Diagenesis of fossil gar fish scales with implications for geochronology and paleoenvironmental applications

Fossil scales from gar fish are made of bioapatite and are common in the continental sedimentary record from the Cretaceous to the present, prompting interest in using these fossil scales as geochemical archives. We investigated the diagenetic characteristics of the two different components of gar scales: ganoine, which has similarities to tooth enamel, and bone. We examined microscopic textural characteristics, spatially resolved trace element patterns, and apparent gar scale ages using the (U-Th)/He and U-Th-Pb systems, in modern scales as well as fossil scales from Cretaceous to Paleogene age strata in the North American Williston and San Juan Basins. Structural and textural characteristics of modern and fossil gar scales suggest that ganoine is more resistant to diagenetic alteration than bone. Trace element concentrations in fossil bone are at least an order of magnitude higher than in ganoine and can be spatially heterogeneous, indicative of a complex and prolonged diagenetic history. Trace element concentrations in fossil ganoine often decrease systematically with distance from the ganoine surface, which may be consistent with a fossilization model in which trace element concentrations are controlled by an inwardly propagating recrystallization front. However, (U-Th)/He dates of ganoine are substantially younger than expected based on basin thermal histories and compared to conventional detrital apatite (U-Th)/He dates from the same localities. The ganoine (U-Th)/He dates are also older than a few Ma for only the lowest effective uranium concentrations. We interpret the young (U-Th)/He dates and the date-effective uranium pattern to be indicative of either protracted or late-stage incorporation of trace elements, which is inconsistent with a recrystallization front model alone. The young (U-Th)/He dates may also reflect microstructurally-controlled low helium retentivity in gar scale ganoine, although this does not explain the observed date- effective uranium systematics. Late-stage incorporation of trace elements is supported by the U-Th-Pb systematics of ganoine, which require addition of U and Th at a time significantly later than the depositional age. Late-stage, or protracted, diagenetic uptake of parent nuclides in gar scale ganoine contrasts with recent studies of bioapatite from conodont elements, which instead appear to exhibit late-stage parent nuclide loss. We conclude that the (U-Th)/He system in gar scale bioapatite is not currently useful as a chronometer and suggest that applications of other trace element proxies to gar scale bioapatite be approached with caution.