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Multiple-phase geobarometry using Rhyolite-MELTS

Sarah L. Smithies 1, Guilherme A. R. Gualda2, Lydia J. Harmon3

  • Affiliations:  1School of Earth and Environment, University of Canterbury, Christchurch, New Zealand; 2Department of Earth and Environmental Sciences, Vanderbilt University, TN, USA; 3 Department of Geology, Occidental College, CA, USA 

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 47

  • Programme No: 1.7.34

  • Theme 1 > Session 7

Abstract

The quartz + feldspar Rhyolite-MELTS geobarometer has been applied to rhyolites worldwide and has proved a useful tool for estimating equilibration pressures. However, it is limited to magma saturated in quartz (mostly, rhyolite). Here, we show how the principles of the Rhyolite-MELTS geobarometer can be applied to estimate the saturation pressure of other phases (e.g. pyroxene and magnetite), estimate oxygen fugacity (fO2), and infer the equilibrium mineral assemblage. This extends the usefulness of the Rhyolite-MELTS geobarometer to a broader range of magmatic compositions. We show examples of (1) storage pressure calculations from quartz-absent rhyodacites to rhyolites from Puyehue-Cordón Caulle, Chile; and (2) equilibration between extracted rhyolitic melt compositions and unknown mush mineral assemblages from the Taupō Volcanic Zone, New Zealand. In both cases, we find orthopyroxene + plagioclase ± quartz pressure solutions that agree with independent pressure and fO2 estimates. Orthopyroxene saturation is sensitive to fO2, so independent fO2 constraint is necessary for calculating plagioclase + orthopyroxene pressures. Alternatively, magnetite is also sensitive to fO2 so searching within fO2 space for plagioclase + orthopyroxene + magnetite pressures can give both pressure and fO2 estimates. These methods are potentially useful for inferring the mineral assemblage a melt composition equilibrated with. Our methods show how Rhyolite-MELTS geobarometry does not need to be limited to three phases. Multiple saturation of a higher number of phases can (1) give further constraints on intensive parameters; (2) yield pressure estimates with smaller uncertainties; and (3) help determine mineral assemblages that equilibrated with a given melt composition.