Voluminous low δ18O magmas in the late Miocene Heise volcanic field, Idaho:: Implications for the fate of Yellowstone hotspot calderas

We report oxygen isotope compositions of phenocrysts and U-Pb ages of zircons in four large caldera-forming ignimbrites and postcaldera lavas of the Heise volcanic field, a nested caldera complex in the Snake River Plain, that preceded volcanism in Yellowstone. Early eruption of three normal delta O-18 voluminous ignimbrites with = 6.4%o and delta O-18(zircon) = 4.8%, started at Heise at 6.6 Ma, and was followed by a 2%o-3%o O-18 depletion in the subsequent 4.45 Ma Kilgore caldera cycle that includes the 1800 km(3) Kilgore ignimbrite, and post-Kilgore intracaldera lavas with delta O-18(quartz) = 4.3% and delta O-18(zircon) = 1.5%. The Kilgore ignimbrite represents the largest known low-delta O-18 magma in the Snake River Plain and worldwide. The post-Kilgore low delta O-18 volcanism likely represents the waning stages of silicic magmatism at Heise, prior to the reinitiation of normal delta O-18 silicic volcanism 100 km to the northeast at Yellowstone. The occurrence of low delta O-18 magmas at Heise and Yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex. Sudden shifts in delta O-18 of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low delta O-18 signature from mantle or crustal sources. Instead, delta O-18 age trends indicate progressive remelting of low delta O-18 hydrothermally altered intracaldera rocks of previous eruptions. This trend may be generally applicable to older caldera complexes in the Snake River Plain that are poorly exposed.