Silicic magma systems are of great scientific interest and societal importance owing to their role in the evolution of the crust and the hazards posed by volcanic eruptions. MELTS is a powerful and widely used tool to study the evolution of magmatic systems over a wide spectrum of compositions and conditions. However, the current calibration of MELTS fails to correctly predict the position of the quartz + feldspar saturation surface in temperature, pressure and composition space, making it unsuitable to study silicic systems. We create a modified calibration of MELTS optimized for silicic systems, dubbed rhyolite-MELTS, using early erupted Bishop pumice as a reference. Small adjustments to the calorimetrically determined enthalpy of formation of quartz and of the potassium end-member of alkali feldspar in the MELTS calibration lead to much improved predictions of the quartz + feldspar saturation surface as a function of pressure. Application of rhyolite-MELTS to the Highland Range Volcanic Sequence (Nevada), the Peach Spring Tuff (Arizona-Nevada-California), and the late-erupted Bishop Tuff (California), using compositions that vary from trachydacite to high-silica rhyolite, shows that the calibration is appropriate for a variety of fluid-bearing silicic systems. Some key observations include the following. (1) The simulated evolutionary paths are consistent with petrographic observations and glass compositions; further work is needed to compare predicted and observed mineral compositions. (2) The nearly invariant nature of silicic magmas is well captured by rhyolite-MELTS; unusual behavior is observed after extensive pseudo-invariant crystallization, suggesting that the new calibration works best for relatively small (i.e. <50 wt %) crystallization intervals, comparable with what is observed in volcanic rocks. (3) Our success with rhyolite-MELTS shows that water-bearing systems in which hydrous phases do not play a critical role can be appropriately handled; simulations are sensitive to initial water concentration, and although only a pure-H 2O fluid is modeled, suitable amounts of water can be added or subtracted to mimic the effect of CO 2 in fluid solubility. Our continuing work on natural systems shows that rhyolite-MELTS is very useful in constraining crystallization conditions, and is particularly well suited to explore the eruptive potential of silicic magmas. We show that constraints placed by rhyolite-MELTS simulations using late-erupted Bishop Tuff whole-rock and melt inclusion compositions are inconsistent with a vertically stratified magma body. © The Author 2012. Published by Oxford University Press. All rights reserved.
CITATION STYLE
Gualda, G. A. R., Ghiorso, M. S., Lemons, R. V., & Carley, T. L. (2012). Rhyolite-MELTS: A modified calibration of MELTS optimized for silica-rich, fluid-bearing magmatic systems. Journal of Petrology, 53(5), 875–890. https://doi.org/10.1093/petrology/egr080
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