HR: 0800h
AN: V41A-1356    [Abstracts]
TI: Studies of Diffusion, Atomic Hopping Frequency and Site Residence Times in Molten SiO2 by Molecular Dynamics
AU: * Gemmell, A L
EM: Alastair.Gemmell@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX26UD United Kingdom
AU: Fraser, D G
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX26UD United Kingdom
AU: Refson, K
AF: Rutherford Appleton Laboratory, Chilton, Didcot, 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. Understanding of diffusion mechanisms and activation energies also provides information on changes in melt structure. In the present paper, the 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 was 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 steps of 1fs depending on the conditions, with data being collected 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. We have previously reported a pronounced non-linearity in the temperature dependence of diffusion in molten SiO$_{2}$ (3). This suggests at least two diffusion mechanisms with differing activation energies which operate to differing extents at lower and higher temperatures. It is this relationship between diffusion coefficients and diffusion mechanism/melt structure that we have investigated in the present paper. In addition, we have extended the study to include a wider range of pressures. Detailed examination of trajectory data, as well as radial distribution function (rdf) and density data provides a picture of the structure and dynamics of SiO$_{2}$ over a range of conditions. We have developed a method for analysing residence times and 'hop' distances under varying conditions. The plots found at the URL accompanying this abstract compare diffusion of an oxygen atom at 3000K and 4000K over a 10$^{6}$ step (1ns) run. Peaks represent 'hopping', troughs residence in 'sites'. At temperatures of 4000K and above the concept of a discreet 'rattle and hop' diffusion mechanism breaks down to be replaced with a plasma-style situation of more continuous random movement. (1) Fraser DG, Cagin T, Demiralp E, Goddard WA, III, "New transferable interatomic potentials for simulating melting of Mg silicates near the base of mantle," A.G.U. 1998. (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. (3) Gemmell AL, Refson K, Fraser DG. Molecular Dynamics Simulations of Diffusion in a Silica Melt. EOS Trans AGU 84(46), Fall Meet. Suppl., Abstract V11D-0528, 2003.
UR: http://www.earth.ox.ac.uk/$\sim$alastair
DE: 8439 Physics and chemistry of magma bodies
DE: 8145 Physics of magma and magma bodies
DE: 5134 Thermal properties
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting