Contrasting types of metasomatism in dunite, wehrlite and websterite xenoliths from Kimberley, South Africa
Introduction
Mantle xenoliths retrieved from kimberlites of the Kaapvaal Craton have been studied intensively to understand the different processes that have modified the Kaapvaal subcratonic upper mantle (e.g. Carswell and Dawson, 1970, Gurney and Harte, 1980, Boyd and Mertzman, 1987, Pearson et al., 1995, Griffin et al., 1999a, Carlson and Moore, 2004, Schmitz et al., 2004). Most Kaapvaal peridotite xenoliths, and especially those from the Kimberley kimberlites have been interpreted to represent formerly strongly depleted mantle that has experienced metasomatic re-enrichment processes at different times by a variety of metasomatising agents (e.g. Erlank et al., 1987, Hawkesworth et al., 1990, Griffin et al., 1999a, Grégoire et al., 2002, Simon et al., 2007).
Dunites, wehrlites and websterites are subordinate to lherzolites and harzburgites in mantle sections, but are found in different tectonic settings throughout the world. These rocks occur as xenoliths in kimberlites and basalts and represent samples of old, Archean mantle (e.g. Kopylova et al., 1999, Lehtonen et al., 2004, Menzies et al., 2004, Viljoen et al., 2005, Aulbach et al., 2007, Beard et al., 2007) and younger Phanerozoic mantle (e.g. Chen et al., 1992, Downes et al., 2001, Peslier et al., 2002, Cvetković et al., 2007, Rehfeldt et al., 2007a). They also occur in ultramafic sections of ophiolites (e.g. Orberger et al., 1995, Koga et al., 2001, Dijkstra et al., 2003) and in orogenic peridotite complexes (Pearson et al., 1991, Takahashi, 1992, Holm and Prægel, 2006). Additionally, websteritic mineral inclusions have been identified in Kaapvaal diamonds (Aulbach et al., 2002). Dunites, wehrlites and websterites with relatively Fe-rich compositions (mostly Fo < 90) are typically interpreted to be cumulate in origin, mostly related to the host rock (e.g. Gurney and Harte, 1980, Chen et al., 1992, Downes, 2001, Holm and Prægel, 2006). In contrast, rocks with higher Mg-number (mostly Fo>90) are generally interpreted as mantle samples that have been depleted by partial melting (e.g. Gurney and Harte, 1980) followed by subsequent re-enrichment by infiltration of basaltic (Koga et al., 2001, Downes et al., 2002, Beard et al., 2007) or carbonatitic melts (Beard et al., 2007) or related to suprasubduction zone processes (e.g. Orberger et al., 1995, Cvetković et al., 2007). The petrogenesis of dunites, wehrlites and websterites is commonly interpreted to be closely related to mantle lherzolites and harzburgites (e.g. Gurney and Harte, 1980, Pearson, 1999, Grégoire et al., 2003, Menzies et al., 2004). Websteritic diamond inclusions have been interpreted to be related to aged, subducted oceanic crust due to reaction of slab-derived melts with surrounding peridotite (Aulbach et al., 2002).
Dunite, wehrlite and websterite xenoliths are rare but important members of the xenolith suite found in the Kimberley kimberlites of South Africa. However, only few studies of these xenolith types have presented geochemical analyses of any type (e.g. Nixon et al., 1981, Aulbach et al., 2002, Grégoire et al., 2003), and their spatial relationship to the more abundant lherzolites and harzburgites has not been described. Here, we report mineral major- and trace-element compositions, whole-rock major-element compositions, oxygen isotope compositions of silicate minerals, and Re–Os systematics of dunite, wehrlite and websterite xenoliths retrieved from the Kimberley kimberlites of South Africa. The results are compared with the well-studied harzburgite and lherzolite xenoliths from the same locality, which gives us the opportunity to constrain spatial variation in metasomatic modification of the Kaapvaal upper mantle.
