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Ab initio molecular dynamics simulations predict that CaSiO3 perovskite melts at 5600 K at 136 GPa, and 6400 K at 300 GPa, significantly higher than MgSiO3 perovskite. The entropy of melting (1.8 kB per atom) is much larger than that of many silicates at ambient pressure and of simple liquids and varies little with pressure. The volume of melting decreases rapidly with increasing pressure, to 3 % at 136 GPa, producing a melting slope that diminishes rapidly with pressure. We determine the melting temperature via the ZW method, combining the Z method, for which we clarify the theoretical basis, with a waiting time analysis. The ZW method results are internally confirmed by integrating the Clausius‐Clapeyron equation, which also yields our results for the entropy and volume of melting. We find the eutectic composition on the MgSiO3‐CaSiO3 join to be x Ca = 0.26 at 136 GPa and that metasilicate melt is denser than coexisting silicates., Key Points Calcium silicate perovskite melts at 5600 K at the core mantle boundaryMetasilicate liquids are denser than coexisting crystalline silicates at the core‐mantle boundaryThe Z method allows precise ab initio melting temperature determination

In this study, we present a modified double-capsule technique to perform experiments on H2O undersaturated, Fe-bearing systems at elevated pressures and temperatures and oxygen fugacities (f(O2)) relevant for natural calc-alkaline magmas. Welded shut, Fe-preconditioned Au90Pd10 capsules were placed in an outer Pt capsule that contains the same starting material. Experiments were performed at 1.0 GPa and 1200 degrees C using a synthetic, hydrous basalt and run with either boron nitride (BN) or MgO surrounding the welded capsules. Optimum results were obtained by using Fe-preconditioned Au90Pd10 inner capsules in combination with MgO assemblies. The application of the modified doublecapsule technique with Fe-preconditioned inner AuPd capsules reduced Fe loss to less than 3% relative, conserved H2O within the error of ion-microprobe analyses, and kept the f(O2) (QFM+1.1) within 1 log unit of the initial value constrained by the Fe2O3/FeO ratio of the starting material (QFM+0.43). These conditions are similar to estimates of f(O2) during the crystallization of natural calc-alkaline magmas.

In silicate melts, the speciation state of altervalent elements depends on the extension and distribution of polymeric units in the system. Since polymerization of silicate melts is driven by acid–base properties of constituting oxides depending on their Lux–Flood reactivity, the bulk optical basicity of molten silicates and glasses allows us to compute the distribution of oxygen among three distinct polarization states (O2−, O−, O0) in conjugation with the polymeric model of Toop and Samis. Data available in literature are employed to investigate the mutual interaction between sulfur and iron in silicate melts to constrain the equilibrium between sulfide and sulfate species. Results put in evidence (i) the role of bulk melt chemistry, affecting through polymerization, and hence the transfer of electrons among the three polarization states of oxygens, the oxidation state of altervalent elements such as Fe and S and (ii) the limits of the Temkin approach implicit in chemical equations that consider sulfide and sulfate in melts as free anions. Improvements of the model’s accuracy for sulfur speciation may be attained by accounting for the various dissociation equilibria of molten sulfide M2/νS, oxide M2/ν and sulfate M2/νSO4 species in their standard state of pure component in the melt phase at temperature and pressure of interest instead of that of complete dissociation required by the Temkin model.

High-temperature Raman spectroscopy and optical microscopic observations have revealed a new metastable polymorph of CaAl2Si2O8 composition, which brings to four the number of known crystalline phases in this system. Similar to the metastable monoclinic pseudo-orthorhombic and pseudo-hexagonal phases, the new polymorph nucleates prior to anorthite, at around 1545 K, and its pseudo-liquidus temperature is 1700 +/- 10 K. It can also be formed from the transformation of the pseudo-hexagonal phase at 1050 K. The actual structure of this new crystalline form is unknown, but its Raman spectrum indicates that it is most likely a 6-membered alumino-silicate framework. We have obtained all three metastable phases as pure single crystals using wire loop heating techniques, and have studied their structures via Raman spectroscopy up to their metastable melting points or transformation temperatures.

High-temperature calorimetry experiments show that the isobaric heat capacity (C(p)) of minerals increases in an anomalously rapid way when the melting point is approached. These premelting effects can begin a few hundred degrees below the melting point, and they should also be observed for other physical properties such as the thermal expansion, compressibility and sound velocities. Premelting could thus affect the properties of partially molten rocks. In this study, new evidence for diopside (CaMgSi2O6), pseudowollastonite (CaSiO3), anorthite (CaAl2Si2O8), akermanite (Ca2MgSi2O7), Na2SiO3, L2SiO3 and CaMgGeO4 olivine obtained by means of calorimetry, electron microscopy and vibrational spectroscopy, demonstrates that premelting represents unquenchable configurational changes within phases remaining crystalline up to the reported melting points. The enthalpy and entropy effects of premelting are consistent with cation disordering over the various crystallographic sites of the structures. More generally, such temperature-induced cation disordering could be at the roots of intrinsic anharmonicity in oxide and silicate compounds.