Investigating the effect of cation substitution and mineralogy on the dual clumped isotope composition of carbonates
Authors: Bernecker Miguel , Bonifacie Magali , Staudigel Philip , Petrash Daniel , Haussühl Eiken , Dietzel Martin , Davies Amelia , Tagliavento Mattia , Siebert Julien , Wehr Nicolas , Fiebig Jens 
Published: March 2024

Dual-clumped isotope thermometry relies on the joint measurement of &#8710;47&#160;and &#8710;48&#160;in CO2&#160;evolved from phosphoric acid digestion of carbonates (Fiebig et al., 2019). The benefit over &#8710;47-only measurements is its capability to identify if &#8710;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 &#8710;63&#160;and &#8710;64&#160;in carbonates are technically not feasible. During acid digestion of carbonates, fractionations of clumped isotopes (&#8710;63&#160;&#8594;&#160;&#8710;47&#160;and &#8710;64&#160;&#8594;&#160;&#8710;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 &#8710;47&#160;&#8211; 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; M&#252;ller 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 >&#160;20&#160;ppm to 7-9&#160;ppm for &#8710;47&#160;(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 &#8710;47&#160;and &#8710;48&#160;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 &#8710;47-&#8710;48&#160;-T framework established for calcite (Fiebig et al., 2021) is extendable to aragonite and dolomite.&#160;&#160;Defliese, W.F. et al.&#160;Chem. Geol.&#160;396, 51&#8211;60 (2015).Murray, S.T. et al.&#160;Geochim. Cosmochim. Acta&#160;174, 42&#8211;53 (2016).M&#252;ller, I.A. et al.&#160;Chem. Geol.&#160;449, 1&#8211;14 (2017).Bonifacie M. et al.&#160;Geochim. Cosmochim. Acta&#160;200, 255-279 (2017).Fiebig, J. et al.&#160;Chem. Geol.&#160;522, 186&#8211;191 (2019).Bajnai, D. et al.&#160;Nat. Commun.&#160;11, 4005 (2020).Fiebig, J. et al.&#160;Geochim. Cosmochim.&#160;Acta&#160;312, 235&#8211;256 (2021).Bernecker, M. et al.&#160;Chem.&#160;Geol.&#160;642, 121803 (2023).