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 sedimentary basins based on the kinetics of clumped isotope reordering of carbonates [1,2,3,4]. The activation energies of dissociation of C-O bonds and O diffusion in the lattice vary in the carbonates with mineral structures, cations, concentration of trace elements and grain sizes. 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 74% (0.392‰) reordering. But witherite and calcite only underwent 52% (0.500‰) and 19% (0.644‰) reordering respectively. We used the disordered kinetic model [5] [6] 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 (µ_E) and 4.5 ± 13.9 kJ/mol (𝜎_E) for strontianite and 95.2 ± 2.9 kJ/mol (µ_E) and 43.4 ± 15 kJ/mol (𝜎_E) for witherite. Their mean activation energies are less than the half of those for calcite and dolomite in the literature [5], 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 suggest 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.

References

[1] Passey and Henkes, 2012.

[2] Stolper and Eiler 2015.

[3] Lloyd et al., 2018.

[4] Chen et al., 2019.

[5] Hemingway and Henkes, 2021.

[6] Hemingway, J.D., isotopylog: open-source tools for clumped isotope kinetic data analysis, 2020, http://pypi.python.org/pypi/isotopylog [online; accessed 2021-03-12].