Geochemistry of a Long In-Situ Section of Intrusive Slow-Spread Oceanic Lithosphere: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge)

Earth and Planetary Science Letters 279 (2009) 110–122

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Earth and Planetary Science Letters

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Geochemistry of a long in-situ section of intrusive slow-spread oceanic lithosphere: Results from IODP Site U1309 (Atlantis Massif, 30°N Mid-Atlantic-Ridge)

M. Godard a,⁎,S.Awajib, H. Hansen c, E. Hellebrand d, D. Brunelli e,f, K. Johnson d, T. Yamasaki g,1, J. Maeda g, M. Abratis h, D. Christie i, Y. Kato b, C. Mariet j, M. Rosner k,l a Géosciences Montpellier, CNRS, Université Montpellier 2, 34095 Montpellier, France b Department of Geosystem Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan c University of Bergen, Department of Earth Science, Allegt. 41, 5007 Bergen, Norway d University of Hawaii, Dept. of Geology and Geophysics, Honolulu, Hawaii, USA e Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, 41100 Modena, Italy f Institute of Marine Sciences, ISMAR-CNR, Via Gobetti, 101, 40129 Bologna, Italy g Division of Natural History Sciences, Hokkaido University, N10 W8, Kita-ku, Sapporo 060-0810, Japan h Institute of Geosciences, University of Jena, D-07749 Jena, Germany i West Coast & Polar Regions Undersea Research Center, University of Alaska, Fairbanks, AK 99775-7220, USA j Laboratoire Pierre Süe CEA — CNRS/UMR 9956 Centre de Saclay, Bât. 637 91191 Gif sur Yvette Cedex, France k University of Bremen, Petrology of the Oceanic Crust, PO Box 330440. 28334 Bremen, Germany l GFZ German Research Centre for Geosciences, Inorganic and Isotope Geochemistry, Telegrafenberg, 14473 Potsdam, Germany article info abstract

Article history: IODP Site U1309 was drilled at Atlantis Massif, an oceanic core complex, at 30°N on the Mid-Atlantic Ridge Received 24 July 2008 (MAR). We present the results of a bulk rock geochemical study (major and trace elements) carried out on Received in revised form 22 December 2008 228 samples representative of the different lithologies sampled at this location. Accepted 23 December 2008 Over 96% of Hole U1309D is made up of gabbroic rocks. Diabases and basalts cross-cut the upper part of the Available online 30 January 2009 section; they have depleted MORB compositions similar to basalts sampled at MAR 30°N. Relics of mantle Editor: R.W. Carlson were recovered at shallow depth. Mantle peridotites show petrographic and geochemical evidence of extensive melt–rock interactions. Gabbroic rocks comprise: olivine-rich troctolites (N70% modal olivine) and Keywords: troctolites having high Mg# (82–89), high Ni (up to 2300 ppm) and depleted trace element compositions (Yb Mid-Atlantic Ridge 0.06–0.8 ppm); olivine gabbros and gabbros (including gabbronorites) with Mg# of 60–86 and low trace oceanic gabbro element contents (Yb 0.125–2.5 ppm); and oxide gabbros and leucocratic dykes with low Mg# (b50), low Ni bulk-hole composition (∼65 ppm) and high trace element contents (Yb up to 26 ppm). Troctolites and gabbros are amongst the most geochemistry primitive and depleted oceanic gabbroic rocks. The main geochemical characteristics of Site U1309 gabbroic ICP-MS rocks are consistent with a formation as a cumulate sequence after a common parental MORB melt, although (lack of systematic) downhole variations indicate that the gabbroic series were built by multiple magma injections. In detail, textural and geochemical variations in olivine-rich troctolites and gabbronorites suggest chemical interaction (assimilation?) between the parental melt and the intruded lithosphere. Site U1309 gabbroic rocks do not represent the complementary magmatic product of 30°N volcanics, although they sample the same mantle source. The bulk trace element composition of Site U1309 gabbroic rocks is similar to primitive MORB melt compositions; this implies that there was no large scale removal of melts from this gabbro section. The occurrence of such a large magmatic sequence implies that a high magmatic activity is associated with the formation of Atlantis Massif. Our results suggest that almost all melts feeding this magmatic system stays trapped into the intruded lithosphere. © 2009 Elsevier B.V. All rights reserved.

1. Introduction

Slow and ultra-slow spreading ridges (b55 mm/yr full spreading rate) represent the most common modern ridge system (about half of ⁎ Corresponding author. Géosciences Montpellier, UMR 5243 CNRS-UM2, Université the ∼67,000 km long volcanic ridge system (Solomon, 1989; Bird, Montpellier 2, cc60, Place Eugène Bataillon, 34095 Montpellier cedex 5, France. Tel.: +33 2003)). Currently available data support hypotheses where magmatic 4 67 14 39 37; fax: +33 4 67 14 36 03. dominated phases of rifting alternate with tectonically dominated E-mail address: [email protected] (M. Godard). 1 Present address: Institute of Geology and Geoinformation, Geological Survery of extension (e.g., (Thatcher and Hill, 1995; Lagabrielle et al., 1998; Buck Japan, AIST, 1-1-1Higashi, Tsukuba, Ibaraki 305-8567, Japan. et al., 2005)), although the architecture of the oceanic crust produced

0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.12.034 M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122 111 in these environments, in particular in its deeper parts, is still poorly Kelemen et al., 2004). The last location, Atlantis Massif (Fig. 1), was constrained. Seafloor sampling along axial zones and median valley drilled during IODP Expeditions 304/305 (Blackman et al., 2006)at walls show that slow spreading centres expose upper mantle rock and Site U1309, which is the object of this communication. lesser gabbro in the axial zone. In some ultra-slow spreading areas, We carried out a geochemical study (major and trace elements) of however, seafloor gabbro has not been sampled and basalt is found in 228 samples representative of the different lithologies sampled at Site proximity to altered peridotite (Dick et al., 2003; Standish et al., 2008). U1309. Our results allow documenting the bulk composition and These seafloor exposures have been interpreted as evidence that part downhole variations at Hole U1309D and provide new insights into of the magmatism is intrusive and that the lithosphere is hetero- the nature of the slow spread lithosphere, magmatic differentiation geneous (Cannat, 1996). This model differs from the layered and melt/rock interactions within gabbroic intrusions and the architecture suggested for intermediate to fast spread oceanic litho- intruded mantle lithosphere. sphere i.e., an oceanic igneous crust comprising (from top to bottom) Mid-Ocean Ridge Basalts (MORB), sheeted dykes and gabbros, which 2. Geological setting overlies residual mantle peridotites. The mechanisms by which occurs the differentiation of primary MORB magmas (produced by mantle Atlantis Massif is located at 30°N on the west side of the MAR axial partial melting (Herzberg et al., 2007)) and how the magmatic valley at the intersection with the Atlantis fracture zone; it extends intrusions are incorporated into the slow spread lithosphere remain ∼15 km N–S, parallel to the ridge, and 8–10 km parallel to spreading largely unknown because of lack of data on the structure and direction, and was formed during the past 1.5–2 m.y (Blackman et al., composition of the deeper parts of oceanic lithosphere. 2006). The massif is dominated by variably serpentinized peridotite at Deep drilling is technically challenging but it remains the only way the surface (Blackman et al., 2004). On the northeastern part of the of documenting long continuous sections of present-day oceanic massif, the hanging wall, a basaltic block, is in contact with the lithosphere. Long-lived oceanic detachment faults, which are inter- footwall (core) of the detachment fault. The southern ridge shallows preted as characterizing tectonically-dominated rifting (Tucholke up to 700 m below sea level (mbsl); it hosts the H2-generating low et al., 1998; Lavier et al., 1999), are inferred to expose the deepest temperature Lost City hydrothermal field (Kelley et al., 2001; Früh- parts of the oceanic crust (Cann et al., 1997; Escartin et al., 2003; Green et al., 2003). Blackman et al., 2004). Detachment faulting close to fracture zones Site U1309 is located on the central part of the massif where the can proceed to a stage where a large and shallow domal structure seafloor coincides with what is interpreted to be a gently sloping, forms an inside corner high, sometimes called megamullion or corrugated detachment fault surface (Fig. 1). During IODP Expeditions oceanic core complex (OCC). Because of these unique features, OCCs 304/305 (Blackman et al., 2006), two deep holes located at ∼20 m have been favored as targets for deep drilling of slow spread from each other were drilled at this site: Hole U1309B (30°10.11′N, lithosphere. Currently, four OCCs have been drilled in the frame of 42°07.11′W; 1642 mbsl), a single-bit pilot hole, and Hole U1309D Ocean Drilling Program (ODP) and the Integrated Ocean Drilling (30°10.12′N, 42°07.11′W; 1645 mbsl) were drilled down to 101.8 mbsf Program (IODP): the deepest hole was drilled at Atlantis Bank, on the and 1415.5 mbsf respectively (Fig. 2). The upper 20.5 m of Hole South West Indian Ridge (ODP Hole 735B – 1500 m deep – (Dick et al., U1309D was not cored; it was cased to provide stable reentry for a 2000)), and the three others are located on the Mid-Atlantic Ridge deep hole. A total of 46.8 m of core was recovered at Hole U1309B and (MAR). The MARK (23°32′N — Sites 921–924) and MAR 15°44′N (Site 1043.3 m at Hole U1309D, with an overall average recovery of, 1275) areas were drilled down to ∼82 and 209 m below seafloor respectively, 46% and 75% (the upper 20.5 m of Hole U1309D (mbsf) respectively, during ODP Legs 153 and 209 (Cannat et al., 1995; excluded). Hole U1309D is the second deepest hole drilled into slow spread lithosphere.

