G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 9, Number October 2008 Q10007, doi: /2008gc ISSN: Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D Guenter Suhr Institut Geologie und Mineralogie, University zu Koeln, Zuelpicher Str. 49b, D Koeln, Germany (guenter.suhr@uni-koeln.de) Eric Hellebrand and Kevin Johnson Department of Geology and, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, Hawaii 96822, USA (ericwgh@hawaii.edu; kjohnso2@hawaii.edu) Daniele Brunelli DST, Università di Modena e Reggio Emilia, Lgo St. Eufemia 19, I Modena, Italy ISMAR-CNR, Via Gobetti 101, I Bologna, Italy (daniele.brunelli@unimore.it) [1] Hole U1309D (Integrated Ocean Drilling Program (IODP) Legs 304/305) penetrated 1415 m into the seafloor of the Atlantis Massif, an oceanic core complex at 30 N, Mid-Atlantic Ridge. More than 96% of all recovered rocks are gabbroic. On the basis of a mineral chemical overview, we suggest that between 800 and 1100 m below sea floor (mbsf), a magmatic unit occurs, ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part. Below 1235 mbsf, massive gabbronorites/oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit. The focus here is on the zone where both units interact and screens, totaling 50 m, of a microstructurally distinct, olivine-rich troctolite occur. We argue that the olivine-rich troctolite is a former mantle rock which was converted to a crust-mantle transition zone dunite at the base of the upper magmatic unit. Later, as melts derived from the lower magmatic unit percolated through it, it was equilibrated to a more evolved chemistry and transformed to a fine-grained, olivine-rich troctolite. Our main arguments against a possible cumulate nature of the olivine-rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine-rich troctolite, its distinctly fine-grained microstructure, the high Cr content of cpx, and its Ni-rich olivine composition. The high NiO for a given Mg/(Mg + Fe) in the olivine-rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt. Evidence for the percolation of evolved melts through the olivine-rich troctolites are Ti-rich, interstitial pyroxenes and, as inclusions in Cr-spinel, highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter-scale noritic veins. The percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust-mantle transition zone. Components: 29,302 words, 15 figures, 6 tables. Keywords: troctolite; IODP Leg 305; melt percolation; olivine. Index Terms: 3614 Mineralogy and Petrology: Mid-oceanic ridge processes (1032, 8416); 3625 Mineralogy and Petrology: Petrography, microstructures, and textures; 3640 Mineralogy and Petrology: Igneous petrology. Received 1 March 2008; Revised 6 August 2008; Accepted 19 August 2008; Published 16 October Copyright 2008 by the American Geophysical Union 1 of 31

2 Suhr, G., E. Hellebrand, K. Johnson, and D. Brunelli (2008), Stacked gabbro units and intervening mantle: A detailed look at a section of IODP Leg 305, Hole U1309D, Geochem. Geophys. Geosyst., 9, Q10007, doi: /2008gc Introduction [2] There are now two deep drill holes into slow spreading oceanic crust, Hole 735B (SW Indian Ridge) and Hole U1309D (Mid-Atlantic Ridge). Both have a core complex setting (see section 2) and both recovered thick (1.5 and 1.4 km, respectively) sequences of almost exclusively gabbro. Gabbro is therefore a prominent component also of slow spreading crust [Coogan, 2007]. If our focus is wider and not on core complex settings alone, the word which describes slow spreading crust best is perhaps heterogeneous [Cannat, 1993; Karson, 1998; Lagabrielle et al., 1998; Cannat et al., 2006]. The lithological arrangement spans the range from a complete, Penrose-type ophiolite crust [Auzende et al., 1989] to isolated, small gabbro bodies within an ultramafic host [Shipboard Scientific Party, 2004]. [3] A remarkable problem encountered in both deep gabbro holes is the question of how to make >1.4 km of gabbro in a slow spreading environment while the in situ observation of melt bodies is in contrast to fast spreading situations [Vera et al., 1990] very rare [Sinha et al., 1997; Singh et al., 2006]. Studies of Integrated Ocean Drilling Program (IODP) Hole 735B have made significant progress to our understanding in showing that the 1.5 km gabbro section can be subdivided into five, m thick magmatic units that display a broadly regular evolution from more primitive troctolite at the base (100 Mg/(Mg + Fe) in olivine 84) toward gabbronorite near the top. These cycles are apparently randomly punctuated by evolved, commonly highly deformed oxide gabbros [Dick et al., 2000, 2002; Natland and Dick, 2001]. How any such single cycle may have formed is still poorly constrained. Natland and Dick [2001] infer an efficient compaction of residual liquid by deformation as the main mechanism to separate crystals and liquid. This residual liquid may then interact with higher level rocks [Dick et al., 2002]. In a review paper, Coogan [2007] argued that gabbro appears to form from episodic, m thick melt batches (p. 29) and that the relation between troctolite and olivine-gabbro is perhaps simply one of additional postcumulus cpx precipitation (p. 22). [4] Another significant problem relates to the mode of accretion of the magmatic units. A common view is that of nested intrusions ranging from isolated bodies within a mantle environment to densely packed bodies forming a more or less solid layer 3 [Lagabrielle et al., 1998; Schwartz et al., 2005]. On the basis of isotopic age data, Schwartz et al. [2005] proposed that on the order of 25% of gabbro intrusions formed at a deep (15 20 km) level within the lower part of the slowspread lithosphere. These deep intrusions became exhumed to a shallow level where they were engulfed and partially resorbed by nested intrusions of the main gabbro-forming stage. For Hole 735B, it was inferred that the deeper magmatic cycles postdate the higher level ones and that along the contact of two cycles, both form an intercalated sequence [Dick et al., 2002]. [5] In this contribution, we look at IODP Hole U1309D [Blackman et al., 2006; Ildefonse et al., 2007], the second deep drill hole into slow spreading oceanic crust. We mainly want to address the problem of what happens as one magmatic unit accretes to another one. Unfortunately, in Hole U1309D, a subdivision into magmatic cycles is less obvious than in Hole 735B. We first show that at least the interval between 800 and 1100 mbsf is likely to represent a regular evolution consistent with upward differentiation from olivine gabbro and troctolite at the base to gabbronorite and oxide gabbro near the top. We then focus on the lower contact of this unit to an underlying, chemically evolved oxide gabbro/gabbronorite. Remarkably, sandwiched between the hangingwall magmatic unit and the footwall gabbronorite, nearly unserpentinized screens of mineral chemically and microstructurally distinct olivine-rich troctolites occur. They are spread out over a 140 m depth interval and are separated by meter-scale units of oxide gabbro, gabbronorite, gabbro, olivine gabbro, and troctolite. Using mineral chemical variation diagrams and results from closely spaced samples along the core, we show that the olivinerich troctolites have not formed by simple fractionation from a primitive mantle melt. Instead, we show by geochemical modeling that they can have been former mantle rocks progressively transformed by melt-rock reaction. We also present microstructural and geological models of how this transformation may have taken place on a sample and the 100 m scale, respectively. We finally 2of31

