Elsevier

Geochimica et Cosmochimica Acta

Volume 72, Issue 23, 1 December 2008, Pages 5722-5756
Geochimica et Cosmochimica Acta

Contrasting types of metasomatism in dunite, wehrlite and websterite xenoliths from Kimberley, South Africa

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Abstract

Dunite, wehrlite and websterite are rare members of the mantle xenolith suite in the Kimberley kimberlites of the Kaapvaal Craton in southern Africa. All three types were originally residues of extensive melt extraction and experienced varying amounts and types of melt re-enrichment. The melt depletion event, dated by Re–Os isotope systematics at 2.9 Ga or older, is evidenced by the high Mg# (Mg/(Mg + Fe)) of silicate minerals (olivine (0.89–0.93); pyroxene (0.88–0.93); garnet (0.72–0.85)), high Cr# (Cr/(Cr + Al)) of spinel (0.53–0.84) and mostly low whole-rock SiO2, CaO and Al2O3 contents. Shortly after melt depletion, websterites were formed by reaction between depleted peridotites and silica-rich melt (>60 wt% SiO2) derived by partial melting of eclogite before or during cratonization. The melt–peridotite interaction converted olivine into orthopyroxene.

All three xenolith types have secondary metasomatic clinopyroxene and garnet, which occur along olivine grain boundaries and have an amoeboid texture. As indicated by the preservation of oxygen isotope disequilibrium in the minerals and trace-element concentrations in clinopyroxene and garnet, this metasomatic event is probably of Mesozoic age and was caused by percolating alkaline basaltic melts. This melt metasomatism enriched the xenoliths in CaO, Al2O3, FeO and high-field-strength-elements, and might correspond to the Karoo magmatism at 200 Ma. The websterite xenoliths experienced both the orthoyproxene-enrichment and clinopyroxene–garnet metasomatic events, whereas dunite and wehrlite xenoliths only saw the later basaltic melt event, and may have been situated further away from the source of melt migration channels.

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)

  • J.B. Dawson et al.

    The MARID (mica–amphibole–rutile–ilmenite–diopside) suite of xenoliths in kimberlite

    Geochim. Cosmochim. Acta

    (1977)
  • P. Deines et al.

    Small-scale oxygen isotope variations and petrochemistry of ultradeep (>300 km) and transition zone xenoliths

    Geochim. Cosmochim. Acta

    (2000)
  • J. Delaney et al.

    Chemistry of micas from kimberlites and xenoliths—II Primary—and secondary-textured micas from peridotite xenoliths

    Geochim. Cosmochim. Acta

    (1980)
  • H. Downes et al.

    Geochemistry of mafic and ultramafic xenoliths from Fidra (Southern Uplands, Scotland): implications for lithospheric processes in Permo-Carboniferous time

    Lithos

    (2001)
  • R.M. Ellam et al.

    A Proterozoic lithospheric source for Karoo magmatism: evidence from the Nuanetsi picrites

    Earth Planet. Sci. Lett.

    (1989)
  • S.F. Foley et al.

    Evidence from Antarctic mantle peridotite xenoliths for changes in mineralogy, geochemistry and geothermal gradients beneath a developing rift

    Geochim. Cosmochim. Acta

    (2006)
  • S.M. Glaser et al.

    Trace element compositions of minerals in garnet and spinel peridotite xenoliths from the Vitim volcanic field, Transbaikalia, eastern Siberia

    Lithos

    (1999)
  • V.A. Glebovitsky et al.

    The thermal regimes of the upper mantle beneath Precambrian and Phanerozoic structures up to the thermobarometry data of mantle xenoliths

    Lithos

    (2004)
  • T.H. Green et al.

    Origin of the calc-alkaline igneous rock suite

    Earth Planet. Sci. Lett.

    (1966)
  • T.H. Green et al.

    SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200 °C

    Lithos

    (2000)
  • M. Grégoire et al.

    Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea Part 2 Trace element characteristics of slab-derived fluids

    Lithos

    (2001)
  • M. Grégoire et al.

    Spinel lherzolite xenoliths from the Premier kimberlite (Kaapvaal craton, South Africa): nature and evolution of the shallow upper mantle beneath the Bushveld complex

    Lithos

    (2005)
  • W.L. Griffin et al.

    The evolution of lithospheric mantle beneath the Kalahari Craton and its margins

    Lithos

    (2003)
  • C.J. Hawkesworth et al.

    Mantle metasomatism: isotope and trace-element trends in xenoliths from Kimberley, South Africa

    Chem. Geol.

