http://www.homepages.ucl.ac.uk/~ucfbls0/stixrudeetal_09.pdf

Thermodynamics of silicate liquids in the deep Earth

We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO_3 glass) and high-density, polycrystalline aggregates (MgSiO_3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Gruneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K' significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg_2SiO_4 liquid, calculated in a similar manner. The Gruneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean.

/pdf/the-mgsio3-system-at-high-pressure-thermodynamic-properties-3ibmrm2xzh.pdf

The MgSiO3 system at high pressure: Thermodynamic properties of perovskite, postperovskite, and melt from global inversion of shock and static compression data

SUMMARY We develop a self-consistent thermodynamic description of silicate liquids applicable across the entire mantle pressure and temperature regime. The description combines the finite strain free energy expansion with an account of the temperature dependence of liquid properties into a single fundamental relation, while honouring the expected limiting behaviour at large volume and high temperature. We find that the fundamental relation describes well previous experimental and theoretical results for liquid MgO, MgSiO3 ,M g2SiO4 and SiO2. We apply the description to calculate melting curves and Hugoniots of solid and liquid MgO and MgSiO3. For periclase, we find a melting temperature at the core–mantle boundary (CMB) of 7810 ± 160 K, with the solid Hugoniot crossing the melting curve at 375 GPa, 9580 K, and the liquid Hugoniot crossing at 470 GPa, 9870 K. For complete shock melting of periclase we predict a density increase of 0.14 g cm −3 and a sound speed decrease of 2.2 km s −1 . For perovskite, we find a melting temperature at the CMB of 5100 ± 100 K with the perovskite section of the enstatite Hugoniot crossing the melting curve at 150 GPa, 5190 K, and the liquid Hugoniot crossing at 220 GPa, 5520 K. For complete shock melting of perovskite along the enstatite principal Hugoniot, we predict a density increase of 0.10 g cm −3 , with a sound speed decrease of 2.6 km s −1 .

/pdf/self-consistent-thermodynamic-description-of-silicate-1gbrov0pgr.pdf

Self-consistent thermodynamic description of silicate liquids, with application to shock melting of MgO periclase and MgSiO3 perovskite

Self-consistent thermodynamic description of silicate liquids, with application to shock melting of MgO periclase and MgSiO3 perovskite (vol 178, pg 162, 2009)

Origin of HIMU and EM-1 domains sampled by ocean island basalts, kimberlites and carbonatites: The role of CO2-fluxed lower mantle melting in thermochemical upwellings

Constant-pressure constant-temperature {\it ab initio} molecular dynamics simulations at high temperatures have been used to study MgSiO$_3$ liquid, the major constituent of the Earth's lower mantle to conditions of the Earth's core-mantle boundary (CMB). We have performed variable-cell {\it ab initio} molecular dynamic simulations at relevant thermodynamic conditions across one of the measured melting curves. The calculated equilibrium volumes and densities are compared with the simulations using an orthorhombic perovskite configuration under the same conditions. For molten MgSiO$_3$, we have determined the diffusion coefficients and shear viscosities at different thermodynamic conditions. Our results provide new constraints on the properties of molten MgSiO$_3$ at conditions near the core-mantle boundary. The volume change on fusion is positive throughout the pressure-temperature conditions examined and ranges from 5% at 88 GPa and 3500 K to 2.9% at 120 GPa and 5000 K. Nevertheless, neutral or negatively buoyant melts from (Mg,Fe)SiO$_3$ perovskite compositions at deep lower mantle conditions are consistent with existing experimental constraints on solid-liquid partition coefficients for Fe. Our simulations indicate that MgSiO$_3$ is liquid at 120 GPa and 4500 K, consistent with the lower range of experimental melting curves for this material. Linear extrapolation of our results indicates that the densities of liquid and solid perovskite MgSiO$_3$ will become equal near 180 GPa.

First principles study of density, viscosity, and diffusion coefficients of liquid MgSiO3 at conditions of the Earth's deep mantle

[1] Constant-pressure constant-temperature ab initio molecular dynamics simulations at high temperatures have been used to study MgSiO3, the major constituent of the Earth's lower mantle to conditions of the Earth's core-mantle boundary. The calculated equilibrium volumes and densities are compared with simulations using an orthorhombic perovskite configuration under the same conditions. For molten MgSiO3, we have determined the diffusion coefficients and shear viscosities at different thermodynamic conditions. Our results provide new constraints on the properties of molten MgSiO3 at conditions near the core-mantle boundary. The volume of the liquid is greater than that of the solid throughout the pressure-temperature conditions examined, and the volume change on fusion ranges from 5% at 88 GPa and 3500 K to 2.9% at 120 GPa and 5000 K. Existing experimental constraints on solid-liquid partition coefficients for Fe suggest that Fe is preferentially partitioned into the liquid. Such enrichment of Fe increases the density of the liquid, thus allowing the possibility of negatively buoyant melts from (Mg,Fe)SiO3 perovskite compositions at deep lower mantle conditions for plausible values of solid-liquid partition coefficients for Fe. At 120 GPa and 4500–5000 K, the diffusion coefficient of liquid MgSiO3 is 2–3 × 10−5 cm 2/s and the diffusion rates of the different chemical species are similar. The shear viscosity is estimated using Zwanzig's formula to be 19–31 cP under these conditions. On the basis of our calculated diffusivities, MgSiO3 is above the glass transition temperature at 120 GPa and 4500 K.

/pdf/first-principles-study-of-density-viscosity-and-diffusion-4qibpsi93x.pdf

Jones Tsz-Kai Wan

Papers

First principles study of density, viscosity, and diffusion coefficients of liquid MgSiO3 at conditions of the Earth's deep mantle

First‐principles study of density, viscosity, and diffusion coefficients of liquid MgSiO3 at conditions of the Earth's deep mantle