Double dating sedimentary sequences using new applications of in-situ laser ablation analysis

Sedimentary rocks can provide important insights regarding the evolution of Earth's surface environments through deep time. Such sequences are pervasive through the geological record and currently cover >70% of the planet's surface. They are also a key repository for energy and mineral resources. However, absolute chronology of sedimentary rocks can be difficult to constrain using conventional methods due to their typically low abundances of radiogenic elements. Establishing chronology is particularly challenging for Precambrian sedimentary rocks, where the lack of a diverse fossil record makes biostratigraphic correlations ambiguous. In this study, we use shale and carbonate samples from the Proterozoic greater McArthur Basin in northern Australia as a case study to demonstrate two emerging in-situ laser-based methods that have the potential to quickly and accurately resolve the minimum depositional age of a sedimentary package. The first method provides a tool to constrain the formation of authigenic clay minerals in shales using in-situ laser ablation Rbsingle bondSr geochronology. The second method demonstrates an approach for dating carbonate sedimentation using Usingle bondPb geochronology via a laser isotopic mapping approach. Laser rasters are compiled into isotopic maps, and this spatial and geochemical information is used to target representative subdomains within the sample. Detrital or altered regions can be avoided by monitoring chemical signatures and pixels, and to the most authigenic domains are then subdivided that give the best spread of data on an isochron. Both approaches provide the key advantage of preserving, and through the mapping approach further resolving, sample petrographic context, which together with complementary geochemical data can be triaged to yield a more appropriate age and interpretation.