RECORDS OF MAGMATIC EVOLUTION OF SMALL AND LARGE VOLUME RHYOLITES IN THE YELLOWSTONE VOLCANIC FIELD

Zircon trace element compositions can serve as records of magmatic evolution via differentiation indices, such as europium anomaly and other incompatible trace elements. Coupled to the titanium-in-zircon thermometer and high-precision U-Pb dating, these trace element signatures can signify the thermochemical history of a silicic magma prior to eruption and allow for distinguishing different zircon populations contained within the eruptive crystal cargo. We use this approach to examine the pre-eruptive differentiation for the small-volume 2.14 Ma Snake River Butte rhyolite and compare this trend to the super-eruption derived Huckleberry Ridge and Mesa Falls Tuffs. The observed trends in zircon thermochemical data for both large and small volume systems suggest a down-temperature differentiation trend over approximately 200˚C of cooling as indicated by a greater europium anomaly and higher incompatible trace element concentrations at lower temperatures. We quantify the degree of differentiation using fractional crystallization models that incorporate temperature-dependent partition coefficients. In models for both types of systems, compositions recorded in autocrystic zircon crystals can be produced from a common parental magma composition following approximately 50-60% crystallization of sanidine ± plagioclase ± quartz. A population of zircon crystals within the Mesa Falls Tuff, characterized by CL-black cores and extreme incompatible trace element concentrations, is modeled by 80-95% total crystallinity. These grains are interpreted as solidification of a precursory magma, and supported by CA-IDTIMS ages up to 50 ka older than the 1.3 Ma eruption age. Yet, such highly fractionated compositions are absent in the earlier Huckleberry Ridge Tuff and Snake River Butte flow. The use of zircon as a proxy for magmatic processes, and providing a quantitative model for the evolution of the magma body, is a novel approach to understanding the similarities and differences between silicic eruptions of various sizes within the Yellowstone Volcanic Field.