Testing the utility of conodonts as a (U-Th)/He thermochronometer
(U-Th)/He thermochronometry is a well-established and powerful hydrocarbon exploration tool that can provide important constraints on fault timing and landscape dynamics. Although the technique can be applied to a wide range of lithologies, marine carbonates and shales present a major challenge because they lack the required accessory minerals. Conodonts have emerged as a novel solution for the application of the method in carbonate-dominated successions. These teeth-like fossils are made of hydroxyapatite with U and Th concentrations similar to magmatic sources of apatite. These fossils are already routinely used in basin studies for biostratigraphic control and as geothermometers via the conodont color alteration index. This project explores the viability this biogenic source of apatite as a (U-Th)/He thermochronometer at two different calibration sites: (1) the Beaver Dam and Mormon Mountains, eastern Nevada and western Utah and (2) the Rebecca K. Bounds and Berexco Cutter KGS #1 core-holes, western Kansas. The study pairs traditional (U-Th)/He analyses of the conodonts with x-ray computed microtomography and laser ablation-inductively coupled plasma mass spectrometry. Major goals of the study are to evaluate the effects of complex morphologies, microstructural heterogeneities, parent isotope distributions and mobility, radiation damage effects, and color alteration index on conodont (U-Th)/He ages. Collaborators include: Andreas Moller (KU), Bradley Cramer (UI), and Daniel Stockli (UT).
Check out our new paper in Chemical Geology!
Figure 1. SEM image of surface mineral characteristics of conodont color alteration index 2.5 grain from the Mormon Mountains, southeastern Nevada.
Figure 2. Laser ablation ICP-MS depth profiles of conodonts showing isotopic enrichment of conodont rims relative to cores. Data also suggest that isotopic concentrations are depressed with increasing CAI.