3 Geosystems G 3 suhr et al.: iodp leg 305 gabbros /2008GC discuss one of the consequences of the model, the possible reason for the still illusive nature of primitive cumulates in Hole U1309D. 2. Geological Setting and Drilling Results [6] IODP Hole U1309D is situated on Atlantis Massif, a topographically elevated rift shoulder at 30 N along the Mid-Atlantic Ridge. Atlantis Massif is a so-called inside corner high, north of the Atlantis Fracture zone and to the west of the ridge axis [Blackman et al., 2002; Karson et al., 2006]. To the south of the drill hole, but still on the inside corner high, the peridotite hosted hydrothermal vent field Lost City is situated. Geological mapping by Karson et al. [2006] showed that Lost City is rooted in a lithosphere consisting of approximately 70% peridotite and only 30% gabbro. In contrast, the 1415 m deep Hole U1309D, 5km north of Lost City, consists of 96% gabbroic lithologies. [7] Inside corner highs are typically linked to longlived normal faults exhuming deeper crustal rocks, i.e., oceanic core complexes [Tucholke et al., 1998]. It currently appears that most core complexes have a gabbroic core with the detachment surface localized in ultramafic rocks [Reston et al., 2002; Escartin et al., 2003; Ildefonse et al., 2007; Dick et al., 2008], the latter representing the weak surface required for long-term partitioning of extensional strain into a single normal fault [Lavier et al., 1999]. This weakness may only develop at temperatures below the serpentine stability field [Escartin et al., 2003; MacLeod et al., 2002] and exhumation up to the serpentine stability should occur along a diverse range of faults [Cannat et al., 1997; Ildefonse et al., 2007]. [8] The very top of Hole U1309D recovered rare talc schists and basalt/diabase. Relative to the rest of the hole, the upper 100 m show increased plastic strain in gabbros, the upper 150 m are marked by an elevated proportion of diabase (Figure 1), and above 225 m, thin peridotite slivers have been encountered [Tamura et al., 2008]. The degree of alteration decreases notably below the last set of major faults, i.e., >780 mbsf [Blackman et al., 2006]. [9] At an average recovery rate of 75%, 96% of all rocks were gabbroic, including olivine-rich troctolites, troctolites, olivine gabbros, gabbros, gabbronorites, oxide gabbros, microgabbros, and felsic veins. Shipboard observations indicate that more evolved lithologies are generally intrusive into less evolved lithologies [Blackman et al., 2006]. A systematic downhole evolution of lithologies is not obvious from the lithological column (Figure 1). However, inspection of shipboard major element data indicated that between 1100 and 800 mbsf an up-section evolution toward more fractionated rocks may be present (Figure 1). Below, abundant screens of olivine-rich troctolites occur. Lithologically, the region between 1100 and 800 mbsf translates to a transition from predominately olivine gabbro and troctolite into gabbro and gabbronorite. Oxide gabbro is more abundant above 900 mbsf. [10] In this study, we analyzed 59 samples spread over the interval from 800 to 1300 mbsf and, with higher spatial resolution, 92 samples from 1194 to 1242 mbsf. The former sample set consists of shipboard-made slides chosen by the U1309D geochemistry team to determine the representative geochemistry of the core. Special settings, e.g., contacts, were largely avoided. Our aim with the shipboard slides was twofold. (1) Test whether the chosen interval shows a differentiation trend in terms of mineral major element chemistry. (2) Evaluate how typical or specific the samples from the detailed interval are if compared to a larger scale. [11] The chosen region of the detailed interval from 1194 to 1242 mbsf marks the lower part of the transition from the presumed 300+ m thick magmatic unit described from 800 to 1100 mbsf into gabbronorite with oxide gabbro below via intervening screens of olivine-rich troctolite. During onboard logging, the olivine-rich troctolites stood immediately out as a distinctly olivine-rich, finegrained lithology, being even dunitic in local patches. It accumulates to a total thickness of 50 m, starting at 1094 mbsf and ending at 1236 mbsf. 3. Petrography [12] Owing to the chosen depth >800 mbsf (beneath the set of faults at around 750 mbsf) the samples are relatively fresh. Exceptions are intervals near or with oxide gabbros which tend to show marginal alteration of cpx to green amphibole and, in certain intervals, more serpentinized, olivinerich troctolites. The order of crystallization as derived from cumulus-intercumulus relationships is generally olivine = > plagioclase = > clinopyroxene (cpx) = > orthopyroxene (opx), i.e., typical for MORB crystallized at low pressure [e.g., 3of31

