Evaluating the Geochemical Consequences of Aragonite-to-Aragonite Diagenesis in Freshwater Fossil Bivalves

Abstract

Carbonate clumped isotope (Δ_(47)) thermometry provides an independent test of the metrics used to assess the preservation of carbonate rocks and fossils. Additionally, Δ_(47) thermometry, when combined with other geochemical and structural measurements, may establish whether diagenesis occurred under closed-system conditions, open-system conditions, or some combination thereof. Here we present an example of diagenesis of Eocene-aged freshwater mollusk fossils (Unionidae) from Wyoming. The fossils yielded Δ_(47) temperatures from 41-46°C, which is physiologically unreasonable. However, X-Ray diffraction (XRD) data indicate that all but one of the fossils contain only aragonite — the carbonate phase unionids deposit; one sample contained small amounts of calcite in powder XRD analysis. We used scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), cathodoluminescence (CL) and secondary electron mass spectrometry (SIMS) of polished thin sections and etched fragments to describe the detailed textural, mineralogical, and trace element characteristics of two of these fossils and one modern unionid shell.