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

Carbonate clumped isotope (Δ₄₇) thermometry provides an independent test of the metrics used to assess the preservation of carbonate rocks and fossils. Additionally, Δ₄₇ 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 Δ₄₇ 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.

SEM images of the etched fossils reveal the presence of secondary mineral overgrowth that partially obscures original boundaries between nacre plates that constitute the framework of the shells; this secondary material is absent in the modern shell. In polished thin sections, both fossils showed evidence of dissolution concentrated at the edges of the nacre plates, and welding along the long-axis boundaries of the plates. CL and EBSD data confirm that one of the fossil samples is composed of aragonite only, and that the other fossil is composed of aragonite with localized calcite. In addition, the EBSD results suggest an overall coarsening of the aragonite grains in the fossils relative to those in the modern shell. Preliminary SIMS results indicate changes to the minor element chemistry of the fossils with progressive alteration. Together these data suggest that diagenesis of metastable primary aragonite involved growth of secondary aragonite (through coarsening and/or deposition of over-growths). These findings suggest that the mechanism of resetting of Δ₄₇ temperatures in these aragonitic fossils during diagenesis may have been recrystallization in addition to (or rather than) solid state diffusion. In future work, we will use nanoSIMS to better characterize the consequences of this diagenesis on trace element and stable isotope distributions.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California