HR: 09:36h
AN: MR41A-08    [Abstracts]
TI: High-pressure alloying of xenon and iron: 'Missing' Xe in the Earth's core?
AU: * Lee, K K
EM: kanani@gps.caltech.edu
AF: California Institute of Technology, Division of Geological & Planetary Sciences, MC 170-25 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Steinle-Neuman, G
EM: G.Steinle-Neumann@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth, 95440 Germany
AB: Noble gas xenon (Xe), with its tendency to not react with other elements, is ideal for studying the evolution of planets through investigation of its daughter products from radioactive isotopes (e.g., $^{244}$Pu (and $^{238}$U) --$>$ $^{136}$Xe, $^{129}$I --$>$ $^{129}$Xe systematics). However, the Earth is 'missing' some xenon in its atmosphere: Xe is much more depleted than expected from chondritic abundances. Mere hydrodynamic escape or impact vaporization, is not enough to account for the missing Xe since even lighter rare gases like argon and neon are less depleted than heavier xenon. Instead, the missing Xe may be hidden deep in the Earth's interior where there is little communication with the surface. To test the possibility of Xe allyoing with iron (Fe) during core formation and its thermodynamic stability as a solvent in the core, we perform density-functional based {\it ab-initio} calculations of Xe incorporation into the hexagonal close packed (hcp) high-pressure phase of iron, $\epsilon$-Fe. To that extent we set up supercells of hcp Fe of various sizes in which we incorporate Xe. Using the projector augmented wave method as implemented in the Vienna {\it Ab-initio} Simulation Package (VASP) we evaluate their energetics and stability relative to the elemental solids. We find that under static conditions up to 1 mol$%$ Xe can be alloyed into Fe at high pressure, suggesting that Xe may have been incorporated into the iron-rich core during core segregation. Substitutional incorporation of Xe into $\epsilon$-Fe causes the hcp structure to expand, depending on pressure and concentration, with 1 mol$%$ (~10$^{4}$ ppm by weight) substitution causing about 1$%$ volume expansion at core pressures. This potential alloying behavior could be crucial for understanding the accretion and evolution of the solid Earth and its atmosphere and could possibly provide an explanation to the Earth's 'missing' Xe.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3900 MINERAL PHYSICS
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting