CHEMGE-15681; No of Pages 18 ARTICLE IN PRESS Chemical Geology xxx (2009) xxx–xxx Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Geochemical and p<strong>et</strong>rographic 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: Received 9 June 2008 Received in revised form 22 December 2008 Accepted 24 February 2009 Available online xxxx Editor: D.B. Dingwell Keywords: Troctolite <strong>LA</strong>-<strong>ICP</strong>-<strong>MS</strong> Olivine Integrated Ocean Drilling Program Mid-ocean ridge Impregnated mantle IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30°N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (~N70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from 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 and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82–88 in olivine-rich troctolites, to 78–83 in gabbros) and is in Mg–Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depl<strong>et</strong>ed in trace elements (e.g., Yb cpx ~5–11×Chondrite), they are in equilibrium with the 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 depl<strong>et</strong>ed 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 <strong>par</strong>t 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 Slow spreading ridges (full-spreading rate b50 mm/yr), such as the Mid Atlantic Ridge (MAR), represent about half of Earth's mid-ocean ridges. In contrast to the layered fast-spread crust (Penrose Conference <strong>par</strong>ticipants, 1972; Ildefonse <strong>et</strong> al., 2007b), slow-spread crust is highly h<strong>et</strong>erogeneous with, at least in some places, discr<strong>et</strong>e gabbroic bodies intruded into serpentinized peridotites (e.g., Lagabrielle and Cannat, ⁎ Corresponding author. E-mail address: Marguerite.Godard@gm.univ-montp2.fr (M. Godard). 1 Present address: Laboratoire Géosciences Reunion, Institut de Physique du Globe de Paris, Université de la Réunion, UMR 7154, CNRS, 15 avenue René Cassin, BP 7151, F-97715 Saint Denis messag CEDEX 9, La Réunion, France. 1990; Cannat, 1993, 1996; Canales <strong>et</strong> al., 2000; Carlson, 2001). Ocean Drilling has provided deep access to gabbroic rocks emplaced at depth in this type of h<strong>et</strong>erogeneous lithosphere in various oceanic core complexes (Ildefonse <strong>et</strong> al., 2007a). During IODP Expeditions 304 and 305 (Blackman <strong>et</strong> al., 2006; Ildefonse <strong>et</strong> al., 2006), a 1415.5 m deep hole was drilled at 30°N near the Mid-Atlantic Ridge (Fig.1). IODP Hole U1309D is the second deepest hole in slow-spread lithosphere, after ODP Hole 735B on the Southwest Indian Ridge (Dick <strong>et</strong> al., 2000). Hole U1309D is almost exclusively made of gabbroic rocks (Blackman <strong>et</strong> al., 2006). Com<strong>par</strong>ed to ODP Hole 735B and other drilled gabbroic series, IODP Hole U1309D gabbroic rocks comprise a relatively large proportion of olivine-rich lithologies. The most olivine rich ones (~N70%) were grouped as olivine-rich troctolites, 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 Please cite this article as: Drouin, M., <strong>et</strong> al., Geochemical and p<strong>et</strong>rographic evidence for magmatic impregnation in the oceanic lithosphere at Atlantis Massif, Mid-Atlantic Ridge (IODP Hole U1309D, 30°N), Chemical Geology (2009), doi:10.1016/j.chemgeo.2009.02.013
Géochronologie U-<strong>Pb</strong> <strong>par</strong> <strong>ablation</strong> <strong>laser</strong> <strong>et</strong> <strong>ICP</strong>-<strong>MS</strong> (<strong>LA</strong>-<strong>ICP</strong>-<strong>MS</strong>): Principes, Complexités & Perspectives Partie D : Références bibliographiques 99