HR: 1340h
AN: T53B-1300    [Abstracts]
TI: Origin of Olivine-Rich Troctolites From IODP Hole U1309D in the Atlantis Massif (Mid-Atlantic Ridge) : Petrostructural and Geochemical Study
AU: Drouin, M
EM: marion.drouin@gm.univ-montp2.fr
AF: Géosciences Montpellier, CNRS, Université Montpellier 2, CC60, montpellier cedex 05, 34095, France
AU: Godard, M
EM: marguerite.godard@um2.fr
AF: Géosciences Montpellier, CNRS, Université Montpellier 2, CC60, montpellier cedex 05, 34095, France
AU: * Ildefonse, B
EM: benoit.ildefonse@um2.fr
AF: Géosciences Montpellier, CNRS, Université Montpellier 2, CC60, montpellier cedex 05, 34095, France
AB: IODP Hole U1309D sampled the Atlantis Massif (western flank of Mid-Atlantic Ridge, 30°N; IODP Expeditions 304 and 305), an oceanic core complex, through an exposed detachment fault. Core U1309D is mostly made of gabbroic rocks; olivine-rich troctolites (Ol >70%) represent 5.4% of the recovered rocks. These troctolites are the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present a petrostructural (EBSD) and in-situ geochemical (EPMA, LA-ICPMS and LA-HR-ICPMS) study of olivine-rich troctolites and associated gabbros from Hole U1309D. Olivine-rich troctolites from Hole U1309D display poikilitic textures, with olivine ranging from coarse-grained subhedral crystals to medium-grained rounded crystals, embedded in large, undeformed clinopyroxene and plagioclase poikiloblasts. Olivine crystallographic preferred orientations are weak, indicating deformation by dislocation creep with activation of the high-temperature (010) [100] slip system. Analysis of misorientation angle distribution in olivine reveals well-developed (100) subgrain boundaries in some grains. Plagioclase has homogeneous composition with La ranging from 0.63 to 2.68 × chondrites, La/Sm of 1.49 and Eu/Eu* of 8.26 in all analysed samples. In contrast, Cpx display strong elemental variations at the scale of the mineral and of the thin section, but these variations do not correspond to changes in the sample lithology. A first group of Cpx (Cpx type 1) has depleted compositions with Yb ranging from 5.37 to 12.25 × chondrites, La/Yb of 0.069. The second group (Cpx type 2) are distinguished by enriched trace element composition (Yb 9.32-17.33 × chondrites), Eu negative anomaly (Eu/Eu* of 0.74) and similar REE fractionation (La/Yb of 0.074) than cpx type 1, which suggests co-precipitation of Cpx 2 and Plag. Olivine in olivine- rich troctolites has low trace element contents (Yb 0.23-0.83 × chondrites) and has strongly fractionated REE patterns (La/Yb of 0.47). Cpx 1 and Plag-Cpx2 assemblages are in equilibrium with depleted MORB, both in olivine-rich troctolites and in neighbouring gabbros. Olivine is characterized by highly variable compositions, displaying a linear correlation between Mg# (82 to 88) and Ni content (1479 to 2294), atypical of cumulate trends. In addition, olivine has highly depleted REE compositions in disequilibrium with both Plag and Cpx. Our results suggest a complex crystallization history in an open system with (i) Cpx and Plag precipitated after the same melt during the same stage of the magmatic evolution that produced Site U1309 gabbroic suite, in both olivine-rich troctolites and in neighbouring gabbros, (ii) olivine in olivine-rich troctolites predate this magmatic process. Hole U1309D olivine-rich troctolites could have been either (i) ultramafic cumulates with a very primitive character suggesting very magnesian source melts, so far unknown at mid-ocean ridges, or (ii) mantle peridotites. We favor an inherited mantle origin, the mantle composition being strongly modified by interaction with, and impregnation by large volumes of MORB-type melt.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3035 Midocean ridge processes
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2007 Fall Meeting