The isotopic composition of secondary dolomites

Abstract The carbon-13 contents of secondary dolomites from various localities in Europe, Canada and the Pacific are very constant within individual deposits and generally fall into the upper range of values obtained from associated limestones. Petrographic evidence indicates precipitation of these dolomites from solution and the constant C 13 C 12 ratios suggest that isotopic equilibrium existed during their formation. The oxygen isotopic composition of these dolomites could thus be controlled by a carbonatewater isotopic equilibrium. Extrapolation from high temperature experiments and observations on sedimentary dolomite-calcite pairs predict an O18 enrichment of 5–7 per mil in the dolomite compared to calcium carbonate deposited in the same environment. Yet there is relatively little difference between the isotopic composition of the two carbonates from these occurrences. This indicates that the isotopic composition of the dolomites was determined by an isotopic equilibrium different from the extrapolated dolomite-water equilibrium. It is proposed that secondary dolomites precipitated as protodolomites and that their oxygen isotopic composition was controlled by the protodolomite-water fractionation. Experimental data demonstrate that the isotopic difference between protodolomite and CaCO3 is considerably smaller than that between dolomite (extrapolated) and water; this could explain the observed small differences of the oxygen isotope composition between secondary dolomites and associated limestones.

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