Paired Δ47 and Δ48 analyses and model calculations constrain equilibrium, experimentally-manipulated kinetic isotope effects, and mixing effects in calcite

Abstract

The high-precision analysis of the abundance of the dominant m/z 47 CO2 isotopologue derived from acid digestion of carbonate minerals (13C18O16O; denoted by Δ47) forms the basis for carbonate clumped isotope thermometry. Since the first measurements were published 16 years ago, considerable effort has gone into characterizing the relationship between Δ47 and carbonate precipitation temperature, and in identifying carbonates that do not achieve isotopic equilibrium. Mass spectrometry is now capable of the paired measurement of the primary m/z 47 and m/z 48-isotopologues (Δ47 and Δ48; 12C18O2 is denoted by Δ48), which has the potential to place additional constraints on kinetic isotope effects in carbonate minerals and trace distinct reaction pathways. Here, we explored factors that contribute to calcite mineral equilibrium and disequilibrium in Δ47 and Δ48 using a combination of experiments and theoretical calculations with three types of models. We precipitated calcite at pH 8.3 with carbonic anhydrase (CA) to approach quasi-isotopic equilibrium in the dissolved inorganic carbon pool and report values for Δ47, Δ48, and oxygen isotopes (δ18O) for calcite grown over a temperature range from 5 to 25 oC and compare our findings to predictions from an ion-by-ion model that support equilibrium precipitation. We also compare results to the Devils Hole slow-growing cave calcite, and other published temperature calibration data. We report the following combined equilibrium calibration relationships: Δ48 CDES 90 = (0.429 ± 0.010) Δ47 CDES 90 - (0.006 ± 0.006); r2 = 0.98; Δ47 I-CDES = (0.037 ± 0.001) × 106T-2 + (0.178 ± 0.009); r2 = 0.99; Δ48 CDES 90 = (0.015 ± 0.0005) × 106T-2 + (0.078 ± 0.006); r2 = 0.98. We used paired measurements of Δ47 and Δ48 to constrain kinetic isotope effects in calcite precipitated at pH ranging from 8.3-11 and temperatures from 5 to 25 oC, with and without CA present, and observe kinetic enrichments in Δ47, negative (hyperstochastic) values for Δ48, and depleted values of δ18O, compared to equilibrium values. Experimentally constrained kinetic trajectories, when compared with an ion-by-ion model and IsoDIC theoretical predictions, are consistent with CO2 hydration/hydroxylation. Mixing drives elevated Δ47 and Δ48 values and was assessed using mixing experiments with endmembers of varying isotopic compositions and compared to a Δ47 and Δ48 mixing model that constrains nonlinear mixing trajectories for calcite. While mixing may induce artifacts in two-component mixtures when endmember bulk compositions differ by more than 7 ‰, or if endmember Δ47 and Δ48 differ by more than 0.03 ‰, this should be detectable and potentially correctible using paired clumped isotope measurements and is unlikely to be important for some materials.

Clumped Isotope Thermometry in Plant‐Derived Carbonates

“…of that expected for dual clumped isotopic equilibrium, with all but one being because Δ 48 values are too high compared to that expected for dual clumped‐isotope equilibrium. These enriched values could be due to the role of kinetic processes during carbonate growth such as CO 2 degassing, isotope exchange between dissolved inorganic species and water, and/or diffusive effects (Guo, 2020) as well as mixing between DIC or carbonate pools with different δ 18 O values (Lucarelli, Purgstaller, et al., 2022). Alternatively, the enriched Δ 48 values could be due to contamination in some samples that affects clumped isotopic measurements that was not identified during our filtering process.…”