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Melting of Bridgmanite Under Hydrous Shallow Lower Mantle Conditions
Author(s) -
Amulele George,
Karato Shunichiro,
Girard Jennifer
Publication year - 2021
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2021jb022222
Subject(s) - stishovite , silicate perovskite , mantle (geology) , geology , silicate , transition zone , mineralogy , post perovskite , olivine , geochemistry , analytical chemistry (journal) , chemistry , quartz , paleontology , organic chemistry , chromatography
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 MgSiO 3 , (Mg, Fe)SiO 3 , (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/SiO 2 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/SiO 2 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.