4 Figure 1. Shipboard data from Legs 304/305, Hole U1309D [Blackman et al., 2006]. Lithological log on left side with core number in center column, whole rock Mg # on right side. The Mg #s suggest that between 800 and 1100 m below seafloor (mbsf), perhaps even between 600 and 1100 mbsf, an upward trend toward more differentiated chemistries is present. Dashed lines are more prominent faults, the one at mbsf being a 10 cm cataclasite, the offset of which is considered not relevant for the lithological column. Detailed section studied is marked by red rectangle. 4of31

5 Figure 2 5of31

6 Bender et al., 1978; Villiger et al., 2007]. An exception might be the olivine-rich troctolite where plagioclase and cpx both occur as interstitial, oikocrystic grains around olivine, normally without a clear indication for the earlier crystallization or higher abundance of one phase or the other. Even in the olivine-rich troctolites, however, interstitial films of cpx postdate plagioclase. A magmatic foliation is weak to moderate in microgabbros, weak in troctolites and olivine gabbros, and negligible in gabbronorites and oxide gabbros (with exception of sample 254R2_023). The overall weak magmatic and plastic foliations argue for crystallization in a lithospheric environment [Hirth et al., 2005]. Detailed petrographic information for each lithology can be found in the work of Blackman et al. [2006]. Here we concentrate on observations most relevant for the ensuing discussion Olivine [13] In the olivine-rich troctolites, olivine grain sizes range broadly from 0.3 to 3 mm. In many olivine-rich troctolites, a typical grain size would be 0.5 mm but a few isolated grains stand out with a size of 3 mm or more (Figure 2a). In the olivinerich troctolites, olivine grains tend to be rounded, polygonal (Figure 2b), or rarely euhedral. Joint optical extinction of adjacent polygonal grains is locally present (Figure 2b). An olivine foliation is sometimes present (e.g., shipboard slide 248R2_96 99 in the work of Blackman et al. [2006]) but more commonly absent. In the olivine gabbros and troctolites, the olivines are commonly 5 mm large, i.e., larger than in the olivine-rich troctolites. Where olivine is rare, it tends to have an interstitial morphology. In some spectacular samples, an olivine grain may extend across the entire 3 cm wide thin section (Figures 2c 2f). These huge olivines may be repeatedly intergrown with plagioclase (Figure 2c), more rarely with cpx grains (Figure 2d). Both olivine and plagioclase/ cpx tend to show homogeneous optical extinction across the grain boundaries, i.e., are single crystals. The large to huge olivines appear restricted to the interval between 249R2_023 and 251R2_006, i.e., about 10 m from 1198 to 1208 mbsf. At least one these samples contains rutile exsolutions in olivine and 10 mm sized, square-shaped Na and Cl-containing crystals (peaks in the energy-dispersive mode of the electron microprobe) scattered over the slide. Cpx occurring between olivine locally assumes a highly irregular, indented outline (Figure 2e). Especially large olivine grains tend to show tilt walls, indicative of recovery from plastic strain (Figure 2f). In most olivine-gabbro norites studied by us in the mbsf interval, olivine occurs as isolated, large patches. Notably, these samples are typically close to the contacts to olivine-bearing lithologies, i.e., troctolites and olivine gabbros. This latter aspect does not apply for the olivine gabbronorites in the overview section mbsf Plagioclase [14] In the olivine-rich troctolites, plagioclase is clearly interstitial to olivine (Figure 2b). Else- Figure 2. Microstructural observations. (a) U1309D-248R4_019. Olivine-rich troctolite with small olivine grain size and interstitial cpx and plagioclase. Crossed nicols, scale bar 10 mm. (b) U1309D-248R2_7 (from Blackman et al. [2006]). Olivine-rich troctolite with polygonal olivine grains having similar optical extinction in shades of blue, separated by interstitial plagioclase. Crossed nicols, scale bar 1 mm. (c) U1309D-250R2_063. Troctolite, bladed plagioclase in olivine, crossed nicols, scale bar 1 mm. (d) U1309D-251R1_ Olivine gabbro (from Blackman et al. [2006]), bladed olivine in cpx; scale bar 1 mm. (e) U1309D-250R2_063. Troctolite with irregular-shaped film of cpx between olivines. The single grain cpx also penetrates as film along the olivine plagioclase grain boundary (red arrow), scale bar 1 mm. (f) Same sample as in Figures 2c and 2e. Huge single olivine grain in near-extinction is intergrown with plagioclase. Elongation of olivine outlines are parallel to tilt walls, rectangles denote areas of photos in Figures 2e and 2c, respectively. Crossed nicols, scale bar 10 mm. (g) U1309D-250R2_91. Olivine gabbro (from Blackman et al. [2006]). Rounded plagioclase within olivine. Crossed nicols, scale bar 1 mm. (h) U1309D- 257R1_109. Olivine gabbro, plagioclase laths enclosed in poikilitic cpx. Crossed nicols, scale bar 0.5 mm. (i) U1309D-250R4_ Olivine gabbro (from Blackman et al. [2006]). Olivine with convex outward outlines against cpx, crossed nicols, scale bar 1 mm. (j) U1309D-248R2_107. Troctolite, films of cpx between olivine and plagioclase. Backscattered electron image, scale bar 100 mm. (k) U1309D-253R1_136. Olivine gabbro, film of cpx transitional to opx between olivine and plagioclase, backscattered electron image, scale bar 30 mm. (l) U1309D- 256R3_52. Noritic vein in olivine-rich troctolite widening along a small jog. Open nicols, scale bar 10 mm. Inset shows vein in crossed nicols, demonstrating that it propagated (and infiltrated the host) along grain boundaries. (m) L450. Troctolite from the Lewis Hills, Bay of Islands Ophiolite. Red lines denote domains of parallel blades of altered plagioclase (rimmed by amphibole) and minor cpx which occur within largely altered olivine, open nicols, scale bar 10 mm. 6of31

