Structure and equation of state of Ti-bearing davemaoite: New insights into the chemical heterogeneity in the lower mantle.

Davemaoite (CaSiO₃ perovskite) is considered the third most abundant phase in the pyrolytic lower mantle and the second most abundant phase in the subducted mid-ocean ridge basalt (MORB). During the partial melting of the pyrolytic upper mantle, incompatible titanium (Ti) becomes enriched in the basaltic magma, forming Ti-rich MORB. Davemaoite is considered an important Ti-bearing mineral in subducted slabs by forming a Ca(Si,Ti)O₃ solid solution. However, the crystal structure and compressibility of Ca(Si,Ti)O₃ perovskite solid solution at relevant pressure and temperature conditions had not been systematically investigated. In this study, we investigated the structure and equations of state of Ca(Si_0.83Ti_0.17)O₃ and Ca(Si_0.75Ti_0.25)O₃ perovskites at room temperature up to 82 and 64 GPa, respectively, by synchrotron X-ray diffraction (XRD). We found that both Ca(Si_0.83Ti_0.17)O₃ and Ca(Si_0.75Ti_0.25)O₃ perovskites have a tetragonal structure up to the maximum pressures investigated. Based on the observed data and compared to pure CaSiO₃ davemaoite, both Ca(Si_0.83Ti_0.17)O₃ and Ca(Si_0.75Ti_0.25)O₃ perovskites are expected to be less dense up to the core-mantle boundary (CMB), and specifically ~1–2% less dense than CaSiO₃ davemaoite in the pressure range of the transition zone (15–25 GPa). Our results suggest that the presence of Ti-bearing davemaoite phases may result in a reduction in the average density of the subducting slabs, which in turn promotes their stagnation in the lower mantle. The presence of low-density Ti-bearing davemaoite phases and subduction of MORB in the lower mantle may also explain the seismic heterogeneity in the lower mantle, such as large low shear velocity provinces (LLSVPs). [ABSTRACT FROM AUTHOR]