• journal_title：Geology
• Contributor：Ilya N. Bindeman ; Kathryn E. Watts ; Axel K. Schmitt ; Lisa A. Morgan ; Pat W.C. Shanks
• Publisher：Geological Society of America
• Date：2007-
• Format：text/html
• Language：en
• Identifier：10.1130/G24141A.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：35
• issue：11
• firstpage：1019

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 δ¹⁸O voluminous ignimbrites with δ¹⁸O_quartz = 6.4‰ and δ¹⁸O_zircon = 4.8‰ started at Heise at 6.6 Ma, and was followed by a 2‰–3‰ δ¹⁸O depletion in the subsequent 4.45 Ma Kilgore caldera cycle that includes the 1800 km³ Kilgore ignimbrite, and post-Kilgore intracaldera lavas with δ¹⁸O_quartz = 4.3‰ and δ¹⁸O_zircon = 1.5‰. The Kilgore ignimbrite represents the largest known low-δ¹⁸O magma in the Snake River Plain and worldwide. The post-Kilgore low δ¹⁸O volcanism likely represents the waning stages of silicic magmatism at Heise, prior to the reinitiation of normal δ¹⁸O silicic volcanism 100 km to the northeast at Yellowstone. The occurrence of low δ¹⁸O magmas at Heise and Yellowstone hallmarks a mature stage of individual volcanic cycles in each caldera complex. Sudden shifts in δ¹⁸O of silicic magmas erupted from the same nested caldera complexes argue against any inheritance of the low δ¹⁸O signature from mantle or crustal sources. Instead, δ¹⁸O age trends indicate progressive remelting of low δ¹⁸O 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.