THE ROLE OF WATER IN THE MELTING OF BRIDGMANITE UNDER SHALLOW LOWER MANTLE CONDITIONS

Abstract:

Evidence of melting under the shallow lower mantle (~750 km depth) is suggested from seismological studies. Such melting may contribute to global water cycling (e.g., (Karato et al., 2020)). It was known that melting occurs under the shallow lower mantle conditions when a substantial amount of water is available. However, compositions of the melt and residual solid have not been well constrained. To investigate this, we conducted high pressure and temperature experiments on bridgmanite under hydrous shallow lower mantle conditions (24 – 25 GPa and 1673 – 1873 K with 5 – 10 wt. % of water in the starting material). Bridgmanite investigated include MgSiO₃, (Mg, Fe)SiO₃, (Mg, Al, Si)O_3,and (Mg, Fe, Al, Si)O₃. Melting was observed in all runs. Upon melting most water goes to the melt, and we determined the water content in both residual solids and “quenched” melt. Melt is enriched in FeO, MgO, and Al₂O₃relative to SiO₂ compared to the starting materials. For all plausible compositions, melts are substantially lighter than co-existing minerals under shallow lower mantle conditions. The residual solids are silica-rich and sometimes containing stishovite as well as bridgmanite. The presence of stishovite may cause seismic wave scattering observed in some locations of the lower mantle. The water content in the residual solids that co-exist with the melt was measured by the FTIR and is 50 – 70 ppm wt. in bridgmanite and 26 – 670 ppm wt. in stishovite, depending on the starting composition. However, bridgmanites in these samples contain inclusions of superhydrous phase B. If the water content of superhydrous phase B is added to that in bridgmanite, the net water content is ~1,000-1,500 ppm wt. that agrees with a previous experimental result on inclusion-free bridgmanite (Fu et al., 2019) and with a theoretical study. This suggests that superhydrous phase B inclusions may have been formed during quenching.

Fu, S., Yang, J., Karato, S., Vasiliev, A., Presniakov, M.Y., Gavriliuk, A.G., Ivanova, A.G., Hauri, E.H., Okuchi, T., Purejav, N., Lin, J.-F., 2019. Water concentration in single-crystal (Al, Fe)-bearing bridgmanite grown from the hydrous melt: implications for partial melting and the water storage capacity in the Earth's lower mantle. Geophysical Research Letters 46.

Karato, S., Karki, B.B., Park, J., 2020. Deep mantle melting, the global water circulation and the stability of the ocean mass. Progress in Earth and Planetary Science submitted.