HR: 14:25h
AN: T22D-04    [PDF]
TI: Al$_2$O$_3$ and its influence on oxygen vacancies in the lower mantle
AU: * Akber-Knutson, S
EM: sofia@eps.berkeley.edu
AF: Earth & Planetary Science, UC Berkeley, Berkeley, CA 94720 United States
AU: Bukowinski, M S
EM: markb@socrates.berkeley.edu
AF: Earth & Planetary Science, UC Berkeley, Berkeley, CA 94720 United States
AB: While commonly accepted mineralogical models of the Earth's lower mantle consist of the three phases: (Mg,Fe)SiO$_3$ perovskite, (Mg,Fe)O (magensiow\"ustite), and CaSiO$_3$ perovskite, little is known about how the minor and rare gas elements fit in. The role of aluminum in MgSiO$_3$ perovskite has received much notice recently. As a trivalent cation, aluminum introduces two likely and interesting substitution mechanisms, one of which involves the formation of oxygen vacancies. Aside from significantly lowering the bulk modulus of MgSiO$_3$ perovskite, these vacancies can provide ``comfortable'' sites for larger, minor elements or noble gases. Our previous theoretical simulations for the MgSiO$_3$-Al$_2$O$_3$-MgO system indicate that as much as one-third of the aluminum enters MgSiO$_3$ via vacancy-forming substitution in the upper part of the lower mantle, and that the number of vacancies is reduced by about a factor of two at lowermost mantle conditions. To better understand the role of aluminum in the lower mantle assemblage, we examine the solubility of aluminum in CaSiO$_3$ perovskite as well. While a small fraction of an already minor element (Al) in a minor phase (CaSiO$_3$) would not significantly affect the elasticity of the lower mantle directly, it could affect its geochemical properties ({\em e.g.} hosting of rare elements, water solubility, melting temperature, etc.). We find that, in contrast to MgSiO$_3$, vacancy-forming substitution is favored in CaSiO$_3$ throughout most lower mantle conditions. However, we also find that aluminum partitions preferentially into MgSiO$_3$. Although we do not yet have an activity-composition relationship for the case of multi-site mixing, a rough estimate of vacancy abundances can be obtained by assuming that the activity coefficients do not deviate significantly from unity. We thus estimate that adding aluminum into the lower mantle assemblage introduces about one oxygen vacancy per 100 O atoms at standard conditions, a result that is consistent with zero pressure data. Since the number of vacancies is only slightly reduced by lower mantle pressure-temperature conditions, these results strongly suggest that O vacancies play a significant geochemical role in the lower mantle. In addition, vacancy formation decreases the amount of MgO present, which may subtly affect the thermal contribution to lateral seismic velocity variations.
DE: 3620 Crystal chemistry
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting