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We report technical and data treatment methods for making accurate, high‐precision measurements of 18O/16O in Ca–Mg–Fe garnet utilising the Cameca IMS 1280 multi‐collector ion microprobe. Matrix effects were similar to those shown by previous work, whereby Ca abundance is correlated with instrumental mass fractionation (IMF). After correction for this effect, there appeared to be no significant secondary effect associated with Mg/Fe2+ for routine operational conditions. In contrast, investigation of the IMF associated with Mn‐ or Cr‐rich garnet showed that these substitutions are significant and require a more complex calibration scheme. The Ca‐related calibration applied to low‐Cr, low‐Mn garnet was reproducible across different sample mounts and under a range of instrument settings and therefore should be applicable to similar instruments of this type. The repeatability of the measurements was often better than ± 0.2‰ (2s), a precision that is similar to the repeatability of bulk techniques. At this precision, the uncertainties due to spot‐to‐spot repeatability were at the same magnitude as those associated with matrix corrections (± 0.1–0.3‰) and the uncertainties in reference materials (± 0.1–0.2‰). Therefore, it is necessary to accurately estimate and propagate uncertainties associated with these parameters – in some cases, uncertainties in reference materials or matrix corrections dominate the uncertainty budget.

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Dating zircon by U-Pb in the Pleistocene Bishop tuff is challenging because of the low concentrations of radiogenic Pb and the relatively large correction required for disequilibrium in the intermediate daughter products. The dates can be difficult to interpret, because the absolute precision on the dates is similar in magnitude to timescales of crystallization. Interpretations of the duration of zircon crystallization span two orders of magnitude and appear to depend on the analytical approach (bulk analysis or microbeam analysis). To reconcile the differing interpretations, we present new SIMS and ID-TIMS zircon Pb/U dates, including some grains that are dated by both techniques. Both the SIMS and ID-TIMS dates have similar distributions as previous results. Normalized to a Th/U melt = 2.81, SIMS spot dates from interior regions of sectioned grains range from 769±31 ka to 845±28 ka (2σ) and ID-TIMS dates have a range from 760±7 ka to 793±6 ka. The double-dated grains demonstrate an average of 31 ka difference between the bulk grain age and the interior spot age, demonstrating 10 ka-scale age variation within individual grains. This level of variability precludes the assignment of a geological meaning to a mean (or weighted-mean) zircon date. A previous ID-TIMS investigation of single BT zircon that showed apparent statistically significant clustering of zircon ages was compromised by an incorrect treatment of the covariance structure of 230 Th-corrected Pb/U data. We show approximate and exact methods for the correct treatment, which demonstrate substantially more scatter in that data than previously recognized. A close investigation of available partition coefficient data show that they are not adequately precise for <±10 ka corrections, and moreover, evidence for disequilibrium uptake of U and Th means that equilibrium partition coefficients may not be useful, even if determined precisely. Mismatches between the U-Th concentration systematics of zircon and of glasses suggest that there are gaps in our understanding of the relationship between the two phases, lending uncertainty to the accuracy of disequilibrium corrections that use measured glass compositions. Given these uncertainties in the disequilibrium correction, caution is urged when correcting dates with nominal precisions that are better than ±10 ka. Nevertheless, the youngest zircon ages very likely constrain the eruption age of the Bishop Tuff to < 775 ka, which conflicts with some estimates of the 40 Ar-39 Ar sanidine age of eruption. In detail, we show that the use of currently available partition coefficients for the correction of the 230 Th deficit limits the absolute precision of Pb/U dates to >±10 ka (2σ) when this method is used, but also show that the amount of trace element uptake (including U, Th, and Hf) varies significantly between crystal faces, and therefore bulk equilibrium may not be a realistic assumption. The liquid line of descent defined by U-Th systematics of glasses are different from the U-Th systematics of zircon, requiring ...

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An excellent record of environmental and paleobiological change around the CretaceousPaleogene boundary is preserved in the Hell Creek and Fort Union Formations in the western Williston Basin of northeastern Montana. These records are present in fluvial deposits whose lateral discontinuity hampers long-distance correlation. Geochronology has been focused on bentonite beds that are often present in lignites. To better identify unique bentonites for correlation across the region, the chemical and Pb isotopic composition of feldspar and titanite has been measured on 46 samples. Many of these samples have been dated by 40 Ar/ 39 Ar. The combination of chemical and isotopic compositions of phenocrysts has enabled the identification of several unique bentonite beds. In particular, three horizons located at and above the Cretaceous-Paleogene boundary can now be traced-based on their unique compositions-across the region, clarifying previously ambiguous stratigraphic relationships. Other bentonites show unusual features, such as Pb isotope variations consistent with magma mixing or assimilation, that will make them easy to recognize in future studies. This technique is limited in some cases by more than one bentonite having compositions that cannot be distinguished, or bentonites with abundant xenocrysts. The Pb isotopes are consistent with a derivation from the Bitterroot Batholith, whose age range overlaps that of the tephra. These data provide an improved stratigraphic framework for the Hell Creek region and provide a basis for more focused tephrostratigraphic work, and more generally demonstrate that the combination of mineral chemistry and Pb isotope compositions is an effective technique for tephra correlation.