Rutile solubility in aqueous sodium salt solutions at high pressures and temperatures: in-situ observations using a diamond anvil cell

Abstract Knowledge of rutile (TiO 2 ) solubility in aqueous fluids at high pressures and temperatures is of primary importance to a quantitative understanding of the high field strength elements (HFSE) transport in subduction zones. We attempted to redetermine rutile solubility in aqueous NaF solution and newly assess rutile solubility in aqueous Na 2 CO 3 and Na 2 SO 4 solutions through in-situ optical observations of the complete dissolution of a rutile grain in a hydrothermal diamond anvil cell (HDAC) at 823–974 °C and a pressure of approximately 0.9–1.8 GPa. The reproducibility of the experimental results in Na 2 CO 3 solution was confirmed across individual experiments and repeated cycles within a single experiment. The rutile solubility in the aqueous sodium salt solutions was significantly higher than the previously reported solubility in pure H 2 O obtained with weight loss techniques using a piston–cylinder apparatus. While the increase in the solubility with added NaF was consistent with previous findings, the present results suggest that the solubility-promoting effect is smaller than previously reported. The high solubility in the aqueous sodium salt solutions may be attributed to the formation of Ti(IV) complexes with potential ligands (e.g., OH − , CO 3 2− , SO 4 2− , and F − ) and sodium that remain to be characterized. Our findings indicate that sodium carbonate and sulfate-bearing fluids in subduction zone lithologies may play a role in efficiently transporting HFSE. A pertinent example of this could be the occurrence of carbonate and sulfate minerals in rutile-rich veins and fluid inclusions from high-pressure metamorphic rocks.