Thermal Equation of State of Aluminum-Enriched Silicate Perovskite

Abstract

Inferences of the chemical homogeneity of Earth's mantle depend on comparing laboratory-derived equations of state of mantle phases with seismically determined properties of the material in situ. A uniform chemical composition of the entire mantle has been found to be consistent with measurements, to date, of these properties for the end-member MgSiO3 perovskite phase. New pressure-volume-temperature data for silicate perovskite containing 5 mole percent Al2O3 has yielded different values of the equation of state parameters, with the bulk modulus being significantly smaller at lower mantle conditions than for aluminum-free perovskite, thus requiring adjustments in other components to match seismic observations.

Subsolidus phase relations and perovskite compressibility in the system MgO–AlO1.5–SiO2 with implications for Earth's lower mantle

“…Observations from four experimental studies indicate that values for the bulk modulus of MgSiO3 perovskite typically used in mineralogic models, about 260 GPa, may be considerably too high [4], [5], [6] and [7]. These studies indicate that even the small amount of Al2O3 in a pyrolite mantle, about 5 mol%, can cause about a 10% reduction in the bulk modulus when substituted into perovskite.…”

The mechanism of solution of aluminum oxide in MgSiO3 perovskite

“…The structure is composed of corner-shared octahedra (nominally occupied by Si 4+) surrounding a larger, eightcoordinated site (nominally Mg2+). Despite a number of studies [Andrault, et al, 1998;Kesson, et al, 1995;Zhang and Weidner, 1999], the proportions of the two site types in the structure that are occupied by A13+ are not well-constrained by experiment, limiting the possibilities for theoretical understanding of the properties of the phase. Atomistic computer simulations [Richmond and BrodhoTt, 1998] predicted that at high pressures, the chargecoupled substitution of two A1 for one Si plus one Mg cation should be energetically favored in the iron-free system; more accurate density functional theory calculations suggested a changeover from an oxygen vacancy mechanism to stoichiometric coupled substitution at about 30 GPa at 0 K, and at perhaps 50 to 60 GPa at mantle temperatures [Brodholt, 2000].…”

“…The most abundant mineral in the earth's lower mantle is thought to be MgSiO 3 with the perovskite structure [KnittTe and deanloz, 1987;Zhang and Weidner, 1999]. After ferrous and/or ferric iron, the most important cation in solid solution in this phase is AI D+.…”

Microscopic properties to macroscopic behaviour: The influence of iron electronic state

“…The isothermal bulk modulus of MgSiO 3 and (Mg,Fe)SiO 3 perovskites have been established to generally fall within the range 250 260 GPa by numerous laboratories (e.g., Funamori et al, 1996;Fiquet et al, 1998;Fiquet et al, 2000;Sinogeikin et al, 2004); however measurements on perovskite compositions incorporating Al (hence stabilising some of the iron, if present, as Fe 3+ ) show dramatically different results, but with a large variation in observed behaviour. Some high pressure measurements show a 5 10% decrease in bulk modulus (Zhang and Weidner, 1999;Kubo et al, 2000;Daniel et al, 2001;Yagi et al, 2004), some show an increase in bulk modulus Ono et al, 2004) and some show no change (Yagi et al, 2004;Jackson et al, 2004). Incorporation of oxygen vacancies according to Equation 5 is predicted to decrease the bulk modulus (Brodholt, 2000;Ross et al, 2002); hence variations in oxygen vacancy concentrations and/or ordering could influence the behaviour, as well as differences in the trivalent cation substitution mechanism, partial amorphisation following sample synthesis, or as yet unidentified factors Ono et al, 2004;Yagi et al, 2004).…”