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.