Differences and controls of carbon and oxygen isotope composition in dolomite and coexisting calcite under deposition conditions

We discussed the composition and factors influencing carbon and oxygen isotopes in dolomites and coexisting calcites in hand specimen mesoscale based on petrography and combined with elemental, X-ray diffraction and carbon and oxygen isotope analyses. Results indicated that dolomites have higher δ¹³C and δ¹⁸O values than coexisting calcites. Two factors may account for the higher δ¹⁸O values of dolomites. (1) The oxygen isotope fractionation factors of a dolomite-water system are higher than those of a calcite-water system. Therefore, oxygen isotope fractionation factors are greater than 1 in a dolomite-calcite system. (2) The fluids of dolomite precipitation have higher salinity and higher ¹⁸O abundance compared to coexisting calcite. In addition, the δ¹³C values of dolomites show the same trends with δ¹⁸O values. That is to say, the carbon isotope fractionation factors in a dolomite-CO₂ system are higher than those in a calcite-CO₂ system. Therefore, carbon isotope fractionation factors are greater than 1 in a dolomite-calcite system during replacement process. Oxygen isotope values varied obviously between syndepositional dolomites and coexisting calcites, while they were similar between hydrothermal/deeply burial dolomites and coexisting calcites.