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Dolomite found in nature is generally thought to be either "primary" in origin, or to be the result of diagenesis of pre-existing calcium carbonate sediments, or else to have been formed under different conditions by both mechanisms. In an effort to elucidate the mechanism of formation of dolomite, attention has been given to the evidence which might be given by the isotopic compositions of dolomite, partially dolomitized limestones, and limestones.
Theoretical studies indicate that there should be little, if any, difference in the carbon isotopic composition of dolomite regardless of whether it is of primary or secondary origin. Measurements of carbon isotope abundance in limestone and continuous dolomitized limestone show no significant differences. Consideration of the circumstances and processes which might affect oxygen isotopic composition indicates that these isotopes are of little help in revealing the mechanism of formation of dolomite.
In theory, study of the magnesium isotopic composition of dolomite and partially dolomitized limestone should indicate whether dolomitization has resulted, in particular cases, from diffusion of magnesium salts in solution into pre-existing calcium carbonates. Measurements have been made of magnesium isotope abundances in carbonate rocks from several different geologic situations, and attempts have been made to interpret the data in terms of the probable mechanism of dolomitization.
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