Direct shock compression experiments on premolten forsterite and progress toward a consistent high‐pressure equation of state for CaO‐MgO‐Al₂O₃‐SiO₂‐FeO liquids

Abstract

[1] We performed shock compression experiments on preheated forsterite liquid (Mg 2 SiO 4 ) at an initial temperature of 2273 K and have revised the equation of state (EOS) that was previously determined by shock melting of initially solid Mg 2 SiO 4 (300 K). The linear Hugoniot, U S = 2.674 ± 0.188 + 1.64 ± 0.06 u p km/s, constrains the bulk sound speed within a temperature and composition space as yet unexplored by 1 bar ultrasonic experiments. We have also revised the EOS for enstatite liquid (MgSiO 3 ) to exclude experiments that may have been only partially melted upon shock compression and also the EOS for anorthite (CaAl 2 SiO 6 ) liquid, which now excludes potentially unrelaxed experiments at low pressure. The revised fits and the previously determined EOS of fayalite and diopside (CaMg 2 SiO 6 ) were used to produce isentropes in the multicomponent CaO-MgO-Al 2 O 3 -SiO 2 -FeO system at elevated temperatures and pressures. Our results are similar to those previously presented for peridotite and simplified "chondrite" liquids such that regardless of where crystallization first occurs, the liquidus solid sinks upon formation. This process is not conducive to the formation of a basal magma ocean. We also examined the chemical and physical plausibility of the partial melt hypothesis to explain the occurrence and characteristics of ultra-low velocity zones (ULVZ). We determined that the ambient mantle cannot produce an equilibrium partial melt and residue that is sufficiently dense to be an ultra-low velocity zone mush. The partial melt would need to be segregated from its equilibrium residue and combined with a denser solid component to achieve a sufficiently large aggregate density.

Solid-liquid density and spin crossovers in (Mg, Fe)O system at deep mantle conditions

“…They all increase considerably with compression thus requiring a strong volume-dependent coefficient B TH . This behavior is in sharp contrast to the case of the solid phase where the thermal pressure is weakly dependent on volume131432. The thermal contributions in the liquid are systematically larger than those in the solid at most compressed volumes.…”

Silicate Melting and Vaporization During Rocky Planet Formation

“…The heat capacity at constant volume is taken from Thomas and Asimow (2013), giving ΔS Liquid-Heating = 559(±80) J/K/kg. Table 5 summarizes the specific entropy at each step and the total at the base of the isentrope.…”

Sound velocity and density of magnesiowüstites: Implications for ultralow‐velocity zone topography

“…For example, Thomas and Asimow [2013] measured the equation of state of silicate liquids to describe the evolution of a crystallizing magma ocean and found that partial melt residues of ambient mantle composition would not be gravitationally stable unless combined with another denser solid. If such ULVZs exist, then inferences could be drawn about isolated chemical reservoirs in the mantle.…”