Structure, thermodynamic and transport properties of liquid MgSiO₃: Comparison of molecular models and laboratory results

• Published in: Geochimica et cosmochimica acta Vol. 75; no. 5; pp. 1272 - 1296
• Main Authors: Spera, Frank J, Ghiorso, Mark S, Nevins, Dean
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.03.2011
• Subjects: activation energy; glass; magnesium; melting; melting point; molecular dynamics; molecular models; oxygen; silicon; spectroscopy; statistics; viscosity; X-radiation
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.12.004

Liquid MgSiO₃ is a model for the Earth’s magma ocean and of remnant melt present near the core–mantle boundary. Here, models for molten MgSiO₃ are computed employing empirical potential molecular dynamics (EPMD) and results are compared to published results including two EPMD studies and three first-principles molecular dynamics (FPMD) models and to laboratory data. The EPMD results derived from the Oganov (OG) potential come closest to the density of MgSiO₃ liquid at the 1-bar melting point inferred from the melting curve. At higher P, EPMD densities calculated from the OG potential and FPMD broadly match shock wave studies, with the OG potential yielding the better comparison. Matsui (M) potential results deviate from other studies above ∼50GPa. Overall, results based on the OG potential compare best to experimental densities over the P–T range of the mantle. Isothermally, upon increasing P the mean coordination numbers (CN¯) of oxygen around Si and Mg monotonically increase with pressure. Tetrahedral Si and octahedral Si monotonically increase and decrease, respectively, whereas pentahedral Si maximizes at 10–20GPa. Tetrahedral Mg decreases monotonically as P increases whereas pentahedral, octahedral and higher coordination polyhedra each show similar behavior first increasing and then decreasing after attaining a maximum; the P of the maximum for each polyhedra type migrates to higher P as the CN increases. Free oxygen and oxygen with one nearest neighbor of either Si or Mg decreases whereas Si or Mg with two or three nearest oxygens (i.e., tricluster oxygen) increases with increasing P isothermally. The increase of tricluster oxygen is consistent with spectroscopy on MgSiO₃ glass quenched from 2000K and 0–40GPa and high-energy X-ray studies constraining the coordination of O around Mg and around Si at 2300K and 1bar. Coordination statistics from FPMD studies for O around Si and Si around O are in agreement with the EPMD results based on the M and OG potentials. Mg self-diffusivity is greater than O and Si self-diffusivities for both the M and OG potentials. All D values monotonically decrease with increasing pressure isothermally and all atoms are more diffusive in the M liquid compared to the OG liquid except at T>∼5000K and P>100GPa. Previously published EPMD diffusivities fall between values given by the M and OG potentials, at least up to 45GPa. The M liquid is generally less viscous than the OG liquid except at P>∼80GPa. Activation energy and volume are around 96kJ/mol and 1.5cm³/mol, respectively. The FPMD viscosity results at 120GPa and 4000 and 4500K are essentially identical to the values from the M and OG potentials. FPMD viscosity results are similar to the OG results for P<60GPa; at higher P, the FPMD viscosities are higher. At 4000K and 100GPa the shear viscosity of liquid MgSiO₃ is ∼0.1Pas. More extensive laboratory results are required to better define the thermodynamic, transport and structural properties of MgSiO₃ liquids and for comparison with computational studies.