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

Abstract

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