Titanium-rich phases in the Earth's transition zone and lower mantle: Evidence from experiments in the system MgO–SiO2–TiO2(±Al2O3) at 10–24 GPa and 1600 °C

Phase relations in the MgSiO3–MgTiO3 and Mg3Al2Si3O12–MgTiO3 systems were studied at 10–24 GPa and 1600 °C using a high-pressure Kawai-type multianvil apparatus.We investigated the full range of starting compositions for the enstatite-geikielite system to derive a P–X phase diagram and synthesize titanium-bearing phases, such as olivine/wadsleyite, rutile, pyroxene, MgTiSi2O7 weberite, bridgmanite and MST-bridgmanite in a wide pressure range. Olivine and pyroxene in run products are characterized by a low titanium content (b0.6 and b0.3 wt% TiO2, respectively) whereas the content of TiO2 inwadsleyite reaches 2 wt% at 12 GPa. The concentration of Ti inMgTiSi2O7 weberite decreaseswith pressure from52wt% TiO2 at 14 GPa to 43wt% TiO2 at 18 GPa. Two perovskite-type structures (MgSiO3 bridgmanite andMg(Si,Ti)O3 bridgmanite)were detected in the studied system. MgSiO3 bridgmanite (Brd) is formed at a pressure of N20 GPa and characterized by significant titanium solubility (up to 13 wt% TiO2 at 24 GPa). Mg(Si,Ti)O3 perovskite is formed at a pressure of N17 GPa. The concentration of TiO2 in this phase varies from29wt% to 49wt%. Itwas found that addition of Ti to the systemmoves the boundaries of Ol/Wad phase transformations to lower pressures. Addition of Al to the starting material allows us to simulate the composition of natural Ti-rich garnets and bridgmanites. It is important to note that garnet in the Prp-Gkl systemis stable throughout awide pressure range (10–24 GPa). Al incorporation does not affect the distribution of titanium between two types of bridgmanite. It is shown that high contents of Ti stabilize bridgmanite-like compounds at considerably lower pressure than that at the lower mantle/transition zone boundary. Our experiments simulate the composition of natural Ti-rich primary garnet found in eclogite from the Sulu ultrahigh-pressure (UHP) terrane.