Abstract
The geochemical and ultrastructural properties of thermally altered
skeletal carbonate are expected to be compromised to varying degrees by
disequilibrium processes between solids and the ambient aqueous fluids.
When assessing the alteration history of carbonates, it is important to
apply models that quantitatively describe these diagenetic processes on
multiple geochemical systems, such that they can be identified in
natural samples. Carbonate clumped isotope analysis provides a unique
tool for validating such models and can be combined with other
geochemical tools/proxies to more comprehensively describe the processes
and products. Here, we have analyzed bivalve shells that have undergone
hydrothermal alteration (experimental diagenesis) in high water/rock
ratio experiments at 130 and 160{degree sign}C, demonstrating that
non-linear changes in ∆47 and ∆48 values can be attributed to
heterogeneous replacement of precursor carbonates. Importantly, this
model predicts decoupled ∆47 and ∆48 values, despite all reactions
occurring at clumped isotope equilibrium with respect to the
experimental temperature. We demonstrate that the rapid, thermally
induced re-equilibration occurs in a “closed system” with minimal
exchange with the ambient fluid, similar to the results of heating
experiments conducted without an extraneous fluid. Later stages of
alteration occur in an “open system” wherein internal fluid is
exchanged with the external fluid at a similar rate to recrystallization
and neomorphism. In these experiments, some oxygen from the original
inorganic-organic composite-biomineral is inherited, indicating
restrictions on the availability of fluid oxygen. Our experiments and
models validate a novel application for dual-clumped isotopes for
reconstructing hydrothermal temperatures and fluid δ18O compositions.