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Abstract: We discuss the geophysical implications of recent first principles molecular dynamics simulations of MgSiO3 liquid, in particular the findings that the Grüneisen parameter increases by a factor of three on two-fold compression, and the predicted melting curve. The 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 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. The increase on compression of the Grüneisen parameter in the liquid state yields 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. [Copyright &y& Elsevier]

Abstract: Melting experiments on a Fe-rich carbonate-saturated pelite were performed at 850–1300 °C and 2.5–5.0 GPa to define melting relations, melt compositions, and the conditions under which carbonates remain residual. In the selected fertile bulk composition, 30 wt.% potassic granite (2.5 GPa) or phonolite (5.0 GPa) melt is generated at the fluid-absent solidus. The temperature of the latter increases from 900 °C at 2.4 GPa to 1070 °C at 5.0 GPa. Phengite+quartz/coesite control initial silicate melting and melt productivity through the reaction phengite+quartz/coesite+clinopyroxene+calcite=silicate melt+kyanite+garnet, which leaves most of the Fe–Mg–calcite in the residue. Na remains compatible in clinopyroxene (D Na cpx/melt =3.1 to 7.3 at the fluid-absent solidus), resulting in silicate melts with K2O/Na2O wt-ratios of 5.8–8.6. Such highly potassic carbonated silicate melts represent ideal metasomatic agents for the source mantle of group II kimberlites. From 3.7 to 5.0 GPa, Fe–Mg–calcite disappears only through the formation of Ca–carbonatite at 1100 °C. The experiments provide a possible source for Ca–carbonatites in combination with alkaline granitic to phonolitic melts at temperatures unlikely to be achieved during ongoing subduction. Large scale carbonate transfer to the subarc mantle can thus only be achieved when burying rates slow considerably down or subducted crust becomes incorporated into the mantle. Consequently, it is likely that carbonates will not be extensively mobilized in a typical subarc region, thus extending and confirming earlier results from subsolidus studies (Connolly, J.A.D., 2005. Computation of phase equilibria by linear programming: a tool for geodynamic modelling and its application to subduction zone decarbonation. Earth Planet. Sci. Lett. 236, 524–541.), that >70–80% of the subducted carbonate will bypass the volcanic arc region and get buried to larger depths. [Copyright &y& Elsevier]