Magma storage and evolution of the most recent effusive and explosive eruptions from Yellowstone Caldera

Abstract

Between 70 and 175 ka, over 350 km³ of high-silica rhyolite magma erupted both effusively and explosively from within the Yellowstone Caldera. Phenocrysts in all studied lavas and tuffs are remarkably homogenous at the crystal, eruption, and caldera-scale, and yield QUILF temperatures of 750 ± 25 °C. Phase equilibrium experiments replicate the observed phenocryst assemblage at those temperatures and suggest that the magmas were all stored in the upper crust. Quartz-hosted glass inclusions contain 1.0–2.5 % H₂O and 50–600 ppm CO₂, but some units are relatively rich in CO₂ (300–600 ppm) and some are CO₂-poor (50–200 ppm). The CO₂-rich magmas were stored at 90–150 MPa and contained a fluid that was 60–75 mol% CO₂. CO₂-poor magmas were stored at 50–70 MPa, with a more H₂O-rich fluid ($X_{{{\text{CO}}_{2} }}$ = 40–60 %). Storage pressures and volatiles do not correlate with eruption age, volume, or style. Trace-element contents in glass inclusions and host matrix glass preserve a systematic evolution produced by crystal fractionation, estimated to range from 36 ± 12 to 52 ± 12 wt%. Because the erupted products contain <10 wt% crystals, crystal-poor melts likely separated from evolving crystal-rich mushes prior to eruption. In the Tuffs of Bluff Point and Cold Mountain Creek, matrix glass is less evolved than most inclusions, which may indicate that more primitive rhyolite was injected into the reservoir just before those eruptions. The presence and dissolution of granophyre in one flow may record evidence for heating prior to eruption and also demonstrate that the Yellowstone magmatic system may undergo rapid changes. The variations in depth suggest the magmas were sourced from multiple chambers that follow similar evolutionary paths in the upper crust.