Carbon isotopes in Kerguelen plume-derived carbonatites: evidence for recycled inorganic carbon

Carbonatites form from deep mantle melts that are believed to incorporate r ecycled crustal carbon. Most of the evidence in favour of this hypothesis i s, however, circumstantial and comes from the study of radiogenic (Nd-Sr-Pb ) isotopes that show HIMU and EM-I mantle signatures. In this work, we pres ent direct evidence for the incorporation of recycled crustal carbon in car bonatites of Eastern India through a study of their stable isotope systemat ics. The Ar-40/Ar-39,ge of one of these coeval complexes is 107.2 +/- 0.8 M a, which suggests that these carbonatites represent late magmatic pulses of the Rajmahal-Bengal-Sylhet flood basalt province. Their age, spatial proxi mity to the Sylhet traps, HIMU-EM I isotopic signatures, and Sr-isotopic si milarity to the 115-105 Ma old Kerguelen Plateau basalts are consistent wit h the hypothesis of their Kerguelen plume origin. The carbon and oxygen iso tope compositions of three of these carbonatite complexes are homogeneous, unlike most of the carbonatites world-wide, and is suggestive of batch crys tallization of these rocks under plutonic conditions. The delta(18)O values of all the complexes are consistent with their derivation in equilibrium w ith mantle silicates, whereas delta(13)C shows higher values than a 'normal ' mantle (delta(13)C = -5.0 to -8.0 parts per thousand). The homogeneity of isotope compositions, absence of O-18 enrichments, co-precipitation of cal cite and dolomite in isotopic equilibrium and absence of any crustal contam ination effects, preclude the possibility of any change in delta(13)C Of th e carbonatite magmas/rocks by magmatic or secondary fractionation process. Therefore, the delta(13)C values of these carbonatites directly reflect the delta(13)C values of their source regions. As all these complexes probably belonged to a single magmatic episode, the higher delta(13)C of the parent magma (average for all the complexes = -3.2 parts per thousand) than that of a 'normal' mantle is clear evidence for incorporation of recycled inorga nic carbon. We suggest that this incorporation is a result of entrainment o f a subcontinental lithospheric mantle, which was already enriched in C-13 derived from subducted ancient oceanic crusts through mantle metasomatism. (C) 1999 Elsevier Science B.V. All rights reserved.