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

Watts, Kathryn; Bindeman, Ilya; Schmitt, Axel

A voluminous (>600 km^3) and long-lived (~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]18O 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 ~520 and 470 ka, exhibit extreme age and oxygen isotopic heterogeneity, requiring derivation from individual parcels of low-[delta]18O melts. We find a progressive increase in zircon homogeneity for rhyolite eruptions from ~260 to 75 ka, with homogeneous low-[delta]18O zircon values of 2.7-2.8 0/00 that are in equilibrium with low-[delta]18O 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]18O melts starting at ~260 ka resulted in progressive homogenization, followed by differentiation and cooling up until the last rhyolite eruption ~75 ka, a trend that we interpret to be characteristic of a dying magma reservoir beneath the Yellowstone caldera.

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

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