Kaapvaal Craton peridotites

The results of intensive study of Kaapvaal Craton peridotites have implications for different craton generation models and hence for the stabilization of the earliest continents. It has been suggested that the opx-rich Kaapvaal lithospheric mantle could have formed from primitive. Si-enriched material in Hadean time (Herzberg 1993). There are no clear indications of Hadean... 

Figure 38 Nd-Sr isotope variation of clinopyroxenes and garnet in peridotite xenoliths. (a) Compares cratonic and noncratonic peridotite xenoliths with continental crust. Inset shows restricted field for oceanic mantle. Arrow points to a peridotite from Lashaine, Tanzania, that lies at an Sr/ Sr value of 0.83. (b) Compares cratonic peridotites from the Kaapvaal, Wyoming, and Siberian cratons.

Figure 43 CHf versus cnj isotope diagrams for lithospheric mantle peridotite minerals. Kaapvaal peridotite data are ah garnets and clinopyroxenes (Simon et al, 2002). Slave peridotite data are garnets and whole rocks from Schmidberger et al (2002). Salt Lake Crater peridotites (Hawaii), Kilboume Hole and Abyssal peridotites, are from Salters and Zindler (1995). Siberian and Mongohan peridotite Held are chnopyroxene data from cratonic and off-craton peridotites (field taken from Ionov and Weis, 2002). Fields for MORE (N-MORB) and OIB are from Nowell et al (1998). Field for Beni Bousera peridotites from Pearson and Noweh (2003).

Bedini R. M., Blichert-Toft J., Boyet M., and Albarede F. (2002) Lu-Hf isotope geochemistry of garnet-peridotite xenoliths from the Kaapvaal craton and the thermal regime of the lithosphere. Geochim. Cosmochim. Acta (Spec. Suppl.) 66S1, A61. 

Irvine G. J., Pearson D. G., and Carlson R. W. (2001) Lithospheric mantle evolution in the Kaapvaal craton A Re-Os isotope study of peridotite xenoliths from Lesotho kimberlites. Geophys. Res. Lett. 28, 2505—2508. 

Saltzer R. L., Chatterjee N., and Grove T. L. (2001) The spatial distribution of garnets and pyroxenes in mantle peridotites pressure-temperature history of peridotites from the Kaapvaal craton. J. Petrol. 42, 2215-2229. 

Herzberg C. T. (1993) Lithosphere peridotites of the Kaapvaal craton. Earth Planet. Set Lett 120, 13-29. 

Pearson, D. G., Carlson, R. W., Shirey, S. B., Boyd, F. R. Nixon, P. H. 1995. Stabilisation of Archaean lithospheric mantle a Re-Os isotope study of peridotite xenoliths from the Kaapvaal Craton. Earth and Planetary Science Letters, 134(3-4), 341-357. 

The effects of post melt-depletion interaction with fluid or melt components in the lithospheric mantle has been extensively documented (e.g. Menzies Hawkesworth 1987, and references therein Harte et al. 1993 Pearson 1999 ) and it is widely accepted that these phenomena dominate the minor element geochemistry of cratonic peridotites. Extensive studies of the effect of metasomatism on the major element chemistry of lithospheric peridotites have also been made (Boyd Mertzman 1987 Keleman et al. 1992, 1998 Walter 1999). To date, most of the discussion has centred around the apparent excess of orthopyroxene, especially in Kaapvaal peridotites. However, major and trace element studies show that it is likely that the abundances of garnet and clinopyroxene are also grossly affected (Burgess Harte 1999 Shimizu 1999). 

To illustrate the appUcation of these methods we have analysed selected samples from different suites of cratonic peridotites for PGE abundances and Re-Os isotopic composition that had been previously analysed for major elements (Table 1). The sample set comprises three peridotites from the Jericho kimberlite, Northern Slave Craton (Irvine et al. 1999 Kopylova Russell 2000) two peridotites from the Kaapvaal Craton (Pearson et al. 1995a Carlson et al. 1999) three peridotites from the Farm Louwrencia kimberlite, Southern Namibia, on the periphery of the Kaapvaal Craton (Pearson et al. 1994, 1998a Hoal et al. 1995 Frantz et al. 1996) three peridotites from the Vitim alkali basalt field, on the southern margin of the Siberian Craton (Ionov et al. 1993 Pearson et al. 19986). 

Table 2. Summary of Re Os model age information for peridotite xenolith suites on and around the Kaapvaal Craton...

Fig. 10. Histograms of Trd ages for on- and off-craton peridotites from southern Africa co-plotted with histograms of olivine mg-numbers. Kaapvaal low-L peridotites xenohths, n = 96. Data from Nixon et al. (1983), Walker et al. (1989), Pearson et al. (1995a), Carlson et al. (1999), Menzies et al. (1999) and Irvine et al. (2002). Lrd ages for peridotite xenoliths from kimberlites from East Griqualand, 170 km from the Lesotho on-craton kimberlites (Pearson et al. 1998) and for peridotites from the Farm Louwrencia kimberlite. Southern Namibia (Pearson et al. 1994 Pearson 1999a). It should be noted that no Archaean Trd ages are observed for off-craton peridotites. Olivine compositional data from Boyd Nixon (1979), Nixon (1987) and Boyd (unpubl. data).

Peridotites from the Newlands kimberlite, in the southern part of the Kaapvaal Craton, have a similar, but less tightly clustered mean Tru age of 2.6 0.4Ga, median 2.6 Ga (Table 2) compared with the Northern Lesotho peridotites. Those from the Kimberley area, in the SW of the craton, show a tightly clustered distribution of Tro and Tma ages with a mean Trd age of 2.6 0.2 Ga, median 2.7 Ga, and a mean Tma age of 2.9 0.1 Ga (Table 2). Peridotites forming lithosphere in the north of the craton (Letlhakane, Bots-... 

Of the on-craton peridotites studied so far, the suite from the Premier mine, in the centre of the Kaapvaal Craton, is the only one to have an average Trd age that is not Archaean (Table 2). 

Fig. 11. Location map of kimberlites on and around the Kaapvaal Craton from which peridotite xenolith suites have been dated by Re-Os isotopes (after Carlson et al. 2000 Irvine et al. 2001). Data sources Pearson et al. (1994, 1995a, 1998) Pearson (1999) Carlson et al. (1999) Janney et al. (1999) Irvine et al. (2001). Kaapvaal Craton is outlined in dark grey with the Archean Limpopo Metamorphic Belt shaded in light grey.

To date, Re-Os isotope studies of peridotite xenolith suites, taken to be representative of the lithospheric mantle beneath continents, reveal that although some lithospheric mantle began to form beneath continents in Palaeo- to Mesoarch-aean time, major crustal differentiation events continued until Neoarchaean time when, at least in southern Africa, significant volumes of lithospheric mantle formed and stabilized the newly accreted continent. On the Kaapvaal Craton, major magmatic events such as that which formed the Bushveld intrusion, substantially modified or introduced new material into the lithospheric mantle (Carlson et al. 1999) and the lower crust (Schmitz Bowring 2000), but the craton retained its integrity for another 2 Ga. 

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