HR: 12:05h
AN: V42A-07 INVITED    [Abstracts]
TI: Silicate-metal fractionation of silicon isotopes at high pressure and temperature
AU: * Schauble, E A
EM: schauble@ucla.edu
AF: Dept. of Earth and Space Sciences, UCLA, Los Angeles, CA 90095, United States
AU: Georg, R B
AF: Earth Sciences, Oxford University, Oxford, OX1 3PR, United Kingdom
AU: Halliday, A N
AF: Earth Sciences, Oxford University, Oxford, OX1 3PR, United Kingdom
AB: Equilibrium ³⁰Si/²⁸Si fractionations between magnesium-silicate perovskite (MgSiO₃) and silicon-bearing iron metal (Fe₃Si) are estimated with first principles lattice dynamical models. The purpose of this study is to investigate possible silicon-isotope fractionation at high pressure during core formation in the Earth-Moon system, recently inferred from high-precision Si-isotope measurements of terrestrial, lunar, and meteorite samples (1). Models use plane-wave basis sets and a combination of norm-conserving and ultrasoft pseudopotentials, with a gradient-correct density functional (PBE). Pressure effects on isotopic fractionation are modeled quasiharmonically, by optimizing each crystal structure at a series of pressures (at 0 K), and applying a thermal pressure correction based on modal Grüneisen parameters. The resulting equations of state are in good agreement with previous measurements and density functional theory models (2). Results indicate that silicate-metal fractionation increases with pressure (with ³⁰Si/²⁸Si 0.5‰ higher in perovskite relative to metal at 25 GPa, 2500 K, and 1.0‰ higher at 140 GPa, 2500 K) but decreases strongly with increasing temperature (from 1.7‰ at 30 GPa, 1500 K to 0.4‰ at 30 GPa, 3000 K). Along the adiabatic portion of the modern lower mantle geotherm, pressure and temperature effects roughly cancel, yielding a nearly constant 1‰ fractionation. A smaller fractionation of ~0.5‰ is expected at liquidus conditions at the base of a deep magma ocean. This fractionation is of the same order of magnitude, but somewhat smaller than the ~1.5‰ fractionation inferred from the Si-isotope composition and mantle Mg/Si ratio of the Earth-Moon system. The presence of IV- and V-coordinated Si in high-pressure silicate magma equilibrated with metal (3) (as opposed to VI-fold coordination in perovskite) might increase ³⁰Si/²⁸Si fractionation.
References: 1) Georg et al. (2007) Nature 447:1102-1106. 2) Karki (2000) Am. Mineral. 85:1447-1451; Hirao et al. (2004) Phys. Chem. Min. 31:329-336. 3) Stixrude and Karki (2005) Science 310:297-299.
DE: 1015 Composition of the core
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3924 High-pressure behavior
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting