Structure and Compressibility of Ti-bearing Davemaoite: New Insight to Seismic Heterogeneity of the Deep Mantle

Davemaoite (CaSiO3 perovskite) is arguably the third most abundant mineral in the pyrolytic lower mantle, and the second most abundant one in the subducted mid-ocean ridge basalt (MORB) according to the Preliminary reference Earth model (PREM). During the partial melting of the mantle and the formation of MORB, titanium (Ti) is enriched into the basaltic magma. Thus, MORB contains a higher concentration of Ti (typically around 8-10%) than the pyrolytic mantle. Davemaoite is considered an important Ti-bearing mineral in subducted slabs. However, the crystal structure and thermoelastic properties of Ca(Si,Ti)O3 perovskite solid solution at high pressure-temperature conditions were not systematically investigated. It was proposed that pure CaSiO3 undergoes tetragonal-cubic phase transition under high pressure-temperature conditions, yet high-resolution X-ray diffraction (XRD) measurements are required to distinguish subtle splitting of diffraction peaks. In this study, we investigated the equation of state (EoS) and structure of Ca(Si0.75,Ti0.25)O3 and Ca(Si0.85,Ti0.15)O3 perovskite at 20-80 GPa and 300 K by synchrotron X-ray diffraction (XRD). Preliminary results show the Ca(Si0.85,Ti0.15)O3 perovskite adopts a tetragonal structure across such pressure range; whereas, the Ca(Si0.25,Ti0.75)O3 perovskite may undergo a subtle phase transition into cubic structure at around 27-30 GPa, 300 K. Based on the lattice parameters of Ti-bearing perovskite, we found that both Ti-bearing Ca(Si0.75,Ti0.25)O3 and Ca(Si0.85,Ti0.15)O3 perovskites is ~ 0.6-0.7% less dense than pure CaSiO3 davemaoite until 70 GPa. Our results suggest the enrichment of Ti-bearing davemaoite phase may reduce the average density of subducting slabs and enhance their stagnationacross the bottom of the mantle transition zone.