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

Abstract

Abstract. Dissolved H20 in silicate melts and glasses plays a crucial role in volcanic eruptions on terrestrial planets and affects glass properties and magma evolution. In this paper, major progress on several aspects of the H20-melt (or glass) system is reviewed, consistency among a variety of data is investigated, discrepancies are evaluated, and confusion is clarified. On the infrared measurement of total H20 and species concentrations, calibration for a variety of glasses has been carried out at room temperature. The measurements for H20 in rhyolitic glasses have undergone the most scrutiny, resulting in the realization that absorptivities for the near-infrared bands depend on total H20 content. Although the variation of the absorptivities does not seem to significantly affect the determination of total H20 , it does affect the determination of molecular H20 and OH species concentrations. Calibration of the infrared technique for H20 in rhyolitic glasses still needs much improvement, especially at high total H20. Furthermore, it is now almost certain that the molar absorptivities also depend on the measurement temperature in in situ studies. Hence it will be necessary to carry out calibrations in situ at high temperatures. On H20 speciation, results from two experimental approaches, the quench technique and the in situ technique, are very different, leading to controversy in our understanding of true speciation. A solution is presented to reconcile this controversy. It is almost certain 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 at high temperatures is necessary for the quantitative application of the in situ technique to H20 speciation in silicate melts and glasses. On H20 solubility in silicate melts, recent experimental work has Significantly expanded 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 model for rhyolitic and quasirhyolitic melts over a wide range of T and P (500 ø-1350øC, 0-8 kbar) by incorporating the role of speciation. The solubility model is able to recover the experimental solubility data and has extrapolative value, although the partial molar volume of H20 derived from the solubility model differs from that derived from density measurements. On H20 diffusion, recent studies on H20 diffusion in a quasi-rhyolitic melt at 800ø-1200øC, 0.5-5 kbar, and up to 7% total H20 not only provide important new diffusion data, but are also challenging earlier understanding of H20 diffusion based on data in rhyolitic glasses at 400ø-550øC, 1 bar, and 0.2-1.8% total H20. A comparison between the earlier model and recent data is made. The recent high-temperature diffusivities at total H20 -< 2% can be predicted by the earlier model. However, at higher total H20 , the earlier model fails. New work is under way t...

Solubility of water in lunar basalt at low pH2O

“…where square brackets indicate mole fractions calculated on a single oxygen basis (e.g., Stolper 1982a; Zhang et al 2007) (Stolper 1982a;Zhang 1999).…”

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

“…Magmatic glasses typically contain both primary (magmatic) and secondary (meteoric) water (Newman et al, 1986;Tuffen et al, 2010;Denton et al, 2012), the latter recording hydration by external water, which diffused into the glass post eruption, plus water adsorbed during sample preparation (Giachetti et al, 2015). It has been assumed previously that, at the low water concentrations characteristic of obsidian (b 1 wt.%), water dissolves in the glass predominantly as hydroxyl groups OH − , as a result of equilibrium speciation (Dixon et al, 1995;Zhang, 1999). However, speciation is dependent on cooling rate, and when lava cools slowly equilibrium speciation cannot be maintained and both OH − and molecular water H 2 O m will contribute to total water content (Dingwell & Webb, 1990;Zhang et al, 1997;Ihinger et al, 1999;Zhang et al, 2000).…”

Infrared Spectroscopy and Stable Isotope Geochemistry of Hydrous Silicate Glasses

“…The first thing to notice is that the fit for a constant (i.e., concentration independent) D(H 2 O) matches the data poorly. That a simple error function fits hydration profiles in silicate melts and glasses poorly is well known and is the basis for our understanding that D ( which water is assumed to be dissolved as both mobile water molecules (which are assumed to have a constant diffusion coefficient) and hydroxyl groups (which are assumed to be immobile) where the concentrations of the two species are related via the reaction H 2 O mol + O 2-= 2OH, which has a constant equilibrium constant, K (Behrens and Nowak, 1997;Zhang, 1999;Zhang and Behrens, 2000;. Figure 2) confirms that this is a robust feature of diffusion of water in silicate melts, extending from highly polymerized compositions such as rhyolite (in which this feature was first demonstrated by Shaw (1974)) to the relatively depolymerized basaltic composition shown here.…”