Effects of mineralogy on Δ47 and Δ48 of carbonate-derived CO2 below analytical resolution

Abstract

Due to the lack of direct methods capable of determining the abundance of isotopologues containing multiple heavy isotopes within the crystal lattice, carbonates are typically reacted with phosphoric acid to produce CO₂ analyte for clumped isotope analyses. This reaction 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 (${\Delta }_{63}$, ${\Delta }_{64}$) and on acid fractionation factors (${\Delta }_{47}^{\ast }$, ${\Delta }_{48}^{\ast }$) is crucial for accurate temperature reconstructions based on clumped isotope measurements (${\Delta }_{47}$, ${\Delta }_{48}$) of CO₂ extracted from various carbonate mineralogies. Previous studies have yielded contradicting results on the effect of carbonate mineralogy on both ${\Delta }_{47}^{\ast }$ acid fractionation factors and the validity of a universal ${\Delta }_{47}$−T relationship, and, so far, a systematic investigation of mineralogy-specific effects on ${\Delta }_{48}$ and ${\Delta }_{48}^{\ast }$ is lacking.

In this study, we have analyzed the dual clumped isotope composition of stochastic and non-stochastic calcites, aragonites, dolomites, witherites‚ and siderites with unprecedented long-term repeatabilities (1SDs) of 8.6 and 28.1 ppm for ${\Delta }_{47}$ and ${\Delta }_{48}$, respectively. In order to facilitate complete acid digestion of dolomite and siderite in a reasonable timeframe, an acid digestion temperature of 110 °C was used for these minerals instead of the 90 °C applied to calcite, aragonite and witherite. A set of calcite samples was reacted at both temperatures to determine the calcite-specific difference in acid digestion-related fractionation factors between 90 and 110 °C, yielding ${\Delta }_{47,110-{90}^{°}C}^{\ast }$ = -0.0147 ± 0.002 % and ${\Delta }_{48,110-{90}^{°}C}^{\ast }$ = -0.0148 ± 0.006 ‰ (2SEs, n = 8). After projecting ${\Delta }_{47}$ and ${\Delta }_{48}$ results from stochastic dolomite and siderite to the carbon dioxide equilibrium scale (CDES90) using the calcite-specific ${\Delta }_{i,110-{90}^{°}C}^{\ast }$, calcite, aragonite, dolomite, witherite and siderite exhibit statistically indistinguishable ${\Delta }_{47,CDES90}$ and ${\Delta }_{48,CDES90}$ values, with weighted averages of 0.1850 ± 0.0042 ‰ and 0.1255 ± 0.0130 ‰ (weighted 2SEs, n = 15), respectively. In addition, ${\Delta }_{47,CDES90}$ and ${\Delta }_{48,CDES90}$ values of non-stochastic aragonites (n = 2), (proto-)dolomites (n = 2) and witherite (n = 1) correspond to calcite equilibrium values predicted by their independently known formation temperatures (Fiebig et al., 2024). Natural dolomites and siderites of unknown formation temperature are also indistinguishable from the calcite equilibrium line. Overall, these results imply that calcite, aragonite, dolomite and witherite share indistinguishable ${\Delta }_{47}^{\ast }$, ${\Delta }_{48}^{\ast }$ and ${\Delta }_{63}$−${\Delta }_{64}$−T relationships. As a consequence, the calcite-specific equilibrium ${\Delta }_{47,CDES90}$−${\Delta }_{48,CDES90}$−T relationships of Fiebig et al. (2024) can be reliably applied to aragonite, dolomite, and witherite. More precipitation experiments under controlled conditions are necessary to clarify with more confidence if these relationships are also valid for siderite.