Water Concentration in Single-Crystal (Al,Fe)-Bearing Bridgmanite Grown From the Hydrous Melt: Implications for Dehydration Melting at the Topmost Lower Mantle

摘要

High-quality single-crystals of (Al,Fe)-bearing bridgmanite, Mg-0.88 Fe0.0653+Fe0.0352+Al0.14Si0.90O3, of hundreds of micrometer size were synthesized at 24 GPa and 1800 degrees C in a Kawai-type apparatus from the starting hydrous melt containing similar to 6.7 wt% water. Analyses of synthesized bridgmanite using petrographic microscopy, scanning electron microscopy, and transmission electron microscopy show that the crystals are chemically homogeneous and inclusion free in micrometer- to nanometer-spatial resolutions. Nanosecondary ion mass spectrometry (NanoSIMS) analyses on selected platelets show similar to 1,020(+/- 70) ppm wt water (hydrogen). The high water concentration in the structure of bridgmanite was further confirmed using polarized and unpolarized Fourier-transform infrared spectroscopy (FTIR) analyses with two pronounced OH-stretching bands at similar to 3,230 and similar to 3,460 cm(-1). Our results indicate that lower-mantle bridgmanite can accommodate relatively high amount of water. Therefore, dehydration melting at the topmost lower mantle by downward flow of transition zone materials would require water content exceeding similar to 0.1 wt%.