Reconstructing a Snake River Plain 'super-eruption' via compositional fingerprinting and high-precision U/Pb zircon geochronology

Despite the potential hazards posed by the largest explosive eruptions, so-called 'super-eruptions', their recurrence rate remains poorly constrained. The younger portion of the Yellowstone-Snake River Plain province is well known for large-scale explosive volcanism; however, the older history within the Snake River Plain remains poorly known and partially obscured by later basaltic volcanism. To address this, we characterised the mineral cargo of four widely spaced rhyolitic ignimbrites from the margins of the Snake River Plain that illustrate a strong similarity revealed in compositions of bulk geochemistry, major phases (e.g., pyroxene and ilmenite), and radiogenic isotopes. To test for the synchronous origin and eruption of these deposits, we used a tandem in-situ and isotope dilution method for U/Pb geochronology of zircon crystals. The youngest sampling of zircons from all four samples is equivalent in age, and defines a pooled weighted mean U-238/Pb-206 age of 11.030 +/- 0.006 (MSWD=1.44, n=24). These results reveal an event with a conservatively estimated erupted volume similar to 1470 km(3), of similar magnitude to the largest explosive events from Yellowstone. Widely dispersed ashfall deposits found across the western portions of North America with geochemical affinities to the Snake River Plain province suggest that other voluminous ignimbrites are awaiting discovery. The improved ability to correlate deposits of an individual eruption shown by this and other recent studies implies that 'super' eruptive events are more common than previously thought.