(U-TH)/HE SYSTEMATICS OF FOSSIL GAR SCALES AND THEIR POTENTIAL FOR BASIN THERMAL HISTORY RECONSTRUCTION

In this thesis, I investigate whether (U-Th)/He thermochronology on the bioapatite of fossilized gar scales can be used to reconstruct the thermal histories of sedimentary basins. I acquired 37 (U-Th)/He dates from fossil gar scale ganoine, two (U-Th)/He dates from fossil gar scale bone, and 18 (U-Th)/He dates from detrital apatite grains within sandstones from the upper Cretaceous and lower Paleogene rocks of two sedimentary basins with distinct thermal histories: the Williston basin and San Juan basin. I also obtained spatially resolved trace element concentrations by laser ablation-inductively coupled plasma mass spectrometry, as well as scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction data to assess the chemical and structural impacts of diagenesis on gar scale bioapatite. Timescales of diagenesis in bioapatite are relevant to (U-Th)/He thermochronology since the parent nuclides are taken up by bioapatite during fossilization but could also be taken up or lost much later, which could complicate (U-Th)/He data interpretation. Trace element concentrations and profiles vary spatially between ganoine and bone. Ganoine trace element profiles commonly show exponentially decreasing concentration profiles from the surface to 20 µm depth, and occasionally display more complex concentration depth profiles. Trace element profiles in bones are complex and spatially heterogeneous. (U-Th)/He data from ganoine and bone show an inverse relationship between (U-Th)/He age and parent nuclide concentrations, indicating open system behavior and late-stage uptake of parent nuclides. Helium loss via diffusion through pore spaces, fractures, and growth layers could also provide a reason for young fossil gar scale (U-Th)/He ages. Detrital apatite (U-Th)/He thermochronology from two Williston Basin samples suggest heating of upper Cretaceous and lower Paleogene rocks to ~100 ℃ at ~50 Ma. Detrital apatite (U-Th)/He thermochronology from the San Juan Basin sample yielded modeled maximum temperatures of ~150 ℃ at ~30 Ma. The discrepancies between fossil gar scale and detrital apatite (U-Th)/He ages suggest that protracted diagenesis of gar scale bioapatite prevents gar scales from being a suitable material for (U-Th)/He thermochronology.