7 where, it forms larger, subhedral laths. A preferred orientation representing a magmatic foliation is rare. The same is true for tapered twins which would point to plastic strain. A common feature is lobate outlines of plagioclase, particularly where in contact with cpx or, as in Figure 2g, with olivine. When enclosed in oikocrystic cpx, plagioclase tends to have very high aspect ratios in cross section or be rounded (Figure 2h). Both rounded plagioclase and rounded olivine in gabbros from the Kane megamullion have recently been attributed to resorption by an evolved melt [Lissenberg and Dick, 2008] Clinopyroxene [15] Cpx shapes range from granular to interstitial to oikocrystic. Oikocrystic grains (Figure 2h) can be exceedingly large (6 cm). Interstitial cpx grains may repeatedly indent a homogeneous olivine grain which thus assumes convex outward outlines (Figures 2e and 2i). Thin (10 30 mm) cpx films at olivine-plagioclase contacts are common (Figures 2e, 2j, and 2k) and tend to be in optical continuity with distal coarser cpx oikocrysts. Deformation structures are not obvious except in shear zones Orthopyroxene [16] Opx is common as film or interstitial grain between olivine and plagioclase (Figure 2k), more rarely between other phases, e.g., along olivineolivine contacts (photo in shipboard collection 227R3_70 72 [Blackman et al., 2006]). It also occurs as inclusions in spinel in the olivine-rich troctolites (samples 256R1_22; 256R2_0; 256R2_122; 256R3_52). Otherwise, opx is granular. The occurrence of opx as thin film does not imply that a granular opx would be present elsewhere in the slide. Only where granular opx was observed did we call a rock gabbronoritic. Gabbronoritic veins also cut the olivine-rich troctolites (Figure 2l). Such veins propagated along (not through) grain boundaries (inset in Figure 2l). Inverted pigeonite was recognized in Fe-rich lithologies (oxide gabbro and gabbronorites, e.g., U1309D-254R1_016) Spinel [17] Chromian spinel occurs only in the olivinerich troctolites as small (50 80 mm) grains with no shape preferred orientation, either enclosed in one of the present phases or along grain boundaries. Abundant orbicular spinel grows around olivine. It contains locally inclusions of opx and brown amphibole, plagioclase, rarely phlogopite and apatite Fe-Ti-Oxides [18] In oxide gabbros, the Fe-rich oxides are typically linked to cpx-rich domains. They are ilmenites, rarely titano-magnetite. In the interval studied in detail, oxide gabbros are mainly undeformed, quite in contrast to the more deformed nature of them in Hole 735B [Dick et al., 2000]. 4. Mineral Major Element Chemistry 4.1. Overview [19] Samples were analyzed on a JEOL 8900 Superprobe at the University of Cologne using wavelength dispersive methods, pure oxide standards for Al, Cr, Fe, and Ni, natural standards for the other elements, and ZAF correction. Conditions were 15 kv acceleration voltage and 20 na beam current, except for olivines where we used 20 kv and 50 na. Counting times were in the s range except for Ni in olivine (70 s) and Ti (50 s). Where possible a 20 mm beam was used for pyroxene analyses; otherwise, a focused beam was applied. Data are always averages of several measurements of different grains within one slide (Tables 1 6; Table 1 also contains lithologies and depth for the samples). In the plots, we mostly show core compositions. For cpx, we could typically also determine a distinct rim composition (Table 1). We subdivided the samples in eight different lithological groups: olivine-rich troctolites, troctolites, olivine gabbros, gabbros, olivinegabbronorites, gabbronorites, oxide-gabbros, microgabbros. Olivine-rich troctolites have more than 50% olivine, gabbronoritic rocks have granular opx, not necessarily >5 vol %. [20] For both sampling intervals, the samples give a good correlation between the An in plagioclase (100 Ca/(Ca + Na)) and the core composition of the Mg # (magnesium number) (100 Mg/(Mg + Fe)) in cpx (Figure 3). In terms of the plotted parameters, the olivine-rich troctolites are least differentiated, whereas the oxide-gabbros are most evolved, followed by gabbronorites. Gabbros are more evolved than olivine-gabbros/troctolites only in terms of the An, not in Mg # (cpx). Most gabbronorites have Mg # (cpx) <80, but a few have Mg # (cpx) up to 85. Olivine-gabbronorites appear to be more differentiated than regular gabbros, but less differentiated than gabbronorites. In both studied intervals, a low density of samples is present at Mg # (cpx) of 7of31

8 Table 1 (Sample). Averaged Clinopyroxene Analyses and One Sigma Standard Deviations for TiO2, Cr2O3 and Mg # a [The full Table 1 is available in the HTML version of this article at Sample Type of Average Rock Type Depth N SiO2 TiO2 TiO2 std Al2O3 Cr2O3 Cr2O3 std FeO MgO MnO NiO CaO Na2O K2O Total Sum Ions Mg# Mg# std mbsf 130R1_55 no cpx oxide gabbro 137R2_133 avg_c oxide gabbro R2_133 avg_r oxide gabbro R2_55 avg gabbronorite R2_55 avg gabbronorite R1_60 avg_c gabbronorite R1_60 avg_r gabbronorite R4_51 avg_c ol-gabbro R4_51 avg_r ol-gabbro R4_88 avg_c ol-gabbro R4_88 avg_r ol-gabbro R1_48 avg_c gabbronorite R1_48 avg_r gabbronorite R3_125 avg_c ol-gabbro R3_125 avg_r ol-gabbro R3_11 avg_c gabbronorite R3_11 avg_r gabbronorite R4_81 avg ol-gabbronorite R4_81 avg ol-gabbronorite R4_94 avg_c gabbronorite R4_94 avg_r gabbronorite R1_54 avg_m microgabbro R1_47 avg_c oxide gabbro R1_47 avg_r oxide gabbro R1_101 avg_c gabbronorite R1_101 avg_r gabbronorite R1_67 avg_c ol-gabbronorite R1_67 avg_r ol-gabbronorite R1_98 avg_c ol-gabbronorite R1_98 avg_r ol-gabbronorite R1_69 avg_c oxide gabbro R1_69 avg_r oxide gabbro R1_69 avg_s oxide gabbro R1_7 avg_c ol-gabbronorite R1_7 avg_r ol-gabbronorite a Averaged clinopyroxene analyses are measured in wt %. Explanations for all tables are as follows: std is one standard deviation; Mg # is 100Mg/(Mg + Fe); Cr # is 100 Cr/(Cr + Al); An = 100Ca/(Ca + Na); Fe2O3c is calculated ferric iron (Fe2O3c), based on spinel stoichiometry; avg_c is average of core analyses; avg_lg-c is average of core analyses of large grains; avg_r is average of rim analyses; avg_i is average of interstitital grains; avg_v is average of grains in a vein with different mineralogy from host; avg_m is average of grains in m-gabbro; avg_s is average of grains in sheared matrix; and N is number of spot analyses in average. 8of31

9 Table 2 (Sample). Averaged Olivine Analyses and One Sigma Standard Deviations for NiO and Mg # a [The full Table 2 is available in the HTML version of this article at Sample N SiO2 TiO2 Cr2O3 FeO MgO MnO NiO NiO std CaO Total Sum Ions Mg# Mg# std mbsf 174R4_ R4_ R3_ R4_ R1_ R1_ R1_ R1_ R2_ R3_ R3_ R3_ R1_70 ol R2_ R1_ R1_ R2_ R2_ R1_ R4_ R3_ R1_ R4_ R1_ R3_ R1_ R3_ R1_ R3_ R2_79 troc part R2_79 gb nor part R2_ R2_ R2_ R2_ R4_ R1_ R2_ R1_ a Averaged olivine analyses are measured in wt %. 9of31

10 Table 3 (Sample). Averaged Plagioclase Analyses and One Sigma Standard Deviations An a [The full Table 3 is available in the HTML version of this article at Sample Type of Average N SiO2 TiO2 Al2O3 Cr2O3 FeO MgO MnO NiO CaO Na2O K2O Total Sum Ions An std An mbsf 130R1_55 avg R2_133 avg R2_55 avg_c R2_55 avg_r R2_55 avg_near v R4_88 avg R1_60 avg_c R1_60 avg_r R4_51 avg_c R4_51 avg_r R1_48 avg_c R1_48 avg_r R3_125 avg_c R3_125 avg_r R3_11 avg R4_81 avg R4_94 avg R1_54 avg R1_47 avg_c R1_47 avg_r R1_101 avg_c R1_101 avg_r R1_67 avg_c R1_67 avg_r R1_98 avg_c R1_98 avg_r R1_69 avg_c R1_69 avg_s R1_7 avg_c R1_7 avg_r R2_34 avg_c R3_103 avg_c R3_103 avg_r R3_66 avg_c R3_66 avg_r R1_38 avg_c R1_38 avg_r a Averaged plagioclase analyses are measured in wt %. 10 of 31

