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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