Structure and properties of forsterite-MgSiO3 liquid interface: molecular dynamics study

Fumiya Noritake,Katsuyuki Kawamura

doi:10.1186/2197-4284-1-14

Abstract

The mechanical properties of partially molten rock, such as their permeabilities and viscosities, are important properties in geological processes. We performed molecular dynamics simulations in terms of structures and diffusivities in forsterite-MgSiO3 liquid interfaces to obtain the nanoscale dynamic properties and structure of the interface. The characteristic structure of the forsterite-MgSiO3 liquid interfaces was observed in the simulations. In the layered structure of the altered surfaces, Si-rich and Mg-rich layers exist alternately in the vicinity of the crystal-liquid interfaces. The layered structure might be formed by the strength difference between Si-O covalent bonds and Mg-O ionic bonds. The difference in the layered structure, indicated by the thickness of the MgSiO3 liquid film, might be caused by the difference in degrees of freedom of the configuration in the liquid film. The two-dimensional diffusivity of oxygen atoms parallel to the interface is controlled by two factors. One factor is the thickness of the liquid film, which decreases oxygen diffusivity with decreasing film thickness. The other is the composition of the sliced layer, where oxygen diffusivity increases with increasing Mg/Si ratio. The effect of the crystal-liquid interface found in this study is negligible in texturally equilibrated rocks. However, the interface can affect the melt flow in deformed samples because a grain boundary melt film with a thickness of several nanometers exists stably in deformed partially molten rock.

Highlights

Knowledge on viscosity and permeability of partially molten rocks is important for understanding volcanism and the thermal history of the earth
The structure and properties of the forsterite-MgSiO3 liquid interface are investigated by application of molecular dynamics simulations
We showed the results of molecular dynamics simulations of forsterite-MgSiO3 liquid, which contribute to our understanding of the nanoscale structure of the interface and diffusivity of atoms in the interface

Summary

Background

Knowledge on viscosity and permeability of partially molten rocks is important for understanding volcanism and the thermal history of the earth. To understand the results obtained by experiments and to estimate physical properties in extreme conditions that are difficult to reproduce in laboratory experiments, it is necessary to know the local structure and the properties of silicate crystal-liquid interfaces. Knowledge on the nanoscale structure and properties of silicate crystalliquid interfaces might be useful in estimating the properties of rocks containing a small degree of melting. Molecular dynamics simulations are widely used to investigate the physical properties and structures of crystals, liquids, gasses, and interfaces. In these simulations, we set the initial positions and velocities of all atoms; the atoms are forced to move according to given force fields under a proper ensemble. It is essential to know the structure and physical properties of forsterite-MgSiO3 liquid interfaces because forsterite is the liquidus mineral of primordial magmas

Methods

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Results and discussion

Findings

Conclusions