Clumped isotope acid fractionation factors for dolomite and calcite revisited: Should we care?

Abstract

During the analysis of the Δ47 values of CO2 derived from carbonates, the acid fractionation factor not only allows Δ47 values produced from carbonates and reacted at different temperatures (usually 25 and 90 °C) to be compared to one another, but also provides an effective tool to better understand the mechanism and rate of isotopic exchange of oxygen with water during the phosphoric acid digestion of carbonate minerals. In this study, experiments have been conducted in which calcite (1 sample) and dolomite (4 samples), with varying Δ47 values, have been digested in phosphoric acid (H3PO4) at five different temperatures between 25 and 90 °C using both the common acid bath and sealed vessel methods. From these experiments, we show that calcite has an acid fractionation factor which is independent of the reaction method. In contrast, all four dolomites show significant differences between the two methods with the sealed vessel reactions showing much smaller acid fractionation factors than the common acid bath. The magnitude of the acid fractionation factor in the dolomites is also dependent upon the absolute Δ47 value of the sample, with samples possessing lower Δ47 values producing lower acid fractionation factors than those with higher Δ47 values. The explanation for the difference in the behavior of acid fractionation factors between calcite and dolomite, in regard to the technique used, is believed to be related to the rate of reaction of the two minerals and thus the time in which the CO2 is in contact with the acid and allowed to exchange with the H2O in the acid and produced during the reaction.