Pressure-Induced Coordination Changes in a Pyrolitic Silicate Melt From Ab Initio Molecular Dynamics Simulations.

N V Solomatova,R Caracas

doi:10.1029/2019jb018238

Abstract

With ab initio molecular dynamics simulations on a Na‐, Ca‐, Fe‐, Mg‐, and Al‐bearing silicate melt of pyrolite composition, we examine the detailed changes in elemental coordination as a function of pressure and temperature. We consider the average coordination as well as the proportion and distribution of coordination environments at pressures and temperatures encompassing the conditions at which molten silicates may exist in present‐day Earth and those of the Early Earth's magma ocean. At ambient pressure and 2,000 K, we find that the average coordination of cations with respect to oxygen is 4.0 for Si‐O, 4.0 for Al‐O, 3.7 for Fe‐O, 4.6 for Mg‐O, 5.9 for Na‐O, and 6.2 for Ca‐O. Although the coordination for iron with respect to oxygen may be underestimated, the coordination number for all other cations are consistent with experiments. By 15 GPa (2,000 K), the average coordination for Si‐O remains at 4.0 but increases to 4.1 for Al‐O, 4.2 for Fe‐O, 4.9 for Mg‐O, 8.0 for Na‐O, and 6.8 for Ca‐O. The coordination environment for Na‐O remains approximately constant up to core‐mantle boundary conditions (135 GPa and 4000 K) but increases to about 6 for Si‐O, 6.5 for Al‐O, 6.5 for Fe‐O, 8 for Mg‐O, and 9.5 for Ca‐O. We discuss our results in the context of the metal‐silicate partitioning behavior of siderophile elements and the viscosity changes of silicate melts at upper mantle conditions. Our results have implications for melt properties, such as viscosity, transport coefficients, thermal conductivities, and electrical conductivities, and will help interpret experimental results on silicate glasses.

Highlights

The Early Earth's mantle was once composed of molten silicate rock—a global magma ocean extending down to the core‐mantle boundary (Stevenson, 1989; Tonks & Melosh, 1993)
With ab initio molecular dynamics simulations on a Na, Ca, Fe, Mg, and Al‐bearing silicate melt of pyrolite composition, we examine the detailed changes in elemental coordination as a function of pressure and temperature
The average cation‐oxygen bond lengths for Si‐O, Al‐O, and Mg‐O gently increase with increasing pressure while the average bond lengths for Fe‐O and Ca‐O increase with increasing pressure more steeply (Figure 2)

Summary

Introduction

The Early Earth's mantle was once composed of molten silicate rock—a global magma ocean extending down to the core‐mantle boundary (Stevenson, 1989; Tonks & Melosh, 1993). Silicate melts exist as lava flows on Earth's surface, as magmas in subterrestial volcanic processes, and are thought to exist at various depths of the mantle, at the base of the upper mantle, at the base of the transition zone, and at the core‐mantle boundary. In the deepest part of the lower mantle, at the boundary between Earth's liquid iron‐nickel core and solid D′′ layer, it is thought that pockets of silicate melts may exist, explaining some of the seismically observed ultra‐low velocity zones (Vidale & Hedlin, 1998; Williams & Garnero, 1996), though it is not yet clear if they would be gravitationally stable (Bower et al, 2011)

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Conclusion