Investigating the effect of cation substitution and mineralogy on the dual clumped isotope composition of carbonates

Dual-clumped isotope thermometry relies on the joint measurement of ∆₄₇ and ∆₄₈ in CO₂ evolved from phosphoric acid digestion of carbonates (Fiebig et al., 2019). The benefit over ∆₄₇-only measurements is its capability to identify if ∆₄₇was affected by rate-limiting kinetics in addition to temperature, and to reconstruct accurate carbonate formation temperatures devoid of this kinetic bias (Bajnai et al., 2020).

Direct measurements of ∆₆₃ and ∆₆₄ in carbonates are technically not feasible. During acid digestion of carbonates, fractionations of clumped isotopes (∆₆₃ → ∆₄₇ and ∆₆₄ → ∆₄₈) occur, but the exact magnitudes of acid fractionation factors (AFFs) are not consistently established and vary across different published sources.

Theoretical models by Guo et al. (2009) indicate cation-dependent differences in AFFs for different carbonate mineralogies. Follow-up empirical studies yielded somewhat inconsistent results for ∆₄₇ – some did not observe any differences in AFFs (e.g., Defliese et al., 2015 for calcite, aragonite, and dolomite; Bonifacie et al., 2017 for calcite and dolomite), whereas others did report differences (e.g., Murray et al., 2016 for calcite and dolomite; Müller et al., 2017 for calcite, aragonite, dolomite, and magnesite).

Advancements in gas source mass spectrometry have led to significant improvements in the long-term external repeatability of clumped isotope measurements, e.g., from > 20 ppm to 7-9 ppm for ∆₄₇ (Bernecker et al., 2023). With this improved analytical set-up, we analyzed an assorted collection of scrambled aragonite, calcite, dolomite, siderite and witherite samples for their ∆₄₇ and ∆₄₈ values. We show that cation substitution and mineralogy have no effect on AFFsfor aragonite , calcite, dolomite and witherite. Moreover, the dual clumped isotope compositions of additionally investigated low-temperature aragonite and dolomite samples plot indistinguishable from the calcite equilibrium line. Altogether these findings strongly imply that the ∆₄₇-∆₄₈ -T framework established for calcite (Fiebig et al., 2021) is extendable to aragonite and dolomite.

 

 

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Murray, S.T. et al. Geochim. Cosmochim. Acta 174, 42–53 (2016).

Müller, I.A. et al. Chem. Geol. 449, 1–14 (2017).

Bonifacie M. et al. Geochim. Cosmochim. Acta 200, 255-279 (2017).

Fiebig, J. et al. Chem. Geol. 522, 186–191 (2019).

Bajnai, D. et al. Nat. Commun. 11, 4005 (2020).

Fiebig, J. et al. Geochim. Cosmochim. Acta 312, 235–256 (2021).

Bernecker, M. et al. Chem. Geol. 642, 121803 (2023).