CO2 is normally isotopically depleted compared to carbonate rocks (e.g. limestone). However, CO2 liberated during decarbonation of carbonate rocks are enriched in both O18 and C13. Why?
Read about CO2 isotopolgues‘ stability as combinations of rare isotopes (13C, 17O, and 18O ) are relatively stable than the abundant ones including 12C and 16O. Check out the various permutations of CO2 isotopologues, you may find answer to your question.
Good luck!
Please have a look over this article:
https://academic.oup.com/petrology/article-abstract/10/2/307/1559479?redirectedFrom=fulltext
ORIGIN OF DISSOLVED MINERAL CARBON
The stable 13C / 12C isotope ratios are expressed in d13C relative to a reference standard (P.D.B.), a belemnite from the Pee Dee formation of the Cretaceous of South Carolina. The d13C of dissolved mineral carbon, bicarbonates (HCO3-) and gases (CO2) allow, when used with care, to trace the origin of carbon. The problem is particularly acute for gaseous mineral sources. The carbonate mineralization of groundwater is commonly acquired by dissolution, on the one hand from soil CO2 resulting from the decomposition by microorganisms of soil organic matter, on the other hand from soil carbonates and rocks in contact with water. Organic matter has an isotopic composition of -27± 5 and biogenic CO 2 slightly enriched with an average value of -22 (+/- 2). The limestone matrix of marine sediments has an average isotopic composition of ± 2 in d13C. If the carbonates are dissolved without isotopic fractionation (or very little fractionation (-1 to -2 at 25 ° C),) the same is not true of the CO2 solution, and especially from its dissociation into bicarbonates which enriches d13C by +8. Thus the d13C of dissolved mineral carbon in groundwater depends on the proportions of dissolved CO2 and bicarbonates and therefore on the partial pressure of CO2 and the pH. It evolves in the aquifer in a closed environment depending on the state of saturation of the water vis-à-vis the carbonates of the reservoir. The d13C of the mineral carbon are under normal conditions between -18 and -14 at saturation. The possible precipitation of calcite slightly depletes the dissolved mineral carbon by balance effect because it is enriched by approximately 2.
Thank you Dr. Ashraf for directing me to the paper. Actually before putting up the question I have referred to it. But I did not find a suitable explanation to my question. If you can explain, it will be nice.
Regards,
Himadri Basu