an equilibratin pressure f abut 10 kbar and MgO cntents f abut 10 wt % [e.g., Presnall and Hver, 1984, 1987; Fujii and Scarfe, 1985]. All f these wrks had as a paradigm a single pressure f equilibratin fr a particular primary magma and attempted t derive all MORB primary magmas frm a restricted range in pressure. A difficulty with the "cnstant pressure" paradigm is that it predicts that MORB primary magmas shuld be related by different extents f melting at a cnstant pressure. Instead, there appears t be a psitive crrelatin between the extent and pressure f melting. This finding leads t a different perspective n primary magmas, where melts are generated in a plybaric melting clumn and where magmas seen at the surface are mixtures f melts frm different pressures within the mantle melting clumn. Thus there may be n single "equilibratin pressure" fr any primary magma, and the mean equilibratin pressure will vary as a functi,n f the pressure f intersectin f the slidus. Furthermre, the fractinatin-crrected basalt data and the partial melting calculatins supprt the idea that there are

22 8110 KLEIN AND LANGMUIR: GLOit AL CORRELATIONS diverse primary magma cmpsitins that can vary hypthesis presented abve. It may be significant, substantially in their majr element cmpsitin and mean hwever, that the fastest spreading ridges are all f average pressures f equilibratin frm regin t regin. Mean chemistry and depth, while the slwest spreading ridges can pressures, where half the melt is generated abve and belw exhibit any depth r chemistry. This suggests that that pressure, fall between apprximately 5 and 16 kbar. The mean MgO cntent f eruptives varies between 10 and 15 wt smehw the fastest ridges may be able t "damp ut" the effects f extremes in temperature variatins. %. On average, there shuld be an inverse crrelatin Implicatins fr Studies f Older Crust between the mean values f MgO (and pressure) and the and Tempral Variatins in Mantle Temperature depth f the spreading axis. This diversity f mean extents and pressures f melting and pled melt cmpsitins may The main results f this paper are the quantitative in part explain the diverse pints f view in the "primary relatinships amng crustal thickness, basalt chemistry, and magma" cntrversy. Indeed, several f the previus studies mantle temperature beneath the ridge. Such relatinships have used ne r a few high MgO samples t represent shuld apply independent f age and thus may prvide a tl MORB primary magmas. Since there appears t be a large fr studying thermal variatins beneath cean ridges as a functin f time. range in MORB primary magma cmpsitins, a perspective frm any small number f samples, even the mst primitive The ptential utility f this apprach can be illustrated nes, will be incmplete. using crustal thickness data n 140 m.y. crust frm the It shuld als be nted that althugh ne can define a western Atlantic and cmparing it with basalt chemical mean pressure fr the pled melts, this mean pressure variatins frm Deep Sea Drilling Prject legs in the shuld nt be cnfused with the isbaric "equilibratin same general regin (within 500 kin) [Byerly and Sintn, pressure" referred t in mst experimental studies [e.g., 1980; Rice et al., 1980]. The seismic data shwed that the Stlper, 1980]. The chemical cmpsitin f the pled Cretaceus crust in this regin is significantly thicker than melt prduced by a particular mean pressure and mean extent zer-age crust n the Mid-Atlantic Ridge suth f the Kane f melting des nt crrespnd t the cmpsitin f the Fracture Zne, althugh the lder crust lies apprximately melt prduced by melting t the same extent at a single alng a flw line emanating frm the latter. The basalt data frm this lder crust shw the chemical characteristics f pressure equal t the mean pressure. Thus, even if the isbaric melting paths n pseud-phase diagrams were mderately thick zer-age crust. The star symbls in perfectly determined, the mean pressure f equilibratin fr Figures 8 and 14 shw the chemical and crustal thickness pled melts culd nt be determined frm such infrmatin data frm this lder regin; these data are cnsistent with alne. the zer-age chemical crrelatins. The data thus suggest Last, we nte that as melting prceeds in an ascending that the thicker crust in this regin is due t higher mantle clumn, a wide range f melt cmpsitins are temperatures