The Perovskite to Post-Perovskite Transition: Atomistic Simulations of Compositions on the MgSiO3-FeSiO3 and MgSiO3-FeAlO3 Joins

Abstract:

Different atomic arrangements of perovskite (pv) and post-perovskite (ppv) in the systems MgSiO₃-FeSiO₃ and MgSiO₃-FeAlO₃ (MS-FS and MS-FA) were examined by Boltzmann statistics and density functional theory (GGA+U). The vibrational contribution to the free energy is calculated within the quasi-harmonic approximation. The FS and FA components partition in opposite directions, towards ppv and pv, respectively. The diverse experimental results on the Fe-partitioning between the two phases are broadly consistent with our theoretical investigation.

We determined a Clapeyron slope for the pv-ppv transition of 9.6 MPa/K for MS, decreasing to 8.4 and 8.1 MPa/K at 6.3 and 12.5 mol% FS and to 8.2 and 7.3 MPa/K at 6.3 and 12.5 mol% FA. An isochemical pv to ppv transition within the investigated solid solution range (up tp 25 mol% FS and FA) is characterized by decreasing bulk and increasing shear modulus. At a reference pressure of ­­­100 GPa, the bulk modulus for pv and ppv increases by 1.4% and 1.0%, respectively, from MS to MS-FS_12.5 and decreases by 0.4% and 0.2%, respectively, from MS to MS-FA_12.5 (subscripts indicate mol%). The shear modulus decreases for both pv and ppv along both of the solid solution series from the pure MS composition, with the largest decrese along the FA-join. For pv and ppv the shear modulus decreases with 2.4% and 1.3%, respectively from MS₁₀₀ to MS-FS_12.5 and with 4.4% and 3.0%, respectively, from MS₁₀₀ to MS-FA_12.5. The results for the MS-FS join are in general agreement with previous DFT-studies (Caracas & Cohen, 2005, GRL; Stackhouse et al. 2006, GRL; Stackhouse & Brodholt 2008, PEPI). Boffa Ballaran et al. (2012, EPSL), however, found a decrease in the bulk modulus for pv on the MS-FS join relative to pure MS. The decreasing bulk modulus along the MS-FA join agrees with Boffa Ballaran et al.

A chemical reaction from FA-rich pv and reduced Fe to form FS-rich ppv produces additional Al₂O₃ and MgO:

2 FeAlO₃ + 3 MgSiO₃ + Fe = 3 FeSiO₃ + Al₂O₃ + 3 MgO

In peridotite, the excess MgO and Al₂O₃ may dissolve in ferropericlase and ppv, respectively. If ppv becomes Al-saturated, Ca-ferrite- (CF) or Ca-titanite- (CT) structured phases with the MgAl₂O₄ component might form. In basaltic rocks MgAl₂O₄ may enter preexisting CF-CT-phases and excess MgO may combine with SiO₂ from silica-dominated phases to the MS-component of ppv.