Melting of Bridgmanite Under Hydrous Shallow Lower Mantle Conditions

High pressure and temperature experiments were carried out on the oxide mixtures corresponding to the bridgmanite stoichiometry under the hydrous shallow lower mantle conditions (24-25 GPa and 1673-1873 K with 5-10 wt. % of water in the starting material). Oxide mixtures investigated correspond to MgSiO3, (Mg, Fe)SiO3, (Mg, Al, Si)O-3, and (Mg, Fe, Al, Si)O-3. Melting was observed in all runs. Partitioning of various elements, including Mg, Fe, Si, and H is investigated. Melting under hydrous lower mantle conditions leads to increased (Mg + Fe)O/SiO2 in the melt compared to the residual solids. The residual solids often contain a large amount of stishovite, and the melt contains higher (Mg,Fe)O/SiO2 ratio than the initial material. (Mg + Fe)O-rich hydrous melt could explain the low-velocity anomalies observed in the shallow lower mantle and a large amount of stishovite in the residual solid may be responsible for the scattering of seismic waves in the mid-lower mantle and may explain the stishovite paradox. Since stishovite-rich materials are formed only when silica-rich source rock (MORB) is melted (not a typical peridotitic rock [bulk silicate Earth]), seismic scattering in the lower mantle provides a clue on the circulation of subducted MORB materials. To estimate hydrogen content, we use a new method of estimating the water content of unquenchable melts, and also propose a new interpretation of the significance of superhydrous phase B inclusions in bridgmanite. The results provide revised values of water partitioning between solid minerals and hydrous melts that are substantially higher than previous estimates. Plain Language Summary Melting is a major process by which Earth evolves chemically. However, the consequence of melting is not well understood under the deep mantle conditions. This study focuses on melting under the shallow lower mantle conditions that occurs with the help of water. Nature of water distribution between the melt and remaining solids was studied using new approaches. We discovered that when melting occurs for a material with composition similar to oceanic crust, the residual solid is nearly completely stishovite (SiO2). Stishovite shows anomalous elastic properties below similar to 800 km depth depending on the composition. Observed strong seismic scattering in this depth range might be due to the presence of stishovite-rich materials formed by melting in the lower mantle.

Automated summary

High pressure and temperature experiments were conducted on oxide mixtures corresponding to bridgmanite stoichiometry under hydrous shallow lower mantle conditions. Melting occurred in all runs, leading to changes in element partitioning between the melt and residual solids. The study suggests that melting under hydrous lower mantle conditions may explain seismic anomalies and the presence of stishovite in the mantle.