A distributed activation energy model for clumped isotope bond
reordering in carbonates
Abstract
Carbonate clumped isotopes (∆47) have become a widely applied method for
paleothermometry, with applicationsspanning many environmental settings
over hundreds of millions of years. However, ∆47-based paleothermometry
can be complicated by closure temperature-like behavior whereby C–O
bonds are reset at elevated diagenetic or metamorphic temperatures,
sometimes without obvious mineral alteration. Laboratory studies have
constrained this phenomenon by heating well-characterized materials at
various temperatures, observing temporal ∆47 evolution, and fitting
results to kinetic models with prescribed C–O bond reordering
mechanisms. While informative, these models are inflexible regarding the
nature of isotope exchange, leading to potential uncertainties when
extrapolated to geologic timescales. Here, we propose a ”disordered”
kinetic framework to circumvent this issue by modeling C–O bond
reordering as a continuum of first-order processes occurring in parallel
at different rates. We show theoretically that all previous models are
specific cases of disordered kinetics; thus, our approach reconciles the
transient defect/equilibrium defect and paired reaction-diffusion
models. We estimate the rate coefficient distributions from published
heating experiment data by finding a regularized inverse solution that
best fits each ∆47 timeseries. Importantly, this approach does not
assume a particular mechanism or energy distribution a priori. Resulting
distributions are well-approximated as lognormal for all experiments on
calcite or dolomite; aragonite experiments require more complex
distributions. Presuming lognormal rate coefficient distributions and
Arrhenius-like temperature dependence yields an underlying activation
energy, E, distribution that is Gaussian with a mean value of μE =
224.3±27.6 kJ mol−1 and a standard deviation of σE = 17.4 ± 0.7 kJ mol−1
(±1σ uncertainty; n = 24) for calcite and μE = 230.3 ± 47.7 kJ mol−1 and
σE = 14.8 ± 2.2 kJ mol−1 (n = 4) for dolomite. These model results are
adaptable to other minerals and may provide a basis for future
experiments whereby the nature of carbonate C–O bonds is altered (e.g.,
by inducing mechanical strain or cation substitution). Finally, we apply
our results to geologically relevant heating/cooling histories and
suggest that previous models underestimate low-temperature alteration
but overestimate ∆47 blocking temperatures.