TITANIUM SOLUBILITY IN COESITE

Experiments have been conducted to determine the temperature and pressure effects on the solubility of Ti in coesite. Amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the coesite stability field from 700 to 1025°C using a piston-cylinder apparatus. Coesite, rutile, and zircon crystallized from silica-, titania-, and zircon-saturated aqueous fluids; Raman microprobe measurements confirm the existence of coesite. Coesite and rutile crystals grew up to ~500 μm in maximum dimension. Cathodoluminescence images and electron microprobe measurements have shown that coesite crystals are relatively homogenous. The slopes of the 3.2 GPa, 3.5 GPa and 4.0 GPa Ti-in-coesite isobars are similar to the Ti-in-quartz isobars; however, the Ti content of coesite crystals are higher than predictions using the Ti-in-quartz calibration (Wark and Watson 2006; Thomas et al. 2010). Additional experiments will be conducted to determine the P-T dependencies of Ti-in-coesite solubility which can be used as a thermobarometer when used in combination with another thermobarometer in a coexisting mineral, an independent P or T estimate, or well-constrained phase equilibria; current experiments are being conducted at 3.0 GPa and 3.2 GPa.