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Voluminous low ¹⁸O magmas in the late Miocene Heise volcanic field, Idaho: Implications for the fate of Yellowstone hotspot calderas

¹ Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA
² Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, California 90095, USA
³ U.S. Geological Survey, Federal Center, Box 25046, MS 973, Denver, Colorado 80225, USA

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 ¹⁸O voluminous ignimbrites with ¹⁸O_quartz = 6.4 and ¹⁸O_zircon = 4.8 started at Heise at 6.6 Ma, and was followed by a 2–3 ¹⁸O depletion in the subsequent 4.45 Ma Kilgore caldera cycle that includes the 1800 km³ Kilgore ignimbrite, and post-Kilgore intracaldera lavas with ¹⁸O_quartz = 4.3 and ¹⁸O_zircon = 1.5. The Kilgore ignimbrite represents the largest known low-¹⁸O magma in the Snake River Plain and worldwide. The post-Kilgore low ¹⁸O volcanism likely represents the waning stages of silicic magmatism at Heise, prior to the reinitiation of normal ¹⁸O silicic volcanism 100 km to the northeast at Yellowstone. The occurrence of low ¹⁸O magmas at Heise and Yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex. Sudden shifts in ¹⁸O of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low ¹⁸O signature from mantle or crustal sources. Instead, ¹⁸O age trends indicate progressive remelting of low ¹⁸O 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.

Key Words: oxygen isotopes • zircon • U-Pb age • caldera • remelting • low ¹⁸O

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