Oxygen Isotope Studies on the Origin of Dolomites

Abstract

Oxygen isotope measurements were made on coexisting dolomite and calcite pairs both from recent sedimentary deposits and from high temperature laboratory syntheses. In the limited number of synthetic carbonate samples thus far available, it appears that there is an oxygen isotope fractionation between dolomite and calcite, similar to that found in naturally occurring hydrothermal calcite-dolomite pairs. There is a lack of such fractionation between dolomite and calcite in young sedimentary deposits. It is therefore concluded that dolomite is a secondary product resulting from addition of magnesium to existing crystalline CaCO_3, which takes place without an isotopic change in the oxygen of the precursor CaCO_3. It is also shown that the O^(18)/O^(16) ratio of dolomite is less susceptible to diagenetic reequilibration with surrounding fluids than is that of calcite.