Crystal scale anatomy of a dying supervolcano: an isotope and geochronology study of individual phenocrysts from voluminous rhyolites of the Yellowstone caldera

A voluminous (> 600 km(3)) and long-lived (similar to 520-75 ka) phase of rhyolitic eruptions followed collapse of the Yellowstone caldera 640 ka. Whether these eruptions represent a dying cycle, or the growth of a new magma chamber, remains an important question. We use new U-Th zircon ages and delta O-18 values determined by ion microprobe, and sanidine Pb isotope ratios determined by laser ablation, to investigate the genesis of voluminous post-caldera rhyolites. The oldest post-caldera rhyolites, erupted between similar to 520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-delta O-18 melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from similar to 260 to 75 ka, with homogeneous low-delta O-18 zircon values of 2.7-2.8aEuro degrees that are in equilibrium with low-delta O-18 host melts for the majority of the youngest eruptions. New sanidine Pb isotope data define separate arrays for post-caldera rhyolites and the caldera-forming tuffs that preceded them, indicating that they were not sourced from a mushy Lava Creek Tuff batholith that remained after caldera collapse. Rather, our new age and isotopic data indicate that the post-caldera rhyolites were generated by remelting of a variety of intracaldera source rocks, consisting of pre-Lava Creek Tuff volcanic and plutonic rocks and earlier erupted post-Lava Creek Tuff rhyolites. Batch assembly of low-delta O-18 melts starting at similar to 260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption similar to 75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera.