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

Abstract

Carbonatites form from deep mantle melts that are believed to incorporate recycled crustal carbon. Most of the evidence in favour of this hypothesis is, 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 present direct evidence for the incorporation of recycled crustal carbon in carbonatites of Eastern India through a study of their stable isotope systematics. The ⁴⁰Ar/³⁹Ar age of one of these coeval complexes is 107.2±0.8 Ma, which suggests that these carbonatites represent late magmatic pulses of the Rajmahal-Bengal-Sylhet flood basalt province. Their age, spatial proximity to the Sylhet traps, HIMU-EM I isotopic signatures, and Srisotopic similarity to the 115-105 Ma old Kerguelen Plateau basalts are consistent with the hypothesis of their Kerguelen plume origin. The carbon and oxygen isotope compositions of three of these carbonatite complexes are homogeneous, unlike most of the carbonatites world-wide, and is suggestive of batch crystallization of these rocks under plutonic conditions. The δ¹⁸O values of all the complexes are consistent with their derivation in equilibrium with mantle silicates, whereas δ¹³C shows higher values than a 'normal' mantle (δ¹³C = -5.0 to -8.0%.). The homogeneity of isotope compositions, absence of ¹⁸O enrichments, co-precipitation of calcite and dolomite in isotopic equilibrium and absence of any crustal contamination effects, preclude the possibility of any change in δ¹³C of the carbonatite magmas/rocks by magmatic or secondary fractionation process. Therefore, the δ¹³C values of these carbonatites directly reflect the δ¹³C values of their source regions. As all these complexes probably belonged to a single magmatic episode, the higher δ¹³C of the parent magma (average for all the complexes = -3.2%.) than that of a 'normal' mantle is clear evidence for incorporation of recycled inorganic carbon. We suggest that this incorporation is a result of entrainment of a subcontinental lithospheric mantle, which was already enriched in ¹³C derived from subducted ancient oceanic crusts through mantle metasomatism.