Crustal Noble Gas Isotopic Characteristics in Low-Permeability Ordovician Sedimentary Rock, Eastern Flank of the Michigan Basin

This study presents noble gas characteristics of porewater in the Ordovician low-permeability drill cores, which were vacuum-sealed in Al-foil bags and collected over a decade ago on the eastern flank of the Michigan Basin. Noble gas ratios and concentrations reveal crustal noble gas features as well as radiogenic He-4, Ar-40, and Xe-136 components. The measured noble gas ratios in this study agree with measurements previously made in the Ordovician brine samples from the western flank of the Michigan Basin. However, unlike water samples from the western Michigan Basin, no mantle-featured noble gas components are found in the Ordovician rock porewater from this site. The Ordovician porewater residence time is quantitatively estimated with both He and Xe radiogenic ingrowth, yielding an average of 251 million years (m.y.). This porewater residence time estimate is comparable with the previous He accumulation time estimate at the same study site that yielded 260 m.y. The radiogenic noble gas ingrowth demonstrates long-term confinement of porewater and concomitant gases within the Ordovician low-permeability rock investigated. The remarkable preservation of gases in these well-sealed vapor-barrier Al-foil bags provides an economical and efficient possibility for noble gas out-diffusion sampling from drill cores.