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Melting of bridgmanite under hydrous shallow lower mantle conditions
  • George Amulele,
  • Shun-ichiro Karato,
  • Jennifer Girard
George Amulele
Case Western Reserve University

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Shun-ichiro Karato
Yale University
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Jennifer Girard
Yale university
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High pressure and temperature experiments were carried out on bridgmanite under the 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)O3 and (Mg, Fe, Al, Si)O3. Melting was observed in all runs. The water content of the melt was estimated from the measurements of the void space using the equation of state of H2O that occupies the void space and agree reasonably well with the initial water content. Melt is enriched in FeO/MgO and Al2O3/SiO2 compared to the starting materials. As a consequence, the residual solids contain a substantial amount of stishovite particularly for the high FeO/MgO and Al2O3/SiO2 starting materials. Stishovite-rich materials will cause seismic scattering when they are transported to ~1100 km depth where stishovite shows anomalous elastic behavior. The water content in the residual solids 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 these inclusions were formed during cooling, water content in bridgmanite co-existing with hydrous melt would be 1000-1500 wt ppm that agrees with the previous experimental results on inclusion-free bridgmanite and with a theoretical study. Some implications of these results are discussed including the nature of chemical evolution associated with melting in the shallow lower mantle.
Sep 2021Published in Journal of Geophysical Research: Solid Earth volume 126 issue 9. 10.1029/2021JB022222