Rutile solubility in H2O, H2O-SiO2, and H2O-NaAlSi3O8 fluids at 0.7-2.0 GPa and 700-1000 °C: Implications for mobility of nominally insoluble elements
The solubility of rutile was measured in H2O, H2O-SiO2 and H2O-NaAlSi3O8 fluids at 700-1000 °C, 0.7-2.0 GPa, in a piston-cylinder apparatus. Solubility was determined by weight loss using a double-capsule method. Rutile solubility in pure H2O shows isothermal increase with pressure (P), isobaric increase with temperature (T), and is low at all conditions investigated (6-118 ppm Ti). Rutile solubility in H2O is given by logcTi° = 6.173 - 5425/T + 178.4P/T, where cTi° is Ti concentration in ppm, T is in K, and P in GPa. This leads to thermodynamic properties of the reaction rutile = TiO2,aq of [Delta]Sr° = 28.6 J/mol K, [Delta]Hr° = 104 kJ/mol, and [Delta]Vr° = - 3.4 cm3/mol. At 800 °C and 1 GPa, addition of SiO2 (up to quartz saturation) did not change rutile solubility relative to that in pure H2O. Determination of rutile solubility in H2O-NaAlSi3O8 fluids was complicated by incongruent dissolution of albite to paragonite or corundum + fluid; however, fluid compositions could be estimated within narrow limits using a mass-balance scheme. The solubility of rutile increases linearly with dissolved Na-Al silicate at fixed P and T, as described by cTi = cTi° + Bws where cTi is ppm Ti, ws is wt.% dissolved silicate and B is given by log B 6.512 - 1.665P - 6224/T + 2215P/T, with T and P again in K and GPa. The results help explain discrepancies among previous studies of rutile solubility in H2O at similar P and T. The new data agree within error with those of Tropper and Manning [Tropper, P., Manning, C.E., 2005. Very low solubility of rutile in H2O at high pressure and temperature, and its implications for Ti mobility in subduction zones. American Mineralogist 90, 502-505.], but give lower solubility than earlier piston-cylinder-based determinations due to suppression of new crystal growth in the present experiments. However, the new data yield higher solubilities than are predicted from a hydrothermal diamond-anvil study, probably because of our longer run times and more complete equilibration. Combination of predicted Ti concentrations in melt-saturated H2O with H2O-saturated albite melts suggests that the melt-vapor partition coefficient for Ti is constant at 9.5 ± 1.5 from 700 to 900 °C at 1 GPa and rutile saturation, implying that an H2O-rich magmatic vapor phase can transport significant Ti in mid- to deep-crustal settings. Because crustal and mantle fluids will contain alkalis, Al and Si, the results in H2O-NaAlSi3O8 fluids provide a better foundation for modeling high-P metasomatic processes than pure H2O values. The strong increase in rutile solubility with dissolved Na-Al silicate suggests that complexing with these constituents promotes Ti mobility and transport during fluid-rock interaction in the lower crust and upper mantle.