Very low solubility of rutile in H2O at high pressure and temperature, and its implications for Ti mobility in subduction zones

The solubility of rutile in H2O has been measured at 1000-1100 degreesC, 1-2 GPa. The data indicate that solubility is very low over the investigated range, with a maximum of 4.7 millimol/kg H2O at 1100 degreesC, 2 GPa. The data were fit with the equation log m(Ti) = 4.892-10470/T + 0.1923P, where m(Ti) is Ti molality, T is in Kelvins, and P in GPa. When Compared to previous results, the new data indicate substantially lower solubility, opposite pressure dependence, and thermodynamic properties of the reaction rutile = TiO2,aq that are now consistent with other oxide hydrolysis reactions. Calculations of Ti transport during mantle metasomatism by H2O in subduction zone environments predict much lower Ti mobility at all conditions. These results offer strong support for models of Ti retention in eclogites during slab devolatilization, and require that examples of significant Ti mass transfer be explained by complexing agents in solution, most likely aluminosilicate complexes.