Plume-Related Olivine-rich Mantle Xenoliths as Products of Large-Scale Melt-Rock Reaction: Examples from the Kerguelen Islands and the French Massif Central

M. Gregoire1, O. Alard1,2, S Y O'Reilly1 and J-L Bodinier2 1. GEMOC, Macquarie, 2. UMR 5569,

"G B E", ISTEEM, Université de Montpellier II

Kerguelen Islands, southern Indian Ocean, and French Massif Central, provide well-documented records of extensive plume activity (³ 25 Ma) in oceanic and continental settings, respectively. In both localities, numerous type I mantle peridotite xenoliths occur in the most alkaline lavas. Among them is found a peculiar rock type, characterized by both high equilibrium temperatures (> 1050ºC) and olivine-rich modes (> 80%). These rocks show coarse-grained, poikiloblastic or highly annealed textures. In spite of their refractory (harzburgitic to dunitic) composition, they\ generally contain significant proportions of interstitial clinopyroxene (up to 7%). The elevated cpx/opx ratio of these refractory peridotites is not consistent with an interpretation as solid residues after high-degree of partial melting. This interpretation is also precluded by the low Fo content of olivine and the composition of clinopyroxene, which is a Mg-augite instead of the typical mantle Cr-diopside. On the other hand, these characteristics may be explained by a melt-rock reaction involving dissolution of orthopyroxene + Cr-diopside, and precipitation of secondary olivine and by Mg-augite.

Whole rocks analyzed by ICP-MS show steadily LREE-enriched, chondrite-normalized REE patterns, sometimes with very low HREE abundance. Cpx display flat or convex upward REE patterns, suggestive of extensive equilibration with large volumes of silicate melt.

These conditions can be achieved either in wall-rocks to magma conduits, or during pervasive infiltration of lower lithospheric peridotites by magmas from a mantle plume. The homogeneous high equilibrium temperatures and the absence of volatile-bearing phases are more consistent with the second hypothesis. In the proposed model, the ol-rich peridotites would result from melt accumulation at the base of lithosphere eroded by a mantle plume.

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