11 Table 4 (Sample). Averaged Orthopyroxene Analyses and One Sigma Standard Deviations for TiO2 and Mg # a [The full Table 4 is available in the HTML version of this article at Sample Type of Average N SiO2 TiO2 TiO2 std Al2O3 Cr2O3 FeO MgO MnO NiO CaO Na2O K2O Total Sum Ions Mg# Mg# std mbsf 137R2_133 avg_c R2_55 avg R1_60 avg_c R1_60 avg_r R4_51 avg_c R4_51 avg_r R4_88 avg R1_48 avg R3_125 avg_c R3_11 avg_c R3_11 avg_r R4_81 avg R4_94 avg_c R1_54 avg_c R1_54 avg_r R1_47 avg_c R1_47 avg_r R1_101 avg_c R1_101 avg_r R1_67 avg_c R1_67 avg_r R1_98 avg_c R1_98 avg_r R1_69 avg_c R1_7 avg R2_34 avg_c R2_34 avg_r R3_103 avg R3_66 avg_c R3_66 avg_r R1_38 avg R2_61 avg_c R2_61 avg_r R3_44 avg R3_62 avg R1_70 avg R3_53 avg R2_38 avg_c R2_38 avg_r a Averaged orthopyroxene analyses are measured in wt %. 11 of 31

12 Table 5. Averaged Cr Spinel Analyses and One Sigma Standard Deviations for TiO 2, Cr #, and Fe 2 O 3 c a Sample N SiO2 TiO2 TiO2 std Al2O3 Cr2O3 FeO MgO MnO NiO CaO Na2O K2O Total Cr# Cr# std Mg# Fe2O3c Fe2O3c std mbsf 256R2_ R1_ R2_ R2_ R2_ R2_ R3_ R3_ R1_ mbsf 257R1_53 to R1_40A R1_22A R3_ R3_ R2_116 to R2_ R2_0 to R1_ R1_ R1_ R1_ R1_ R1_ R1_ R1_ R4_ R4_ R4_16 to R4_ R3_ a Averaged Cr spinel analyses are measured in wt %. 12 of 31

13 Table 6. Single Analyses of Amphibole Inclusions in Cr Spinel and One Phlogopite in Oxide Gabbro Sample SiO2 TiO2 Al2O3 Cr2O3 FeO MgO MnO NiO CaO Na2O K2O Total Mg # Inclusions in Cr Spinels in Olivine-Rich troctolite 248R2_7 inclusion in oxide R2_7 inclusion in oxide R2_0 oxide 10 inclusion in sp R1_22 bwn inclusion in oxide R1_39 inclusion in oxide Phlogopite in Oxide Gabbro 258R1_ Olivine-rich troctolites form a continuous array to the troctolites in the An Mg # (cpx) space. The few microgabbros sampled plot in the intermediate differentiation range. An outlier of the olivine-rich troctolites in the detailed interval contains a noritic vein which apparently still affected the mineral chemistry >1 cm (used for the data point) away from it Downhole Variation of Mineral Chemistry [21] In Figure 4 the mineral chemistry of the suspected magmatic unit above 1100 mbsf is shown together with data from 150 m further down. While the scatter in the data is large, all trends are consistent with an upward fractionation trend. The trend for Cr 2 O 3 is most tightly constrained. Even on this coarser scale it becomes apparent that the largest variation exists at ±1200 mbsf, i.e., the range covered by the detailed profile. [22] In Figure 5 the downhole variation along the detailed section from 1242 to 1194 mbsf is given. Here we also show differences between compositions of cores, rims, interstitial grains, and veins, where applicable and discriminated during microprobe analysis. As discussed further in section 4.5, the differences between these positions within one slide is most pronounced for Cr and Ti in cpx. At this stage, we refer to core compositions only but show the compositions of the other positions for reference. The suggested units and intrusive sequence shown at the right side of the graph are mainly defined on the basis of Cr 2 O 3 in cpx cores as well as lithology. This interpretative part is discussed further in section 5.2. [23] Most importantly, within these units, a systematic internal evolution is not obvious. This is perhaps best seen in the topmost olivine gabbro troctolite unit from 1197 to 1208 mbsf which has flat trends for all components over a length scale of 11 m. An exception could be a slight NiO (olivine) trend toward lower concentrations for the base of the olivine-rich troctolite at 1235 mbsf. Other observations are (1) olivine-gabbro norites at 1213 mbsf and 1223 mbsf occur at the contacts of gabbronorites and olivine-rich lithologies; (2) oxide gabbros are spatially bound to gabbronorite occurrences; (3) microgabbros tend to cluster, (4) further above, at 1122 mbsf (Cores 232R to 234R, not covered in Figure 5) a gradational contact over 10 m from a footwall olivine-rich troctolite to a hangingwall olivine gabbro occurs [Hellebrand and Suhr, 2007]. 13 of 31

