CHALLENGES ASSOCIATED WITH THE CONODONT (U-TH)/HE METHOD: A CASE STUDY FROM THE MORMON MOUNTAINS, TULE SPRING HILLS, AND BEAVER DAM MOUNTAINS, SOUTHEASTERN NEVADA AND SOUTHWESTERN UTAH

(U-Th)/He thermochronology is a well-established dating technique used to understand the temperature-time histories of rocks that contain specific accessory minerals like apatite or zircon. Our objective was to determine if biogenic apatite from conodonts could be used to expand the (U-Th)/He method to marine carbonates and shales. We performed (U-Th)/He thermochronometry, LA-ICPMS depth profiling, and scanning electron microscopy on color alteration index (CAI) 1.5 – 3 conodonts that were extracted from 15 carbonate samples collected from the footwalls of low-angle normal faults in the Mormon Mountains, Tule Spring Hills, and Beaver Dam Mountains.

Our conodonts produced widely variable data. (U-Th)/He dates had high scatter but were often reproducible to within 20% of sample means. All CAI 1.5 – 2.5 conodonts produced (U-Th)/He dates of 20 to 130 Ma, consistent with thermal resetting of samples; however, most CAI 3 conodonts dated substantially older than deposition. Average U, Th, and rare earth element (REE) concentrations depend on conodont tissue type (albid, hyaline, or basal tissue) and range from <10 to 100s of ppm in concentration. Parent isotope concentrations are especially low in CAI 3 conodonts, and there is an inverse relationship between these concentrations and (U-Th)/He dates. The majority of REEs are concentrated within the outer 5 microns of the conodont elements and frequently show 10x enrichment of rims relative to cores. Rim enrichment is also depressed with increasing CAI. SEM imaging shows a shift in the orientation of apatite microcrystallites from perpendicular to parallel to the major axis of the conodont elements at CAI 3.

We believe microstructural changes associated with increasing CAI influence the (U-Th)/He dates. Parent isotope loss occurs during the post-cooling stage, either in the outcrop or in the laboratory. Our hypothesis is that the double-buffered formic acid procedure for dissolving dolomitized carbonates may accelerate this loss in higher CAI conodonts. Conodont tissue type and size with respect to surface area and volume have lower influences on (U-Th)/He dates in this data set.