Uranium–lead phosphate chronostratigraphy: A proof of concept from the mid-Carboniferous boundary

Abstract

Sedimentary phosphates, chiefly composed of apatite group minerals, contain exceptionally high concentrations (10–200 ppm) of U relative to other sedimentary precipitates, making them an attractive target for U–Pb dating. Phosphate-rich sedimentary rocks are common from the Neoproterozoic onwards and commonly occur at key points in the geological record, often coinciding with periods of significant environmental flux or facies change. There is thus a clear rationale for dating phosphatic rocks. In this study, we demonstrate how spatially-resolved U–Pb dating of sedimentary apatite by laser-ablation quadrupole inductively-coupled-plasma mass-spectrometry (LA-Q-ICPMS) can provide constraints on early phosphate diagenesis and reworking in ancient rocks. Our samples are from two locations in a well-characterized stratigraphic sequence containing phosphorites, and phosphate-rich carbonates and shales in western Ireland, which span the Mississippian–Pennsylvanian (mid-Carboniferous) boundary. Sedimentary phosphate preserves precise (<0.7% uncertainty) U–Pb ages that record early diagenesis, or sediment reworking, depending upon the facies sampled. These ages are consistent with age constraints derived from Carboniferous conodont biostratigraphy from the same sections. Phosphorite deposition, occurring on the sediment-starved margin of an epeiric basin, is constrained by U–Pb dating of apatite to the lower part of the Bashkirian stage at 320.7 ± 2.0 Ma (MSWD = 1). However, we identify a significant time-gap between the phosphorite and the immediately underlying carbonate stratigraphy, and thus attribute phosphorite accumulation to forced regression accompanying a widely recognized and significant glacial advance in Gondwana that took place at the mid-Carboniferous boundary. Our two sampling sites are several kilometers apart, broadly along strike of the basin margin, but preserve U–Pb isotopic and trace element compositional information that is strikingly consistent at the same stratigraphic levels. If used appropriately, integrated phosphate U–Pb dating and trace element analysis could provide age constraints for key parts of the Neoproterozoic and Phanerozoic, aiding reconstruction of evolutionary and paleoenvironmental records.