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A metasomatic origin for late Archean eclogitic diamonds: Implications from internal morphology of diamonds and Re-Os and S isotope characteristics of their sulfide inclusions from the late Jurassic Klipspringer kimberlites

Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch 7701, South Africa, e-mail: kjw{at}geology.uct.ac.za

J.J. Gurney

Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch 7701, South Africa, e-mail: jjg{at}geology.uct.ac.za

R.W. Carlson

Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC, 20015 USA, e-mail: carlson{at}dtm.ciw.edu

S.B. Shirey

Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC, 20015 USA, e-mail: shirey{at}dtm.ciw.edu

E.H. Hauri

Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC, 20015 USA, e-mail: hauri{at}dtm.ciw.edu

S.H. Richardson

Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch 7701, South Africa, e-mail: shr{at}geology.uct.ac.za

The majority of the Klipspringer eclogitic diamonds display complex crystal zonation with zones of N- and H-rich cubic diamond alternating with zones of N-poor octahedral diamond. These growth features were likely related to different levels of carbon supersaturation in the diamond forming fluid(s). The diamonds are younger than their host rock and formed through differential flux of fluids into the eclogite.

Re-Os isotope systematics of sulfide inclusions yield an isochron age of 2554 ± 150 Ma. The age overlaps the break-up of the Vaalbara supercontinent, the formation of the Ventersdorp lavas and a major re-activation of the Thabazimbi-Murchinson lineament into which the Klipspringer kimberlites intruded 148 ± 4 Ma ago. While a separate, relatively older generation of Klipspringer diamonds (Westerlund and Gurney, 2004) may be related to a proposed ~2.9 Ga craton-wide eclogitic diamond formation event (Shirey et al., 2001; 2002), a temporal connection between the current diamonds and the Ventersdorp lavas is inferred.

Sulfur isotope compositions (³⁴S = –1.8 to +2.4 ) of the sulfide inclusions overlap mantle values and large S isotope excursions indicative of altered MORB and associated sediment protoliths are absent. The data are compatible with slightly altered oceanic basalt or sub-lithospheric mantle sources for the S in these inclusions.

The radiogenic initial Os isotope composition of the sulfide inclusions (¹⁸⁷Os/¹⁸⁸Os of 0.187 ± 0.046; _Os = +69) is not compatible with an origin of the PGE in the sulfides from the Ventersdorp lavas. The diamond growth characteristics along with the S and Os isotopic composition of their sulfide inclusions could be accounted for by the remobilization of local fluids in or proximal to the pre-existing eclogite host rock. We propose that the event that generated the Ventersdorp lavas may have raised the ambient lithospheric temperature and triggered fluid remobilization and the formation of the diamonds.

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