3. Methods

The primary mineralogy (igneous modes, grain size and textures), alteration and structure of the core were described on-board (Fig. 2), on both hand samples (Visual Core Description —VCD-) and thin sections (74 thin sections were made from Hole U1309B samples and 632 at Hole U1309D). Detailed VCD and thin section descriptions are provided in Expeditions 304/305 Proceedings (Blackman et al., 2006). A total of 62 and 770 igneous units were deﬁned at Hole U1309B and Hole U1309D respectively. Each unit is distinguished on the basis of igneous contacts and variations in primary modes and grain size. 243 samples, representative of the drilled material, were chosen for geochemical analysis onboard of the drillship Joides Resolution by Expeditions 304/305 scientiﬁc party. Thin sections were made wherever a geochemistry sample was taken. Due to the high petrographic variability of the core, even at the scale of a single igneous unit, the rock-names of the geochemistry samples were determined on the basis of the detailed descriptions of these thin sections and not only using the VCD. Loss on Ignition (LOI) was determined on each sample. Inductive coupled plasma atomic emission spectroscopy (ICP-AES) was used for determining major and some trace element (Sr, V, Cr, Co, Ni, Cu, Sc, Y, Zr, Ba and Nb)

concentrations and gas chromatography for H2O and CO2. The sample preparation procedure (which involved grinding in a shatterbox with a tungsten carbide —WC- barrel), ICP-AES and gas chromatography Fig. 1. Map of Atlantis Massif, showing location of Site U1309, where Holes U1309B and U1309D were drilled, and of Lost City Hydrothermal Vent (LCHV) (after Ildefonse et al., analytical techniques (including accuracy and precision) and results 2007). are reported in Blackman et al. (2006). 112 M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122

Trace element concentrations (Li, Cd, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, element composition (this study) in Supplementary Material 2 Sr, Y, Zr, Nb, Cs, Ba, Rare Earth Elements (REE), Hf, Ta, Pb, Th, U and W) (Table A2 in the Appendix). W and Ta were contaminated during were determined on-shore using Inductively Coupled Plasma-Mass grinding in the WC container as well as Co in a few samples: these Spectrometry (ICP-MS) on a subset of 228 samples in seven three elements are eliminated from the ﬁnal dataset. laboratories: University Montpellier 2 (France), University of Jena (Germany), University of Tokyo (Japan), University of Bergen (Nor- 4. Composition of the different rock types sampled at Site U1309 way), Washington State University (USA), Max-Planck-Institute for Chemistry (Mainz, Germany), and Pierre Sue CEA-CNRS Laboratory With the exception of a few meters composed of altered (France). We used two certiﬁed reference materials, serpentinite UB-N peridotites, diabase bodies and basaltic dykes at Hole U1309B and in and basalt BIR-1, to identify interlaboratory and method biases for the upper 200 m of Hole U1309D, the basement drilled at Atlantis trace element analyses and to determine the accuracy of the analyses. Massif consists almost entirely of gabbroic rocks (Fig. 2). Alteration, In addition, we used four in-house standards to test the reproduci- mainly in the greenschist facies, is moderate and becomes low bility and repeatability of trace element analyses. The in-house towards the bottom of the borehole, except near fault zones and in standards are four samples from Hole U1309D that we considered as areas where olivine-rich (locally serpentinized) rock-types are representative of the dominant lithologies at this borehole: SR1, an observed. Recovered samples are characterized by a marked hetero- olivine gabbro, SR2, a gabbro, SR3, an oxide gabbro and SR4, an geneity in texture, mode and chemical composition. The different orthopyroxene-bearing gabbro. Reproducibility of these references rock types sampled at Site U1309 (after Blackman et al., 2006) and samples is generally better than 5% for concentrations N1 ppm; it is their main geochemical characteristics are detailed below (Table 1; better than 10% for concentrations N0.5 ppm; it is within 10–20% and Figs. 3–7). 20–40% for concentrations of 0.01–0.5 ppm and less than 0.01 ppm respectively. Details on the ICP-MS analytical methods and results of 4.1. Diabase and basalt UB-N, BIR-1 and in-house rock standards analyses are provided as Supplementary Material 1 in the Appendix. Diabase bodies and basaltic dykes cross-cut the other rock types; The mode and major element composition of these 228 samples they occurred late in the formation of Atlantis Massif. At Hole U1309B, (from Blackman et al. (2006)) are reported together with their trace they represent almost half of recovered samples; at Hole U1309D, they

Fig. 2. Downhole plots along Hole U1309D of (left to right): lithology (20 m running average, white on the right hand side=no recovery), total alteration (in %, averaged over 5 m intervals), crystal–plastic deformation and magmatic foliation intensity (intensity, averaged over 5 m intervals), bulk rock Mg# (cationic ratio: 100×Mg/(Mg+Fetotal), all Fe as FeO), Yb content (in ppm). Deformation (fabric) intensity scale: 0=undeformed, 1=weakly foliated, 2=strongly foliated. Symbols in inset. Lithology, alteration estimates and Mg# data from Blackman et al. (2006). M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122 113

Table 1 Mean compositions and densities of the different rock-types sampled at Site U1309 and estimates of the bulk composition and density of Hole U1309D

Peridotite Basalt Gabbroic rock Bulk hole Bulk hole diabase compositions compositions including leucocratic dykes (LD) Hole B Hole D Olivine-rich Troctolite Olivine Gabbro Oxide Leucocratic troctolite gabbro gabbro dyke Total Gabbro Gabbronorite BH BG BH:LD BG:LD N 2 2 23 16 11 79 75 51 24 13 3 % vol. Hole 2.9 5.4 2.7 25.5 55.7 n.d. n.d. 7 n.d. U1309D Density 2.92 2.93 2.84 2.91 2.94 n.d. n.d. 3.02 n.d. 2.93 2.94 –– (g/cm3) wt.%

SiO2 44.90 43.96 50.59 41.82 44.36 48.84 51.66 51.39 52.28 43.41 52.05 49.55 50.18 49.68 50.27

Al2O3 0.85 0.53 14.32 4.61 13.51 17.51 17.03 17.33 16.34 11.16 15.96 15.92 16.69 15.93 16.65

Fe2O3 9.64 10.44 11.94 12.42 8.29 6.16 6.34 6.07 6.97 20.47 5.30 7.75 7.44 7.62 7.34 MgO 45.05 43.42 7.83 37.33 25.81 12.98 9.56 9.60 9.46 6.15 4.41 12.20 10.26 11.81 9.97 MnO 0.10 0.09 0.17 0.16 0.12 0.10 0.12 0.11 0.14 0.27 0.09 0.13 0.13 0.13 0.12 CaO 0.31 1.16 10.89 3.30 7.82 12.96 13.20 13.45 12.65 10.83 13.74 12.26 12.94 12.34 12.98

