LONGEVITY OF THE MESA FALLS TUFF MAGMA SYSTEM RECORDED BY ZIRCON PETROCHRONOLOGY AND SANIDINE 40AR/39AR DATES

The joint zircon petrochronologic and sanidine ⁴⁰Ar/³⁹Ar approach to examining a magmatic system allows for constraining the thermal and chemical evolution, autocrystic growth and antecrystic recycling, and deposition age of an eruptive unit. The 1.3 Ma Mesa Falls Tuff (MFT), the second and volumetrically smallest of the Yellowstone Plateau caldera-forming eruptions, offers a prime natural laboratory to decipher the petrochronologic history of a large silicic system and explore the longevity of magma batches preceding eruption. Approximately 450 LA-ICPMS spots collected from 320 zircon crystals from five pumice blocks and two welded ash flow tuff samples provide a trace element concentration and Ti-in-zircon thermometry database complemented by ²³⁸U/²⁰⁶Pb CA-TIMS dates from over 50 of those grains. Sanidine from two of the pumices were analyzed by incremental step-heating or total fusion ⁴⁰Ar/³⁹Ar dating techniques performed on single crystals using a Noblesse multi-collector mass spectrometer, yielding an eruption age of 1.300 Ma. The thermochemical data show a distinct differentiation trend defined by probable autocrystic zircon spanning 200˚C of cooling and associated increases in Eu anomaly and incompatible trace element concentrations. Zircon crystallization temperatures and trace element concentration trends are similar to those in the Huckleberry Ridge Tuff (HRT), however MFT geochronology demonstrates only rare recycling of crystals from the HRT or other rhyolites into the Mesa Falls magma system. CA-TIMS dates define a predominant trend of decreasing age with differentiation between 1.33 and 1.30 Ma. Isolated antecrysts as old as 1.37 Ma may have been derived from pre-caldera volcanics or non-erupted intrusives. A subset of composite grains comprising older cores with extremely differentiated compositions overgrown with younger MFT-like rims likely represent early solidified sidewall or roof accumulations later remobilized within the main MFT magma. The spread of sanidine and zircon dates indicates that the entire pre-caldera to climatic Mesa Falls magmatic system was active for <70 ka, with the bulk of zircon crystal nucleation and growth occurring within 30 ka of eruption.