Carbon and Oxygen Isotope Geochemistry of the Amba Dongar Carbonatite Complex, Gujarat, India

A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sövite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in ¹³C coupled with a sizeable increase in ¹⁸O. From an average of −4.6 ± 0.4 ‰ PDB] for the least differentiated (coarse) sövite member, δ¹³C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ¹⁸O rises from 10.3 ± 1.7 ‰ SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ¹³C and δ¹⁸O over the δ¹⁸O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of ¹⁸O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of ¹⁸O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic ¹³C/¹²C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.