Assessing the preservation and provenance of Sr and Nd isotopic signatures in Cretaceous volcanic ash beds

Abstract

Bentonite, a claystone formed from the devitrification of volcanic ash, demonstrates varying amounts of alteration in elemental concentrations through the transition from glass to clay. Many studies have sought to characterize and correlate bentonites using various elemental signatures, but the information garnered from this can be misleading since 1) volcanoes produce highly variable compositions during single events, making tephra classifications reliant on the position within the bed and the distance from the source, and 2) the elemental concentrations may vary significantly due to devitrification. Strontium and neodymium isotopic signatures offer a powerful dataset to supplement these efforts, but the extent to which the isotopic ratio is preserved during devitrification and post-depositional diagenesis is not well documented. Here we perform a detailed isotopic, geochemical, and mineralogical examination of sixteen bentonites preserved in Cenomanian and Campanian strata (Late Cretaceous) from South Dakota in order to characterize the diagenetic factors that may influence the ash geochemistry, while checking for the fidelity of the isotopic signatures to the magmatic source. A mixing model between the isotopic composition of bentonite and seawater can be generated to determine the original endmember composition of each deposit, since Sr was likely derived from both sources during devitrification. Initial 87Sr/86Sr ratios from Cenomanian samples ranged higher than the Campanian, demonstrating a negative correlation with εNd values that point towards magmatic signatures with the influence of crustal assimilation. X-Ray diffraction was utilized to determine the speciation of the clay mineral and sedimentary input, displaying a complete lack of smectite illitization that would indicate the presence of a hydrothermal influence. Bentonite isotopic chemistry, therefore, is reliably traced to the magmatic source under the appropriate diagenetic conditions. With careful consideration for diagenesis, the isotopic composition of the Cretaceous bentonites can be used for provenance identification and points towards magmatic emplacement within an evolved crustal source on the Laurentian craton. This isolates the most likely provenance of Cenomanian bentonites as being the Idaho batholith, while Campanian bentonites were likely derived from the Elkhorn Volcanic complex.