beneath the ridge in the Cretaceus than exist generated in the clumn. Thus there is the ptential fr the nw at zer-age alng the same flw line. An increase in ccasinal eruptin f magmas frm substantially higher r crustal thickness by underplating, serpentinizatin r ther lwer pressures than the mean pressure. The glbal age-dependent mechanisms is nt required and indeed may be systematics f reginal averages d nt preclude the pssibility f eruptin f spradic diverse and unusual cmpsitins. Spreading Rate A remarkable feature f the glbal variatins in majr element chemistry is the lack f an bvius crrelatin with spreading rate. The slwest spreading ridges erupt bth the highest and lwest Na20 basalts, and have bth the thicknest and thinnest crust (Figure 14). These findings appear at first t cnflict with the cnclusins f Reid and Jacksn [1981], wh fund that crustal thickness crrelates with spreading rate. It shuld be nted, hwever, that Reid and Jacksn regarded ht spts as anmalus and based their cnclusins n "nrmal" cean ridges. Althugh instances have been nted abve where ht spt centers deviate frm the glbal trends (see, e.g., Figures 2b and 2c), a fundamental finding presented herein is that many ht spts shw the same relatins between chemistry, depth, and crustal thickness as nn-ht spt spreading centers (see, e.g., Figures 2a and 14). Hwever, it is clear that at very slw spreading rates, cnductive cling will affect the upwelling mantle, and the assumptin f adiabatic ascent will n lnger be valid [Bttinga and Allegre, 1978; Reid and Jacksn, 1981]. This may be a partial explanatin fr the ccurrence f thin crust at the slwest spreading rates, and under these cnditins, prvides a pssible alternative t the "mantle temperature" difficult t recncile with the chemical data. These results raise the pssibility that tempral variatins in mantle temperature beneath the ridge can be studied by multichannel seismic prfiling f lder crust r by chemical sampling. Further evidence f the crrelatin between reginal crustal thickness and basalt chemistry, hwever, must be btained by additinal drilling with cmplementary seismic experiments. CONCLUSIONS Examinatin f a glbal data base shws that there are systematic glbal variatins in basalt chemistry that crrelate with axial depth. Shallw regins, such as the Klbeinsey ridge, shw the fllwing characteristics: high CaO, lw A120 3 (high CaO/A1203), lw Na20, high FeO, lw SiO 2, high Sc, lw Sm/Yb, and lw Ni. In cntrast, basalts frm deep regins such as the Australian-Antarctic Discrdance r Mid-Cayman Rise, shw the ppsite chemical trends. Between these shallw and deep endmembers, a cntinuum f cmpsitins is bserved. The bserved chemical systematics are remarkably cnsistent with the results f experimental studies n the melting f peridtitic cmpsitins and with cmpatible trace element mdeling, suggesting that basalts ccurring n shallw ridge segments are derived by larger extents f melting at greater mean pressures f melt segregatin. Mre extensive experimental data are needed, hwever, t test this pssibility rigrusly. Sme ht spts (e.g., Azres, Jan

23 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS 8111 Mayen, Galapags) appear t be chemically anmalus in a glbal cntext and may represent evidence f majr element surce hetergeneity. Calculatins based n the chemistry f the end-member basalt cmpsitins suggest that the mean extents f melting range frm apprximately 8 t 20% and the mean pressures f melting range frm 5 t 16 kbar. Calculated crustal thicknesses prduced by this range in extent f melting are cnsistent with thse inferred frm seismic studies. These calculatins als suggest that between the end-member basalt cmpsitins, temperature differences in the sub-slidus mantle may be as much as 2500C. Calculatins f predicted depth ranges depend n the depth f isstatic cmpensatin, crustal thickness, residue thickness, and mantle temperature. Fr a cmpensatin depth f km, the relative depth changes fr the isstatic mdel are cnsistent with bserved depths, but sme ht spt centers may be anmalusly shallw, r alternatively their surces may be anmalusly high in Na2. Thus there appears t be cnsistency amng three f the majr bservable features f the glbal system f cean ridges, chemical cmpsitin, crustal thickness, and water depth, all f which are diverse manifestatins f the temperature variatins in