TRACKING MAGMA EVOLUTION WITHIN THE YELLOWSTONE VOLCANIC FIELD USING INTEGRATED 40AR/39AR DATING AND ZIRCON GEOCHEMISTRY

Recent advancements in ⁴⁰Ar/³⁹Ar multi-collector instrumentation and astronomical calibration of widely-used neutron fluence monitors coupled with detailed morphological and chemical characterization of zircon crystals has helped improve our understanding of Late Cenozoic magmatic processes over fine temporal scales. We apply these methods to rhyolite units within the Yellowstone Volcanic Field in order to re-evaluate volcanic stratigraphy and interpret timescales of magmatic evolution and rejuvenation. This work focuses on six small volume rhyolite flows, domes, and tuffs known as the Big Bend Ridge Rhyolites, which erupted along the ring-fracture margins of the Island Park and Henry’s Fork calderas. These units were previously correlated with either the caldera-forming eruptions of the 2.08 Ma Huckleberry Ridge or 1.30 Ma Mesa Falls Tuffs based on whole rock geochemical affinities and imprecise K/Ar ages. The lack of precise eruption ages, along with obscured field relationships, resulted in stratigraphic ambiguity thus inhibiting our interpretations of rejuvenation timescales within this part of the volcanic field. Here we present a revised eruptive sequence for the Big Bend Ridge Rhyolite group based on new high-precision ⁴⁰Ar/³⁹Ar dating of each unit and use zircon petrochronology to establish potential genetic relationships between individual magma batches. Our results indicate that the extrusion of these rhyolites was significantly punctuated, with as much as 150 ka passing between effusive and caldera-forming eruptions. Further, morphological and multi-point in-situ trace element and thermometric data on hundreds of zircon grains capture a complex history of thermochemical evolution of these magmas, and are used to delineate the longevity of storage, differentiation, and rejuvenation. These data suggest relatively rapid timescales of magma assembly that preceded and followed the caldera-forming eruptions and significant recycling of previously crystallized material. Geochemical trends within large and small volume eruptions indicate that Yellowstone rhyolitic magmas follow similar evolutionary paths, yet the nature of the source area and chemical heterogeneity continue to be investigated.