Na2O ––2.86 0.18 0.68 1.61 2.29 2.24 2.39 2.41 4.34 1.96 2.10 2.08 2.22

K2O ––0.04 0.02 0.06 0.03 0.03 0.03 0.02 0.04 0.08 0.03 0.03 0.03 0.03

P2O5 0.04 0.06 0.11 0.06 0.04 0.03 0.03 0.03 – 1.20 1.54 0.12 0.13 0.19 0.20

TiO2 0.02 0.03 1.76 0.07 0.10 0.23 0.35 0.33 0.38 4.73 3.18 0.62 0.67 0.75 0.80 Total 100.92 99.69 100.51 99.96 100.80 100.45 100.61 100.59 100.62 100.67 100.70 100.44 100.51 100.55 100.57 Mg# 90 89 57 86 86 81 75 76 73 37 62 76 73 75 73 Ca# 68 91 86 82 76 77 74 71 64 78 77 77 76 ppm Li 0.4 1.4 0.8 3.3 5.0 2.1 1.3 1.3 1.4 1.7 0.4 1.8 1.6 1.7 1.5 Cd ––0.10 ––28.7 0.05 0.05 ––––––– Sc 8.9 7.8 38.8 11.6 10.3 27.5 36.4 34.9 39.5 48.8 37.1 33 35 33 35 Ti 60.3 123.0 9069 375 520 1431 2053 1970 2237 24561 17597 3438 3752 4146 4405 V 41.3 52.3 307.9 57.8 45.0 107.8 156.1 145.6 182.4 994.1 – 198 211 –– Cr 2545 2603 – 1284 1146 850 334 321 358 74 – 525 460 –– Ni 2249 2360 99 1581 1124 326 142 158 105 73 55 292 189 280 182 Cu 2.7 4.0 16.5 76.6 79.5 61.0 44.4 41.4 51.3 70.1 9.8 53 51 51 49 Zn ––69.8 47.6 153.5 35.6 34.7 31.8 41.6 173.6 – 49 47 –– Rb 0.020 0.033 0.144 0.094 0.420 0.220 0.174 0.164 0.195 0.239 0.422 0.19 0.19 0.20 0.20 Sr 1.25 14.29 96.93 13.56 49.99 81.17 91.39 90.23 93.86 93.58 230.4 83.4 88.6 90.7 95.7 Y 0.291 0.950 41.18 1.59 2.83 7.33 10.10 9.68 10.99 89.39 252.1 14.6 15.8 26.5 27.6 Zr 0.181 0.615 74.49 2.39 3.22 7.65 9.46 9.12 10.24 151.68 1006.6 19.1 20.5 68.4 69.9 Nb 0.022 0.036 2.44 0.044 0.11 0.19 0.19 0.18 0.22 3.99 13.28 0.47 0.50 1.11 1.14 Cs 0.001 0.001 0.004 0.004 0.012 0.010 0.031 0.037 0.016 0.023 0.018 0.02 0.02 0.02 0.02 Ba 0.299 0.087 5.76 0.448 2.62 2.69 3.21 2.95 3.77 5.51 8.79 3.07 3.25 3.36 3.53 La 0.042 0.079 3.01 0.071 0.20 0.37 0.39 0.38 0.42 5.23 19.56 0.72 0.78 1.67 1.72 Ce 0.120 0.284 10.24 0.245 0.66 1.22 1.34 1.31 1.40 21.44 72.26 2.73 2.94 6.21 6.41 Pr 0.017 0.046 1.76 0.050 0.12 0.24 0.27 0.27 0.29 4.08 13.58 0.53 0.57 1.18 1.22 Nd 0.078 0.246 10.64 0.283 0.68 1.42 1.73 1.70 1.80 25.90 80.78 3.35 3.61 7.22 7.47 Sm 0.021 0.068 3.87 0.127 0.25 0.61 0.79 0.77 0.83 9.67 28.33 1.35 1.46 2.70 2.80 Eu 0.012 0.008 1.40 0.062 0.16 0.36 0.48 0.46 0.52 2.82 8.96 0.59 0.64 1.01 1.05 Gd 0.031 0.094 5.77 0.212 0.39 0.96 1.29 1.24 1.38 13.82 39.60 2.06 2.22 3.93 4.09 Tb 0.006 0.016 1.01 0.042 0.07 0.18 0.25 0.24 0.27 2.30 6.97 0.37 0.40 0.70 0.72 Dy 0.048 0.126 6.99 0.278 0.49 1.29 1.77 1.70 1.92 14.98 45.68 2.52 2.71 4.67 4.86 Ho 0.012 0.030 1.45 0.067 0.11 0.28 0.39 0.37 0.43 3.10 9.53 0.54 0.58 0.99 1.03 Er 0.043 0.100 4.19 0.182 0.31 0.80 1.10 1.06 1.20 8.00 25.84 1.47 1.58 2.69 2.79 Tm 0.008 0.015 0.61 0.031 0.04 0.12 0.16 0.15 0.18 1.08 3.63 0.21 0.22 0.38 0.39 Yb 0.056 0.105 3.80 0.201 0.30 0.77 1.08 1.03 1.18 7.00 22.70 1.37 1.47 2.44 2.53 Lu 0.011 0.019 0.59 0.036 0.05 0.12 0.16 0.15 0.18 1.02 3.31 0.20 0.22 0.36 0.37 Hf 0.006 0.023 2.38 0.080 0.11 0.28 0.36 0.35 0.39 3.89 24.25 0.58 0.63 1.76 1.81 Pb 0.030 b0.001 0.16 0.042 0.06 0.09 0.10 0.09 0.13 0.27 0.17 0.11 0.11 0.11 0.12 Th 0.001 0.003 0.159 0.007 0.019 0.016 0.029 0.021 0.045 0.059 0.352 0.03 0.03 0.04 0.04 U 0.002 0.352 0.040 0.003 0.009 0.008 0.012 0.011 0.014 0.027 0.258 0.012 0.012 0.024 0.025

N represents the number of samples (considered as independent observations) used to calculate the mean composition of each rock-type. The volume of each rock-type in Hole U1309D (noted % vol. Hole U1309D) and their mean density values are from Blackman et al. (2006). Bulk hole estimates were calculated on the gabbroic suite by itself (diabases and peridotites are not included in the calculations). The bulk hole composition (BH) is the sum of the mean composition of each gabbroic rock-type (from olivine rich troctolite to oxide gabbro) weighted by their respective mass fraction in Hole U1309D. The bulk gabbro composition (BG) is the sum of the mean composition of olivine–gabbro, gabbro (including gabbronorite) and oxide gabbro weighted by their respective mass fraction. The mass fraction of each rock-type is its volume fraction in Hole U1309D multiplied by its mean bulk density; to calculate BH and BG, these values are normalized so that the sum of the mass fractions is 1. Bulk hole densities are the sum of the mean density of each rock type multiplied by their respective volume fractions, assuming that the total volume is 1. We calculated also the bulk compositions of gabbroic rocks and of gabbros taking into account a contribution of leucocratic dykes (LD). Although leucocratic dykes and veinlets were observed throughout the whole borehole, the volume fraction of this rock-type has not been estimated by the Expedition 304/305 shipboard scientiﬁc party; we report here the results of calculations carried out assuming that leucocratic dykes represent 5% of the total mass of gabbroic rocks (BH:LD) or of gabbro (BG:LD) sampled at Hole U1309D, which are in a close match to the estimates of primary magmas of MORB (see Fig. 9). Notes: n.d. = not determined. 114 M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122

