HR: 11:25h
AN: V22A-05    [Abstracts]
TI: Geochemical Study of Mid Atlantic Ridge Peridotites From 15\deg N: Preliminary Results From ODP Sites 1272 and 1274
AU: * Godard, M
EM: margot@dstu.univ-montp2.fr
AF: Tectonophysique, Universit‚ Montpellier 2, Montpellier, 34095 France
AU: Seyler, M
AF: G‚osciences Marines, IPGP, Paris, 75252 France
AU: Harvey, J
AF: Dept. Earth Sciences, Open Univ., Milton Keynes, MK7 6AA United Kingdom
AU: Alard, O
AF: Tectonophysique, Universit‚ Montpellier 2, Montpellier, 34095 France
AB: During ODP Leg 209, eight sites were drilled along the Mid-Atlantic Ridge from 14\deg43 to 15\deg44 N, allowing recovery of ca. 354 meters of residual mantle peridotite intruded by gabbroic rocks (up to 25% of recovered samples). We present here the results of a whole rock major and trace element study of 28 peridotites selected among the less altered samples (those which preserved enough primary features to allow detailed petrographic and mineralogical studies), at Site 1272 and Site 1274 (respectively S and N of the 15\deg20 fault zone). Site 1272 and Site 1274 peridotites are mainly harzburgites (cpx $<$ 5%) with a few dunites. Harzburgites are characterized by highly variable opx contents (10-30 vol.%). Some samples show evidence of trapped melt (up to 3-5%). A few samples at Site 1274 show textural evidence of abundant cpx crystallization, resulting in the formation of lherzolites. All studied peridotites have been modified by alteration ($>$70% serpentinisation), leading to the addition of significant amounts of volatiles (loss on ignition $>$10 wt%). Nevertheless, except for alkali-earth elements and U, major and trace element compositions show no evidence of additional compositional changes during alteration. Site 1272 and Site 1274 peridotites are characterized by high Mg# (100 x molar Mg/[Mg + Fe]) (90.3- 92.2) and low Al$_{2}$O$_{3}$ contents ($<$0.3 wt% in dunites and ranging from 0.6 to 0.9% in harzburgites). Site 1272 and Site 1274 peridotites display flat to light Rare Earth Element (REE) depleted patterns (harzburgites : Ce$_{N}$$<$0.015; Yb$_{N}$$<$0.26 - dunites : Ce$_{N}$$<$0.001; Yb$_{N}$$<$0.04-0.08- $_{N}$: chondrite normalised). The most depleted harzburgites are observed at the southernmost Site 1272 (Ce$_{N}$$<$0.003; Yb$_{N}$: 0.045-0.1). REE allow to distinguish two harzburgites groups at Site 1274. The first one shows patterns similar to those of Site 1272 peridotites yet with slightly higher REE content (Ce$_{N}$: 0.0015-0.015; Yb$_{N}$: 0.14-0.26). The second displays on average lower heavy REE contents (Yb$_{N}$: 0.1-0.18) and more variable light REE contents (Ce$_{N}$: $<$0.002-0.05). These samples are found at the bottom of Hole 1274A where cpx display the highest Na- and Ti contents among the studied peridotite suite. Site 1272 and Site 1274 peridotites composition is similar to that of the most depleted peridotites sampled in ophiolites (e.g., Oman, Cuba) and in oceans (e.g., Izu-Bonin-Marianna). They represent the most refractory peridotites yet sampled at a slow-spreading ridge.
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting