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 19–26 GPa and 700–2000 K 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 22–26 GPa and 1000–1400 K. 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 20–24 GPa and temperatures above 1500 K 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 10.0 wt% 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 660 km depth discontinuity.

Research highlights
► High-pressure and high-temperature phase relations of Ca3Al2Si3O12 grossular garnet.
► Garnet-to-perovskite transition of Ca3Al2Si3O12.
► Orthorhombic crystal symmetry of Al-bearing CaSiO3 perovskite.
► Formation of the CAS phase at high-pressure and high-temperature.