Formation of low-delta super 18 O rhyolites after caldera collapse at Yellowstone, Wyoming, USA.

Abstract

We present a new model for the genesis of low-delta super 18 O rhyolites of the Yellowstone caldera based on analyses of zircons and individual quartz phenocrysts. Low-delta super 18 O rhyolites were erupted soon after the massive caldera-forming Lava Creek Tuff eruption 602 ka, approximately 1000 km super 3 and contain xenocrysts of quartz and zircon inherited from precaldera rhyolites. These zircons are isotopically zoned and out of equilibrium with their host low-delta super 18 O melts and quartz. Diffusion modeling predicts that magmatic disequilibria of oxygen isotopes persists for as much as tens of thousands of years following nearly total remelting of the hydrothermally altered igneous roots of the depressed cauldron, in which the alteration-resistant quartz and zircon initially retained their delta super 18 O values. These results link melting to caldera collapse, rule out rapid or catastrophic magma-meteoric water interaction, and indicate wholesale melting rather than assimilation or partial melting.

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