Magma storage and evolution of the Central Plateau Member Rhyolites, Yellowstone caldera, USA

Abstract:

We use volatile and trace element contents from quartz-hosted melt inclusions to place constraints on the pre-eruptive storage and evolution of 6 lavas and 3 tuffs from the Central Plateau Member Rhyolites, Yellowstone. Overall, inclusions contain 1.0-2.5% H₂O and 50-600 ppm CO₂. Individual units have more restricted volatile contents, and define regions in H₂O and CO₂ space with internal variability of ±0.5 wt.% and ±100 ppm, respectively. Water contents in both lavas and pyroclastic units are similar, but CO₂ contents vary and can be used to separate the units as CO₂-rich and CO₂-poor. CO₂-rich units contain 300 to 600 ppm CO₂ (Tuff of Bluff Point, Pitchstone Plateau, Trischmann Knob, Buffalo Lake, Summit Lake). CO₂-poor units contain 50 to 200 ppm CO₂ (Solfatara Plateau, West Yellowstone, Tuff of Cold Mountain Creek, unnamed tuff on Douglas Knob). Volatile contents do not correlate with eruption age, volume, or style. Inclusions from individual units contain incompatible trace-element concentrations that range from primitive to evolved. Incompatible elements within each unit increase systematically from primitive inclusions (units differ by <20%), to more evolved inclusions, and finally to highly evolved matrix. The inclusions preserve a systematic evolution produced by crystal fractionation, which we estimate to range from 38±8 wt.% to 54±11 wt.%. Inclusions from the Tuffs of Bluff Point and Cold Mountain Creek display similar evolution, except that matrix glass in those tuffs is less evolved than the inclusions. We infer the magmas were volatile-saturated at depth because H₂O and CO₂ do not correlate positively with incompatible elements. If true, then the CO₂-rich magmas were stored at 90 to 150 MPa and contained a fluid that was 60-75 mol.% CO₂, whereas CO₂-poor magmas were stored at 50 to 70 MPa and contained a more H₂O-rich fluid (X_CO2 = 40-60%). The variations in pressure/depth suggest the magmas were at least temporarily stored as separate batches that followed similar evolutionary paths in the shallow crust.