the mantle. This apparent cnsistency needs t be tested by further sampling f areas that shw substantial zer-age depth and crustal thickness variatins. The same principles relating chemistry and crustal thickness shuld als apply t lder cean crust, which raises the pssibility f petrlgically cnstraining the rigins f bathymetric anmalies thrughut the cean basins. AP 'END X and the ttal clum height t Dmax is Dmax = Dw + Dc + Dr +Dm (A2) where subscripts w, c, r, and m refer t water, crust, residual mantle, and fertile mantle, respectively, and p and D refer t density and thickness. The demities and thicknesses f the cmpnents f each clumn will vary with cmpsitin, temperature, and pressure. Each f these are cntrlled by the mantle temperature prir t crust frmatin, since mantle temperature cntrls the pressure and temperature f intersectin f the slidus and hence the thickness and cmpsitin f bth crust and residue. In the fllwing, relative differences in water depth (Dw) are calculated as a functin f differences in mantle temperature beneath the ridge. Temperature differences cntrl the pressure at which melting begins in an upwelling mantle diapir. The temperature T and pressure P at which melting begins are related by an equatin apprximating the mantle slidus [Takahashi and Kushir, 1983]: T = 1150øC + 12øC/kb * P (A3) T O and P cntrl the extent f melting and hence the resulting melt cmpsitin, assuming a hmgeneus mantle. Calculated melt cmpsitins, presented in Table 3, can be used t calculate crustal densities by determining mlecular nrms and then applying the partial mlar vlumes fr the nrmative minerals [e.g., Rbie et al., 1979] t calculate densities. An empirical equatin fr the calculated crustal densities as a functin f P, which recvers values t better than 0.1% within the range f basalt cmpsitins cnsidered, is Pc = * P (0'038) (A4) Crustal thickness, D c, described by equatin (8) in the text, is in units f kilbars. Cnversin t kilmeters can be accmplished by using the density frm equatin (A4) and the fact that 1020 g/cm 2 is equivalent t 1 bar at the earth's surface. Then, allwing fr cnversin frm bars t kilbars and centimeters t kilmeters, D in kilmeters, is Dc = * (P-pf)2, 10.2/pc (A5) The residual mantle is that prtin f the mantle between the initial (P )and final (Pf)pressures f melting. The density f the residual mantle varies with pressure, temperature, cmpsitin, and mineralgy. The effects f the latter tw parameters can be mdeled as a functin f the extent f melting. We take the 1 atm, 25øC densities f fertile plagiclase lherzlite and garnet-spinel lherzlite t be 3.27 g/cm3 and 3.34 g/cm3, respectively. The bundary between plagiclase and spinel lherzlite is taken t be 8 kbar [Presnall et al., 1979]. Density f the mantle is assumed t vary linearly as the fractin f melting (F=0.012*AP) increases frm 0 t 0.3, between the initial In the fllwing, we present a methd fr calculating differences in water depth between isstatically balanced unit value fr unmelted mantle and a value f g/cm 3 fr clumns extending t a specified depth f cmpensatin, residual harzburgite [Oxburgh and Parmentier, 1977]; when Dma x, as a functin f mantle temperature differences. Each clumn cnsists f water, crust, residual mantle that has F exceeds 0.3, density is maintained at g/cm 3. Thus a different density is calculated fr each facies depending n melted t prduce the crust, and fertile mantle extending the mean fractin f melting that ccurs within that facies. frm the base f the residual mantle t D max (Figure A1). Thus the ttal mass fr each clumn, M T, is The mean fractins f melting within plagiclase and garnet-spinel facies (subscripts pl r gt-sp) are cntrlled by MT = Pw Dw + pcdc + prdr + PmDm (A1) P and Pf, analgus t equatin (6), and are given by FpI = * (2P - Pf- 8) (A6) F gt-sp = * (P-8) (A7) Then, the 1 atm, 25øC densities (dented by primed superscripts) within each facies can be calculated as a functin f the mean extents f melting within each facies: P'pl = Fpl * [( )/0.3] (A8) P'gt-sp = Fgt_sp * [( )/0.3] (A9) prvided F is less than 0.3, with a value fr p' f integrated int the result fr thse prtins where F exceeds 0.3. Thicknesses f residual mantle within each facies can be determined, as in (A5), ignring the slight change in the earth's gravitatinal field with depth ver the depth range cnsidered: Dpl = (8- Pf) * (10.2/p'p/) D gt-sp = (P- 8) * (10.2/p'gt_sp) (AlO) (All)

