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

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 suitab...