CHEMISTRY AND STRUCTURE OF FOSSIL GAR SCALES WITH IMPLICATIONS FOR BIOAPATITE (U-TH)/HE THERMOCHRONOLOGY

To explore the viability of gar fish scale bioapatite for (U-Th)/He thermochronometry, we investigated the character and timescales of diagenesis during gar scale fossilization. Gar scales are made of two structurally distinct components: ganoine, a dense enamel-like tissue, and bone, which is more porous. During fossilization, these components undergo diagenesis that involves loss of organic material and addition of authigenic apatite. Trace elements such as uranium and thorium are thought to diffuse and adsorb onto co-precipitating authigenic apatite in bone on timescales ≥ 10⁵ years, whereas ganoine is thought to recrystallize and become a closed system more rapidly due to lower porosity and crystallite surface area. We investigated the structure and chemistry of fossil gar scales from Cretaceous–Paleogene strata in the Williston Basin (WB) and San Juan Basin (SJB) to assess their diagenetic history and estimate the timescales over which recrystallization and trace element uptake occurred. LA-ICP-MS trace element data suggest complex uptake of uranium and other trace metals controlled by the pore spaces within the bone. Ganoine shows simpler uptake of uranium and other trace elements, consistent with diffusion-adsorption models. Gar scale trace element concentrations differ by an order of magnitude between the WB and SJB, in both ganoine and bone, reflecting the distinct fluid and thermal histories of the two basins. PXRD data from modern and fossil gar scale ganoine suggest hydroxyapatite crystallites maintain a similar orientation and size during fossilization. Similar structural and chemical components between modern and fossil ganoine suggests that ganoine is more resistant to diagenesis, and recrystallizes more rapidly than bone, making it more attractive as a (U-Th)/He thermochronometer. These findings will inform forthcoming (U-Th)/He measurements and helium diffusion experiments on gar scale bone and ganoine from the WB and SB.