24 8112 KLEIN AND LANGMUIR: GLOBAL CORRELATIONS,-.,8 k b--), P(SOl idus) ; Depth f Cmpensatin w z w WATER CRUST Plagiclase Stability Spinel r Grnet Stbility FERTILE MANTLE Be Dm "-- Dmax Fig. A1. Schematic representatin f isstatically balanced clumns, cnsisting f fertile mantle, residual mantle, crust, and water, as discussed in the text. The mean density f the residual mantle at 1 att and 25øC is then given by the relative cntributins within each facies: P'r = [Dpl[(Dpl+Dgt-sp)l*P l + [D gt-sp/(dpl+d gt-sp)]*p'gt-sp (A12) The mean in situ density f the residual mantle depends n the mean temperature and pressure f the residual mantle. Assuming that pressure decreases linearly between P and Pp mean pressure in the residual mantle is given by Pr = (P + PjO/2 (A13) Mean temperature depends n T and the temperature decrease that ccurs thrugh melting by adiabatic Ryan, D. Frnari, E. Bnatti, K. Kastens, N. Bgen, L. decmpressin, which we take t be apprximately Cathles, L. Viereck, T. Plank, and Y. Zhang. Frmal 6øC/kbar [Cawthrn, 1975]. Mean temperature in the reviews by B. Hager, D. Presnall, and J. Simn cntributed residual mantle is then given by greatly t the substance and clarity f the manuscript. Discussins with D. Christie were very helpful in Tr = [T + T- 6øC/kb * (P- PjO]/2 understanding the systematics f mantle melting. In = T - 3(P -Pf) (A14) additin, a Saturday discussin with D. McKenzie during the final stages f preparatin f the submitted versin f this Using values f 3 x 10-5/ C and 10-3/kbar fr the vlume manuscript was bth enjyable and illuminating. We are cefficients f thermal expansin ([) and cmpressibility ( ) [Birch, 1952], the mean density f the residual mantle is given by Pr = P'r * [1 - a*(tr - 25øC) + *Pr] (A15) The thickness f the residual mantle, as in (A5), is then given by is taken as that f spinel r garnet lherzlite (3.34 g/cm3). As in (A15), the mean in situ density f the fertile mantle depends n the mean temperature (Tm) and pressure (Pm) f the fertile mantle: Pm = 3.34 * [1 - a*(tm - 25øC) + *Pm] (A17) Fr a given maximum depth f cmpensatin (Dmax), the mean pressure f the fertile mantle (Pm)is given by the difference in pressure between that at Drnax (Pmax) and the pressure where melting begins, P: Pm = (Pmax + P)[2 (A18) The mean temperature f the fertile mantle (Tm) is given by T and the adiabatic gradient frm P t Pmax: Tm = T + 2øC/kb * [(Pmax - P)/2] (A19) Equatins (A17)-(A19) can be evaluated nce Pmax is determined. Pmax, in turn, depends upn a knwledge f the ttal clumn mass Mr If, hwever, we specify that fr a particular P (taken as 40 kbar), water depth (Dw) is zer, then fr this P, the thickness f the fertile mantle, frm equatin (A2), is D m = D max - Dr - D c ( A20 ) since Dw=O fr P=40 kbar. Ttal clumn mass and pressure at D ma x can then be determined iteratively. Using an initial, arbitrary value fr P m (e.g., 3.34 g/cm 3) and equatins (A1) and (A3)-(A16), Mr and Pmax (=Mr/10.2) are calculated; P max is then used t recalculate P m using equatins (A1) and (A3)-(A16), and new values f M r and Pmax are determined. These calculatins are repeated until cnvergence is btained fr bth M T and Pmax. Because M r and Pmax are cnstant fr each isstatically balanced clumn independent f variatins in P, water depth can be calculated fr all ther specified P 'S using equatins (A1)-(A19). The results f these calculatins, presented in Table 3 and Figure 15a, thus prvide differences in water depth and crustal thickness as a functin f mantle temperature and depth f cmpensatin. Acknwledgments. The authrs are indebted t the fllwing peple fr generusly giving f their thughts and expertise: D. Walker, R. Buck, C. Lesher, J. Bender, A. Watts, J. Andrews, W. Haxby, J. Natland, B. Parsns, W. als indebted t R. Glynn fr suffering multiple revisins, and t P. Catanzar fr her careful drafting f the figures. This wrk was supprted by Natinal Science Fundatin grants OCE and OCœ Lamnt-Dherty Gelgical Observatry Cntributin Dr = (P - Pf)*lO.2/pr (A16) The mean density f the fertile mantle at 1 atm and 25 C Ahem, J.L., and D.L. Turctte, Magma migratin beneath an cean ridge, Earth Planet. Sci. Lett., 45, , Andrews, J.A., W.F. Haxby, and W.R. Buck, Variatins in geid

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