Oxygen isotope fractionation during the dolomitization of calcium carbonate |
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| Authors: | Alan Matthews Amitai Katz |
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| Institution: | Department of Geology, The Hebrew University of Jerusalem, Israel |
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| Abstract: | The oxygen isotope fractionation accompanying the hydrothermal dolomitization of CaCO3 between 252 and 295°C has been investigated. Dolomitization (which occurs via the crystallization of one or more intermediate magnesian calcite phases) is characterised by a progressive lowering in δ8O, which smoothly correlates with the change in the Mg/(Mg + Ca) and the ratios and with the sequential phase formation. The data support the proposals of Katz and Matthews (1977) that (a) all reaction occurs by solution and reprecipitation, (b) intermediate phases and dolomite form sequentially and (c) the intermediate phases form within limited solution zones surrounding the dissolving precursor. Calculated volumes of the solution zone for the aragonite → low magnesian calcite transformation are within the range 3.7–6.7 × 10?5 liters (out of 5 × 10?3 liters, the volume of the bulk solution used in the present study), and agree well with those calculated from strontium and magnesium partitioning data. Dolomite precipitates in apparent isotopic equilibrium with the bulk solution. The temperature dependence of the fractionation is defined by the equation 1000 InαD-H2O = 3.06 × 106T?2 ? 3.24 Dolomite-water fractionations from this equation are significantly lower than those obtained by extrapolation of the Northrop And Clayton (1966) calibration. The reaction zone model can be applied to explain near zero dolomite-calcite oxygen isotope fractionations reported by Epsteinet al. (1964). |
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