Fig. 3. Compositions of the different rock types recovered at Site U1309 illustrated on (a) Ca# versus Mg#, (b) Ni (ppm) versus Mg# and (c) FeO (wt.%) versus MgO (wt.%) diagrams. FeO in c stands for all Fe recalculated as FeO. A compilation of basalts sampled along the MAR (55°S–52°N, PetDB –20-Aug.-2007–) is shown for comparison. The range of composition of the gabbros sampled on the SWIR (Hole 735B (Dick et al., 1991, 1999; Hertogen et al., 2002; Holm, 2002; Miller and Cervantes, 2002; Niu et al., 2002) and in the Atlantis Fracture Zone area (Coogan et al., 2001)) and on the MAR (MARK (Agar et al., 1997; Barling et al., 1997); 15°20 Fracture Zone and ODP Site 1275 OCC (Kelemen et al., 2004)), and of peridotites (refractory and impregnated samples of the 15°20 Fracture Zone (Kelemen et al., 2004; Paulick et al., 2006; Godard et al., 2008); MARK (Casey, 1997)) are illustrated. Also shown are the composition of the gabbros and an estimate of the serpentine composition (point with error bars) from the southern wall of the Atlantis Massif (near LCHV (Boschi et al., 2006)). Thin grey lines in c indicate iso-Mg# lines for Mg# ranging from 10 to 95. Variations of Fe–Mg compositions of olivine (OL), clinopyroxene (CPX) and plagioclase (PLAG) in Site U1309 gabbroic suite are represented by thick dashed lines (OL, CPX) and a white rectangle (PLAG) (Data from Miller et al., in press). Symbols of Site U1309 samples and the compiled dataset are in inset. are observed mainly in the upper 120 m of the borehole (Fig. 2). and basalt is the same as that of the average of MAR MORB sampled Basaltic dykes are a few centimeter thick. Diabase bodies are at 30°N (Figs. 4 and 7). interpreted as being emplaced as groups of 2–8 m thick sills (Blackman et al., 2006). 4.2. Peridotites Site U1309 diabase and basalt have mostly tholeiitic compositions

(SiO2 =49–53 wt.%; Na2O+K2O=2.2–3.7 wt.%) and Mg# (100×Mg/ Highly altered relics of mantle were recovered at shallow depth at (Mg+Fetotal)∼56) (Fig. 3). They have depleted MORB compositions Site U1309: a ∼3.5 m thick interval of relatively undeformed with Yb of 14–30×C1-chondrites (Sun and McDonough, 1989), light serpentinized harzburgite was sampled at ∼60 mbsf at Hole rare earth element (REE) depleted patterns (La/Yb=0.66–0.83; N- U1309B, and a 88 cm interval of serpentinized harzburgite and dunite MORB=0.81 (Sun and McDonough, 1989)), Pb negative anomaly and was recovered at ∼172 mbsf at Hole U1309D. Similar (b10 s cm thick) a strong depletion in highly incompatible elements (Pb/Ce∼0.02 — ultramafic intervals were drilled throughout the upper 225 m of Hole N-MORB=0.04; Nb/Zr∼0.03 — N-MORB=0.03) (Figs. 4 and 7). Site U1309D; some may also represent relics of mantle, but they were too U1309 diabase and basalt are characterized by a strong scatter in serpentinized for their origin to be reliably inferred. When recovered, major and trace element content (e.g., Fig. 3). This compositional the boundary of peridotite intervals is in direct contact with the variability may be related to the pervasive greenschist alteration neighboring gabbro or diabase, without evidence of chilled margins in observed to different extents in all diabase and basalt samples, in the latter. Peridotite intervals are cross-cut by narrow gabbroic dykes particular when mobile elements such as Na2O, Rb or U are and dykelets, in the vicinity of which petrographic evidence of considered. It may also indicate local differentiation within larger intensive melt impregnation (late precipitation of plagioclase and diabase bodies: variations in SiO2, CaO and Al2O3 and REE content or clinopyroxene) is observed. This suggests that these gabbroic intru- in Eu/Eu⁎ (0.8–1.1) or Sr/Ce (0.8×N-MORB (Sun and McDonough, sions took place at temperatures close to the peridotite solidus 1989)) may reflect the plagioclase/clinopyroxene ratio in fine- (Blackman et al., 2006; Tamura et al., 2008). grained diabases and/or locally a higher degree of differentiation of We analyzed two harzburgites from Hole U1309B, one dunite and the crystallizing melt. The average composition of Site U1309 diabase one harzburgite from Hole U1309D and one ultramafic rubble (a talc M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122 115

Hole U1309B reﬂects mainly its talc-rich secondary mineralogy (e.g.,

SiO2 =60.2 wt.%); nonetheless this sample has high Mg# and Ni and trace element contents similar to Site U1309 peridotites (Figs. 3 and 4). We note also that the peridotites and the ultramaﬁc rubble sampled at Hole U1309B are enriched in Eu and in Pb (Eu/Eu⁎=1.3– 2.4; Pb/Ce=1.1–4×Primitive Mantle (PM (Sun and McDonough, 1989))), while Hole U1309D peridotites are strongly depleted in Pb and Eu (Eu/Eu⁎=0.2–0.3; Pbb0.01 ppm). High Eu/Eu⁎ in peridotites are classically attributed to trace amounts of plagioclase, however Eu/Eu⁎ values in the studied samples show no correlation with Sr concentrations (in these samples, Sr is controlled by carbonate

addition as indicated by correlation with CO2 content). Consequently, it seems more likely that the variations of Eu (and in Pb, which shows similar trends) result from interaction with hydrothermal ﬂuids during serpentinization, probably under highly reducing and high temperature conditions, similar to those associated with

Fig. 4. REE compositions normalized to CI chondrites of (a) basalts and diabases and of (b) peridotites sampled at Site U1309. Normalizing values were taken from Sun and McDonough (1989). The average composition of basalts sampled along the MAR in the Atlantis Massif area (thick black line – determined after a compilation of PetDB dataset – PetDB -20-Aug.-2007–) and the composition of refractory harzburgites (grey ﬁeld noted Mantle peridotites — (Godard et al., 2008)) and of reacted and impregnated peridotites (Paulick et al., 2006) sampled in the 15°20 Fracture Zone area are shown for comparison. Symbols are indicated in insert.

tremolite schist) sampling the top of Hole U1309B. Peridotites have high loss on ignition (LOIN10%) indicating that high amounts of volatile constituents were added to the original ultramaﬁc assemblage during alteration; nevertheless they have preserved compositions typical of mantle harzburgites with low Al2O3 (∼0.8 wt.%) and high Ni (N2300 ppm) and MgO contents (N43.4 wt.%). Although they are highly refractory, Site U1309 harzburgites have Mg# (89–90) overlapping with that of fertile abyssal peridotites (Fig. 3b). These Mg# values reﬂect the peridotites high Fe contents (up to 10.44 wt.%

Fe2O3); this suggests that they have undergone late Mg–Fe chemical equilibration with basaltic melts (Berger and Vannier, 1984; Tommasi et al., 2004). Site U1309 peridotites have high trace element contents (e.g., Yb=0.20–0.66×C1-chondrites) and selectively light-REE (LREE) enriched to linear LREE depleted chondrites-normalized patterns ((La/

Yb)N =0.4–0.8; (La/Sm)N =0.6–2.6- N=normalized to C1-chondrite). These compositions are similar to that of highly reacted and impregnated peridotites previously sampled along the MAR (Paulick et al., 2006)(Fig. 4), and to that of peridotites from the mantle/crust transition zones in ophiolites (e.g., (Godard et al., 2000; Dijkstra et al., 2003)); together with textural evidence, this suggests that the slivers Fig. 5. REE composition of gabbroic rocks sampled at Site U1309 normalized to CI of mantle sampled at Site U1309 have undergone melt/rock reactions chondrites (Sun and McDonough,1989): (a) Olivine-rich troctolite and troctolite, (b) olivine gabbro, gabbro and gabbronorite (orthopyroxeneN5%) and (c) oxide gabbro and at high melt/rock ratios. leucocratic dyke. The average composition of Site U1309 basalt and diabase and the Locally, alteration supersedes magmatic signatures. For instance, composition of Site U1309 peridotites are shown for comparison (thick grey lines). Symbols the major element composition of the ultramaﬁc rubble sampled at are indicated in insert. 116 M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122

