Ferric iron and aluminum partitioning between MgSiO₃- and CaSiO₃-perovskites under oxidizing conditions

Abstract

Ferric iron and aluminum partitioning between MgSiO₃-dominant perovskite (Mg-Pv) and CaSiO₃-dominant perovskite (Ca-Pv) was investigated at 27-100 GPa and 1600-2500 K under oxidizing conditions using a starting composition of 0.85CaMgSi₂O₆ + 0.15Fe³⁺AlO₃ employing a combined laser-heated diamond anvil cell, synchrotron radiation, and analytical transmission electron microscopy. The product phases were Mg-Pv, Ca-Pv, and a small amount of stishovite at pressures below 30 GPa, while a two-phase assemblage of Mg-Pv and Ca-Pv was identified at pressures above 30 GPa. The appearance of stishovite below 30 GPa indicates the partial formation of an oxygen-deficient perovskite from 2ASiO₃ + Fe³⁺AlO₃ = 2AFe³⁺_0.5Al_0.5O_2.5 (oxygen-deficient perovskite) + 2SiO₂ (stishovite), where A = Mg or Ca. The composition of Mg-Pv and Ca-Pv show that ferric iron and aluminum were predominantly incorporated into Mg-Pv under oxidizing conditions, indicating that Mg-Pv is the dominant host phase for ferric iron and aluminum even under highly oxidizing lower mantle conditions.