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 differe ntiation series sovite --> alvikite --> ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in C-13 coupled with a sizeable increase in O-18. From an average of -4.6 +/- 0.4 parts per thousand [PDB] for the least differentiated (coarse) sovite member, delta( 13)C values slowly increase in the alvikite (-3.7 +/- 0.6 parts per thousan d) and ankeritic fractions (-3.0 +/- 1.1 parts per thousand), whereas delta (18)O rises from 10.3 +/- 1.7 parts per thousand [SMOW] to 17.5 +/- 5.8 par ts per thousand over the same sequence, reaching extremes between 20 and 28 parts per thousand in the latest generation of ankeritic carbonatite. Whil e an apparent correlation between delta(13)C and delta(18)O over the delta( 18)O range of 7 - 13 parts per thousand conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of O-18 notably in the ankeritic member is d emonstrably related to a late phase of low-temperature hydrothermal activit y involving large-scale participation of O-18-depleted groundwaters. As a w hole. the Amba Dongar carbonatite province displays the characteristic C-13 /C-12 label of deep-seated (primordial) carbon, reflecting the carbon isoto pe composition of the subcontinental upper mantle below the Narmada Rift Zo ne of the Indian subcontinent.