CRYSTAL-SCALE RECORD OF SILICIC MAGMA EVOLUTION BENEATH YELLOWSTONE CALDERA

Despite the immensity and environmental impacts of silicic volcanism from the Yellowstone hotspot, little is known about the magmatic evolution that leads to these voluminous eruptions. The high-silica rhyolites of the Central Plateau Member (CPM) at Yellowstone caldera erupted over an ~100 k.y. interval and provide sequential “snapshots” in space and time into an evolving subcaldera reservoir. To quantify the thermal and compositional history of magma in the CPM reservoir, we have performed crystal-scale geochronology and thermometry using ion and electron microprobe analyses of major and accessory minerals from several CPM lavas erupted at intervals on the order of 10³ years, as well as trace element analyses of groundmass glasses. With decreasing eruption age, glasses contain higher Rb, Y, Nb, U, LREE and Th, and lower Eu, Sr, and Ba. Clinopyroxene and sanidine phenocrysts are more evolved with decreasing eruption age. However, most individual sanidine and clinopyroxene display reverse compositional zoning. Temperatures for CPM quartz from ion probe TitaniQ-thermometry (with aTiO₂ calculated from groundmass and inclusion glasses) range from ~800-900°C. In general, quartz rims yield lower temperatures (by ~10-20°) than cores, although quartz from a single ~100 ka rhyolite contains rims yielding higher (up to 40°) temperatures than cores. Zircon trace element concentrations mirror the trace element co-variations of groundmass glasses. Coupled Ti-in-zircon thermometry and ²³⁸U-²³⁰Th and U-Pb geochronology reveals that the youngest CPM zircons yield the lowest crystallization temperatures. In general, zircon rims yield temperatures that are ~10-20° lower than their cores. Taken together, the secular variation of mineral and glass compositions indicate that rhyolitic magma in the CPM reservoir evolved via crystal-melt fractionation to generally cooler and more-evolved compositions. Periodic episodes of reheating and/or magma mixing led to reverse zoning in sanidine and clinopyroxene. Although explosive volcanism characterized ≤10% of CPM eruptions, the cooling-induced differentiation and punctuated reheating/magma mixing recorded by CPM crystals may characterize the magmatic evolution that leads to voluminous rhyolitic eruptions from calderas along the Yellowstone hotspot trace.