14 Figure 3. Anorthite content in plagioclase versus Mg # of cpx cores in (a) the interval from 800 to 1300 mbsf and (b) the interval from 1194 to 1242 mbsf. Generally, the correlation is good and the different sampling scales yield similar results. Further details in text. [24] Note also the large difference in NiO (olivine) between the olivine-rich troctolites and the second most primitive unit, the olivine-gabbro/troctolites, i.e., 0.26 and 0.16 wt %, respectively. The same is true for the other compatible component plotted, Cr 2 O 3 in the core of cpx: olivine-rich troctolites are at 1.3 wt % and the topmost olivine-gabbro/ troctolite at 0.9 wt % Contacts [25] In this section we present the variation of the mineral chemistry on the thin section to meterscale as taken across contacts and along a 3 m section of olivine-rich troctolite. Figure 6 emphasizes that Cr 2 O 3 in cpx, NiO in olivine and the Mg # (cpx) change sharply across both the lower and upper contact of the 3 m thick olivine-rich troctolite to the adjacent gabbronorites. NiO in olivine and Cr 2 O 3 and Mg # in cpx are remarkably high in the 2 cm of the lower gabbronorite adjacent to the olivine-rich troctolite. Below, olivine disappears and reappears further down with much lower values in NiO. TiO 2 (cpx) trends in Figure 6 are drawn as to connect the typical minimum value in a given thin section, since higher concentrations tend to be related to cpx rims and interstitial grains, as discussed further in section 4.5. This trend shows quite some variation along the profile. TiO 2 in cpx changes systematically along the upper contact to higher values. Internally, Cr, Ni, and Mg # are very constant in the olivine-rich troctolite with no systematic trend toward enrichment or depletion with depth. Half-core pictures in Figure 6 demonstrate that plagioclase is enriched in schlieren within the olivine-rich troctolite. This feature, however, seems to be uncorrelated with the mineral chemistry. [26] Starting at core 256R3 at mbsf a near vertical 3 5 mm wide noritic vein can be traced in the core. An element distribution map for Ti in cpx across this vein is shown in Figure 7. The host chemistry is affected by the vein over 1 2 widths of the vein but chemically not detectable further away from the contact, which is particularly well illustrated by substantially more elevated Ti contents along cpx rims. [27] In Figure 8, another contact of a footwall gabbronorite against a hangingwall olivine-rich troctolite is shown. There is a 2 cm wide transition of olivine-gabbro between both units. NiO in olivine drops (correlated with Mg # in olivine) systematically from 0.24 wt % to 0.15 wt % over a distance of 7 cm in the olivine-rich troctolite plus the olivine gabbro, Mg # (cpx) and An (plagioclase) drop sharply in the gabbronorite, Cr 2 O 3 in cpx is bumpy but the drop is dramatic from >1 wt % in the olivine-rich troctolite to 0.1% in the gabbronorite. Na 2 O in cpx has a flat signal with a slight tendency toward higher concentrations in the olivine-rich troctolite, and TiO 2 in cpx clearly increases as the gabbronorite is reached but is again variable on the local scale Oxide Variation Diagrams for Regional and Detailed Interval [28] The mineral chemistry of the constituent phases is generally well correlated, as already demonstrated in Figure 3. In this section, we 14 of 31

15 Figure 4. Downhole variation of mineral chemistry in the interval from 800 to 1300 mbsf. Data of the detailed section mbsf are not included. Hexagons denote that in this sample, a gabbroic veins or a contact is present. Core compositions only are plotted. We suggest that between 800 and 1100 mbsf, a magmatic unit showing upward differentiation is present. further show the similarity of the two sections (Figure 9) and discuss internal variations. [29] An obvious feature in all diagrams of Figure 9 is that outliers from the general trends can be traced back to special circumstances in the core. Most commonly, the outliers turn out to be linked to nearby veins, i.e., gabbronorite veins within olivine-rich troctolites being the most widespread case. Vice versa, a thin vein might plot out of the typical position of its lithology because it partially equilibrated with a different host matrix. [30] Olivines alone show a gap between olivinerich troctolites and the rest of the lithologies for NiO and perhaps Mg # (Figures 9a and 9b). Note that this gap is not obvious for Mg # in cpx (Figures 9c and 9d) which explains why the trend appears continuous in Figure 3. In other words, whereas NiO in olivines of the olivine-rich troctolites form a distinct group set aside from the other lithologies, plagioclase, and cpx of the olivine-rich troctolites form smooth trends linking them to olivine-gabbros and troctolites (Cr in cpx is an exception, see Figures 9e and 9f). In two oxide gabbros, small grains of more iron-rich olivines were found (Figure 9b). While a distinction was made between olivine gabbros and troctolites based on abundance of olivine, this discrimination seems to have little reflection in the mineral chemistry trends. [31] Compatible (Cr in cpx) and incompatible (Ti in cpx) elements are negatively correlated, as expected (Figures 9e and 9f). Beyond a certain Cr-concentration, this correlation breaks down, however, and high Cr-concentrations in cpx are at constant or even variable Ti-concentration. This latter case was already emphasized by the bumpy behavior of Ti in cpx along the profiles of Figures 6 and 8. A typical minimum concentration of TiO 2 in cpx is slightly below 0.4 wt % (Figures 9e and 9f). An exceptional sample is U- 1309D-189R3_103 at mbsf. This troctolite has a strong magmatic foliation, a feature that is rare in the Core. It plots at high Ti-concentrations and low Cr-concentrations in cpx. Microstructurally, in this sample all grains are classified as interstititial or films between olivine and plagio- 15 of 31

