A newly calibrated plagioclase-liquid hygrometer for rhyolites

Rhyolite is the most differentiated silicate magma type on Earth and makes up some of the largest explosive eruptions (100-1000's km3), including those at Yellowstone and Long Valley calderas. Understanding the origin and evolution of large-volume rhyolitic magmatic systems is of considerable interest because their formation must fundamentally re-constitute and differentiate continental crust. The mineral phases in rhyolites often provide a rich opportunity to examine pre-eruptive temperatures, oxidation states, and melt water concentrations, as well as time scales for melt accumulation in the upper crust. Given the wealth of information that can be derived from various mineral phases in rhyolites, it is perhaps surprising that so few phase-equilibrium experiments exist for natural high-silica rhyolite melt compositions. The limitation in the experimental data set for rhyolites was a problem encountered by Lange et al. (2009) during their calibration of the plagioclase-liquid hygrometer. Available high-quality experiments (e.g., those with glass totals, including H2O, ≤ 97%) were restricted to those with plagioclase ≥ An37. Results from the experimental study of Couch et al. (2003) on a low-SiO2 rhyolite (71.4 wt%) were included in the Lange et al. (2009) calibration, whereas experiments on a rhyolite (75.2 wt% SiO2) from a more recent study (Tomiya et al., 2010) were published afterwards. Therefore, application of the 2009 plagioclase-liquid hygrometer to rhyolites with sodic plagioclase (<An35) has required an extrapolation beyond the calibration. Although melt water concentrations can be determined from trapped melt inclusions in quartz, not all rhyolites contain quartz phenocrysts and it would be highly useful to have the plagioclase-liquid hygrometer well calibrated for rhyolites. Moreover, at fluid-saturated, shallow crustal conditions (100-250 MPa) plagioclase often saturates at higher pressures than quartz in many rhyolites, and therefore the plagioclase-liquid hygrometer has the potential to record higher melt water concentrations than melt inclusions entrapped in quartz, if crystallization is occurring under fluid-saturated conditions. To address this issue, Waters et al. (2012) conducted a series of water-saturated phase equilibrium experiments from 300-100 MPa and 750-900°C on three rhyolite and rhyodacite samples with phenocrysts of sodic plagioclase (An9-50). Their experimental results, along with those from the literature on both rhyolite and rhyodacite (e.g., Larsen, 2006; Coombs and Gardner, 2002; Tomiya et al., 2010), have been used to re-calibrate the plagioclase-liquid hygrometer. When the new hygrometer is applied to the Bishop Tuff (early and late samples with An13 and An24, respectively), it predicts melt water concentrations of 6.6 and 4.6 wt%, respectively. Although this is slightly higher than the concentrations found in quartz melt inclusions (5.7 and 4.1 wt% H2O; Wallace et al., 1999), this may reflect saturation of plagioclase prior to quartz during degassing-induced crystallization.