NEW 40AR/39AR SANIDINE DATES AND ZIRCON IMAGING FOR THE GREEN CANYON FLOW, YELLOWSTONE VOLCANIC FIELD

The combination of precise dating techniques along with zircon geochemistry can be used to further determine the age of an eruptive unit while also describing important information on the evolution of the magma system as a whole. We employ this approach within the Yellowstone Volcanic Field to determine the chronologic relationship between pre- and post-collapse rhyolitic lavas and tuffs and larger caldera forming eruptions. Green Canyon Flow is a member of the Big Bend Ridge rhyolites. These small volume effusive lavas erupted in between two caldera-forming eruptions: the Huckleberry Ridge and Mesa Falls Tuffs. Green Canyon Flow in particular sits on the rim of the caldera formed by the Mesa Falls eruption. Therefore, to evaluate the published K-Ar age for this unit as well as the relationship between this small volume rhyolite flow and the larger caldera formed by the Mesa Falls eruption, we present results from new high-precision 40Ar/39Ar single-crystal sanidine incremental heating experiments. The new ages suggest that the Green Canyon Flow erupted in close temporal proximity to the Mesa Falls Tuff and ~150 ka after eruption of the Bishop Mountain Flow and Lyle Spring Tuff. In addition, the zircon zoning patterns and morphologies obtained through use of cathodluminescence imaging allowed us to obtain qualitative information on crystallization and other processes that occurred within the magmatic system. This data along with forthcoming trace element geochemistry and high precision U/Pb TIMS dating will allow us to place constraints on the timescales of magma rejuvenation and further determine the relationships amongst the various rhyolitic units within the Yellowstone Volcanic Field.