TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz

Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940A degrees C in a piston-cylinder apparatus to explore the potential pressure effect on Ti solubility in quartz. A systematic decrease in Ti-in-quartz solubility occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for the P-T dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments to the simple expression RTln X-TiO2(quartz) = -60952 + 1.520 . T(K) - 1741 . P(kbar) + RTln a(TiO2) where R is the gas constant 8.3145 J/K, T is temperature in Kelvin, X-TiO2(quartz) is the mole fraction of TiO2 in quartz and aTiO(2) is the activity of TiO2 in the system. The P-T dependencies of Ti-in-quartz solubility can be used as a thermobarometer when used in combination with another thermobarometer in a coexisting mineral, an independent P or T estimate of quartz crystallization, or well-constrained phase equilibria. If temperature can be constrained within +/- 25A degrees C, pressure can be constrained to approximately +/- 1.2 kbar. Alternatively, if pressure can be constrained to within +/- 1 kbar, then temperature can be constrained to approximately +/- 20A degrees C.