Oxygen-Isotope Fractionations in Systems Containing Dolomite

Abstract

Oxygen-isotope fractionation was determined in the laboratory at various temperatures in the range 200°-800° C. in the following systems: dolomite-water, dolomite-calcite, dolomite-carbon dioxide, and calcite-carbon dioxide. Over the range 300°-510° C. the dolomite-water equilibrium fractionation can be represented by the empirical equation 1000 in $$a_{dolomite-water} = 3.20 \times 10^{6} T^{-2 } - 2.00$$ and the dolomite-calcite fractionation by the equation 1000 in $$a_{dolomite-water} = 0.50 \times 10^{6} T^{-2 }$$. In all the systems investigated, the exchange rates and stability of dolomite were less than for calcite. Consequently, a procedure was developed for determination of equilibrium fractionation factors from sets of experiments involving incomplete isotopic exchange. Oxygen exchange was found to occur between dry, solid carbonate phases in the range 500°-800° C, but at a rate much slower than cation exchange in the same system. The resulting isotopic fractionations were consistent with measurements in hydrothermal systems. Exchange between carbonates and carbon dioxide at low pressures was investigated from 200° to 400° C. and was found to be controlled by a surface layer whose isotopic composition and exchange properties differed from those of the bulk carbonate. Exchange rates were low, and equilibrium between the solid and gas was not attained.