HR: 0830h
AN: V11D-0528    [PDF]
TI: Molecular Dynamics Simulations of Diffusion in a Silica Melt
AU: * Gemmell, A
EM: Alastair@earth.oxford.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX13PR United Kingdom
AU: Fraser, D
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX13PR United Kingdom
AU: Refson, K
AF: Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX110QX United Kingdom
AB: Computer modelling of silicate melts enables the study of pressure-temperature conditions not easily obtainable by traditional experimentation (e.g. 1). Diffusion in melts under various conditions is critical to our understanding of a variety of processes such as melt crystallisation, magma mixing and the behaviour of trace elements during magma ascent that underpins the field of igneous petrogenesis. It can also provide information on melt structure via diffusion mechanisms and their activation energies. In the present paper, the magnitude and mechanism of diffusion of silicon and oxygen in molten silica has been investigated by molecular dynamics using a modified BKS potential (2). A range of melt temperatures and pressures were studied with a view to understanding the relationship between temperature, pressure, diffusion and melt structure. At each P-T point studied, the system was equilibrated for between 1 million and 40 million 1fs steps depending on the conditions, with data collection over the same time range. The potential was adjusted to overcome problems with instability in the particle velocities at high temperature. The simulations were run at the Oxford University Supercomputing centre, UK. Systems of 144, 288, 576 and 1152 particles were investigated. In addition, two different sets of periodic boundary conditions were used - cubic and truncated octahedral. The latter was found to provide a better ratio of simulated time to compute time. The present data extend the range of data available and indicate a pronounced non-linearity in the temperature dependence of diffusion, as shown by plots of log10D with 1/T. The second derivative is greatest around 3500K. At least two different diffusion mechanisms may operate at lower and higher temperatures with differing activation energies. Comparison with previous molecular dynamics data shows broad agreement with most studies. Although the data presented do include some low temperature runs, comparison with experimental data is still difficult because of the differences in temperatures and pressures that are attainable by experimentation and computation. However, the extrapolated data agree reasonably well. (1) Fraser DG, Cagin T, Demiralp E, Goddard WA (1998) New transferable interatomic potentials for simulating melting of Mg silicates near the base of the mantle. A.G.U. abstract (2) Van Beest BWH, Kramer GJ, Van Santen RA (1990) Force fields for silicas and aluminophosphates based on ab-initio calculations. Phys Rev Lett 64: 1995
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
DE: 3949 Thermal expansivity
DE: 5120 Plasticity, diffusion, and creep
DE: 5134 Thermal properties
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting