Aqueous fluids as transport medium at high pressure and temperature: Ti4+ solubility, solution mechanisms, and fluid composition

The stability of Ti-bearing crystalline phases such as rutile and ilmenite in the Earth's interior can be dependent on the solubility behavior of TiO2 in aqueous fluids. Natural and experimental evidence indicate that significant TiO2 mobility is possible in this environment. Other solutes affect Ti solubility and solution mechanisms in the aqueous solutions. Here, the solubility behavior and the influence of other solutes on Ti solubility have been addressed with experiments in an externally-heated diamond anvil cell by using rutile coexisting with aqueous fluid in the TiO2-H2O, TiO2-SiO2-H2O, TiO2-(Na2O center dot 5SiO(2))-H2O, and TiO2-(CaO center dot 5SiO(2))-H2O systems in the 450-900 degrees C and 220-2100 MPa temperature and pressure range. Micro-Raman spectroscopy was employed to probe the samples while these were at the temperatures and pressures of interest. The TiO2 solubility in all fluid compositions studied increased with increasing temperature and pressure. The solubility is the lowest, between similar to 10 and similar to 50 ppm, in TiO2-H2O fluids. Addition of SiO2 enhances the TiO2 solubility to concentration in excess of 150 ppm at the highest temperature and pressure (900 degrees C/1335 MPa). The solubility in TiO2-(Na2O center dot 5SiO(2))-H2O, and TiO2-(CaO center dot 5SiO(2))-H2O fluids is more than twice that in TiO2-SiO2-H2O fluids. The TiO2 solubility in the Na-system is about 50 ppm greater than in the Ca-system. The enthalpy of solution of TiO2 in the aqueous fluids is in the 15-28 kJ/mol range. The local oxygen environment surrounding Ti4+ in TiO2-H2O and TiO2-SiO2-H2O fluids resembles that in the rutile structure (i.e., 6-coordinated Ti4+). With Na+ or Ca2+ in the fluid, additional complexes are formed with a smaller number of oxygen in the coordination sphere. The bulk composition of hydrated rocks in the Earth's interior governs the pressure/temperature-stability of hydrous phases and, therefore, solute content of a fluid released when the stability limits of the hydrous phases in the rocks are exceeded and aqueous fluid is released. This solute and, therefore, the source rock bulk composition, controls titanium solubility. For example, the distribution of Ti-bearing phases in equilibrium with aqueous fluid in subduction zone environments depends on the depth and thermal profile because the temperature and pressure stability of the relevant hydrous minerals (e.g., serpentine, lawsonite, hornblende, phengite) are quite different.