The kinetics of clumped isotope reordering of synthetic inorganic carbonates

In the last decade, the reordering of carbonate clumped isotopes has been applied in reconstructing thermal histories of basins based on the kinetics of clumped isotope reordering of carbonates. We systematically evaluate how mineral structures and types of cations affect the reordering rate and associated activation energy by conducting heating experiments on synthetic calcite, aragonite, strontianite and witherite at temperatures ranging from 185 to 485 under dry conditions. The initial 47 values of carbonate precipitates vary between 0.732 and 0.752. The order of the initial reordering rate of carbonates at all temperatures is strontianite > witherite > aragonite > calcite. For the 385 (30-minute) heating experiment, strontianite underwent 76% (0.387) reordering. But witherite and calcite only underwent 52% (0.503) and 19% (0.644) reordering respectively. Beyond our expectation, witherite shows a slower reordering rate than the other carbonates in the latter period of heating, causing its apparent equilibrium 47 values to be far off the 47equilibrium values, and varying little with temperature. We used the disordered kinetic model to calculate the mean and standard deviation of the activation energy distributions for strontianite and witherite. The model yields 84.3 8.1 kJ/mol (E) and 4.5 13.9 kJ/mol (E) for strontianite and 95.2 2.9 kJ/mol (E) and 43.4 15 kJ/mol (E) for witherite. Their mean activation energies are less than half of those for calcite and dolomite in the literature, leading to the huge difference in reordering rates among the minerals. For example, the reordering rates of strontianite and witherite are 4 - 6 orders of magnitude faster than those of calcite and dolomite at 385 . The significant difference in the reordering activation energies between strontianite and witherite with orthorhombic crystal structure and calcite and dolomite with trigonal crystal structure suggests that crystal structure is the main contributor to the difference in the activation energy. In future, we will further explore the reordering mechanism at the atomic scale by combining modeling of molecule dynamics with reordering kinetic parameters obtained from heating experiments.