MELT AND FLUID INCLUSIONS IN BASALTS AND XENOLITHS FROM TAHAA-ISLAND, SOCIETY ARCHIPELAGO - EVIDENCE FOR A METASOMATIZED UPPER MANTLE

Peridotites and basalts from Tahaa (Society Archipelago, French Polynesia), have unusual REE contents. The peridotites exhibit LREE enrichments, whereas some of the basalts have HREE enriched patterns. The study of melt and fluid inclusions trapped in peridotitic and basaltic minerals indicate two different types of trapped melts. Type I melt inclusions contain daughter minerals (Fe-Ti oxide, salite and plagioclase An48-65) and occur in olivine and pyroxene phenocrysts from basalts. After homogenization (T = 1200-degrees-C), glass composition is similar to the average whole-rock composition of the host basalt (SiO2 < 52 wt%, MgO > 6.5 wt%, TiO2 > 2.5 wt%, K2O/Na2O = 0.6, S almost-equal-to 1200 ppm and Cl < 200 ppm). These inclusions are primary inclusions and correspond to trapped basaltic melt. Type II inclusions occur as secondary inclusions in olivine and pyroxene porphyroclasts from peridotite nodules. They contain microlites of kaersutite (TiO2 > 7.3 wt%), pargasite, rutile, ilmenite, salite, carbonate, apatite and plagioclase (An88). After homogenization (T = 1220-degrees-C), the silicate melt has the following composition: SiO2 > 60 wt%, TiO2 < 1 wt%, K2O > 6 wt%, K2O/Na2O > 1 and high contents of H2O, CO2 and Cl. Co-genetic relationships between CO2 fluid inclusions and type II melt inclusions yield a minimum trapping pressure of 7 kbar at 1220-degrees-C, corresponding to entrapment at oceanic upper mantle depths. This silicate melt is interpreted as being a trapped metasomatic melt resulting from a partial melting event of the peridotitic assemblage induces by deep seated fluids (H2O + CO2 + Cl).