Carbon- and oxygen-isotope variations in Indian carbonatites

Abstract

Carbon- and oxygen-isotope data are presented for several Indian carbonatites. These carbonatites are classified, based on mineralogy and petrography, as: Ambadungar (holocrystalline sövite and alvikite, with porphyritic calcite carbonatite, brecciated carbonatite, ferro-carbonatite, silicocarbonatite, and tuff carbonatite), Mundwara (holocrystalline sövite), Kala Doongar (holocrystalline sövite), Borra (calcite dolomite/crystalline limestone?), Koratti-Sevathur (holocrystalline sövite and ferro-carbonatite), and Samalpatti (holocrystalline sövite, ankeritic carbonatite, dolomitic carbonatite, and porphyritic carbonatite). Values for δ¹³C and δ¹⁸O of the Mundwara and Koratti-Sevathur carbonatites suggest a deepseated origin. The Ambadungar carbonatites have δ¹³C values similar to the Primary Igneous Carbonatite (PIC) but are enriched in δ¹⁸O. Kala Doongar, Borra, and Samalpatti carbonate rocks contain concentrations of both carbon and oxygen isotopes that are higher than those of the PIC. The mantle δ¹³C values and enrichment of δ¹⁸O in the Ambadungar carbonatites suggest involvement of post-magmatic processes, particularly interaction with meteoric water or isotopic exchange at low temperatures. The Rayleigh fractionation model together with secondary processes, either magmatic or non-magmatic, can explain the enrichment of δ¹³C and δ¹⁸O values in Kala Doongar and Samalpatti carbonatites. The range of δ¹³C and δ¹⁸O values in Borra carbonate rocks suggests a metamorphic, rather than an igneous, origin.