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Chemical Geology 264 (2009) 71–88

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Chemical Geology 264 (2009) 71–88 Chemical Geology 264 (2009) 71–88 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Geochemical and petrographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge (IODP Hole U1309D, 30°N) Marion Drouin a,1, Marguerite Godard a,⁎, Benoit Ildefonse a, Olivier Bruguier a, Carlos J. Garrido b a Géosciences Montpellier, CNRS & Université Montpellier 2, F-34095 Montpellier cedex5, France b Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC & UGR, Facultad de Ciencias, Fuentenueva sn. 18002 Granada, Spain article info abstract Article history: IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow Received 9 June 2008 spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~N70% olivine), possibly the most Received in revised form 22 December 2008 primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element Accepted 24 February 2009 study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from Editor: D.B. Dingwell olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poikilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene Keywords: and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly Troctolite variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# LA-ICP-MS augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging Olivine from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82–88 in olivine-rich Integrated Ocean Drilling Program troctolites, to 78–83 in gabbros) and is in Mg–Fe equilibrium with clinopyroxene. Clinopyroxene cores and Mid-ocean ridge plagioclase are depleted in trace elements (e.g., Yb ~5–11×Chondrite), they are in equilibrium with the Impregnated mantle cpx same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges. © 2009 Elsevier B.V. All rights reserved. 1. Introduction 1990; Cannat, 1993, 1996; Canales et al., 2000; Carlson, 2001). Ocean Drilling has provided deep access to gabbroic rocks emplaced at depth Slow spreading ridges (full-spreading rate b50 mm/yr), such as the in this type of heterogeneous lithosphere in various oceanic core Mid Atlantic Ridge (MAR), represent about half of Earth's mid-ocean complexes (Ildefonse et al., 2007a). ridges. In contrast to the layered fast-spread crust (Penrose Conference During IODP Expeditions 304 and 305 (Blackman et al., 2006; participants, 1972; Ildefonse et al., 2007b), slow-spread crust is highly Ildefonse et al., 2006), a 1415.5 m deep hole was drilled at 30°N near heterogeneous with, at least in some places, discrete gabbroic bodies the Mid-Atlantic Ridge (Fig.1). IODP Hole U1309D is the second deepest intruded into serpentinized peridotites (e.g., Lagabrielle and Cannat, hole in slow-spread lithosphere, after ODP Hole 735B on the Southwest Indian Ridge (Dick et al., 2000). Hole U1309D is almost exclusively made of gabbroic rocks (Blackman et al., 2006). Compared to ODP Hole 735B ⁎ Corresponding author. and other drilled gabbroic series, IODP Hole U1309D gabbroic rocks E-mail address: [email protected] (M. Godard). comprise a relatively large proportion of olivine-rich lithologies. The 1 Present address: Laboratoire Géosciences Reunion, Institut de Physique du Globe de N Paris, Université de la Réunion, UMR 7154, CNRS, 15 avenue René Cassin, BP 7151, F-97715 most olivine rich ones (~ 70%) were grouped as olivine-rich troctolites, Saint Denis messag CEDEX 9, La Réunion, France. and represent 5.4% of the rocks recovered at Hole U1309D (Blackman 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.02.013 72 M. Drouin et al. / Chemical Geology 264 (2009) 71–88 et al., 2006). Such a large proportion of primitive crustal lithologies had and its relationship with the formation of the oceanic crust in these never been drilled before in present-day mid-ocean ridge crust. environments. Geochemical studies show that MORB have compositions too evolved to represent primary mantle melts (e.g., Bender et al., 1978; 2. Geological setting Fujii and Bougault, 1983; Falloon and Green, 1987). Primitive MORB melts are saturated in olivine, and should crystallize olivine-rich Atlantis