Abstract
The oxygen isotope fractionation accompanying the hydrothermal dolomitization of CaCO, between 252 and 295°C has been investigated. Dolomitization (which occurs via the crystallization of one or more intermediate ma~esian calcite phases) is characterised by a progressive lowering in 6'*0, which smoothly correlates with the change in the Mg/(Mg + Ca) and the Sr/(Mg + Ca) ratios and with the sequential phase formation. The data support the proposals of KATZ and MATTH~VS (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 magne~an calcite transformation are within the range 3.7-6.7 x 10d5 liters (out of 5 x lo-' 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 100 in aD-Hzo = 3.06 x lo6 T-= - 3.24 Dolomite-water fractionations from this equation are significantly lower than those obtained by extra- polation 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 EKXEIN et al. (1964).
Published Version
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