西南印度洋留尼旺洋岛玄武岩的地幔源区岩性制约OA

Upwelling mantle plumes can transport deep mantle materials into the upper mantle.Consequently,identifying the lithological compositions of mantle plumes is essential for constraining deep mantle reservoirs and related geodynamic processes.Seismic tomography reveals that the root of the Réunion mantle plume is connected to the African large low-shear-velocity province in the lower mantle,indicating a lower-mantle origin.However,the lithological composition of this plume remains poorly constrained.Ocean island basalts(OIBs)from Réunion Island,generated by partial melting of the Réunion plume over the past two million years,provide a unique opportunity to investigate its lithological makeup.This study presents whole-rock major element compositions of Réunion OIBs,along with the chemical compositions of olivine phenocrysts within these basalts.We first estimated the primary magma compositions of the studied samples,which were found to be rich in MgO and TiO2 but deficient in SiO2,CaO,and Al2O3.Comparison with experimental melts of different mantle rocks shows that these primary magmas predominantly fall within the range of melts derived from silica-deficient pyroxenite.To further evaluate this proposal,we analyzed the chemical compositions of olivine phenocrysts in the Réunion OIB samples.The results indicate that the Fo values range from 80.1 to 90.1.Notably,high-Fo olivine phenocrysts exhibit elevated Ni,depleted Ca and Mn contents,and high Fe/Mn ratios.These compositions resemble those found in Loihi(Hawaii)OIBs,interpreted to have a source containing silica-deficient pyroxenite,and are distinctly different from olivine phenocrysts in equilibrium with peridotite-derived melts.These data suggest a significant contribution of pyroxenites to the mantle source of Réunion OIBs.Both whole-rock and mineral chemistry indicate that the mantle source of the Réunion OIBs is dominated by silica-deficient pyroxenite.This demonstrates that the Réunion mantle plume contains significant pyroxenite components rather than being composed exclusively of peridotite.