lithologies (average modes in Supplementary Material 3 (Table A3 in the Appendix)), as: (1) Olivine-rich troctolite and troctolite Olivine-rich troctolite comprises N70% olivine (±clinopyroxene) and b30% plagioclase; it groups the most olivine-rich rock-types of the gabbroic rock series (dunite, werhlite and plagioclase-poor troctolite). Troctolite comprises 30%–70% olivine, 30%–50% plagioclase, and up to 5% clinopyroxene. Spinel is observed in both rock-types. Olivine-rich troctolite displays subhedral to rounded medium-grained olivine and interstitial to poikilitic plagioclase and clinopyroxene in variable proportions. Troctolites are characterized by cumulate textures, irregularly seriate with local poikilitic clinopyroxene occurrences. Plagioclase/olivine ratios vary on a decimeter scale, and there are frequent irregular gradations into troctolitic gabbro and even olivine gabbro. Olivine-rich troctolite and troctolite are typically interlayered with gabbro and represent 5.4% and 2.7%, respectively, of the lithologies recovered at Hole U1309D. Olivine-rich troctolite and troctolite have relatively high Mg# (82– 89). In detail, they show two different evolutions in Mg–Fe content: most olivine–troctolite samples show variable Fe-enrichments, at relatively high MgO contents, in line with the trend defined by impregnated peridotites (Fig. 3c), while troctolite Mg–Fe content mainly reflects changes in the olivine mode. Also, at constant Mg#, olivine-rich troctolite is distinguished by higher Ni content (1200–2300 ppm) compare to troctolite (760–1600 ppm) reflecting higher Ni content in olivine in olivine-rich troctolite (Drouin et al., in revision). Olivine-rich troctolite and troctolite have low trace element contents (Yb 0.06– 0.8 ppm) (Fig. 6); they have LREE-depleted yet linear chondrites- normalized patterns (on average La/Yb ∼0.3–0.4×C1-chondrite). Troc- tolites have positive Eu and Sr anomalies (Eu/Eu⁎=1–4.2; Sr/Sr⁎=1–11)

Fig. 6. Trace element compositions of peridotites and of gabbroic rocks recovered at Site U1309 illustrated on (a) Yb (ppm) and (b) Nb (ppm) versus Ce (ppm) diagrams. A compilation of gabbros sampled on the SWIR (Hole 735B (Hertogen et al., 2002; Holm, 2002; Miller and Cervantes, 2002; Niu et al., 2002); Atlantis Fracture Zone area (Coogan et al., 2001)) and on the MAR (MARK (Agar et al., 1997; Barling et al., 1997; Ross and Elthon, 1997); 15°20 Fracture Zone and ODP Site 1275 OCC (Godard and Ildefonse, unpublished data)), and of peridotites (refractory and impregnated samples of the 15°20 Fracture Zone (Paulick et al., 2006; Godard et al., 2008); MARK (Casey, 1997)) are shown for comparison. Also shown are the composition of the gabbros and an estimate of the serpentine composition (point with error bars) from the southern wall of the Atlantis Massif (Boschi et al., 2006). Symbols are the same as in Fig. 3.

