α-β-γ transformations in Mg₂SiO₄ in Earth's transition zone

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.