Calibration of the dolomite clumped isotope thermometer from 25 to 350°C, and implications for a universal calibration for all (Ca, Mg, Fe)CO_3 carbonates

Abstract

Carbonate clumped isotope thermometry is based on the temperature-dependent formation of ^(13)C^(18)O^(16)O_2^(2-) ion groups within the lattice of solid carbonate minerals. At low temperatures the bonds between rare, heavy ^(13)C and ^(18)O isotopes are thermodynamically favored, and thus at equilibrium they are present in higher than random abundances. Here we calibrate the use of this temperature proxy in a previously uncalibrated carbonate phase — dolomite [CaMg(CO_3)_2] — over a temperature range that extends to conditions typical of shallow crustal environments, by determining the Δ_(47) values of CO_2 extracted from synthetic or natural (proto)dolomites grown at known temperatures from 25 to 350°C and analyzed in two different laboratories using different procedures for sample analysis, purification and post-measurement data treatment. We found that the Δ_(47) – 1/T^2 dependence for (proto)dolomite is linear between 25 and 350°C, independent of their Mg/Ca compositions or cation order (or the laboratory in which they were analyzed), and offset from, but parallel to, the theoretical predictions of the Δ_(63) dependence to temperature of the abundance of the ^(13)C^(18)O^(16)O_2 isotopologue inside the dolomite and calcite mineral lattices as expected from ab-initio calculations (Schauble et al., 2006). This suggests that neither the equilibrium constant for ^(13)C–^(18)O clumping in (proto)dolomite lattice, nor the experimental fractionation associated with acid digestion to produce CO_2, depend on their formation mechanism, degree of cation order and/or stoichiometry (ie., Mg/Ca ratio) and/or δ^(18)O and δ^(13)C compositions (at least over the range we explored). Thus, we suggest the following single Δ_(47) – 1/T^2 linear regression for describing all dolomite minerals: with T in kelvin and Δ_(47) in the Carbon Dioxide Equilibrium Scale (CDES) of Dennis et al. (2011) and referring to CO_2 extracted by phosphoric acid digestion at 90°C. The listed uncertainties on slope and intercept are 95% confidence intervals. The temperature sensitivity (slope) of this relation is lower than those based on low temperature acid digestion of carbonates, but comparable to most of those based on high temperature acid digestion (with however significantly better constraints on the slope and intercept values, notably due to the large range in temperature investigated and the large number of Δ_(47) measurements performed here, n = 67). We also use this dataset to empirically determine that the acid fractionation factor associated with phosphoric acid digestion of dolomite at 90°C (Δ∗_(dolomite90)) is about + 0.176‰. This is comparable to the Δ∗_(calcite90) experimentally obtained for calcite (Guo et al., 2009), suggesting that the acid fractionation Δ∗ for acid digestion of dolomite and calcite are the same within error of measurement, with apparently no influence of the cation identity. This hypothesis is also supported by the fact that our dolomite calibration data are indistinguishable from published calibration data for calcite, aragonite and siderite generated in similar analytical conditions (ie., carbonate digested at T ⩾ 70°C and directly referenced into CDES), demonstrating excellent consistency among the four (Ca,Mg,Fe)CO_3 mineral phases analyzed in seven different laboratories (this represents a total of 103 mean Δ_(47) values resulting from more than 331 Δ_(47) measurements). These data are used to calculate a composite Δ_(47)–T universal relation for those carbonate minerals of geological interest, for temperatures between -1 and 300°C, that is found to be statistically indistinguishable from the one described by dolomite only: Thus, in order to standardize the temperature estimates out of different laboratories running high temperature digestion of (Ca,Mg,Fe)CO_3 carbonate minerals, we recommend the use of this single Δ_(47)-T calibration to convert Δ_(47CDES) data into accurate and precise temperature estimates. More widely, this study extends the use of the Δ_(47) thermometry to studies of diagenesis and low-grade metamorphism of carbonates with unprecedented precision on temperature estimates based on Δ_(47) measurements — environments where many other thermometers are generally empirical or semi-quantitative.

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