Guidelines for Accurate and Precise Stable Isotope Analysis of Calcite, Dolomite, and Magnesite Using a Carbonate Device for Continuous Flow-Isotope Ratio Mass Spectrometry (CF-IRMS)

Rationale: Carbonate minerals are one of the most popular samples for an automated sample preparation system for CF-IRMS, such as GasBench II and iso FLOW, but no standardized analytical protocols exist. This study gives guidelines on optimal analytic conditions for carbon and oxygen isotope analysis of Ca-Mg carbonates when using the carbonate-phosphoric acid reaction method.

Methods: Calcite (CaCO₃-McMaster Carrara), dolomite (CaMg(CO₃)₂-MRSI Dolomite), and magnesite (MgCO₃-ROM Brazil Magnesite) with two grain size fractions (< 74 and 149-250 μm) were reacted with 103% (specific gravity of 1.92) phosphoric acid under He atmosphere in 12-mL borosilicate glass vials to examine the full δ¹³C and δ¹⁸O evolution of acid-liberated CO₂ for an extended reaction time of up to 12-30 days at 25°C and up to 3-7 days at 72°C.

Results: At 25°C, the optimal reaction time of calcite is 1 day for both grain size fractions while the optimal reaction time of 2-10 day is suggested for dolomite with a grain size of < 74 μm. At 72°C, 30-min to 12-h or 45-min to 12-h reaction is optimal for calcite with < 74-μm or 149- to 250-μm grain size fraction, respectively, whereas dolomite requires 12-h to 1-day reaction for both grain size fractions. The only optimal condition for magnesite is 6-7 days of reaction with < 74-μm grain size at 72°C.

Conclusions: To determine precise and accurate δ¹³C and δ¹⁸O values of a carbonate mineral using the carbonate-phosphoric acid reaction method, an optimal reaction time must be assessed for a given analytical condition to avoid nonequilibrium isotope effects and unnecessary oxygen isotope exchange of acid-liberated CO₂ in the carbonate reaction vessel. Our experimental result provides a guideline for the accurate and precise stable isotope analysis of Ca-Mg carbonate minerals.