Résumé
Geochemical studies have shown that, in addition to thermal erosion, plume/lithosphere interaction may induce significant changes in the lithosphere chemical composition. In order to constrain the extent of these processes in an oceanic environment and their consequences on the lithosphere seismic properties, we carried out a study of 12 mantle xenoliths (lherzolites, harzburgites and dunites) from the Austral-Cook, Society and Marquesas archipelagos in French Polynesia. In this study, we investigate the relationship between petrological processes and microstructure by coupling electron microprobe analyses and crystallographic orientation data obtained by indexation of Electron BackScattered Diffraction (EBSD) patterns. Olivines of the studied samples are characterized by a wide range of compositions (Fo# = 84-90), the lowest forsterite contents being found in Marquesas dunites. Yet, the high Ni content (1500-3500 ppm) of these olivines precludes a cumulate origin. These rocks are interpreted as resulting from melt/rock reactions involving olivine precipitation and pyroxene dissolution, the formation of dunites being indicative of high melt-rock ratios. Most lherzolites are also characterized by abnormally high clinopyroxene/orthopyroxene ratios (>>1), which suggest precipitation of clinopyroxene at the expenses of orthopyroxene. A similar reaction was recently described in the Ronda massif and interpreted as a near-solidus melt-freezing reaction occurring at the boundary of a partial melting domain developed at the expenses of lithospheric mantle (Lenoir et al. 2000). These results suggest that the oceanic lithosphere above mantle plume undergoes a complex sequence of magmatic processes that induce significant modifications of the upper mantle chemical and modal composition. Yet, the EBSD study shows that the crystallographic orientations and thus the seismic anisotropy of these peridotites are little affected by these processes. Nevertheless, the decrease in olivine Fo# may result in up to 2% of decrease in seismic velocity. The chemical variations induced by melt/rock interaction may thus explain the low velocity anomalies observed beneath large igneous provinces, like the Decan, Parana, and Ontong Java (Kennett & Widiantoro 1999, Van Decar et al. 1995, Richardson et al. 2000).