Carbonate clumped isotope bond reordering and geospeedometry

Carbonate clumped isotope thermometry is based on the preference of ¹³C and ¹⁸O to form bonds with each other. At elevated temperatures such bond ordering is susceptible to resetting by diffusion of C and O through the solid mineral lattice. This type of bond reordering has the potential to obscure primary paleoclimate information, but could also provide a basis for reconstructing shallow crustal temperatures and cooling rates. We determined Arrhenius parameters for solid-state reordering of C-O bonds in two different calcites through a series of laboratory heating experiments. We find that the calcites have different susceptibilities to solid-state reordering. Reaction progress follows a first order rate law in both calcites, but only after an initial period of non-first order reaction that we suggest relates to annealing of nonequilibrium defects when the calcites are first heated to experimental temperature. We show that the apparent equilibrium temperature equations (or "closure temperature" equations) for carbonate clumped isotope reordering are analogous Dodson's equations for first order loss of daughter isotopes. For each calcite, the sensitivity of apparent equilibrium temperature to cooling rate is sufficiently high for inference of cooling rates within a factor of ~5 or better for cooling rates ranging from tens of degrees per day to a few degrees per million years. However, because the calcites have different susceptibilities to reordering, each calcite defines its own cooling rate-apparent equilibrium temperature relationship. The cooling rates of Carrara marble inferred from carbonate clumped isotope geospeedometry are 10 ^-6-10 ^-3 degrees per annum and are in broad agreement with rates inferred from thermochronometric methods. Cooling rates for ¹³C-depleted calcites from the late Neoproterozoic Doushantou cap carbonates in south China are on the order of 10 ²-10 ⁴ degrees per annum, consistent with rapid cooling following formation of these calcites by a short-lived hydrothermal event (Bristow et al., 2011, Nature v. 474, p. 68-71). Most of the uncertainty in these estimates relates to uncertainty in Arrhenius parameters for different calcites. Thus, while the carbonate clumped isotope geospeedometer shows promise for recording cooling rates in settings and lithologies where other geospeedometers may not be applicable, the uncertainty in cooling rate will be large without independent knowledge of the reordering kinetics of each study material. Thus the full potential of the method will only be realized if reordering kinetics can be accurately determined for each study material, or predicted on the basis of mineral composition, texture, or other observable parameters.