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Molten Earth: Magma in the Deep Mantle

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We discuss recent first principles molecular dynamics simulations of silicate liquids and their geophysical implications1,2. We find that the Grüneisen parameter increases rapidly on compression, yielding liquid state isentropes that are much hotter than previously thought: a potential temperature of only 2450 K is sufficient to melt the entire mantle, and a magma ocean will begin crystallization at mid-mantle depths, rather than at the base of the mantle. Our predicted melting temperature of MgSiO3 perovskite at 136 GPa is 5400±600 K, which if lowered by freezing point depression by 1300 K, yields a mantle solidus temperature of 4100 K, identical to recent estimates of the temperature at the base of the mantle. We argue on this basis that partial melting at the base of the present-day mantle is plausible. While MgSiO3 perovskite is denser than the isochemical liquid, reasonable values of iron partitioning and bulk iron contents, leads to the conclusion that a melt at the base of the multi-component mantle would be denser than its surroundings and therefore buoyantly stable. We predict the bulk sound velocity of this melt to be 10.9 km s-1, less than that of ultra-low velocity zones, and consistent with the explanation of these regions as being due to partial melting. We also find that water and silicate melt are completely miscible over nearly the entire mantle pressure regime, suggesting a mechanism for storing accreted water at depth early in Earth’s history.

References 1. Stixrude, L. and B. Karki, Structure and freezing of MgSiO3 liquid in Earth’s lower mantle, Science, 310, 297-299, 2005. 2. Mookherjee, M., L. Stixrude, and B. Karki, Hydrous silicate melt at high pressure, Nature, 452, 983-986, 2008, doi: 10.1038/nature06918.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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