Phase transformations of Ca₃Al₂Si₃O₁₂ grossular garnet to the depths of the Earth's mantle transition zone

High-pressure and high-temperature phase transformations of Ca₃Al₂Si₃O₁₂ grossular garnet were examined at 19-26GPa and 700-2000K using Kawai-type multi-anvil apparatus coupled with in situ X-ray diffraction (XRD). Recovered samples were analyzed by a combination of micro-focused X-ray diffraction (μ-XRD) and transmission electron microscopy (TEM). The results show that grossular garnet gradually transforms to an Al-rich CaSiO₃ perovskite at 22-26GPa and 1000-1400K. The transition boundary can be expressed as P (GPa)=-0.0082×T (K)+33.05. When the garnet completely disappears, we observed orthorhombic CaSiO₃ perovskite with a grossular composition. At 20-24GPa and temperatures above 1500K the CAS phase with the composition CaAl₄Si₂O₁₁ appears to accommodate excess Al from the perovskite along with two distinct populations of Al-bearing CaSiO₃ perovskites, with Al content of 3.7 and 10.0wt% Al₂O₃, respectively. The pressure and temperature of these transitions correspond to the lowermost part of the transition zone and therefore it suggests that Ca-rich aluminosilicates could provide alternative candidates to explain multiple seismic reflections near the 660km depth discontinuity.