Section snippets
Mantle metasomatism in relation to magmatic events in the Kimberley area
The Kimberley kimberlite cluster is situated in the western part of the Kaapvaal Craton in southern Africa (Fig. 1). At ∼2.9 Ga two main domains of the Kaapvaal Craton, the Kimberley Block to the west, consisting of 3.2–2.8 Ga old gneisses (e.g. Anhaeusser and Walraven, 1999, Poujol et al., 2002, Schmitz et al., 2004), and the Witwatersrand Block to the east, were juxtaposed close to the Colesberg Lineament (de Wit et al., 1992, Schmitz et al., 2004). Age constraints on the assembly of the
Samples and analytical methods
Six dunite, eight wehrlite and four websterite xenoliths were analysed petrographically and geochemically. Most samples are part of a new collection, whereas samples AJE362, AJE400 and AJE401 were provided by the University of Cape Town (RSA) and are part of the A.J. Erlank collection. Both collections are from the Boshof Road dump, Northern Cape Province, South Africa, and are by-products of diamond mining believed to be derived from the Bultfontein kimberlite pipe.
Kimberlites of the Kimberley
Petrography and mineral composition
Dunite, wehrlite and websterite xenoliths comprise olivine, clinopyroxene, orthopyroxene and garnet in different amounts, plus accessory oxides and phlogopite. Both dunites and wehrlites show an olivine-rich mineralogy and similar metasomatic histories and so they are described together. Only three of the wehrlites have >10 vol % clinopyroxene but otherwise form a homogeneous group with rocks with <10 vol % clinopyroxene, so that we treat the latter, although technically dunites (Le Maitre, 1989
Melt depletion
Several lines of evidence indicate that the protolith of the dunite and wehrlite xenoliths are residues of extensive partial melt extraction during formation of the cratonic lithosphere in the Archean. A similar protolith is suggested for one olivine-bearing websterite (DJ0216), but has probably been completely eliminated from the other olivine-free websterites by thorough metasomatic overprinting. The evidence for an early depletion event includes: (i) high abundances of olivine in dunite and
Conclusions
The kimberlites of Kimberley, South Africa have carried large quantities of mantle peridotites of diverse lithologies. These xenoliths reveal a complex history of melt depletion and multiple re-enrichment of the Kaapvaal upper mantle. Silica-rich and alkaline basaltic silicate melts overprinted large areas of the Kaapvaal mantle, as evidenced by dunite, wehrlite and websterite as well as orthopyroxene-rich peridotite xenoliths. The evolution of dunite, wehrlite and websterite xenoliths from the
Acknowledgments
Samples with D.J. suffixes were collected by D.E.J. together with J. Robey (DeBeers, Kimberley), who provided expert assistance. Additional samples were provided to us by the University of Cape Town (A.J. Erlank collection; A.J.E. suffixes). M. Bozovic helped during sample preparation. The help of the laboratory managers B. Schulz-Dobrick and M.G. Barth (Mainz University), G. Brügmann (MPI Mainz), A. Kronz (Göttingen University), M. Horan and T.D. Mock (Carnegie Institution of Washington) is
References (151)
- et al.
Rb–Sr age determinations on South African kimberlite pipes
Phys. Chem. Earth
(1975) - et al.
Episodic granitoid emplacement in the western Kaapvaal Craton: evidence from the Archaean Kraaipan granite-greenstone terrane, South Africa
J. Afr. Earth Sci.
(1999) Characterization of spinel peridotites by olivine–spinel compositional relationships: review and interpretation
Chem. Geol.
(1994)- et al.
Partial melting in Archean subduction zones: constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonalitic melts under upper mantle conditions
Precambrian Res.
(2002) - et al.
Geochemistry of xenolithic eclogites from West Africa, part I: a link between low MgO eclogites and Archean crust formation
Geochim. Cosmochim. Acta
(2001) - et al.
Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula (Russia): evidence from spinel lherzolite and wehrlite xenoliths
Lithos
(2007) Compositional distinction between oceanic and cratonic lithosphere
Earth Planet. Sci. Lett.
(1989)- et al.
Ultramafic nodules from the Kimberley pipes, South Africa
Geochim. Cosmochim. Acta
(1978) - et al.
Modification of the subcontinental mantle beneath East Serbia: evidence from orthopyroxene-rich xenoliths
Lithos
(2007) A fertile harzburgite–garnet lherzolite transition: possible inferences for the roles of strain and metasomatism in upper mantle peridotites
Lithos
(2004)