Phase transformations between garnet and perovskite phases in the Earth’s mantle: A theoretical study

Using first-principles theories, we show that the stability of garnet and perovskite phases in an Al-free system is strongly influenced by both pressure and temperature, giving rise to a sequence of phase changes. Around 17 ± 3  GPa pressure, the (Mg1−y Cay)3(MgSi)Si3 O12 majorite garnet dissociates into Ca- and Mg-perovskites. This divariant transition is associated with structural, density and elastic changes, and for y ≈ 0.13 it has a width of ∼ 0.6 GPa. In CaSiO3 plus MgSiO3 aggregate, a (Mg, Ca)SiO3 solid solution with an intermediate orthorhombic perovskite structure can be formed. The (Mg1−x Cax)SiO3 solid solution with x ≈ 0.04 – 0.06 is calculated to be stable at the transition zone base and uppermost lower mantle conditions, and with increasing pressure it separates into perovskite end-members. The pressure–temperature stability limit for the perovskite solid solutions is close to the mantle geotherms, suggesting the appearance of structural and chemical inhomogeneities driven by temperature anomalies within the Earth’s lower mantle.