Hf isotope systematics of kimberlites and their megacrysts: New constraints on their source regions

Nowell, G.M; Pearson, D.G; Bell, D.R; Carlson, R.W; Smith, C.B; Kempton, P.D; Noble, S.R

Abstract

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.

Citation

Nowell, G., Pearson, D., Bell, D., Carlson, R., Smith, C., Kempton, P., & Noble, S. (2004). Hf isotope systematics of kimberlites and their megacrysts: New constraints on their source regions. Journal of Petrology, 45(8), 1583-1612. https://doi.org/10.1093/petrology/egh024

• Journal Article Type: Article
• Publication Date: 2004-08
• Deposit Date: Feb 20, 2007
• Journal: Journal of Petrology
• Print ISSN: 0022-3530
• Electronic ISSN: 1460-2415
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 45
• Issue: 8
• Pages: 1583-1612
• DOI: https://doi.org/10.1093/petrology/egh024
• Keywords: isotopes; kimberlites; lutetium–hafnium; megacrysts.
• Public URL: https://durham-repository.worktribe.com/output/1575643