Ryan B. Ickert

Chemical and Pb isotope composition of phenocrysts from bentonites constrains the chronostratigraphy around the Cretaceous-Paleogene boundary in the Hell Creek region, Montana

An excellent record of environmental and paleobiological change around the Cretaceous-Paleogene 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 40Ar/39Ar. 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.

Model selection during sample-standard-bracketing using reversible jump Markov chain Monte Carlo

For instrument calibration where interpolation between reference materials is required, there exists a need for a generally applicable technique to determine: (1) the value of the calibration (e.g., the mass bias at an arbitrary time); (2) the uncertainty of this value; and, (3) the degree to which the uncertainties in the reference material analyses account for their scatter about the calibration. Here, we show that an implementation of the reversible-jump Markov chain Monte Carlo (rj-MCMC) technique can provide all three values for a reasonable range of complexity. Using this algorithm we treat a drifting calibration value as a function of time by a series of straight line segments. The benefit of the rj-MCMC technique is that the number of straight line segments does not need to be specified a priori, but is a parameter that is estimated. This technique is also able to simultaneously determine the presence and magnitude of overdispersion (the amount of scatter in the data not accounted for by estimated uncertainties) even in the presence of complex, non-linear drift. The result of this data treatment is a probability distribution in calibration-time space that, despite having an origin in line segments, smoothly follows the data and therefore yields the calibration value and its uncertainty. We validate this technique using synthetic data from prescribed distributions, and demonstrate its utility and flexibility by applying it to data collected by multi-collector inductively coupled plasma mass spectrometry that display complex non-linear drift.