Géochronologie U-Pb par ablation laser et ICP-MS (LA-ICP-MS ...

CHEMGE-15681; No of Pages 18
ARTICLE IN PRESS
Chemical Geology xxx (2009) xxx–xxx
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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:
Received 9 June 2008
Received in revised form 22 December 2008
Accepted 24 February 2009
Available online xxxx
Editor: D.B. Dingwell
Keywords:
Troctolite
LA-ICP-MS
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 depleted 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 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
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
participants, 1972; Ildefonse et al., 2007b), slow-spread crust is highly
heterogeneous with, at least in some places, discrete 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 et al., 2000; Carlson, 2001). Ocean
Drilling has provided deep access to gabbroic rocks emplaced at depth
in this type of heterogeneous lithosphere in various oceanic core
complexes (Ildefonse et al., 2007a).
During IODP Expeditions 304 and 305 (Blackman et al., 2006;
Ildefonse et 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 et al., 2000). Hole U1309D is almost exclusively made
of gabbroic rocks (Blackman et al., 2006). Compared 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., et al., Geochemical and petrographic 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