A multi-radioisotope approach to dating sedimentation : applying Re-Os organic-rich shale and U-Pb authigenic xenotime dating to the Shannon and Pennine Basins

Abstract

Sedimentary rocks are the most abundant hthologies on the Earth, covering over 70% of the planet’s surface. Quantitative dating of sedimentary rocks (especially those devoid of fossils) is critical to understanding the past events and rates and duration of processes that shaped the Earth. Recent advances in geochronology of sedimentary rocks, such as Re-Os dating of organic-rich shales and U-Pb dating of authigenic xenotime, allow precise depositional and early diagenetic ages to be obtained, respectively, with uncertainties potentially as low as 0.7% (2σ). However, compared to the techniques commonly used in constructing the geological timescale (U-Pb dating of zircon and 40Ar-39Ar dating of volcanic rocks), they are still in the early stages of development. One of the major triggers for the interest in direct dating of sedimentary rocks is the ability to constrain further the ages of globally distributed Neoproterozoic glacial deposits, which provide one of the key lines of evidence in assessing the Snowball Earth hypothesis. This study investigated the Upper Carboniferous Shannon and Pennine Basins (which are well constrained biostratigraphically) to test the applicability of a multi-radioisotope approach to dating sedimentation in basins subject to elevated (250-360°C) and moderate (~ I40°C) palaeotemperatures, respectively. Two methods were employed: Re-Os dating of organic-rich shales by TIMS and U-Pb dating of authigenic xenotime overgrowths on detrital zircon grains by SIMS and LA-ICP-MS to 1) assess whether precise and accurate dates can be obtained from rocks that experienced palaeotemperatures > 300°C and 2) estimate how these ages relate to a) the age of deposition of the sedimentary rocks, b the age of tectonism and fluid flow and c) each other.