Sg. Viladkar et M. Schidlowski, Carbon and oxygen isotope geochemistry of the Amba Dongar carbonatite complex, Gujarat, India, GONDWANA R, 3(3), 2000, pp. 415-424
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.