Nd and Sr isotopes in kimberlites and lamproites from Western Australia: an enriched mantle origin

Abstract

Diamond-bearing kimberlites and lamproites from the Fitzroy area of the West Kimberley region of Western Australia have isotopic compositions (ɛNd = −7.4 to −15.4, 87Sr/86Sr (I) = 0.7104 to 0.7187) indicating derivation from ancient (> 1,000Myr) highly enriched (high Rb/Sr, Nd/Sm) mantle sources. The Nd and Sr isotopic ratios of the kimberlites overlap those of the lamproites, consistent with the petrological and geochemical gradation between the two groups, and implies a genetic relationship. Mixtures of highly enriched mantle like that reported here with depleted MORB-type mantle can, in principle, account for both the enriched and depleted portions of mantle array. Here we present new Nd and Sr isotopic data for a suite of kimberlites and associated lamproites from the Fitzroy area which indicate that the magmas were derived from extremely enriched (high Rb/Sr, Nd/Sm) mantle and cast doubt on the validity of a primitive chondritic Sm/Nd source for kimberlites. We also present a model which accounts for the apparent clustering of many kimberlites near ɛNd∼0, and for the covariation of Nd and Sr isotopes in oceanic basalts (that is, the mantle array).