“Logatchev” type ultramafic rock hosted hydrothermal vents (Douville et al., 2002; Allen and Seyfried Jr., 2005; Paulick et al., 2006). In contrast, Hole U1309D peridotites are enriched in U and Li (Hole U1309D peridotites: U/ThN150 × PM–LiN1.1 ppm; Hole U1309B peridotites: U/Th=∼9×PM–Li∼0.5 ppm). U and Li are considered as mobile in hydrothermal fluids under low temperature and oxidizing conditions (Seyfried et al., 1998; Niu, 2004). These contrasting variations between Holes U1309B and U1309D suggest highly localized and complex sequences of alteration in the residual mantle sampled at Atlantis Massif. Detailed geochemical studies should allow determining whether alteration predates or postdates the emplacement of the gabbroic series. Fig. 7. Mean trace element composition, normalized to primitive mantle values, of the different rock-types sampled at Site U1309: (a) basalt and diabase and (b) gabbroic 4.3. Gabbroic rocks rocks. Normalizing values were taken from Sun and McDonough (1989). The average composition of basalts sampled along the MAR in the Atlantis Massif area (thick black Gabbroic rocks span a wide range of modes, textures and line – determined after a compilation of PetDB dataset – PetDB -20-Aug.-2007–) and estimates of primary MORB melts (grey field) are shown for comparison in a. Estimates geochemical compositions (Figs. 3, 5–7). For simplicity, the various of the primary MORB magmas were calculated as the composition of melts formed by rock types provided in the visual core and thin section descriptions 6% aggregated fractional melting of a depleted to enriched Depleted MORB Mantle were grouped by Expeditions 304/305 Scientific Party (Blackman (after Workman and Hart, 2005). The average composition of Site U1309 basalt and et al., 2006), from the most olivine-rich to the most differentiated diabase is shown for comparison in b. Symbols are indicated in insert. M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122 117 that reflect plagioclase accumulation (Fig. 5). Olivine–troctolites have are observed in almost all gabbroic rock types. Leucocratic dykes variable Eu and Sr anomalies (Eu/Eu⁎=0.4–3; Sr/Sr⁎=0.3–3.2), these consist mostly of medium-grained albitic plagioclase, (magmatic?) scattered compositions reflect the effect of late alteration processes in amphibole and minor quartz. Accessory minerals such as apatite and some samples, although to a lesser extent than in Site U1309 peridotites. zircon are observed in oxide gabbro and leucocratic dyke. Except for one U-enriched highly altered olivine-rich troctolite Oxide gabbros and leucocratic dykes represent the most evolved (wehrlite) sampled at the top of Hole U1309D, olivine-rich troctolite end-members of Site U1309 gabbroic series. and troctolite are depleted in highly incompatible elements (Figs. 2 Oxide gabbro is characterized by very low Mg# (23–50), which and 7); they display a slight depletion in Zr and Hf relative to REE (Zr/ reflect high iron concentration (Fe2O3 =7–30 wt.%), and high TiO2 Sm=10–20 in troctolite and 10–27 in olivine-rich troctolites — PM=25 contents (up to 8 wt.%). Oxide gabbros are trace element enriched (Sun and McDonough, 1989)). They have compositions close to (and (Yb=1.2–20 ppm) (Figs. 6 and 7), but they have LREE depleted patterns even overlapping with) that of serpentinites sampled on the southern ((La/Yb)N ∼0.4) and they are depleted in highly incompatible elements wall of Atlantis Massif (Boschi et al., 2006). (Nb/Zr 0.02–0.03) and Zr and Hf relative to REE (Zr/Sm∼19) similar to the other gabbroic rocks sampled at Site U1309. However, in contrast (2) Olivine gabbro and gabbro to the latter, they have on average slight negative Eu, Sr and Pb anomalies (Eu/Eu⁎∼0.97; Sr/Sr⁎∼0.65; Pb/Ce∼0.05). Some samples Gabbro and olivine gabbro are the most abundant rock types represent the most Fe- and trace element-enriched oxide gabbros yet sampled at Site U1309: they represent, respectively, 56% and 28% of analyzed at a slow spreading ridge (Fig. 6). the lithologies recovered at Hole U1309D. Olivine gabbro (N5% olivine) Leucocratic dykes have high SiO (52–53 wt.%) and Na O contents is characterized by strong modal variations (5%–50% olivine, 25%–80% 2 2 (3.4–5 wt.%); one sample is distinguished by particularly high SiO plagioclase, and 10%–70% clinopyroxene); it displays decimeter- to 2 (63 wt.%) and Na O contents (7.9 wt.%) (Fig. 3). Leucocratic dykes meter-scale variations in clinopyroxene content and grades locally 2 represent the most trace element enriched samples at Site U1309 into troctolitic gabbro. Olivine gabbro is commonly spatially asso- (Yb=12–26 ppm); they have incompatible element depleted patterns ciated with troctolites. Gabbro, olivine-bearing (b5% olivine) and ((La/Yb) ∼0.8; Nb/Zr∼0.02) (Figs. 6 and 7). They are characterized by orthopyroxene-bearing (b5% orthopyroxene) gabbro, microgabbro N negative Eu, Sr and Pb anomalies (Eu/Eu⁎∼0.8; Sr/Sr⁎∼0.2; Pb/Ceb0.01) and disseminated oxide gabbro (b2% Fe–Ti oxides) are grouped as and, in contrast to the other gabbroic samples from Site U1309, a positive gabbro. Gabbro mode is dominated by plagioclase and clinopyroxene. Zr–Hf anomaly (Zr/Sm∼36). They represent the most differentiated Gabbronorite (5–30% orthopyroxene±inverted pigeonite) is present gabbroic rocks sampled at Site U1309 series. In detail, we note that some mainly in the lower part of Hole U1309D (depthN620 mbsf). In leucocratic dykes, although they are trace element rich similar to oxide contrast to other gabbroic rocks, gabbronorite abundances were not gabbros, have TiO contents too low (b0.4 wt.%) to represent high estimated as this rock-type is difficult to identify in the core (without 2 degrees of differentiation; they are also the most SiO rich samples (53– thin section). Olivine–gabbro and gabbro are characterized by wide 2 63 wt.%). These samples may represent products of gabbro anatexis, variations in texture; for instance, gabbro and gabbronorite exhibit triggered by water intrusion into the crystallizing gabbroic series significant variation in grain size (b1mmtoN10 cm), sometimes (Koepke et al., 2007). within a single section of core. Site U1309 olivine gabbro has high Mg# (73–83) and Ca# 5. Downhole variations at Hole U1309D (100×cationic Ca/(Na+Ca)=72–91), overlapping that of troctolite, while gabbro (including gabbronorite) displays lower Mg# (62–82) Hole U1309D shows strong variations in lithology, degree of and Ca# (63–85) values (Fig. 3). Olivine–gabbro and gabbro have low alteration and composition with depth, whereas deformation stays Ni content (∼400 ppm and ∼140 ppm in olivine–gabbro and gabbro low to moderate (Fig. 2). When preserved, intrusive contacts are respectively) and high incompatible trace element contents (Yb indifferently sharp or diffuse, although the later is more common in the 0.125–2.5 ppm) compared to troctolite (Figs. 3 and 5). Site U1309 lower part of the borehole (Blackman et al., 2006). The more evolved gabbro and gabbronorite are characterized by a strong overlap in gabbro rock types are generally intrusive into more the olivine-rich composition for both major and trace elements (Figs. 3–6). Because of rock types. Systematic change of composition is not observed across its highly variable mode, olivine–gabbro is characterized by markedly structural boundaries, or related to changes in paleomagnetic inclina- more dispersed compositions, compared to gabbro. Nevertheless, tion or to variations of the magmatic and plastic fabric intensity olivine–gabbro, gabbro and gabbronorite display on average similar (Blackman et al., 2006; Ildefonse et al., 2006). Compositional variations trace element fractionations. They have LREE depleted patterns with at Site U1309 reveal mainly changes in the igneous lithologies and the positive Eu and Sr anomalies (La/Yb∼0.3–0.4×C1-chondrite; Eu/ complex intrusive relationships between these different rock types at Eu⁎ ∼ 1.6; Sr/Sr⁎ ∼ 4), a slight depletion in highly incompatible the scale of the borehole. Contact relationships and downhole changes elements (e.g., average Nb/Zr∼0.02–0.03) and in Zr and Hf relative in lithology and composition indicate that the gabbroic sequence is to REE (Zr/Sm∼12), and a slightly negative Pb anomaly (Pb/Ce∼0.08– built by a series of interfingered units, of highly variable composition, 0.09 — PM=0.1) (Fig. 7). The composition of olivine gabbro and gabbro that vary in thickness from centimeters to several meters. is little affected by alteration. Olivine gabbro and gabbro sampled at Even though Hole U1309D lithostratigraphy is dominated by a Site U1309 have highly depleted trace element composition, in strong scatter in composition, we observe at 600–740 mbsf and at particular with respect to the most incompatible elements, compared 1250–1340 mbsf abrupt and significant changes in the bulk rock Mg# to previously analyzed gabbroic series (Fig. 6). and trace element contents of gabbros and olivine gabbros (Fig. 2). The zone delimited by these two intervals is distinguished by (i) the (3) Oxide gabbro and leucocratic dyke common occurrence of orthopyroxene in gabbros (in fact, almost all Oxide gabbros (N2% Fe–Ti oxide) mostly occur as undeformed, gabbronorites in this study come from the lower part of the borehole), generally coarse-grained gabbro with randomly dispersed patches of (ii) the abundance of olivine-rich troctolites: the longest section of oxides. Oxides are also found as discrete dykelets or layers cutting olivine-rich rock types in Hole U1309D, which is marked by high Mg# other rock types with either sharp or diffuse boundaries, and and low trace element content on downhole plots, was sampled associated with intervals of ductile deformation (b20% of oxide between 1030 and 1220 mbsf (Fig. 2). Furthermore, radiometric dating gabbro). Leucocratic gabbros and felsic veins (thereafter grouped as of zircons from oxide gabbros and leucocratic dykes (Grimes et al., leucocratic dykes) represent a minor component of Site U1309 2008) indicates that gabbroic series below 600 mbsf began to gabbroic suite; yet, narrow leucocratic intrusions or diffuse veinlets crystallize ∼200 ka earlier than in the upper part of the borehole, 118 M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122 even if melt injections continued throughout the whole sequence fractional crystallization of a common parental MORB melt (e.g., until at least 1.08±0.07 Ma (the youngest ages found in Hole U1309D (Hart et al., 1999; Dick et al., 2000)). Modal variations suggest low samples). Hence, the chemical variations observed at 600–740 mbsf pressure crystallization as generally inferred for oceanic gabbros and at 1250–1340 mbsf may mark the transition between magmatic (0.2 GPa–∼6km( Grove and Bryan, 1983)): precipitation of olivine, domains formed at different pressure and temperature conditions then plagioclase, clinopyroxene and orthopyroxene. Geochemical during the construction of Site U1309 gabbroic series and/or local variations reflect these modal changes – higher plagioclase or olivine changes in the composition of the crystallizing magma; these abundance leading to elevated Al2O3 or MgO respectively (Fig. 3) –, hypotheses are discussed below. and the progressively more evolved compositions in crystallizing minerals (Miller et al., in press). The progress of melt crystallization is 6. Discussion marked by increasing bulk rock trace element contents (Fig. 6). With the exception of the most evolved lithologies (leucocratic dykes and to 6.1. Site U1309: documenting the building of an intrusive lower crust a lesser extent oxide gabbros), all gabbroic samples display positive Eu and Sr anomalies as well as negative Zr and Hf anomalies (Fig. 7), Holes U1309B and U1309D were drilled through series of gabbroic which can be accounted for by plagioclase accumulation (plagioclase rocks locally cross-cut by diabases; gabbroic rocks and diabases are characterized by a strong Zr–Hf depletion relative to REE (Drouin represent ∼96.3% and ∼2.9%, respectively, of the core sampled at Hole et al., in revision). U1309D. Small intervals of mantle peridotites were recovered, Site U1309 gabbroic series encompass the whole range (and more) preserved into Site U1309 gabbroic series, but only in the upper of modes and major and trace element compositions previously 200 m of the boreholes (Blackman et al., 2006). They provide the first reported for oceanic gabbro series (Figs. 3 and 6). They are evidence (to our knowledge) of incorporation of slivers of mantle into characterized by (i) the abundance of recovered olivine-rich litholo- an oceanic gabbro series formed at a spreading centre. gies: troctolites and olivine-rich troctolites represent 8.1% of the core The detailed geochemical study of the harzburgitic interval recovered at Hole U1309D (in comparison with 1.9% of troctolite and sampled at Hole U1309B indicates the progressive reequilibration of troctolitic gabbros sampled at Hole 735B (Dick et al., 1999)), (ii) high peridotites with neighboring gabbros (Tamura et al., 2008). In contrast, Mg-contents and (iii) an overall trace element depletion in troctolites the small mantle peridotite interval sampled at Hole U1309D is too and gabbros compared to previously analyzed troctolites and gabbros modified by melt–rock interactions to provide irrefutable evidence of (Figs. 3 and 6). Actually, the most primitive end-members of Site magmatic interaction with the gabbroic intrusions: it represents either U1309 gabbroic series (olivine-rich troctolites, troctolites and olivine– an impregnated mantle, similar to that sampled at mantle/crust gabbros) have compositions intermediate between that of oceanic transition zones (e.g., Boudier and Nicolas (1995)), cross cut by the gabbros and that of abyssal peridotites (even overlapping the later for gabbroic series (without further melt/rock interactions), or a residual some samples). These observations suggest that drilling of Site U1309 mantle modified during the emplacement of the gabbroic intrusion. gabbroic series allowed to sample, for the first time, a complete These observations, together with the significantly different type of oceanic cumulate suite from its most primitive to its most evolved alteration affecting Hole U1309B and Hole U1309D peridotites end-members. In addition, Site U1309 gabbroic rocks are character- ((Blackman et al., 2006), this study), suggests that the preserved ized by a strong depletion in highly incompatible trace elements peridotite relics may have been intruded at different stages of (and similar to Site U1309 diabases and basalts and MORB sampled at MAR maybe at different depths during) the formation of the gabbroic series. 30°N (e.g., Nb/Zr=0.02–0.03) that suggests that the whole igneous The recovered mantle relics do not mark major magmatic suite derive from a common parental melt. transitions in the gabbroic sequence. This observation together with In detail, however, some samples have compositions that do not the complex intrusive relationships observed over more than 1400 m seem consistent with this simple accumulation model. Some olivine- at Hole U1309D led us to posit that the gabbroic suite sampled at Site rich troctolites have textures and compositions that suggest they U1309 was formed by multiple magma injections. The volume and comprise products of melt–rock reaction and assimilation of olivine- size of each magmatic intrusion is difficult to assess, as downhole rich material (Drouin et al., in revision). The role of assimilation and observations do not provide conclusive evidence for broad scale incorporation of the wall–rock into the ascending magmas at mid- modal or cryptic layering, as expected within series of separate oceanic ridges had been little explored (Bédard, 1993; Bédard et al., intrusions. This suggests that each magmatic intrusion produced a 2000) until the recent works of Lissenberg and Dick (2008) and differentiation suite characterized by the formation of a sequence of Kvassnes and Grove (2008) who invoke a possible control of melt– progressively more evolved intrusive melt lenses, thus producing a rock reaction on the composition of crystallizing melts. Also, Site geometry comparable to the linked sill model proposed by several U1309 gabbronorites have compositions similar to gabbros (Figs. 3–6) authors for the formation of lower crustal oceanic gabbro sections in contrast to most mid-ocean ridge gabbroic suites in which (e.g., (Marsh, 1989; Bédard, 1991; Sinton and Detrick, 1992; Kelemen gabbronorites (when present) have on average more evolved (trace and Aharonov, 1999)), in contrast to the hypothesis whereby the lower element enriched) compositions than associated gabbros (e.g., Hess crust would be formed by several magma chambers having different Deep (Mevel et al., 1993; Pedersen et al., 1996); MARK area (Barling et crystallization history (e.g., (Nisbet and Fowler, 1978; Dick et al., al., 1997)). At depth (N0.3 GPa; ∼10 km), orthopyroxene precipitate 2000)). Radiometric dating of zircons and structural data indicates with/before clinopyroxene (e.g., (Ghiorso, 1985)), which is then that melt injections occurred at random depths during at least characterized by high Al2O3 (∼6% Al2O3 (Stolper, 1980)). However, ∼200 ka (Grimes et al., 2008). Several periods of episodic magmatism clinopyroxene in Site U1309 gabbros and gabbronorites have were recorded, during which the whole (crystallizing or crystallized) compositions typical of oceanic gabbro suites (∼2.2 wt.% Al2O3 (Miller gabbroic series as well as the lithospheric mantle were intruded. et al., in press) thus precluding formation at high pressure.

