Extremely high solubility of rutile in chloride and fluoride-bearing metamorphic fluids: An experimental investigation

Rutile (TiO2) is an important high field strength element sequestering phase, and thus understanding its solubility in metamorphic fluids is important if we are to understand terrestrial trace element cycling. Titanium is often assumed to be fluid immobile, and previous experimental data suggest that rutile has very low solubility in H2O. However, fluids in high-temperature metamorphic rocks are known to be more complex in composition. The solubility of rutile in chloride and fluoride-rich hydrous fluids was determined experimentally at 0.5 GPa and high temperatures. The experimental results indicate much higher rutile solubility in brines compared to pure H2O. In chloride brines solubility was 2-4 times higher than in H2O, and in fluoride brines solubility was elevated 20-100 times. The results imply that titanium may be much more mobile in metamorphic fluids than previously anticipated, as solubility is critically dependent on fluid chemistry. We also present a possible mechanism for rutile precipitation in metamorphic veins due to a change in fluid chemistry, following the precipitation of halogen-rich minerals such as apatite.