Identifying the asthenospheric component of kimberlite magmas from the Dharwar Craton, India

Bulk-rock samples of kimberlites from the Narayanpet and Wajrakarur kimberlite fields of the Dharwar Craton, India have been analysed for major and trace element composition and Nd and Hf isotope ratios. Despite the effect of post-emplacement alteration on a number of elements, the dataset allows inferences to be made regarding kimberlite petrogenesis and the nature of the magma source. It is argued that no significant compositional differences exist between kimberlites of the Narayanpet and Wajrakarur kimberlite fields, despite evidence for significantly different sub-continental lithospheric mantle domains beneath each field. In addition, Nb/U and Ce/Pb ratios are distinct from lithospheric xenoliths, but similar to sub-lithospheric mantle-derived rocks, lending support to models of kimberlite genesis involving direct derivation from asthenospheric depths. We argue that the observed Nd and Hf isotopic compositions of Indian kimberlites were not detectably impacted by the assimilation of sub-continental lithospheric mantle material, and faithfully record the characteristics of an asthenospheric source. Further, based on low εHf_i relative to εNd_i of samples we conclude that the asthenospheric component must have originated from within or below the transition zone, and may represent ancient subducted oceanic crust.