Several linked sill magma plumbing systems may have been produced Alternatively, water in the crystallizing magma may favor early SiO2- during these episodes, but they were probably short-lived compared saturation (e.g., (Boudier et al., 2000; Nonnotte et al., 2005; Koepke et to the duration of the construction of the gabbroic sequence. al., 2007)). Nonnotte et al. (2005) proposed that hydrated melts could be generated by re-melting of altered mantle at ridges, and favor 6.2. Site U1309: a typical section of oceanic gabbros? precipitation of orthopyroxene early in the crystallization sequence; but the resulting gabbronorites are then signiﬁcantly trace element The mode and composition of gabbroic series sampled at Site depleted, while at Site U1309, gabbronorites and gabbros have the U1309 is typical of cumulate series formed by the progressive same compositions. Alternatively, (local) changes in the crystallizing M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122 119 melt water content may have occurred if altered lithospheric material (mantle or cumulates) was assimilated into the gabbroic sequence. Olivine-rich troctolites and gabbronorites are observed mainly in the lower part of the borehole where melt intrusions occur over a longer time period (Grimes et al., 2008), which may have favored, locally, impregnation and complete assimilation of slivers of the intruded (crustal or mantle) lithosphere, in contrast to the upper part of the borehole where the relics of mantle peridotites were preserved.

6.3. The bulk composition of Hole U1309D: an insight on the nature of the lower crust in magma-starved areas

We calculated the bulk composition of Hole U1309D, on the basis of the estimates of the mean composition and density of the different gabbroic rock types sampled at Site U1309 and of their volume fraction in Hole U1309D (see caption of Table 1). Mantle peridotites and late diabases were not taken into account in the calculations. Fig. 9. Estimates of the bulk trace element composition of Hole U1309D normalized to Results are presented in Table 1 and Figs. 8 and 9. primitive mantle values (Sun and McDonough, 1989). Estimates of the primary MORB magmas (grey ﬁeld – after Workman and Hart (2005) – see caption Fig. 7) and estimates of the bulk trace element composition of Hole 735B from Hart et al. (1999) are shown for comparison. Hole 735B “protolith estimate” was determined on the basis of analyses conducted on a small suite of fresh and unaltered samples representative of the different lithologies characteristic of Hole 735B; the “strip average” estimate was obtained using a strip-sampling technique (sampling of 1 to 4 m long continuous 1 cm strip samples of the core to take into account small scale lithologic variability, patchiness of veins, alteration and thus obtain a possibly more representative sampling). The bulk composition of Hole U1309 (BH) is characterized by low trace element contents, with signatures indicative of plagioclase accumulation (positive Eu and Sr and negative Zr and Hf anomalies). Models giving results consistent with estimates of the Mg#–Ca# composition of primary MORB magmas (see Fig. 8) are shown for comparison: BG (a proxy to back stripping of the olivine contribution), addition of 10% oxide gabbro or 20% basalt give also typical cumulate compositions. Only models taking into account the contribution of leucocratic dykes in the bulk hole estimates give results consistent with primary magma compositions. It should be noted that the same difference is observed between bulk hole estimates at Hole 735B: calculations taking into account only representative samples (without the contribution of evolved trondjhemitic or altered end-members) give bulk hole compositions having cumulate signatures while the strip average method (which can be approximated by a 90%:10% mix between the bulk hole average and trondjhemite) gives compositions consistent with primary magma compositions.