16 Figure 5. Downhole variation of the mineral chemistry between 1194 and 1242 mbsf. In addition, the fraction of recovered core and a possible intrusive scenario is shown on the left- and right-hand sides, respectively. Core compositions are shown in round symbols, rim compositions in squares, triangles indicate the composition of interstitial grains, and vein-hosted grains are shown in hexagons (rare). The suggested units are best defined via the Cr 2 O 3 contents in cpx cores. Gradual transitions between units are not present or on a scale <10 cm. The interpretation on the right implies that at least the olivine-rich troctolites and the olivine gabbros once formed coherent bodies. clase, i.e., in this sample, no good core composition away from a rim composition could be defined. The Mg # in olivine of this sample (81.8), on the other hand is the second highest of any nonolivine-rich troctolites in our sample set. Correlations with Na in cpx are generally weak, as already indicated in the detailed profiles (Figures 6 and 8). Ignoring the olivine-rich troctolites, a negative correlation seems nevertheless to exist with An in plagioclase (Figures 9g and 9h). However, while Ti in cpx cores varies by a factor of more than 2 (0.4 to 0.8 wt % or more), Na 2 O in cpx varies by less than 50% (0.3 to 0.45%) Core-Rim Variation [32] Nearly all our samples from Hole U1309D are marked by cpx rim compositions which differ markedly from the core compositions. As a typical example, a cpx grain in contact with olivine and minor plagioclase is shown in Figure 10. Ti is enriched toward the rim, Cr and Al are depleted toward the rim, and even for Fe and Na, a slight increase toward the rim is notable. No significant zoning was detected in olivine grains. Weak plagioclase zoning (higher albite component toward rim, about 2 units) was sometimes but not always observed. Figures 5 and 10 further demonstrate that cpx grains located at an interstitial position (e.g., the film-like cpx; Figure 2e) show even more enrichment than cpx rims. A result of this behavior is that samples which only have small grains with interstitial shapes will tend to have higher TiO 2 core concentrations since a core completely unaffected by the high-tio 2 rims cannot be defined. It appears likely that the TiO 2 variation found in cpx grains of the olivine-rich troctolites, marked by interstitial to oikocrystic cpx, can be attributed to this effect. 16 of 31

17 Figure 6. Detailed view of the olivine-rich troctolite in cores 256R and 257R plus its lower and upper contact. Half-core pictures at left from shipboard documentation [Blackman et al., 2006]. Depth in centimeters. Absolute depth mbsf. [33] In Figure 11 the core/rim variation is plotted as a function of lithology. All values are averages. Where an interstitial grain average was available, we plotted this instead of average rim composition. It is clear that gabbronorites have only a weak Figure 7. Ti-distribution map across a noritic vein (red lines) in sample 257R1_22B. At left, optical picture with crossed nicols is shown. In the core reference frame, the vein is near-vertical (see Figure 6). 17 of 31

18 Figure 8. (top) Detailed profile along the contact of a footwall gabbronorite into a hangingwall olivine-rich troctolite via an intervening 1.5 cm of olivine gabbro. (bottom) Thin sections analyzed are shown. Location is U1309D-248R4_54 to 63 ( mbsf). enrichment in Cr 2 O 3 in the core over the rim (Figure 11a), oxide gabbros have no enrichment at all, gabbros, olivine-gabbros, and troctolites have the strongest depletion in the rim, and olivine-rich troctolites have only a moderate one (but large cpx grains are absent). For the incompatible element Ti in cpx, the lithological trends are similar, just the sign is reversed (Figure 11b). In oxide gabbros, even a slight enrichment in TiO 2 in the core seems to be present. Trends for the mbsf section (not shown) are overall quite comparable. 5. Discussion 5.1. The mbsf Interval [34] All parameters plotted in Figure 4 show a broad evolutionary trend up-section from 1100 to 800 mbsf. This is most clearly visible in the decrease of Cr 2 O 3 in the cores of cpx, but also Mg # in cpx and An content in plagioclase decrease in this interval and TiO 2 in cpx slightly increases. Also, variations in mineral composition (Figures 3 and 9) are consistent with differentiation processes. The most straightforward, first-order interpretation is one involving differentiation within a single magmatic cycle. In Figure 12, a comparison is made to one of the five cycles proposed for Leg 735B [Dick et al., 2002] using the same scale for depth and Mg # in cpx. The slope on the Mg # numbers is nearly the same in both Holes. Hole U1309D data show a greater scatter, partly because the 735B data set does not include the mineral compositions for gabbronorites. The overall large scatter in the Mg # of cpx is consistent with the geological information from the U1309D core, i.e., the logging of 130 magmatic units defined between 800 and 1300 mbsf by the Igneous Team on board of Leg 305 [Blackman et al., 2006]. Given the large number of magmatic units, the view of one single fractionation cycle is clearly simplistic, but to a first order it appears viable to us. There must be additional processes to explain the data scatter but at this stage we do not have the spatial resolution to further define them. Another 18 of 31