H2O in rhyolitic glasses and melts: Measurement, speciation, solubility, and diffusion

Dissolved H2O in silicate melts and glasses plays a crucial role in volcani c eruptions on terrestrial planets and affects glass properties and magma e volution. In this paper, major progress on several aspects of the H2O-melt (or glass) system is reviewed, consistency among a variety of data is inves tigated, discrepancies are evaluated, and confusion is clarified. On the in frared measurement of total H2O and species concentrations, calibration for a variety of glasses has been carried out at room temperature. The measure ments for H2O in rhyolitic glasses have undergone the most scrutiny, result ing in the realization that absorptivities for the near-infrared bands depe nd on total H2O content. Although the variation of the absorptivities does not seem to significantly affect the determination of total H2O, it does af fect the determination of molecular H2O and OH species concentrations. Cali bration of the infrared technique for H2O in rhyolitic glasses still needs much improvement, especially at high total H2O. Furthermore, it is now almo st certain that the molar absorptivities also depend on the measurement tem perature in in situ studies. Hence it will be necessary to carry out calibr ations in situ at high temperatures. On H2O speciation, results from two ex perimental approaches, the quench technique and the in situ technique, are very different, leading to controversy in our understanding of true speciat ion. A solution is presented to reconcile this controversy. It is almost ce rtain that the quench technique does not suffer from a quench problem, but interpretation of in situ results suffered from ignoring the dependence of the molar absorptivities on measurement temperature. Accurate calibration a t high temperatures is necessary for the quantitative application of the in situ technique to H2O speciation in silicate melts and glasses. On H2O sol ubility in silicate melts, recent experimental work has significantly expan ded the T-P range of solubility measurements, and recent solubility models fill a gap for predicting solubility for a wide range of melt compositions. I present a solubility modelfor rhyolitic and quasirhyolitic melts over a wide range of T and P (500 degrees-1350 degrees C, 0-8 kbar) by incorporati ng the role of speciation. The solubility model is able to recover the expe rimental solubility data and has extrapolative value, although the partial molar volume of H2O derived from the solubility mode differs from that deri ved from density measurements. On H2O diffusion, recent studies on H2O diff usion in a quasi-rhyolitic melt at 800 degrees-1200 degrees C, 0.5-5 kbar, and up to 7% total H2O not only provide important new diffusion data, but a re also challenging earlier understanding of H2O diffusion based on data in rhyolitic glasses at 400 degrees-550 degrees C, 1 bar, and 0.2-1.8% total H2O. A comparison between the earlier model and recent data is made. The re cent high-temperature diffusivities at total H2O less than or equal to 2% c an be predicted by the earlier model. However, at higher total H2O, the ear lier model fails. New work is under way to understand the diffusion mechani sms at high H2O contents.