    (1990)
  • K.O. Hoal

    Samples of Proterozoic iron-enriched mantle from the Premier kimberlite

    Lithos

    (2003)
  • P.M. Holm et al.

    Cumulates from primitive rift-related East Greenland Paleogene magmas: petrological and isotopic evidence from the ultramafic complexes at Kælvegletscher near Kærven

    Lithos

    (2006)
  • D.E. Jacob

    Nature and origin of eclogite xenoliths from kimberlites

    Lithos

    (2004)
  • G.A. Jenner et al.

    Determination of partition coefficients for trace elements in high pressure–temperature experimental run products by laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS)

    Geochim. Cosmochim. Acta

    (1993)
  • P. Kelemen et al.

    Silica enrichment in the continental upper mantle via melt/rock reaction

    Earth Planet. Sci. Lett.

    (1998)
  • S.E. Kesson et al.

    Slab–mantle interactions, 1 Sheared and refertilised garnet peridotite xenoliths—samples of Wadati-Benioff zones?

    Chem. Geol.

    (1989)
  • O. Klein-BenDavid et al.

    Mantle fluid-evolution—a tale of one diamond

    Lithos

    (2004)
  • O. Klein-BenDavid et al.

    Fluid inclusions in diamonds from the Diavik mine, Canada and the evolution of diamond-forming fluids

    Geochim. Cosmochim. Acta

    (2007)
  • J. Konzett et al.

    The timing of MARID metasomatism in the Kaapvaal mantle: an ion probe study of zircons from MARID xenoliths

    Earth Planet. Sci. Lett.

    (1998)
  • M.L. Lehtonen et al.

    Layered mantle at the Karelian Craton margin: P–T of mantle xenocrysts and xenoliths from the Kaavi-Kuopio kimberlites, Finland

    Lithos

    (2004)
  • D. Lowry et al.

    Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine, RSA

    Geochim. Cosmochim. Acta

    (1999)
  • I.D. MacGregor

    Petrologic and thermal structure of the upper mantle beneath South Africa in the Cretaceous

    Phys. Chem. Earth

    (1975)
  • D. Mattey et al.

    High-precision oxygen isotope microanalysis of ferromagnesian minerals by laser-fluorination

    Chem. Geol.

    (1993)
  • D. Mattey et al.

    Oxygen isotope composition of mantle peridotite

    Earth Planet. Sci. Lett.

    (1994)
  • W.F. McDonough et al.

    The composition of the Earth

    Chem. Geol.

    (1995)
  • B.I.A. McInnes et al.

    Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: petrology and geochemistry of fluid-metasomatised mantle wedge xenoliths

    Earth Planet. Sci. Lett.

    (2001)
  • T. Meisel et al.

    Osmium isotopic composition of mantle xenoliths: a global perspective

    Geochim. Cosmochim. Acta

    (2001)
  • A. Menzies et al.

    Peridotitic mantle xenoliths from kimberlites on the Ekati Diamond Mine property, NWT, Canada: major element compositions and implications for the lithosphere beneath the central Slave craton

    Lithos

    (2004)
  • P. Agrinier et al.

    Oxygen-isotope constraints on serpentinization processes in ultramafic rocks from the Mid-Atlantic Ridge (23°N)

    Proc. ODP Sci. Results

    (1997)
  • A. Armstrong

    CITZAF—A package of correction programs for the quantitative electron microbeam X-ray-analysis of thick polished materials, thin-films, and particles

    Microbeam Anal.

    (1995)
  • S. Aulbach et al.

    Origins of xenolithic eclogites and pyroxenites from the Central Slave Craton, Canada

    J. Petrol.

    (2007)
  • S. Aulbach et al.

    Eclogitic and websteritic diamond sources beneath the Limpopo Belt—is slab-melting the link?

    Contrib. Mineral. Petrol.

    (2002)
  • M. Becker et al.

    Geochemistry of South African on- and off-craton, Group I and Group II kimberlites: petrogenesis and source region evolution

    J. Petrol.

    (2006)
  • D.R. Bell et al.

    Silica and volatile-element metasomatism of Archean mantle: a xenolith-scale example from the Kaapvaal Craton

    Contrib. Mineral. Petrol.

    (2005)
  • F.R. Boyd et al.

    Composition and structure of the Kaapvaal lithosphere, southern Africa

  • S.R. Burgess et al.

    Tracing lithosphere evolution through the analysis of heterogeneous G9–G10 garnets in peridotite xenoliths, II: REE chemistry

    J. Petrol.

    (2004)
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