Experimental study of titanium-in-coesite solubility

Experiments are being conducted to determine the effect of pressure and temperature on the solubility of Ti in coesite. Silica glass, anatase and water were added to silver capsules that were run in the coesite stability field from 700 to 940C using a piston-cylinder apparatus. Coesite and rutile crystallized from silica- and titania-saturated aqueous fluids. Raman microprobe measurements confirm that the silica polymorph is coesite. Coesite and rutile crystals range up to ~150 μm in maximum dimension. Cathodoluminescence images and electron microprobe measurements show that coesite crystals are relatively homogenous. In P-T space, the slope of the 35 kbar Ti-in-coesite isobar is similar to the Ti-in-quartz isobars; however, Ti contents of coesite crystals are higher than predictions using the Ti-in-quartz calibration (Wark et al. 2006; Thomas et al. 2010). Additional experiments will be conducted to determine the P-T dependencies of Ti-in-coesite solubility that 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. Thomas et al. (2010) Contributions to Mineralogy and Petrology 160, 743-759 Wark et al. (2006) Contributions to Mineralogy and Petrology 152, 743-754