The kinetics of clumped-isotope reactions in calcite and apatite from natural and experimental samples

Measurements of clumped isotopes of carbonate-bearing minerals are a powerful tool for reconstructing past surface temperatures and thermal histories of shallow crustal rocks. Because the clumped-isotope thermometer is based on homogenous-phase equilibrium, a sample's clumped-isotope temperature is susceptible to resetting through, for example, intracrystalline diffusion and redistribution of C and O isotopes during (re)heating or slow cooling. Quantitative knowledge of the kinetics of this resetting have received increasing attention (1-3) and is critical for understanding the meaning of clumped-isotope temperatures of samples with complex burial histories. To better constrain these kinetics and complement previous work (1-3) we performed heating experiments (400-700°C) on optical calcites and carbonate-bearing apatites. As previously observed (2-3), calcites exhibit non-first-order kinetics. The data were fit using a model that incorporates both diffusion and isotope-exchange reactions (4). The kinetics derived with this model using the optical-calcite heating experiments of (2) and those measured here are indistinguishable. The model predicts that subtle changes (>10°C) in measured calcite clumped-isotope temperatures can occur at burial temperatures between 60-100°C on million-year timescales. Though small, such changes may have an impact on clumped-isotope-based reconstructions of past surface temperatures and thermal histories. The derived kinetics were compared to clumped-isotope measurements of cogenetic calcites and apatites from slowly cooled carbonatite intrusions. Apparent temperatures are 70-140°C for apatites and 120-190°C for calcites. Measured temperatures for calcites match modeled temperatures using reasonable geological cooling rates. Natural apatite samples yield lower apparent temperatures than predicted based on the model. We propose that this difference is the result of annealment of structural damage in apatites (e.g., generated by radiation damage) during experiments. (1) Dennis and Schrag, 2010; (2) Passey and Henkes, 2012; (3) Henkes et al., 2014. (4) Zhang et al., 1995.