Oxygen Isotope Fractionation between Coexisting Calcite and Dolomite in the Freshwater Upper Carboniferous Freeport Formation

Abstract

HYDROTHERMAL experiments of Epstein, Graf and Degens¹ have suggested that dolomite forms from pre-existing calcite by a replacement mechanism in which the ¹⁸O/¹⁶O ratio of the original calcite remains unchanged. In an investigation of natural sedimentary dolomite-calcite rocks, Degens and Epstein² observed no fractionation of the oxygen isotopes between coexisting calcite and dolomite in relatively young deposits and concluded that all dolomite is secondary, that is a replacement product of pre-existing calcite. Older carbonate rocks examined by those authors, however, did contain dolomite which was enriched in oxygen-18 with respect to the associated calcite. This phenomenon was explained by possible isotopic equilibration of the calcite with intrastratal fluids (which are generally enriched in oxygen-16). Dolomite is believed to be more resistant to equilibration. Hence, according to Degens and Epstein, such calcite rocks are partly replaced by diagenetic dolomite without alteration of the isotopic composition of the carbonate. With time, oxygen isotope exchange between formation waters and the calcite, but to a negligible extent between the waters and the dolomite, results in a higher ¹⁸O/¹⁶O ratio in the dolomite. Thus the difference in δ¹⁸O_dol and δ¹⁸O_ct observed in ancient carbonate rocks does not represent original isotopic equilibrium between these two minerals.