High-pressure and high-temperature titanium solution mechanisms in silicate-saturated aqueous fluids and hydrous silicate melts

In the system Na₂O-Al₂O₃-SiO₂-H₂O-TiO₂, the behavior of Ti-containing structural complexes has been determined in H₂O-saturated silicate melts and in coexisting silicate-saturated aqueous fluids as well as in silicate-rich supercritical fluids to 900 °C and 2225 MPa. Titanium speciation in aqueous fluids in the system TiO₂-H₂O was also characterized. All measurements were carried out in situ at the desired temperature and pressure using confocal microRaman and microFTIR spectroscopy. The experiments were carried out in an Ir-gasketed hydrothermal diamond-anvil cell (HDAC) with K-type thermocouples for temperature measurement and the Raman shift of ¹³C synthetic diamond to monitor pressure.

In the system Na₂O-Al₂O₃-SiO₂-H₂O-TiO₂, four or five O atoms surround a central Ti⁴⁺ cation in melts, fluids, and supercritical fluids. In this environment, the titanium solubility mechanism is the same for melt, fluid, and single-phase liquid and can be described with the equation, 4Q¹_Si(Na)4H₂O + TiO₂ ↔ 4Q⁰_Si(HNa) + Q⁰_Ti(Na). Subscripts denote the nature of central cation, superscripts, the number of bridging oxygens, and the symbol(s) in parentheses the type(s) of cation(s) that form bonds with nonbridging oxygen in the Q-species. The ΔH of this reaction is on the order several tens of kJ/mol. Because of the structural similarity of Ti-complexes in hydrous silicate melts and silicate-rich aqueous fluid, the fluid/melt partition coefficients also resemble one another. The partition coefficient is between 0.1 and 1 and is positively correlated with temperature, pressure, and Al/(Al+Si) of the silicate melt or aqueous fluid. In the chemically simpler system TiO₂-H₂O, titanium in aqueous fluid occupies the central position in oxygen polyhedra surrounded by approximately six O atoms. Here, the enthalpy change for the solution reaction is between 50 and 60 kJ/mol absent pressure corrections for volume differences between TiO₂ in rutile and in aqueous solution. The Ti concentration in aqueous fluid is on the order of fractions to a few tens of parts per million. This solubility is orders of magnitude lower than in Ti solubility in silicate-saturated aqueous fluid in the system Na₂O-Al₂O₃-SiO₂-H₂O-TiO₂ at similar pressures and temperatures.

In natural environments such as high-grade metamorphic terranes and subduction zones, aqueous fluids are silicate-saturated. Such fluids are more efficient solvent refractory oxides, perhaps by 2–3 orders of magnitude for an oxide such as TiO₂, than inferred from their solubility in pure H₂O fluids.