Stable isotope analysis of multiple carbonate samples using selective acid extraction

Abstract

Selective chemical separation techniques for isotopic analysis of coexisting calcite, dolomite, siderite and magnesite have been investigated in this study. Comparison of reaction rates of all pure carbonate minerals with 100% phosphoric acid at 25° and 50°C demonstrated that a uniformly fine grain size (<200 mesh) is required. Tests with fine-grained (<200 mesh) pure calcite, dolomite, siderite and magnesite reacted at 25°C (calcite) and 50°C (others) show that δ 18O of evolved CO 2 increases during the course of reaction. This is probably due to a slight kinetic isotope effect associated with rapid dissolution of sub-micrometer carbonate crystallites adhered to the outer surfaces of the very fine-grained crystals. Isotopic analysis of carbonates based on CO 2 from incomplete acid reactions is accurate only after minimum reaction times. Conversely, much longer reaction times than the minima are not required. Experiments conducted on variable mixtures of calcite-dolomite, dolomite-siderite and calcite-dolomite-siderite demonstrated the magnitudes of cross-contamination on the inferred isotopic compositions of the individual minerals. The isotopic compositions and magnitudes of their uncertainty can be evaluated for individual carbonates selectively extracted from natural mixtures by variable time-temperature protocol.