Journal of Petrology Advance Access originally published online on July 1, 2004
Journal of Petrology 2004 45(8):1583-1612; doi:10.1093/petrology/egh024
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Journal of Petrology 45(8) © Oxford University Press 2004; all rights reserved
Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions
1 ARTHUR HOLMES ISOTOPE GEOLOGY LABORATORY, DEPARTMENT OF EARTH SCIENCES, DURHAM UNIVERSITY, SOUTH ROAD, DURHAM DH1 3LE, UK
2 ARIZONA STATE UNIVERSITY, CHEMISTRY AND BIOLOGY, PO BOX 871604, TEMPE, AZ 85287, USA
3 CARNEGIE INSTITUTION OF WASHINGTON, 5241 BROAD BRANCH ROAD NW, WASHINGTON, DC 20015, USA
4 DEBEERS CONSOLIDATED MINES, GEOSCIENCE CENTRE, PO BOX 82232, SOUTHDALE GAUTENG 2135, SOUTH AFRICA
5 NERC ISOTOPE GEOSCIENCES LABORATORY, KINGSLEY DUNHAM CENTRE, KEYWORTH, NOTTINGHAM NG12 5HH, UK
* Corresponding author. Telephone: +44 (0) 191 3342339; Fax: +44 (0) 191 3342301. E-mail: g.m.nowell{at}durham.ac.uk
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 
Hf values. Group I kimberlites have Hf values varying from –1.2 to –10.1. 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 (Hf values –1.0 to –9.0) 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 (Hf values 3.6 to –2.6). The data indicate a genetic link between kimberlites and the low-Cr megacryst suite. The negative 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.
KEY WORDS: isotopes; kimberlites; lutetium–hafnium; megacrysts
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