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Voluminous low delta (super 18) O magmas in the late Miocene Heise volcanic field, Idaho; implications for the fate of Yellowstone Hotspot calderas



Voluminous low delta (super 18) O magmas in the late Miocene Heise volcanic field, Idaho; implications for the fate of Yellowstone Hotspot calderas



Geology (Boulder) 35(11): 1019-1022



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

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