Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 5: Principles for multiple-phase geobarometry with examples from plagioclase + orthopyroxene ± quartz ± magnetite assemblages

The quartz + feldspar rhyolite-MELTS phase-equilibrium geobarometer is a useful tool for calculating equilibration pressures of rhyolitic magmas. However, it is limited by requiring quartz saturation in magma. Here, we employ the principles from Parts 1-4 to move beyond modeling a specific mineral assemblage. We demonstrate methods for carefully interpreting the rhyolite-MELTS geobarometry results to constrain equilibration pressure in quartz-undersaturated dacites to rhyolites, and where quartz saturation is uncertain. We show examples of storage pressure calculations from quartz-absent rhyodacites to rhyolites from Puyehue-Cordón Caulle (PCC), Chile; and examples of equilibration between extracted rhyolitic melt compositions and unknown mush mineral assemblages from the Taupō Volcanic Zone, New Zealand. In this case, orthopyroxene + plagioclase pressures can be used. However, orthopyroxene saturation pressure results are higher at lower modelled oxygen fugacity. This can be resolved by modelling at independently constrained fO2, or by modelling at a range of fO2 to search for orthopyroxene + magnetite + feldspar co-saturation. We show that orthopyroxene + magnetite + feldspar pressures for PCC are consistent with results from other geobarometers and occur within error of the fO2 calculated from Fe-Ti oxides. If quartz saturation is uncertain, quartz + feldspar pressures are a maximum and pyroxene-bearing pressures at low fO2 are a minimum. For uncertain mineral assemblages, the coincidence of multiple phases (3) saturating together at reasonable fO2 could be used to infer the equilibrium mineral assemblage. Careful inspection of rhyolite-MELTS geobarometry results therefore gives nuanced information about equilibration pressure, mineral assemblage, and fO2.