The carbon and oxygen isotopic composition of carbonates from the Oka carbonatite complex, Quebec, Canada

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The Oka complex is a carbonatite belonging to the alkaline intrusive suite of the Monteregian petrographic province. The carbon and oxygen isotopic composition of carbonates from this complex were studied and found to vary by 7%. for carbon and about 13%. for oxygen. The mode of the C 13 - O 18 distribution for the carbonatite lies at C 13 = - 5% . vs. PDB and O 18 = 7.4%. vs. SMOW. The isotopic composition variability could not be related to weathering or hydrothermal alteration. The frequency distributions (650 samples) of carbon and oxygen isotopic compositions are highly skewed toward increasing C 13 and O 18 concentrations. The carbon isotopic composition frequency distribution may be interpreted as the result of a Rayleigh fractionation process taking place during the formation of the carbonatite. The variations in O 18 content are greater than would be expected from such a process; part of the increased variability may be due to isotopic exchange with or the incorporation of the intruded gneisses. Relationships between isotopic composition and rock type could be demonstrated in several cases. With respect to sövites, rauhaugites show a distinct, and carbonates from the alkaline ultrabasic rocks a slight enrichment in C 13 . The oxygen isotopic compositions of sövites and rauhaugites are consistently lighter than those of carbonates from the alkaline ultrabasic rocks, and the carbonates from fenites and silicate rocks, for which a derivation by ultra-fenitization is suggested on petrographic grounds, show the highest average O 18 concentrations. Small gradients in isotopic composition exist in cross sections through the carbonatite complex.

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