The bulk composition of Hole U1309D (noted BH) is characterized by high Mg# (76) compared to the different estimates of the bulk composition of Hole 735B (Fig. 8). It has cumulate-like characteristics with low trace element contents (Yb∼1.4 ppm), positive Eu–Sr and negative Zr–Hf anomalies, similar to the most depleted estimate of the bulk composition of Hole 735B (calculated using the weighted average of representative lithologies — noted “protolith” on Figs. 8 and 9). To investigate the mass balance between Site U1309 gabbroic series and associated basalts, which are supposed to constitute an igneous suite formed after a common parental melt, we mixed various fractions of basalts with BH (BH:B model) and compared the results to estimates Fig. 8. Estimates of the bulk composition of Hole U1309D illustrated on a Ca# versus of the primary magma of MORB (Figs. 8 and 9). Adding 20% basalts to Mg# diagram. The bulk composition of Hole U1309D is calculated taking into account all gabbroic rocks (BH) or only gabbros (BG) (see also caption of Table 1). The grey ﬁeld 80% BH produces major element compositions consistent with indicates values of the primary magmas of MORB from Kinzler and Grove (1993), which primary MORB magma estimates (Fig. 8), but the resulting trace represent the most primitive basaltic compositions (high Mg) expected at a spreading element composition is too cumulate-like (strong Zr–Hf anomalies) centre. Estimates of the bulk composition of Hole 735B (Hart et al., 1999; Dick et al., and too depleted (Fig. 9) compared to primary MORB compositions. 2000) as well as the mean compositions of basalts at Site U1309 and Hole 735B are BH composition is not that of the complementary igneous product of shown for comparison. The estimate of the bulk composition of Hole U1309 (BH) is characterized by high Mg# compared to primary magma compositions. These high Mg# MORB emplaced at MAR 30°N. may indicate a cumulate composition complementary to that of MORB, but also an over- BH composition may reﬂect the over- or under-estimation of the estimation of the mass of the most primitive end-member of the gabbroic suite, or an contribution of one of the components of Site U1309 gabbroic series under-estimation of the mass of its most evolved end-members in our calculations. due to incomplete core recovery and lateral heterogeneities in the These hypotheses are tested using various models: (i) BH:Ol: 5% of olivine (Fo89 and Fo92) is subtracted from BH; (ii) BH:OG: mix between BH and oxide gabbro, numbers distribution of the drilled lithologies. For instance, oxide gabbros indicate the mass fraction of the latter; (iii) BH:B: mix between BH and Site U1309 represent only 7% of the gabbroic rocks sampled at Site U1309, by basalts, numbers indicate the mass fraction of the latter; (iv) BH:LD: a 95%:5% mix of BH comparison with almost 25% of the rocks recovered at Hole 735B and leucocratic dyke; (v) BG:LD: a 95%:5% mix of BG and leucocratic dyke. Several of (Natland and Dick, 2002). Also, assimilation of olivine-rich rocks, as these models are consistent with estimates of the composition of primary MORB proposed by Drouin et al. (in revision), may have resulted in a bulk magmas: back stripping of olivine contribution (analogue to BG), addition of 10% oxide gabbro or 20% basalt, addition of 5% LD to the bulk hole estimate taking into account hole composition more olivine-rich and trace element depleted than only gabbro. Symbols are indicated in insert. expected in a pure MORB accumulation sequence. 120 M. Godard et al. / Earth and Planetary Science Letters 279 (2009) 110–122

To test how these different scenarios could modify our bulk dykes; trace element geochemistry indicate that they have the same composition estimates, we carried out different mixing and mass parental melts as the late diabase and MORB sampled at the ridge 30°N balance calculations between BH and various fractions of representa- MAR. Downhole (lack of systematic) variations indicate that the tive end-members, as illustrated on Figs. 8 and 9. Addition of 10% gabbroic series were built by multiple magma injections. There is no oxide gabbro to BH (BH:OG) balances the Fe depletion and Mg excess or little deformation of the gabbroic section, yet dating of late leucocratic that characterizes BH when compared to primary MORB estimates dykes and oxide gabbros suggest that magma intrusion and the (Fig. 8). The new bulk composition has Mg# consistent with primary functioning of the large fault that brings the gabbro body to the surface MORB values, but although it is trace element enriched relative to BH, are contemporaneous (Grimes et al., 2008). The bulk composition of it keeps a depleted cumulate signature (strong Zr–Hf negative gabbroic rocks sampled at Hole U1309D is consistent with that of anomaly) relative to primary MORB estimates (Fig. 9). We calculated primitive MORB mantle melts as proposed in the literature, without any an olivine-depleted bulk hole end-member using only olivine gabbro, of the trace element fractionation typically attributed to cumulate gabbro and oxide gabbro (BG) to estimate the effect of an over- sequence (e.g. positive Eu and Sr anomalies). This implies that no (or sampling of the most primitive end-members of the gabbroic series. very little) melt was extracted from the gabbroic series. To cancel the effect of assimilation of olivine-rich rocks into the The formation of a large gabbroic body, such as the one sampled at gabbroic sequence on its bulk composition, the contribution of up to Atlantis Massif, indicates a high magmatic activity in this area, 5% olivine (Fo89–Fo92) was subtracted from the BH estimate. To a first previously considered as magma-starved because of the lack of order, the two calculations produced similar results, therefore we basaltic crust. It is consistent with recent models indicating that considered BG as a proxy accounting for the maximum effect that formation of OCC demands relatively elevated magmatic activity reactions involving olivine-rich lithologies may have had on the bulk (Ildefonse et al., 2007; Tucholke et al., 2008). Our results suggest that composition of the emplaced gabbroic sequence. BG has Mg–Fe melts feeding this magmatic system were mostly trapped into the compositions close to that expected for primitive mantle melts (∼73). intruded lithosphere. Yet, at 30°N, the extrusive crust and the gabbroic On the other hand, the bulk hole trace element composition is very body have the same parental melt. There must be a complementary little modified by variations in the fraction of olivine-rich rocks, cumulate for the extrusive crust. We posit that, similar to what is because of their depletion in these elements: for example, the proposed at ultra-slow spreading centres (Cannat et al., 2008), the contribution of olivine-rich troctolites and troctolites to BH incompa- along-axis variations of the depth of the thermal boundary control tible element budget in less than 2%. Hence, taking into account a magma emplacement: at segment-ends, a thick thermal boundary possible under-sampling of highly evolved oxide gabbros or over- layer favors the formation of a heterogeneous intrusive lithosphere, sampling (or assimilation) of olivine-rich lithologies produces com- while a layered crust forms at the centre of segments (Cannat, 1996). position similar to that of primary MORB magma, but it cannot fully In this system, no melt is extracted at the end of segments while explain the trace element characteristics of Hole U1309D gabbroic magma input at the middle of the segment feeds the whole extrusive sequence compared to basalts and their primary magma estimates. crust, for instance, via dykes (Dick, 1989; Smith and Cann, 1999). Leucocratic gabbros have been considered as a minor component Further studies of the lateral crustal heterogeneities sampled by of Site U1309 gabbroic series and the fraction of the volume of the Atlantis Massif should provide further insights into the interplay of core they represent was not determined, and therefore they were not tectonic extension and magmatism during the formation of OCCs. taken into account in our calculations. Yet, because of their high trace element contents, they could represent an important component of Acknowledgements the bulk trace element budget. The underestimation of the trondjhemite-type end-member in oceanic gabbro series was We thank the captain and crew of the JOIDES Resolution, the pointed out by Hart et al. (1999),whoshowedthatupto10% Integrated Ocean Drilling Program (IODP) staff and the members of trondjhemite should be added to the bulk hole composition IODP Expeditions 304–305 Shipboard Scientific Party. MG thanks calculated using only representative lithologies to obtain a repre- S. Pourtales and O. Bruguier for the technical support during ICP-MS sentative average composition of Hole 735B, comparable to that analyses (AETE – Research Department T2E – University Montpellier obtained using a strip-sampling technique (Fig. 9). At Hole U1309D, 2(http://www.aete-t2e.univ-montp2.fr/)). MA thanks D. Merten for including a few percent of leucocratic components (up to 5%) to BH technical assistance on ICP-MS and acknowledges support by the (BH:LD) has little effect on the bulk hole major element composition German IODP council to join the expedition. This research used samples yet it produces trace element compositions very similar to that and data provided by the Integrated Ocean Drilling Program (IODP). proposed for primary MORB melts (Fig. 9). Funding was provided by INSU (Institut National des Sciences de Calculations carried out to test the effects of biases of sampling and l'Univers – France – MG and DB), PRIN-COFIN 2007 (Italy — DB), N.S.F. and of extended assimilation processes resulted in bulk hole compositions the U.S. Science Support Program of J.O.I. (KJ). This work benefited from close to that of a primary MORB magma. This suggests that although discussionswithJ.Lissenberg,J.-M.Dautria,M.DrouinandB.Ildefonse. troctolites and gabbros form a cumulate series indicating there has We are grateful to C. Garrido and an anonymous reviewer for helpful been local separation of melt and solids, the most evolved end- comments and suggestions and R. Carlson for editorial handling. members of the Site U1309 gabbroic series are complementary to the cumulate series. We interpret these results as evidence that there was Appendix A. Supplementary data no large scale removal of melts from this gabbro section. Supplementary data associated with this article can be found, in 7. Conclusion the online version, at doi:10.1016/j.epsl.2008.12.034.

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