Stoichiometry effect on stable isotope analysis of dolomite

Dolomite admixed with calcite in partially dolomitized limestones is often nonstoichiometric. Precise stable isotope compositions of these two minerals in such samples are difficult to determine. Differential acid extraction and weak-acid leaching are the two common methods used. In the present study, we evaluate the validity of these methods. Our results show that the reaction rate between dolomite and phosphoric acid is sensitive to not only grain size but also stoichiometry of dolomite. Nonstoichiometric calcium-rich dolomite is more liable to react with acid than stoichiometric dolomite. This stoichiometry effect increases the difficulty in adopting the differential acid extraction method to recover reliable stable isotope compositions of partially dolomitized samples. Acceptable results could be expected only if sample grain size is well confined and the sample-acid reaction time for calcite-CO2 is properly reduced based on predetermined dolomite chemical compositions. Moreover, the data also imply that differential leaching of dolomite in the weak-acid leaching method is quite probable and may induce additional errors. Such an effect on analytical results must be taken into consideration before making meaningful interpretation on isotopic data of coexisting calcite and dolomite.