Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions

Kimberlites from Southern Africa, along with their low-Cr megacrysts, have unusual Hf-Nd isotopic characteristics.Group I and Transitional kimberlites define arrays trending oblique to, and well below, the Nd-Hf isotope 'mantle array', defined by oceanic basalts, i.e. they have negative De Hf values.Group I kimberlites have De Hf values varying from À1.2 to À10.1.Low-Cr megacryst suites from individual Group I kimberlites have compositions that overlap those of their host kimberlites.The trend for all Group I kimberlite megacrysts (De Hf values À1.0 to À9.0) shows a striking correspondence to that of the Group I kimberlite field.Group II kimberlites and their low-Cr megacrysts plot on or close to the mantle Nd-Hf array (De Hf values 3.6 to À2.6).The data indicate a genetic link between kimberlites and the low-Cr megacryst suite.The negative De Hf characteristics of Group I kimberlites and their megacrysts require a source component that is ancient (> 1 Ga), and has evolved with low time-integrated Lu/Hf relative to Sm/Nd.Our preferred option is that this component originates beneath the lithosphere, from a reservoir of ancient, deeply subducted oceanic basalt that became incorporated into the convecting mantle source region for Group I and Transitional kimberlites.