Massif, a ~2 Ma old Ocean Core Complex (OCC), is located primitive cumulate first. However, the occurrence of these primitive at 30°N at the inside corner of the eastern intersection of the Mid- cumulates has not been reported yet in slow-spread oceanic crust Atlantic Ridge (MAR) with the Atlantis Transform Fault (Fig. 1a). This (e.g., MARK area at 23°N on the Mid-Atlantic Ridge, and ODP Hole OCC extends approximately 20 km parallel to the ridge and 15 km 735B on the Southwest Indian Ridge; Ross and Elthon, 1997; Bloomer across (Fig. 1a); it is assumed to result from low-angle detachment et al., 1989; Dick et al., 2000; Coogan et al., 2001). Because of their faulting (Cann et al., 1997, 1998; Blackman et al., 2002; Schroeder and very primitive composition, Hole U1309D olivine-rich troctolites are John, 2004; Karson et al., 2006; Ildefonse et al., 2007a). potential candidates for being these “missing” cumulates, and they IODP Hole U1309D was drilled into the central part of the massif may provide new insights on the early stages of MORB differentiation. during IODP Expeditions 304 and 305 at 30°10.1′N, 42°07.11′W(Black- On the other hand, some olivine-rich troctolites have textures sug- man et al., 2006). This borehole penetrated 1415.5 m below seafloor gesting melt impregnation and extensive melt-rock reaction pro- (mbsf). The recovered section comprises mainly of gabbroic rocks cesses (Blackman et al., 2006). The latter are shown to be important (N96%), although minor mantle peridotites (bb1%) and diabases (2.9%) processes in both the lower crust (e.g., Bédard et al., 2000; Ridley were also sampled. Peridotite occurs at 172 mbsf as a 88 cm interval et al., 2006; Kvassnes and Grove, 2008; Lissenberg and Dick, 2008) of alternating dunites and harzburgites, with evidence of local melt and the upper mantle (e.g., Boudier and Nicolas, 1995; Niu and impregnation in the vicinity of cross cutting gabbroic dykelets (Black- Hekinian, 1997; Godard et al., 2000; Niu, 2004; Paulick et al., 2006; man et al., 2006; Tamura et al., 2008). ~10–20 cm thick ultramafic Takazawa et al., 2007). intervals were drilled at 61,155, and 224 mbsf; some may also represent We carried out an in situ geochemical study of troctolites and serpentinized peridotite, but they were too altered for their origin to be olivine-rich troctolites sampled in four intervals of Hole U1309D in reliably inferred. Most diabases were observed in the upper 130 m of the which these lithologies are dominant, and of gabbros sampled next to borehole where they occur as 50 cm to 10 m thick intrusive intervals these intervals. This study aimed at providing geochemical constraints (Fig. 1b); they have MORB compositions (Blackman et al., 2006; Godard on the origin of the olivine-rich lithologies in slow-spread lithosphere et al., 2009). Fig. 1. (a) Bathymetric map of Atlantis Massif with location of Hole U1309D drilled during IODP expedition 304–305. (b) Lithology proportions in Hole U1309D, 20 m vertical running average (Blackman et al., 2006) and localization of the four sampled olivine-rich intervals. (c) Streckeisen diagram of studied samples, and other samples from IODP Hole U1309D. Note that olivine-rich troctolites are characterized by their proportion of olivine (~N70%), and by their texture (see text for further details). M. Drouin et al. / Chemical Geology 264 (2009) 71–88 73 The gabbroic rocks sampled during Expeditions 304–305 are interstitial plagioclases. Coarse-grained gabbros show sharp contacts characterized by a broad range of modal compositions: troctolite, with olivine-rich troctolites. Olivine-rich troctolites are moderate to olivine gabbro (including troctolitic gabbro), gabbro and oxide gabbro highly altered compared to gabbro due to serpentinization of olivine. represent 2.7%, 28%, 56% and 7% respectively of the lithologies In the fourth interval, from core 227R1 to 248R4 (~105 m), we recovered at Hole U1309D (Blackman et al., 2006). The shipboard collected 40 samples (28 olivine-rich troctolites, 2 troctolites, 4 olivine– scientific party grouped an olivine rich subset of gabbroic samples as gabbros and 6 gabbros).
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