Phase relations in α-β-γ Mg2SiO4 have been investigated by first principles quasi-harmonic free energy computations. The computed phase boundaries obtained using the local density approximation (LDA) and the generalized gradient approximation (GGA) bracket the experimental ones, with LDA (GGA) calculations giving the lowest (highest) bound, while the Clapeyron slopes are in good agreement with the experimentally determined ones. This is the same trend displayed by previous similar computations. Further analyses reveal that despite the uncertainties in phase boundary determination, the calculated discontinuities in density, bulk modulus, and bulk sound velocity are quite insensitive to pressure and have small uncertainties and useful accuracy to discriminate potential sources of discontinuities in the mantle. We verify that ∼ 3% density discontinuity at 410-km depth can be produced primarily by the α to β transition in an aggregate with pyrolite composition, i.e. ∼ 60 vol.% of Mg2SiO4. However, the 1.3-2.9% density discontinuity observed in some places at 520-km depth cannot be accounted for solely by the β to γ transition but also requires changes in the coexisting pyroxene/garnet/Ca-perovskite system.
Bibliographical noteFunding Information:
We thank L. Li for allowing us to compare our forsterite results with hers, C. R. S. da Silva for comments on the manuscript, and S. Zhang for improving the performance of the first principle codes on the blade system at the Minnesota Supercomputing Institute (MSI). We also thank S. V. Sinogeikin and A. Saikia for providing us references which helped the comparison of this study with theirs. This research was supported by NSF/EAR 013533, 0230319, 0635990, and NSF/ITR 0428774 (VLab). Computations were performed at MSI and on the Big Red Cluster at Indiana University.
Copyright 2011 Elsevier B.V., All rights reserved.
- phase transition
- transition zone