Rutile solubility in hydrous rhyolite melts at 750–900 °C and 2 kbar, with application to titanium-in-quartz (TitaniQ) thermobarometry

Abstract

The solubility of rutile in water-saturated haplogranite melts with molar Al/(Na+K)-ratios ranging from 0.84 to 1.25 was determined at 750–900°C and 2kbar in cold-seal pressure vessel experiments. Due to the low diffusivity of Ti at these conditions a new method was developed to determine TiO2 solubility. In this method, glasses with TiO2 gradients were used as starting material, and after the experiments the TiO2 content was measured at the contact between rutile-bearing and rutile-free glass. The glasses were either directly equilibrated at the desired P–T conditions (i.e., crystallization experiments), or first treated at 50–150°C lower temperatures and then subjected to the desired conditions (i.e., dissolution experiments). The results obtained in crystallization and dissolution experiments agree well with each other, suggesting that equilibrium was attained.Rutile solubility in peralkaline melts strongly increases with temperature and the amount of excess alkalies according to the relation: logTiO2(wt%)=(1.8∗ΔANK-0.53)∗10,000/T-(12.8∗ΔANK-4.3)in which ΔANK is the deviation of the molar Al/(Na+K)-ratio from unity and T is given in Kelvin. Excess alumina does not seem to promote TiO2 solubility. For natural melt compositions it was found that if Ca, Mg and Fe are assumed to be 10 times less effective in promoting TiO2 solubility than excess Na and K, good fits to previous solubility data in silicic (⩾70wt% SiO2) melts at upper crustal pressures (⩽10kbar) are obtained.Application of this extended TiO2 solubility model to natural melt inclusions in quartz phenocrysts from five silicic volcanic systems returns TiO2 activities that are similar to those calculated with a previous experiment-based model and to those calculated from pairs of coexisting Fe–Ti-oxides, but are up to 2.9 times higher than those calculated with MELTS and rhyolite-MELTS. Pressures calculated from Ti concentrations in the host quartz using the most recent calibration of the TitaniQ thermobarometer are in good agreement with independent pressure estimates, suggesting that at upper crustal conditions this calibration is valid.