Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS—Part 3: Application to the Peach Spring Tuff (Arizona–California–Nevada, USA)

Abstract

250 ± 50 MPa) and on glass SiO 2 (255-275 MPa). Amphibole geothermobarometry gives much wider pressure ranges (temperature-independent: ~65-300 MPa; temperature-dependent: ~75-295 MPa; amphibole-only: ~80-950 MPa); average Anderson and Smith (Am Mineral 80:549-559, 1995) + Blundy and Holland (Contrib Miner Petrol 104:208-224, 1990) or Holland and Blundy (Contrib Miner Petrol 116:433-447, 1994-Thermometer A, B) pressures are most similar to phase-equilibria results (~220, 210, 190 MPa, respectively). Crystallization temperatures determined previously with rhyolite-MELTS (742 °C), Zr-in-sphene (769 ± 20 °C), and zircon saturation (770-780 °C) geothermometry are similar, but temperatures from amphibole geothermometry (~450-955 °C) are notably different; the average Anderson and Smith + Holland and Blundy (1994-Thermometer B; ~710 °C) temperature is most consistent with previous estimates. The rhyolite-MELTS geobarometer effectively culls glass compositions affected by alteration or analytical issues; Peach Spring glass compositions that yield pressure estimates reveal a tight range of plausible Na 2 O and K 2 O contents, suggesting that low Na 2 O and high K 2 O contents of many Peach Spring samples are due to alteration. Use of altered whole-pumice compositions in rhyolite-MELTS simulations is likely the cause of the incorrect crystallization sequence reported previously for Peach Spring compositions. Using the rhyolite-MELTS geobarometer, we estimate a more realistic composition for Peach Spring Tuff high-silica rhyolite, and the calculated composition finds close matches with some analyzed rocks and results in the expected sequence of crystallization.Abstract Establishing the depths of magma accumulation is critical to understanding how magmas evolve and erupt, but developing methods to constrain these pressures is challenging. We apply the new rhyolite-MELTS phase-equilibria geobarometer-based on the equilibrium between melt, quartz, and two feldspars-to matrix glass compositions from Peach Spring Tuff (Arizona-California-Nevada, USA) high-silica rhyolite. We compare the results to those from amphibole geothermobarometry, projection of glass compositions onto the haplogranitic ternary, and glass SiO 2 geobarometry. Quartz + 2 feldspar rhyolite-MELTS pressures span a relatively small range (185-230 MPa), consistent with nearly homogeneous crystal compositions, and are similar to estimates based on projection onto the haplogranitic ternary Communicated by Gordon Moore. Electronic supplementary materialThe online version of this article (

Climbing the crustal ladder: Magma storage-depth evolution during a volcanic flare-up

“…Magma crystallization depths were calculated with the rhyolite-MELTS geobarometer ( 27 , 28 ) using the measured major-element glass compositions, as the pressures at which quartz and plagioclase are in equilibrium with a melt of the analyzed composition ( 27 ). A total of 88 of 144 compositions yielded crystallization pressures, consistent with previous applications of the rhyolite-MELTS geobarometer ( 10 , 27 , 29 , 30 ).…”

“…The quality of the resulting analysis was checked by analyzing glass synthesized in the laboratory using the U.S. Geological Survey reference powder RGM-1 ( 54 ); results by Pamukcu et al . ( 30 ), using the same instrument and conditions, demonstrate that our EDS analyses compare very well with reference values for RGM-1.…”

“…Magma crystallization depths can be calculated using the rhyolite-MELTS ( 28 ) geobarometer ( 27 , 29 , 30 ), by calculating the pressure at which an assemblage of quartz and feldspars last equilibrated with melt of composition equal to that of observed glass. We interpret these pressures as those at which crystal-poor, eruptible magma was stored and crystallized before eruption.…”

Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models

“…It been used successfully to investigate magma storage conditions in metaluminous and peralkaline systems (e.g. Fowler et al 2007;Fowler and Spera, 2010;Cannatelli, 2012;Pamukcu et al, 2015;Stock et al, 2016).…”

Hydrothermal Cooling as a Requirement for Short Storage of Silicic Magmas

“…(2012), Pamukcu, Gualda, Ghiorso, et al. (2015), and Gualda et al. (2018) in using rhyolite‐MELTS to model the change in enthalpy (total heat loss) during crystallization of a magma body at constant pressure.…”