Oxygen isotope fractionation in carbonate and sulfate minerals

Oxygen isotope fractionations involving carbonates and sulfates have been controversial for a long time. There are important unresolved conflicts among the results of theoretical calculations, experimental measurements and empirical estimates. In this paper, the increment method is adapted to systematically evaluate oxygen isotope fractionations in the carbonates and sulfates. The following sequence of O-18-enrichment in carbonate minerals is obtained: siderite > ankerite > magnesite greater than or equal to dolomite > calcite > aragonite > strontianite > cerussite greater than or equal to witherite. The sequence of O-18-enrichment in sulfate minerals is predicted as follows: anhydrite > celestite > barite > anglesite. The internally consistent fractionation factors for the systems carbonate-water and sulfate-water are acquired for a temperature range of 0 to 1200 degrees C, which are in fair agreement with existing experimental and/or empirical data. The present calculations suggest that dolomite should behave isotopically like calcite; equilibrium fractionation between dolomite and calcite is only 0.56 parts per thousand at 25 degrees C. Aragonite is predicted to be significantly depleted in O-18 relative to calcite; equilibrium fractionation between calcite and aragonite is 4.47 parts per thousand at 25 degrees C. It is possible that polymorphic transition from aragonite to calcite could proceed through an essentially intact oxygen structure without isotopic resetting. As a result, the temperature dependence of oxygen isotope partitioning in aragonite could be conveyed to calcite. Oxygen isotope inheritance in calcite formation by the polymorphic transition may be of critical importance in attempts to resolve dilemma involving fractionations in aragonite-calcite-dolomite-water systems.