Experimental Calibration of an OH-in-Rutile Oxybarometer

Hydroxyl substitution in the rutile structure has the potential to act as a barometer of oxygen fugacity due to the coupling of OH with Ti^{4+} reduction via the reaction 2Ti^{4+}O2 + H2O = 2Ti3+O(OH) + 1/2O2. We have experimentally investigated this reaction at 0.5-2.0 GPa and 600- 900°C. Pure, synthetic rutile and H2O were sealed in a Pt inner capsule and placed within a Au outer capsule containing an fO2 buffering assemblage (NNO or HM). Experiments employed a piston-cylinder apparatus and NaCl- graphite furnaces; run times were 10-70 hr. Estimated equilibration time was <2 hr. Equilibrated rutile crystals were made into doubly polished thin sections, and OH concentrations were determined using IR spectroscopy. The experiments revealed a strong fO2 dependence of OH solubility in rutile. The concentration of OH increases from 165 to 585 ppm H2O when the oxygen fugacity is decreased from log fO2 = -9.4 (HM) to log fO2 = -13.9 (NNO) at 800°C and 1 GPa. Hydroxyl substitution increases with increasing T. At 1 GPa and NNO, the OH solubility increases from 158 ppm H2O at 600°C to 748 ppm H2O at 900°C. For rutile held at the HM buffer and 1 GPa, the concentration of OH increases from 84 to 267 ppm H2O from 600 to 900°C. Hydroxyl concentration in synthetic rutile shows no detectable dependence on H2O activity in the range 1.0-0.1. The fact that the concentration of OH in rutile is not effected by water activity, but is strongly dependent upon oxygen fugacity, suggests it can be used as a highly accurate oxygen barometer in geologic environments where pressure and temperature are known.