Petrology, mineralogy, and geochemistry of upper mantle/lower crustal xenoliths and their host basalts from Tahiti, Society Islands, French Polynesia

Xenoliths in alkali basalt from Tahiti consist of an intriguing suite of rocks. The study of these samples has not only unraveled a complicated petrogenetic history of the xenoliths but also provided some crucial information for understanding the origin and evolution of Tahitian volcanic rocks and the structure of the lithosphere of this region. The xenoliths are divided into two types, Type I and Type II, based on texture and mineralogy. The Type I xenoliths are peridotite, composed of olivine (Fo_86-91), orthopyroxene (Wo_1-2En_84-88), clinopyroxene (Wo_43-47En_47-50), and Cr-spinels. There is a systematic variation of whole-rock compositions. The xenoliths appear to be residues from partial melting of a Iherzolite source in the mantle. Partial melting occurred at The East Pacific Rise (EPR), and 19-38% tholeiite was extracted. The residues were metamorphosed at temperatures ~ 883-1106°C and pressures between 9 and 21 Kbars and were cryptically metasomatized before picked up by Tahitian magma. They were drifted to the present site by sea-floor spreading. The Type II xenoliths consist of peridotite, pyroxenite, and kaersutitite, composed of olivine (Fo₈₈ to Fo₇₇), clinopyroxene (Wo_46-52En_32-46), spinel minerals (Cr-spinel to titanomagnetite), and kaersutite. They typically have cumulate textures and appear to be segregated from Tahitian magmas earlier than the host. Segregation may have occurred both in magma chambers and conduits at pressures ranging from 1 to 25 kbars. Dunite formed first, followed by wehrlite, clinopyroxenite, and kaersutitite. Segregation of dunite may continued throughout the sequence. Segregation of the Type II xenoliths controlled the evolution of Tahitian lavas. The majority of the Tahitian basalts can be generated by fractional crystallization of about 52% olivine, clinopyroxene, and spinels from picrobasalt, an assumed primary magma. The more evolved trachyte and phonolite may require up to 90% fractionation, and other cumulus phases are required. The lithosphere beneath the Island of Tahiti consists of unevenly distributed partial melting residues and cumulates. Cumulate rocks prevail at certain levels in the lithosphere.