NEW INSIGHTS INTO THE GENESIS OF INDIAN KIMBERLITES FROM THE DHARWAR CRATON VIA IN SITU Sr ISOTOPE ANALYSIS OF GROUNDMASS PEROVSKITE

Abstract

In situ Sr isotopic analyses of kimberlitic perovskite are more representative of primary magmatic compositions than conventional bulk-rock analyses of the same samples, because the latter are variably compromised by contamination and alteration processes. Bulk-rock Sr isotopic data obtained for 18 intrusions from the adjacent Narayanpet and Wajrakarur kimberlite fields of the Dharwar Craton, India, exhibit a high degree of scatter (˜0.701-0.709) and have indistinguishable initial isotope ratios. In contrast, laser ablation perovskite results display strikingly uniform and distinct initial 87Sr/86Sr compositions for each field of 0.70312-0.70333 and 0.70234-0.70251, respectively. The increased resolution provided by these new data permits the evaluation of key aspects of kimberlite genesis. It is argued that lithospheric and crustal contamination had a negligible impact on the perovskite Sr isotope compositions, and that these values are representative of the primary melt component in each field. The results cast doubt on models of kimberlite formation that invoke either the small degree melting of metasomatized subcontinental lithospheric mantle, or derivation from unusually enriched asthenospheric mantle. The data are more compatible with a source region for the Dharwar kimberlites that was similar to a common mantle component such as prevalent mantle (PREMA) or focal zone (FOZO).