Rutile solubility and mobility in supercritical aqueous fluids

Abstract

Experimental and thermodynamic data and the apparent immobility of Ti under metamorphic conditions suggest that rutile is very insoluble in aqueous fluids at upper crustal conditions. New solubility measurements at 1.0–2.93 GPa and 800–1200°C show, however, that under certain pressure and temperature conditions rutile is quite soluble in H₂O. Solubilities were estimated from the measured weight loss of a single crystal equilibrated with a known mass of fluid in a piston cylinder apparatus. Measured solubilities in H₂O range from 0.15 wt% (wt loss crystal/wt fluid) at 2.93 GPa and 1000°C to 1.9% at 1.0 GPa and 1100°C. Solubility increases with increasing temperature and with decreasing pressure in a manner given by the following fit to the experimental data:

$$\log _{10} m_{Ti} = - 7049/T - (0.589* P)/T + 5.14$$

wherem _Ti is the molality of Ti in the fluid,T is in degress Kelvin andP is in MPa. The effect of fluid composition on rutile solubility was also examined at 1.0 GPa and 1000°C for H₂O-CO₂, 1m NaCl, and 1m HF fluids. Kesults suggest that solubility depends on the mole fraction of H₂O in the fluid but is independent of ionic strength and fluid pH. This behavior implies that Ti dissolves as the neutrally-charged hydrolysis product Ti(OH)₄. The free energy of this species was calculated for each set of experimental conditions. TheP-T dependence of rutile solubility suggests that aqueous fluids derived from subducted oceanic lithosphere would dissolve rutile or other Ti-rich minerals from the deepest portion of the mantle wedge and precipitate them at higher levels. Subsequent melting of the base of the mantle wedge would form HFSE-depleted IAB.