2nd international workshop on clumped isotopes, London, August 10-12 2011
Geochemical and Mineralogical Consequences of Diagenesis
of Aragonitic Fossil Bivalves
Kathryn E. Snell1, 2*, John M. Eiler2, David L. Dettman3, Paul L. Koch1
1) University of California Santa Cruz, Earth and Planetary Sciences Department
2) California Institute of Technology, Division of Geological and Planetary Sciences
3) University of Arizona, Department of Geosciences
* Corresponding author: ksnell@caltech.edu
Carbonate clumped isotope (47) thermometry is a useful tool for characterizing
diagenesis in carbonate rocks, as it provides an independent test of the value and
meaning of metrics commonly used to assess the preservation of carbonate rocks
and fossils used for paleoenvironmental reconstructions. Additionally, 47
thermometry, when combined with other geochemical and structural measurements,
can help establish whether diagenesis occurred under closed-system conditions (i.e.,
reordering of the isotopes while retaining the original bulk chemical isotopic
composition) or open-system conditions (i.e., 47 values and bulk chemical isotopic
composition are both affected). It is less clear whether re-setting of 47 thermometry
occurs by recrystallization versus solid state diffusion, but it is possible one could
gain insight into this by combined 47, textural and X-ray and/or electron diffraction
study of variably altered materials.
Here we present an example of cryptic diagenesis of Eocene-aged freshwater
mollusk fossils (family Unionidae) from the Bighorn Basin in Wyoming. The fossils
yield 47 temperatures between 41°C and 46°C, which is too high to be
physiologically reasonable (growth rates decrease significantly above 30°C in
modern unionid bivalves). However, X-Ray diffraction (XRD) data indicate that all
but one of the fossils contains only aragonite — the carbonate phase unionids
deposit; the exception is one sample that contains small amounts of identifiable
calcite in powder XRD analysis. To better understand the diagenetic changes the
samples underwent, we used scanning electron microscopy (SEM), including
electron backscatter diffraction (EBSD), and cathodoluminescence (CL) of polished
thin sections and etched fragments to describe the textural characteristics of two of
these fossils and one modern unionid shell. In both fossils, SEM images of the
etched samples reveal the presence of secondary mineral overgrowth that partially to
completely obscures the 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,
while the long-axis boundaries between the plates are commonly indistinct. CL and
EBSD analyses support the XRD results: one of the fossil samples is composed of
aragonite with little to no calcite, while the other fossil is composed dominantly of
aragonite with small amounts of calcite dominantly in the gaps between microband
sets. In addition, the EBSD results suggest an overall coarsening of the aragonite
grains in the fossils relative to those in the modern shell. Together these data
suggest both dissolution of primary aragonite and, more importantly, growth of new,
diagenetic aragonite (either through coarsening or deposition of over-growths). In
other words, diagenesis of metastable primary aragonite involved growth of
secondary aragonite in addition to growth of stable calcite. In addition, these findings
suggest that the mechanism of resetting of carbonate clumped isotope temperatures
in these aragonitic fossils during diagenesis may have been recrystallization rather
than solid state diffusion. Future work will add high-resolution trace element and
stable isotopic analyses to these samples using SIMS, in an effort to better
characterize the geochemical consequences of this style of diagenesis.
1