Petrology and Geochemistry of Al-Augite and Cr-Diopside Group Mantle Xenoliths from Tahiti, Society Islands

Abstract

Al-augite group xenoliths and subordinate Cr-diopside group mantle xenoliths occur as inclusions in Tahitian basalts. Al-augite group xenoliths are orthopyroxene free and consist of dunite, wehrlite, and clinopyroxenite. Dunites have metamorphic textures, but wehrlites and clinopyroxenites have igneous textures. Mineral compositions vary systematically with modal olivine content, with Fe and Ti contents in spinel and clinopyroxene increasing with decreasing modal olivine. Cr-diopside group xenoliths have porphyroclastic to equigranular-mosaic textures and consist of nearly equal proportions of lherzolite and harzburgite. The mineral compositions and textures of the different Cr-diopside group lithologies are similar, but are clearly distinguished from Al-augite group xenoliths by their higher Mg#s and lower Ti and Fe contents. Equilibrium temperatures, determined by coexisting orthopyroxene and clinopyroxene in Cr-diopside group xenoliths, range from 880° to 1100°C. These temperatures, at pressures suitable for spinel peridotite, are at least 150°C higher than would be expected for conductively cooled, old oceanic crust. Such high temperatures indicate that the Tahitian lithosphere has been reheated. The rare-earth-element (REE) patterns of clinopyroxene separates and bulk rocks of Al-augite group xenoliths are light rare-earth-element (LREE) enriched, and the calculated REE patterns of liquids in equilibrium with A-augite group xenoliths are similar to Tahitian basaltic lavas. Sr and Nd isotopic compositions of Al-augite xenoliths overlap the range measured for Tahitian basalts. These geochemical data are consistent with this group of xenoliths crystallizing from magmas similar in composition to Tahitian lavas. Fractionation of dunite, wehrlite, and clinopyroxenite from a Tahitian picritic basalt can generate most of the chemical variations observed among Tahitian basaltic lavas. Based upon the phase petrology and mineral compositions of the Cr-diopside group xenoliths, this group of xenoliths is interpreted to represent residues from partial melting in the upper mantle, perhaps from mid-ocean-ridge basalt extraction. The LREE enrichment in clinopyroxene separates is the result of later metasomatism, and the depletion signature of isotopic compositions may have been changed by this metasomatic event.