NEW 40AR/39AR AGES OF THE BISHOP MOUNTAIN FLOW FROM THE YELLOWSTONE VOLCANIC FIELD

The super-eruptions of the Yellowstone Volcanic Field have been studied extensively, but little is known about the relationship between pre- and post-collapse effusive rhyolitic volcanism and the caldera-forming eruptions. Prior to the eruption of the Mesa Falls Tuff and the formation of the Henry’s Fork Caldera, five small-volume rhyolitic flows, referred to as the Big Bend Ridge Rhyolites (BBRR), erupted along what would later become the caldera boundary. The three youngest of these flows have been interpreted to either represent eruptions immediately preceding the Mesa Falls Tuff that signaled the start of the second volcanic cycle, or eruptions of a smaller, isolated system unrelated to the eventual Mesa Falls Tuff. In order to resolve this ambiguous relationship, samples of the Bishop Mountain rhyolite flow were collected based on published geologic maps. To improve upon the published K/Ar eruption age, sanidine phenocrysts were separated for new ⁴⁰Ar/³⁹Ar age single crystal incremental heating analyses, which constrains the eruptive age of the Bishop Mountain Flow with respect to the eruption of the Mesa Falls Tuff. One of the unique characteristics of these super-eruptions is the rapid accumulation and release or retreat of the magma that produces them. Because of this and the significant time difference (~150 ka) between the Bishop Mountain Flow and Mesa Falls Tuff, it is difficult to postulate a relationship between these two erupted magmas. With this data, and forthcoming geochemical data and U-Pb zircon ages, we can constrain the chemical evolution of the Bishop Mountain Flow and other BBRR magmas through time and how they contribute to the wider development of the magmatic system that generates super-eruptions within the Yellowstone Volcanic Field.