Effects of mineralogy on carbonate clumped isotope composition below analytical resolution

Phosphoric acid digestion of carbonates is associated with fractionations of both bulk oxygen and clumped isotopes. Accurate knowledge of the effect of cation substitution on the degree of isotopic clumping in the carbonate phase (∆₆₃, ∆₆₄) and on acid fractionation factors (∆^*₄₇, ∆^*₄₈) is crucial for accurate temperature reconstructions based on clumped isotope measurements (∆₄₇, ∆₄₈) of the extracted CO₂. Previous studies have yielded contradicting results whether a universal ∆^*₄₇ acid fractionation factor and ∆₄₇-T relationship is valid for all carbonate mineralogies, and a systematic investigation of mineralogy-specific effects on ∆₄₈ and ∆^*₄₈ is still lacking.

We have determined ∆₄₇ and ∆₄₈ values of stochastic (i.e, ∆₆₃ = ∆₆₄ = 0) and non-stochastic calcites, aragonites, dolomites, witherites‚ and siderites with outstanding precision. We demonstrate that stochastic calcite, aragonite, dolomite, witherite and siderite exhibit statistically indistinguishable ∆_{47, CDES90} and ∆_{48, CDES90} values. In addition, ∆_{47, CDES90} and ∆_{48, CDES90} values of non-stochastic aragonites, (proto-)dolomites and witherite correspond to calcite equilibrium values^[1] predicted by their independently known formation temperatures. These results provide evidence that calcite, aragonite, dolomite and witherite share indistinguishable ∆^*₄₇, ∆^*₄₈ and equilibrium ∆₆₃-∆₆₄-T relationships. Consequently, the calcite-specific equilibrium ∆₆₃-∆₆₄-T relationships^[1] can be reliably applied to aragonite, dolomite, and witherite. More investigations are necessary to clarify its validity for siderite.

^[1]Fiebig, J. et al. Chem. Geol. 670, 122382 (2024)