Rutile solubility in albite-H 2O and Na 2Si 3O 7-H 2O at high temperatures and pressures by in-situ synchrotron radiation micro-XRF

Abstract

Rutile is an important mineral host for high-field strength elements, so its solubility in geologic fluids at high pressure and temperature plays an important role in the crustal and mantle processes that control the terrestrial cycling of these elements. However, experimental measurements of rutile solubility are in conflict by a factor of more than 100 at most studied conditions. We performed new measurements of rutile solubility in H 2O-albite and H 2O-Na 2Si 3O 7 (NS3) fluids by in-situ synchrotron radiation X-ray fluorescence spectroscopy using modified Bassett-type hydrothermal diamond-anvil cells. Minimum detection limits were 1.9 and 2.3 ppm Ti by weight for the two cells. Three albite-H 2O experiments at starting bulk compositions of 2.7, 6.7 and 10.3 wt.% albite involved spectral acquisition at rutile saturation in the presence of albite crystals, melt, or a single homogeneous fluid phase; after accounting for the additional phases, corrected fluid compositions were 0.6 to 7.5 wt.% dissolved silicate over the run conditions. At ≤ 2.7 wt.% albite, rutile dissolution rate was slow and steady state was not achieved at 600–800 °C; however, at higher dissolved albite contents, constant solubility with time was observed. Rutile solubilities in the presence of a single fluid phase at 700 °C, 0.79 GPa, 5.4 wt.% albite, and at 800 °C, 1.10 GPa, 6.7 wt.% albite, were 37 ± 2 and 156 ± 6 ppm, respectively. These data agree with results acquired using hydrothermal piston-cylinder methods with long run times and suppression of new crystal growth, but not with data derived from visual observation in hydrothermal diamond-anvil cells. This discrepancy is likely due to lack of equilibrium in the latter approach. Two experiments in 10 and 30 wt.% NS3 at 660–800 °C, 0.5 ± 0.1 GPa, show extreme concentration-dependent enhancement of rutile solubility to ∼ 4500 ppm. The data indicate a strong positive correlation between rutile solubility and Na/Al. Because (Na + K)/Al is likely to be greater than unity in aqueous fluids at high pressure and temperature due to incongruent dissolution of albite and micas, the increase in rutile solubility along the albite-NS3 join points to the possibility of significant Ti transport by silicate-bearing aqueous fluids in the lower crust and upper mantle.