Stable carbon and oxygen isotopic compositions of Indian carbonatites

Abstract

Stable carbon and oxygen isotopic compositions of carbonatites are results of fractionation caused by various magmatic and post-magmatic processes during their generation and evolution. In the present work, we review available stable isotopic data from Indian carbonatites that span in age from Precambrian to Cretaceous. We explain the observed variations using various theoretical models and attempt to decipher the nature and temporal evolution of the mantle source(s) of these carbonatites. As observed elsewhere, δ¹⁸O variations are larger compared to those of δ¹³C. However, the average and mode of δ¹³C distributions in Indian carbonatites (-4 %.) are clearly higher than the global average. In general, δ¹³C and δ¹⁸O variations of Indian carbonatites can be grouped into (1) primary, unaltered carbonatites and (2) secondary, altered carbonatites. Primary variations are results of either batch crystallization under plutonic conditions, as observed in Hogenakal and northeastern Indian carbonatites, or fractional crystallization from CO2+H2O fluid-rich parent magmas, as observed in the rest. Secondary isotopic variations in all the carbonatites are apparently results of low temperature alteration by either meteoric water or CO₂-bearing aqueous fluids. Estimated δ¹⁸O values of the mantle sources of Indian carbonatites (5.3-7.5%.) show the expected normal mantle signatures, but δ¹³C values appear to be more variable (-6 to -3.1%.) than expected for a normal mantle. The younger carbonatites (<107 Ma) in particular appear to have been derived from ¹³C enriched sources. Combined study of δ¹³C and ⁸⁷Sr/⁸⁶Sr data suggests that Indian carbonatites were derived from enriched mantle sources, and the enrichment probably took place some time in the Archean (~2.4 Ga). We suggest that the Indian sub-continental mantle, which was metasomatized by fluids from subducted oceanic crusts around that time, has remained a continuous source of carbonatites.