Conodont (U–Th)/He thermochronology: A case study from the Illinois Basin

Abstract

Acquisition of thermochronologic data in carbonates and shales has traditionally been elusive owing to the paucity of dateable minerals in these lithologies. Conodont apatite (U–Th)/He (CAHe) thermochronology has the potential to fill this need. We acquired 50 (U–Th)/He dates for conodonts with CAI values ≤1.5 from seven Pennsylvanian shale and limestone samples from two drillcores in the Illinois Basin. We also obtained X-Ray microcomputed tomography (MicroCT) results for 8 conodonts to evaluate the accuracy of alpha-ejection corrections. The simplified geometric corrections yield corrected dates within 5% of those derived from 3D characterization using MicroCT. Nearly all of the conodont CAHe dates are substantially younger than their depositional age, indicating that maximum post-depositional temperatures of ≤90 °C caused He loss over geologic timescales. The youngest and most reproducible dates consist of whole platform elements from shales, and may record a regional Late Cretaceous–early Tertiary cooling and erosion event. The remainder of the data exhibit strong negative date-U and date-Th correlations, characterized by higher and more variable Th/U than the conodonts with reproducible dates. These patterns are best explained by U loss, with more limited Th loss. The results suggest that whole platform elements and higher U–Th conodont materials are the most promising targets for CAHe analysis.