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

Abstract The quartz + feldspar rhyolite-MELTS phase-equilibrium geobarometer is a useful tool for calculating equilibration pressures of rhyolitic magmas. However, it is somewhat limited by typically 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 rhyolite-MELTS geobarometry results to constrain multiple-phase 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 these cases, orthopyroxene + plagioclase pressures can be used. However, orthopyroxene saturation pressure results are higher at lower modelled oxygen fugacity ( f O2 ). This can be resolved by modelling at independently constrained f O2 , or by modelling at a range of f O2 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 at f O2 values within error of the f O2 calculated from Fe-Ti oxides. If quartz saturation is uncertain, quartz + feldspar pressures are a maximum and pyroxene-bearing pressures at low f O2 are a minimum. For uncertain mineral assemblages, the coincidence of multiple phases (≥3) saturating together at reasonable f O2 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 f O2 .