Clumped isotope bond reordering in dolomite: new experimental constraints based on low temperature sedimentary dolomites

Carbonate clumped isotope thermometry can be used to constrain the formation temperature and the oxygen isotope composition of the fluids involved in the precipitation of carbonate minerals. It exploits the preference of ¹³C-¹⁸O bonds in carbonate molecules to form with decreasing temperature. This method has important applications in reconstructing the temperature history of the ocean through time, paleoaltimetry and diagenesis. Dolomite in the rock record can have multiple origins. It can form as a primary precipitate in seawater, during early diagenesis or as a late burial diagenetic phase. Depending on its origin thus dolomite can provide information on earth surface temperatures, or on the diagenetic history of carbonate sequences, particularly in successions and times in earth history where calcite is less abundant.

The use of clumped isotopes to reconstruct dolomitization conditions in ancient sequences, requires determining if the temperatures reconstructed from clumped isotopes reflect the original temperature of formation and how resistant clumped isotope signals are against bond reordering at elevated temperatures during burial. In this contribution we will present a series of heating experiments at temperatures between 360 and 480 °C with runtimes between 0.125 and 426 hours we used to determine bond reordering kinetic parameters. In contrast to the only existing previous study¹, which used millimeter-sized hydrothermal dolomite, we used fine grained sedimentary dolomites to test the influence of grains size, surface area-to-crystal volume ratio (S/V), and cation ordering on bond reordering kinetics. Specifically, we analysed a lacustrine dolomite with poor cation ordering and compare it to a replacement dolomite with high cation ordering, both being almost perfectly stoichiometric. Experimental results show a higher susceptibility to solid state bond reordering as well as stable isotope depletion in the lacustrine sample, whereas the replacement dolomite is comparatively resistant, similar to previously studied coarse-grained hydrothermal dolomite. We compare our experimental results to previous work on dolomite and different calcites and derive robust, dolomite-specific kinetic parameters for the disordered kinetic model of Hemingway and Henkes². We show that Δ47 reordering in dolomite, similar to calcite, is material specific. Furthermore, in contrast to crystallographically well-ordered dolomite, disordered and microcrystalline dolomite with high S/V ratios shows a rapid depletion in stable-isotope and Δ47 values. The application of existing reordering models to our experimental data stresses the need for further experimental temperature-time series experiments to properly constrain dolomite Δ47 reordering over geologic timescales for different dolomite types.

 

1:    Lloyd MK, Ryb U, Eiler JM (2018) Experimental calibration of clumped isotope reordering in dolomite. Geochim Cosmochim Acta 242:1–20

2:    Hemingway, J., D. and G., H. Henkes, (2021) A disordered kinetic model for clumped isotope bond reordering in carbonates. Earth and Planetary Science Letters, 566, 116962.