HR: 1340h
AN: V53A-0620    [Abstracts]
TI: A Numerical Model of Chemical and Soret Diffusion at Crystallizing Boundaries
AU: * Sonnenthal, E L
EM: elsonnenthal@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, MS90-1116, Berkeley, CA 94720 United States
AB: The effect of thermal or Soret diffusion on differentiation in layered intrusions has recently been reexamined for explaining marginal reversals (Latypov, 2003). Previously, Lesher (1986) and Lesher and Walker (1986) provided a detailed analysis of the potential role of Soret diffusion in magmatic systems and published experimentally determined Soret coefficients for different composition melts. In these papers, and more directly in numerical studies done by Cygan and Carrigan (1992), the effect of Soret diffusion compared to chemical diffusion was shown to be relatively minor, because of the roughly three order of magnitude smaller Soret coefficient and the relatively small thermal gradients expected in magmas above the liquidus temperature. It was suggested by the latter authors that the effects would be even less under subliquidus conditions. In this contribution, a numerical study is presented that considers the coupled effects of Soret and chemical diffusion to heat transfer and crystallization for a multicomponent system. The model accounts for the full chemical diffusion tensor in a twelve-component system, including the Soret coefficients for the diagonal components. Simulations performed using estimated Skaergaard Intrusion liquid compositions indicate that that the more rapid chemical diffusion of water and alkalis ahead of the crystallization front leads to changes in the liquidus temperature of the magma, thus allowing for a temperature gradient that would not be present without chemical diffusion. This temperature gradient gives rise to Soret diffusion, accentuating the transport of alkalis into the hotter magma. The ratio of Soret to chemical diffusion becomes greater than that predicted without the consideration of changes to phase equilibria. While this work does not imply that Soret diffusion is a dominant mechanism for differentiation in mafic magmas, it does suggest that the effects may be greater than previously recognized and could play a role in the observed phase appearances, compositions, and modal proportions.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting