U-PB TIMS-TEA: A NEW TOOL FOR UNDERSTANDING ZIRCON CRYSTALLIZATION HISTORIES IN MAGMATIC SYSTEMS AND INTERPRETING ASHBED GEOCHRONOLOGY

U-Pb geochronology has become a critical tool for calibrating the geologic timescale because high-U minerals are commonly found in volcanic ashes interbedded within stratigraphic intervals that record bioevolutionary, climatic, and geologic events. Advances in high-precision ID-TIMS U-Pb geochronology have resulted in closed-system single zircon dates with uncertainty as low as ±0.05% of the age, and much better on weighted-mean dates. As a result, it is increasingly common to obtain populations of zircons from ashbeds with dates that do not overlap and may predate eruption by zero to millions of years. Though in young magmatic systems such pre-eruptive zircons are easily resolvable, in older ashbeds where absolute precision decreases, it is difficult to know whether spreads in zircon dates are due to expected analytical scatter or real age differences. Recently, we have developed a new tool for ID-TIMS U-Pb geochronology that allows us to measure both U-Pb date and trace element geochemistry of the same volume of zircon by analyzing the remaining undated dissolved zircon by high-sensitivity solution ICP-MS (U-Pb TIMS-TEA). We present U-Pb TIMS-TEA data from both plutonic and volcanic zircon from rocks ranging in composition from gabbroic to granitic and show how these data can improve interpretations of ashbed zircon dates and lead to a richer understanding of the geochemical evolution of magmatic systems with time. These data are coupled with numerical modeling and in situ trace element analyses from zircon to build a generalized analytical protocol for generating robust geochronological datasets of cogenetic zircons from ashbeds. This method provides a means to test whether zircons from a dated population are cogenetic (e.g. crystallized from the same batch of magma prior to eruption) or whether they were more likely inherited from the earlier batches of magma or during eruption. Identifying cogenetic zircon populations provides independent justification for using weighted-mean statistics to arrive at higher precision eruption dates to better than 0.05% precision, which is increasingly necessary to resolve questions correlating chemostratigraphic and facies proxies for environmental change with biostratigraphic data worldwide within framework of high-precision time.