Oxygen Isotope Studies on the Origin of Dolomites

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