Osmium isotopic composition of carbonatites and associated rocks from the Amba Dongar carbonatite complex, Gujarat reveals presence of old oceanic crust in the source

Amba Dongar carbonatite complex (65±0.3 Ma) is located in Gujarat, India within the Deccan Flood Basalt Province. It intruded into the Bagh beds (~90 Ma). The complex comprises calciocarbonatites, ferrocarbonatites, and alkaline rocks namely nephelinites and phonolites. This study reports ¹⁸⁷Os/¹⁸⁸Os ratios of Amba Dongar rocks with an objective to investigate the carbonatite-source characteristics as well as melt-lithosphere interaction. Interestingly, carbonatites have higher ¹⁸⁷Os/¹⁸⁸O_i (1.45 to 4.32) than the mantle (0.11 to <0.16). Radiogenic isotope mixing (Os vs. Sr and Os vs. Nd) modelling results indicate presence of ~2 Ga old recycled oceanic crust in the source of Amba Dongar carbonatite complex. The stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions of these carbonatites and Bagh limestone track differentiation pathways for carbonatite melts and point to the source of the inherited carbon and oxygen. Composition of melts originating from the mantle (δ¹³C_PDB: -7‰ to -5‰; δ¹⁸O_SMOW: +5‰ to +8‰) sources may be modified by various processes such as fractionation during ascent, isotope effects in magma crystallization, crustal contamination, and post magmatic alteration processes. Amba Dongar calciocarbonatites show a spread from -5.57 to -0.96‰ for δ¹³C and +6.74 to +16.31‰ for δ¹⁸O, and several calciocarbonatites fall inside or near to the mantle derived primary carbonatite field suggests their mantle origin. Assimilation of the host Bagh limestone by ~30% can produce these trends observed for the calciocarbonatites. The δ¹³C and δ¹⁸O values obtained from the carbonate mineral separates (e.g. calcite and ankerite) present within the ferrocarbonatite show negligible isotopic shift as compared to whole rock δ¹³C and δ¹⁸O compositions of same ferrocarbonatite. Contamination by recycled crust having a δ¹⁸O value of 10.5‰ can also play a role in this enrichment, but cannot account for the complete range of δ¹⁸O values shown by the carbonatites. Ferrocarbonatites show much more positive δ¹³C (-3‰ to +0.8‰) and δ¹⁸O (+9.85‰ to +23.77‰) values, which can be attributed to late stage differentiation, hydrothermal alteration as well as influence of varying amounts of sedimentary assimilation and/or recycled crustal components.