Collaborative Research: Experimental Investigation of Actinide Partitioning in Zircon and its Applications to Geochronology

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One of the most accurate and useful ways of determining the age of rocks that formed more than about 500,000 years ago is uranium-lead (U-Pb) geochronology. Earth scientists use U-Pb geochronology to put together the geologic history of entire regions and of specific events, like the mass extinction of all non-avian dinosaurs about 66 million years ago or the catastrophic eruptions of supervolcanoes like the one currently centered at Yellowstone. The mineral zircon is often utilized because it is abundant, durable, and readily incorporates uranium into its crystal structure. But it excludes thorium, whose isotope 230Th is part of the naturally occurring isotopic decay chain from 238U to 206Pb. Calculating a date from the relative abundances of 206Pb and 238U therefore requires a correction for the missing 230Th. Existing experimental and observational constraints on the way U and Th behave when zircon crystallizes from a melt are imprecise, so we propose to quantify this behavior in a series of controlled laboratory experiments. The results of this study will reduce the largest source of uncertainty in dating young zircons and improve the accuracy of U-Pb dates, improving our ability to tell time during geologic processes. The attainment of more accurate timing of the geologic timescale is important to geologists of all disciplines, from paleontology to planetary cosmochemistry to geobiology. The data from this study will be broadly disseminated to the geoscience community. The data will be packaged and presented in an interactive visualization, hosted on the web, to illustrate why a correction must be made to geochronology data, how to calculate the magnitude of the correction, and how the correction will vary with compositional parameters. The results of this grant will also support the admission and mentoring of students in underrepresented groups and support the professional and laboratory development of two early career investigators. Synthetic zircon will be grown at high temperature from melts doped with U and Th that mimic natural magmas at a range of temperatures, pressures, and compositions. The liquids will be quenched so that the melt turns into glass. Zircons will be separated from their coexisting glass and using high precision and high-spatial-resolution techniques, the abundance and distribution of U and Th in each phase will be measured. The experiments proposed will result in precise determination of the zircon/melt uranium and thorium partition coefficients under a wide variety of naturally occurring conditions. This data will be fit to a multidimensional surface using maximum likelihood regression techniques, so that the ratio of partition coefficients can be calculated for any set of known parameters. Measurement uncertainties from the isotope dilution analyses, as well as any observed experimental scatter will be incorporated into these uncertainties, and we expect that they will conservatively be on the order of 1-5%. This is compared to the existing experimental data, which has yielded uncertainties on the order of 50-100%.