Emergence of supervolcanism at Valles caldera as recorded by changes in oxygen fugacity and crystal cargo

Hydrous high-silica rhyolites at Valles caldera typically contain either fayalite (F-type) or biotite (B-type); two phases that are often stable at different oxygen fugacities (fO2). Interestingly, F-type rhyolites only erupted immediately before and during caldera-forming eruptions. Fayalite crystallization may likewise be associated with a shift in magma storage conditions from system perturbations that induce supervolcanism. We investigated how storage conditions (temperature, pressure, fO2) of F and B-type rhyolites differ through fO2 -buffered phase equilibrium experiments. Powdered samples of representative F and B-type rhyolites were run at fO2 conditions ranging from the manganosite-hausmannite (~nickel-nickel oxide +4 log units) to the quartz-fayalite-magnetite (QFM) buffer curves. Critically, both the F-type and B-type mineral assemblages were reproduced simply by varying fO2. The F-type mineral assemblage (quartz + sanidine + clinopyroxene + fayalite + Fe-Ti oxides) was only reproduced, with or without biotite, at QFM conditions. In contrast, the B-type assemblage (quartz + sanidine + biotite + Fe-Ti oxides ± clinopyroxene ± hornblende ± plagioclase) was reproduced without plagioclase and hornblende at fO2 ~NNO. Furthermore, storage pressures and temperatures show little difference between F and B-type rhyolites. Combining our experimental data with geothermometry and H2O-CO2 melt inclusion pressure estimates, both F and B-type storage is constrained between and 690-750°C and 50-200 MPa. Therefore, the only change in storage conditions between F and B-type rhyolites is a difference in fO2. The ephemeral nature of F-type rhyolite eruptions suggests that the normal state of the system is at higher fO2 (~NNO) conditions. System reduction may be driven by the flux of reducing fluids from the well documented recharge event that mixed in dacitic enclaves with the rhyolite that erupted as the Tshirege tuff during caldera formation. Recharge intensity evidently waned following caldera collapse, as recorded by the absence of any F-type rhyolites in the post-caldera eruption record. Fayalite versus biotite may therefore indicate the nature of system rejuvenation, with fayalite appearing only during intense recharge that may lead to supervolcanism.