THE EFFECT OF CATION SUBSTITUTION ON HYDROGEN SOLUBILITY IN RUTILE

A recently developed OH-in-rutile oxybarometer can be used to determine oxidation state in igneous and metamorphic environments, provided temperature and pressure are constrained (Colasanti 2007 GCA 71(15s) A181). The oxybarometer is based on the redox equation: Ti⁴⁺O₂ + ½H₂O = Ti³⁺O(OH) + ¼O₂ for pure rutile. However, cation substitution (e.g. M³⁺O(OH)) in natural rutile crystals may affect hydrogen solubility. To quantify the effect of trace element substitution on hydrogen solubility in rutile, synthetic rutile crystals were doped with Al, Cr, and Fe by exchange with cryolite melts. Doped rutile crystals were equilibrated at 10 kbar and 500°C at the hematite-magnetite (HM) buffer in the presence of water in the experimental laboratory at UCLA. OH concentrations and trace element concentrations of run products were quantified using the Varian Digilab Excalibur FTS3000 FTIR spectrometer and JEOL JXA-8900R WD/EDS microanalyzer in the Department of Mineral Sciences, Smithsonian Institution. OH solubility ranges from 50 ppm H₂O for pure TiO₂ to 1700 ppm for Al-doped rutile. The position of the OH band shifts to higher wavenumbers in cation-doped rutile. The most pronounced change is the shift from 3278 cm^-1 for pure rutile to 3325 cm^-1 for Al-doped rutile. Molar hydrogen concentrations show a 1:1 correlation with molar concentrations of total trace cations (Cr+Al+2Fe), consistent with data obtained from natural rutile (Vlassopoulos Am Min 78 1881). The solubility of OH will be higher in rutile containing trace elements than for pure rutile under identical temperature and pressure conditions. Oxidation state can still be estimated for natural samples by quantifying and correcting for trace cation substitution. Diffusive loss of hydrogen from rutile may be implied if the molar hydrogen concentration is much lower than total trivalent cation concentration.