Kathryn E. Watts

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 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.

Linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved Yellowstone-type supervolcanoes

The geological record contains evidence of volcanic eruptions that were as much as two orders of magnitude larger than the most voluminous eruption experienced by modern civilizations, the A.D. 1815 Tambora (Indonesia) eruption. Perhaps nowhere on Earth are deposits of such supereruptions more prominent than in the Snake River Plain–Yellowstone Plateau (SRP-YP) volcanic province (northwest United States). While magmatic activity at Yellowstone is still ongoing, the Heise volcanic field in eastern Idaho represents the youngest complete caldera cycle in the SRP-YP, and thus is particularly instructive for current and future volcanic activity at Yellowstone. The Heise caldera cycle culminated 4.5 Ma ago in the eruption of the ∼1800 km3 Kilgore Tuff. Accessory zircons in the Kilgore Tuff display significant intercrystalline and intracrystalline oxygen isotopic heterogeneity, and the vast majority are 18O depleted. This suggests that zircons crystallized from isotopically distinct magma batches that were generated by remelting of subcaldera silicic rocks previously altered by low-δ18O meteoric-hydrothermal fluids. Prior to eruption these magma batches were assembled and homogenized into a single voluminous reservoir. U-Pb geochronology of isotopically diverse zircons using chemical abrasion–isotope dilution–thermal ionization mass spectrometry yielded indistinguishable crystallization ages with a weighted mean 206Pb/238U date of 4.4876 ± 0.0023 Ma (MSWD = 1.5; n = 24). These zircon crystallization ages are also indistinguishable from the sanidine 40Ar/39Ar dates, and thus zircons crystallized close to eruption. This requires that shallow crustal melting, assembly of isolated batches into a supervolcanic magma reservoir, homogenization, and eruption occurred extremely rapidly, within the resolution of our geochronology (103–104 yr). The crystal-scale image of the reservoir configuration, with several isolated magma batches, is very similar to the reservoir configurations inferred from seismic data at active supervolcanoes. The connection of magma batches vertically distributed over several kilometers in the upper crust would cause a substantial increase of buoyancy overpressure, providing an eruption trigger mechanism that is the direct consequence of the reservoir assembly process.

Supereruptions of the Snake River Plain: Two-stage derivation of low-δ18O rhyolites from normal-δ18O crust as constrained by Archean xenoliths

Supereruptive volumes of low-δ 18 O rhyolites define the Snake River Plain–Yellowstone Plateau volcanic province, begging the question of the sources and the processes by which 18 O-depleted magmas are generated. New analyses show that Archean crustal xenoliths from the central and eastern Snake River Plain have normal-δ 18 O signatures of 6‰–9‰, precluding them as a low-δ 18 O source in the genesis of >10,000 km 3 of low-δ 18 O (δ 18 O 18 O rhyolites have variable crust (∼30%–50%) and mantle (∼50%–70%) proportions that are specific for each eruption. Low-δ 18 O rhyolites can be traced along a genetic array of mixing lines from normal-δ 18 O rhyolite end members to a low-δ 18 O (∼–1‰) source. The data support a two-stage magma genesis process, in which normal-δ 18 O rhyolites are generated by partial melting and hybridization of the crust by mantle-derived basalt, and low-δ 18 O rhyolites tap ∼20%–80% of hydrothermally altered portions of normal-δ 18 O rhyolitic rocks. This two-stage magma genesis process may be applicable to other caldera systems around the world for which the characteristic O isotope depletions are either less pronounced or undiscovered.

Revised Wonoka isotopic anomaly in South Australia and Late Ediacaran mass extinction

The global Late Ediacaran Shuram–Wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean, assuming that all Ediacaran carbonate is marine. Disregarding carbonate in South Australia shown here to be palaeosol or palaeokarst, the synchronous marine organic–carbonate excursion is only −8‰ for δ13C organic and −6‰ for δ13C carbonate, and lasted less than a million years. This revised magnitude and duration is comparable with perturbations across the Permian–Triassic boundary, and correlative with a global Late Ediacaran acritarch mass extinction. Like Permian–Triassic isotopic excursions, the revised organic–carbonate Wonoka excursion may also have been a greenhouse palaeoclimatic warm spike, which terminated valley incision and glacioeustatic drawdown during the mid-Ediacaran Fauquier Glaciation, and preceded chill of the Late Ediacaran Billy Springs Glaciation. Supplementary material: Measured sections and tables of mineral and grain-size proportions, major element and stable isotope analyses are available at www.geolsoc.org.uk/SUP18756.