| dc.description.abstract | The element Th, and its isotope 230Th, an intermediate daughter isotope in the 238U decay series, is often excluded relative to U from the mineral zircon during crystallization. Zircon is geochronologically important because it incorporates U, but when Th is excluded, a deviation from secular equilibrium occurs. The return to secular equilibrium, where the parent nuclide and intermediate daughter products have equal activities, results in a deficit of stable daughter 206Pb. This causes measured zircon 206Pb/238U dates to be up to ~109 ka too young, necessitating a correction for the initial 230Th deficit by comparing the Th/Uzircon to an estimated Th/Umelt and calculating a fractionation factor. For many Cenozoic zircon analyses, uncertainty in this factor makes up a majority of the uncertainty budget because existing Th-U partitioning studies of synthetic and natural samples are imprecise (e.g. Burnham and Berry, 2012; Luo and Ayers, 2009; Rubatto and Hermann, 2007). We have synthesized zircon from a range of temperatures (1200 to 1350ºC), melt compositions (basaltic andesite to andesite), and oxygen fugacities (QFM-4 to QFM+4), all doped with 500-1000 ppm U and Th. By analyzing whole crystals using isotopic dilution methods and high-spatial resolution SIMS analyses, we have reduced the uncertainty on DTh/DU to 1-5 %. With higher resolution, we found that DTh/DU generally decreases with fO2 and increases with temperature and melt composition. The variation in our datasets we attribute to the sector zoning that can be seen in cathodoluminescence (CL) images of our samples. Dark sectors generally had higher DTh/DU ratios than light sectors. We were able to sample light and dark sectors with EMPA and SIMS spots and observed that weighted mean DTh/DU of our ID-ICPMS analyses existed inside the range of the ratios found in light and dark sectors. Using these measurements, we demonstrate that trace element partitioning varies on a micron-scale in zircon, but that whole-crystal DTh/DU can be applied to natural samples by estimating the volume ratio of light dark sectors. These DTh/DU ratios can be used to calculate 230Th corrections for zircon grown under similar oxygen fugacities, melt compositions, and temperatures to improve analytical uncertainties on 206Pb/238U dates. These higher precision dates can then be used to resolve rapid geologic processes like incremental pluton assembly (e.g., Mills and Coleman, 2013) or better constrain timescales of mass extinction and biotic recovery (e.g., Schoene et al., 2015). | |
