HIGH-TEMPERATURE SILICIC MAGMAS OF THE BRUNEAU-JARBIDGE ERUPTIVE CENTER FROM 12.7 – 8.0 MA, YELLOWSTONE HOTSPOT: QUARTZ AND PYROXENE THERMOMETRY

The ten units of the Cougar Point Tuff (CPT) and subsequent lavas of the Bruneau-Jarbidge eruptive center of the Yellowstone hotspot define an episode of exceptionally high-temperature, large-volume silicic magmatism that persisted from 12.7 to ~8.0 Ma. The titanium-in-quartz thermometer, with activity of TiO₂ estimated at 0.5, yields pre-eruptive magma temperatures that overlap with pyroxene temperatures. In the CPT, the temperature range within individual ashflow tuff units is typically 30-100°C as recorded by each thermometer. In some units (e.g. CPT V, VII, IX, XII, XVb), upper vitrophyres record higher temperatures than lower vitrophyres, whereas in others temperatures from upper and lower vitrophyres do not differ detectably. CPT units that are polymodal with respect to pyroxene and glass compositions provide evidence for simultaneous eruption of multiple discrete magmas, each with a distinctive temperature range based on pyroxene thermometry. In these units, core-to-rim variations of Ti in quartz phenocrysts (diameter =0.1-1.0 mm) and of major elements in pyroxene (diameter =0.1-0.7 mm) reveal individual grains that have experienced different thermal histories. Core-to-rim profiles in some crystals preserve the full spectrum of temperature variation observed within the entire unit, whereas others record modest but detectable normal or reverse zonation over 20-30°C, and some are unzoned. Some tuffs and all lavas are homogeneous and unimodal with respect to glass and/or pyroxene compositions. In these units, single crystals of quartz and pyroxene typically display limited zonation but collectively describe overall temperature variations of 25-125°C as follows: CPT IX (915-990°C), CPT XIII (820-940°C), and for the lavas, Mary's Creek (880-1005°C), Cedar Tree (905-985°C), Long Draw (845-900°C), Bruneau-Jasper (860-940°C), Sheep Creek (930-1000°C), and Dorsey Creek (905-985°C), the youngest of the lavas. In the tuffs, some instances of intra-crystal compositional and thermal zonation are consistent with possible transfer of crystals among coexisting, discrete magma volumes. In unimodal lavas, the thermometry results provide evidence for reservoirs that are nonetheless heterogeneous with respect to their thermal structures to nearly the same extent as the polymodal tuffs.