HR: 0800h
AN: T41B-1195    [Abstracts]
TI: Solubility of hydrogen and aluminum in CaSiO$_3$-perovskite and the partitioning of water among silicates in the lower mantle
AU: * Panero, W R
EM: wpanero@umich.edu
AF: Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 United States
AU: Akber-Knutson, S
EM: sofia@gps.caltech.edu
AF: Geological and Planetary Science Division, Caltech, Pasadena, CA 91125 United States
AB: The incorporation of water into nominally anhydrous silicates is associated with mechanical weakening and lowered melting points such that small amounts of water in mantle minerals can serve to alter planetary dynamics. The effects of minor phases have been largely neglected, yet CaSiO$_{3}$-perovskite, which may contain a large vacancy population, could serve as a repository for significant quantities of trace volatile elements. Here we present first principles, pseudopotential calculations of fully relaxed structures on CaSiO$_{3}$-perovskite with aluminum and hydrogen substitutions. We assume the hydrogen enters the structure via the coupled substitution of Si$^{4+}$ for Al$^{3+}$ + H$^{+}$. The most stable hydrogen location has an OH bond length of 1.02 $\AA$ and is virtually independent of pressure. This substitution is associated with a 1.5% softening of the bulk modulus for a structure containing 3.1 mol% H$_{2}$O and Al$_{2}$O$_{3}$, a lesser effect than recently reported results for Si$^{4+}$ for Al$^{3+}$ + H$^{+}$ softening in MgSiO$_{3}$-perovskite [Panero and Stixrude, EOS trans. AGU 85(17), 2004] or SiO$_{2}$-stishovite [Panero and Stixrude, EPSL 2004]. We calculate that the enthalpy of solution of Ca$^{2+}$+Si$^{4+}$=2Al$^{3+}$ is about half that of the Si$^{4+}$ for Al$^{3+}$ + H$^{+}$ reaction, with both solution enthalpies generally increasing with pressure between 0 and 100 GPa. This is in contrast to MgSiO$_{3}$ in which the solution enthalpies for equivalent reactions are nearly equal and generally constant with pressure. Assuming ideal entropy of solution, at 2000 K we find Ca-perovskite saturated in aluminum and water to contain 100 ppm wt H$_{2}$O and 1.7 wt % Al$_{2}$O$_{3}$ at the top of the lower mantle. Due to the differing behavior of the solution enthalpy with pressure, the partitioning of aluminum and hydrogen between MgSiO$_{3}$ and CaSiO$_{3}$ perovskite increasingly favors MgSiO$_{3}$ under water- and aluminum- undersaturated conditions. Aluminum partitions preferentially into MgSiO$_{3}$ with K$_{Al}^{Mgpv/Capv}$ $\sim$ 10 at 25 GPa and 2000 K, consistent with Irifune (1994) and Murakami et al., (2002), increasing to $\sim$ 50 at 100 GPa. The effect of pressure on water partitioning is more dramatic with K$_{H}^{Mgpv/Capv}$ increasing from $\sim$15 at 25 GPa to $\sim$ 120 at 100 GPa.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3620 Crystal chemistry
DE: 3670 Minor and trace element composition
DE: 3900 MINERAL PHYSICS
DE: 3924 High-pressure behavior
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting