SPATIO-TEMPORAL CLUSTERING OF POST-CALDERA ERUPTIONS AT YELLOWSTONE: IMPLICATIONS FOR VOLCANIC HAZARDS AND PRE-ERUPTIVE MAGMA RESERVOIR CONFIGURATION

The Yellowstone Plateau Volcanic Field is well-known for its caldera-forming eruptions at 2.08 Ma, 1.30 Ma, and 0.631 Ma. While such catastrophic events pose Yellowstone’s greatest volcanic hazard, they are rare and represent its least likely eruption type. More commonly, Yellowstone erupts rhyolites as small to large volume lava flows (0.5 to 70 km³) within prior caldera structures (i.e., intracaldera). Since formation of Yellowstone caldera 631 ka, at least 27 intracaldera rhyolite eruptions occurred, thus understanding the frequency and duration of intracaldera eruptions is essential for characterizing volcanic hazards currently posed by Yellowstone. We present new ⁴⁰Ar/³⁹Ar eruption ages for the most recent episode of intracaldera rhyolitic volcanism at Yellowstone, the Central Plateau Member rhyolites, which consists of 21 eruptions from ~160 ka to ~70 ka. These eruptions occurred along two linear vent zones within Yellowstone caldera that appear to be extensions of extracaldera Basin and Range faults. The Central Plateau Member rhyolites erupted in 5 brief clusters with group mean ages of 160.3±1.0 ka (2σ), 150.5±1.8 ka, 111.2±1.0 ka, 104.1±0.8 ka, and 70.8±0.7 ka. Rhyolites within each group have indistinguishable ⁴⁰Ar/³⁹Ar ages suggesting brief eruption durations. Deposits of each group are also spatially clustered, with only one of the two linear vent zones being active during an eruption group. Using the nine 160 ka rhyolites as a case study, we apply paleomagnetic and geochemical analyses to interrogate the duration of these eruption groups and their pre-eruptive magma reservoir configuration. Paleomagnetic analyses suggest that the nine rhyolite eruptions ca. 160 ka spanned no more than 400 years. Glass and sanidine compositions of the 160 ka rhyolites are nearly identical suggesting they sourced from a large (~130 km³), interconnected melt-rich body within Yellowstone’s otherwise crystal-rich magma reservoir. These characteristics differ from intracaldera eruptions near the caldera ring fracture system, which tend to be compositionally disparate with less temporal clustering. Thus, a relationship may exist between eruption recurrence, the structures controlling volcanic vent location, and the positions within the caldera system that are important for hazard preparedness.