Water solubility in trachytic melts

New data on water solubility in trachytic melts at pressures from 20 to 200 MPa and 850 °C are reported. Three trachytes, which differ mainly in Na/K ratio, were studied. The glasses obtained from water saturated experiments were analysed using both infrared spectroscopy (FTIR) and Karl Fischer Titration (KFT). The independent KFT data on total water contents were used to calculate linear molar absorptivity values for infrared bands at ∼4500 cm−1 (hydroxyl groups) and ∼5200 cm−1 (molecular water). Water contents in the three trachytic melts studied are higher than those observed in rhyolitic melts at similar pressures and approach 8 wt.% H2O at 200 MPa. Increasing Na2O content in the trachyte compositions investigated is found to correlate positively with melt water content, similar to the effect of Na2O in haplogranitic systems. The new solubility data, when compared with two theoretical models for water solubility [Papale, P., 1997. Thermodynamic modeling of the solubility of H2O and CO2 in silicate liquids. Contrib. Mineral. Petrol. 126, 237–251; Moore, G., Vennemann, T., Carmichael, I.S.E., 1998. An empirical model for the solubility of H2O in magmas to 3 kilobars. Am. Mineral. 83, 36–42], show that these models tend to systematically underestimate water solubilities in trachytic melts, probably because much of the available solubility data concern relatively alkali-poor, calc-alkaline melt compositions.