STABLE CARBON AND OXYGEN-ISOTOPE ANALYSIS OF NATURAL CALCITE AND DOLOMITE MIXTURES USING SELECTIVE ACID-EXTRACTION

Accurate analysis of coexisting calcite and dolomite for delta(13)C an d delta(18)O is generally hindered by the difficulty in the physical s eparation of these minerals. The difference in the reaction rates of c alcite and dolomite with phosphoric acid, has been used to chemically separate these carbonates. Previously established procedures are only suitable for samples containing 1:1 calcite-dolomite pairs. However, t hese are sometimes tedious and time-consuming. Using mixtures of calci te and dolomite, two different selective acid extraction procedures (f or reactions at 25 degrees C), one for calcite-rich and the other for dolomite-rich;carbonates, have been developed. Reduction of grain size (to 37 mu m - 45 mu m) increased the reaction rare and hence decrease d the total reaction time to a reasonable interval (less than or equal to 72 hours for dolomite); Applying these procedures, delta(13)C and delta(18)O of calcite and dolomite/ankerite of carbonatites' from Amba Dongar and Sung Valley carbonatite complexes of India were measured a nd their temperatures of crystallization determined. Accurate isotopic compositions of individual carbonates of calcite-dolomite mixtures ca n be determined for geological interpretations by these procedures.