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Origin of Archean Subcontinental Lithospheric Mantle: Some Petrological Constraints

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Origin of Archean Subcontinental Lithospheric Mantle: Some Petrological Constraints Lithos 109 (2009) 61–71 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos Origin of Archean subcontinental lithospheric mantle: Some petrological constraints N.T. Arndt a,⁎, N. Coltice b, H. Helmstaedt c, M. Gregoire d a LGCA, UMR 5025 CNRS, Université de Grenoble, 1381 rue de la Piscine, 38401 Grenoble, France b Laboratoire de Sciences de la Terre, Université de Lyon, Université Lyon1, Ecole Normale Supérieure de Lyon, CNRS, 2 rue Raphaël Dubois, 69622 Villeurbanne Cedex, France c Department of Geological Sciences, Queen's University, Kingston, Canada d Observatoire Midi-Pyrenées, Université de Toulouse 4 Ave. E. Belin 31400, Toulouse, France article info abstract Article history: The longevity of the continental lithosphere mantle is explained by its unusual composition. This part of the Received 9 June 2008 mantle is made up mainly of forsterite-rich olivine (Fo92–94), with or without orthopyroxene, and it is Accepted 17 October 2008 essentially anhydrous. The former characteristic makes it buoyant, the latter makes it viscous, and the Available online 5 November 2008 combination of these features that allow it to remain isolated from the convecting mantle. Highly forsteritic olivine is not normally produced during mantle melting. Possible explanations for its abundance in old Keywords: Archean subcontinental lithospheric mantle include: (a) high-degree mantle melting in a plume or at an Mantle Lithosphere Archean ocean ridge; (b) accretion of this material to older lithosphere and its reworking in a subduction Olivine zone; (c) redistribution of material to eliminate high-density, low-viscosity lithologies. Following an Archean evaluation of these models based on petrological and numerical modeling, we conclude that the most likely explanation is the accumulation of the residues of melting of one or more mantle plumes following by gravity-driven ejection of denser, Fe-rich components. © 2008 Elsevier B.V. All rights reserved. 1. Introduction–the scientific problem billions of years after it initially formed therefore requires one or more of the following conditions: (a) melting under highly unusual condi- In most of the Archean subcontinental lithospheric mantle, the tions, (b) a petrological/tectonic process that transforms less-magnesian dominant mineral is olivine that has an unusually magnesian compo- olivine and other mantle minerals into forsterite-rich olivine, and/or (c) sition, with forsterite contents (Fo=mole fraction MgO/(MgO+FeO)) a process that physically separates forsterite-rich olivine from less in the range 92 to 94. In many regions, the magnesian olivine is magnesian olivine and other mantle minerals. In this contribution we accompanied by orthopyroxene with about the same Mg/Fe ratios, to first investigate the models that have previously been proposed to produce a rock with harzburgitic bulk composition (Boyd and explain the composition of old subcontinental lithospheric mantle, then Mertzman, 1987; Griffin et al., 1999); more rarely the rock consists we develop a modified version of these models that best accounts for the only of olivine and is a highly refractory dunite (Berstein et al., 1997). features of the subcontinental lithospheric mantle. Highly magnesian olivine and orthopyroxene, if anhydrous, have low densities and high viscosity, features that enhance the chance that a 2. Summary of the composition, structure, physical properties and lithosphere composed mainly of these minerals survives as a layer history of old subcontinental lithospheric mantle above the convecting mantle (Lenardic and Moresi, 1999). The long- term stability of old subcontinental lithospheric mantle is therefore Many recent papers (e.g. (Griffin et al., 1999; Gaul et al., 2000; directly linked to its particular composition. Poudjom Djomani et al., 2001; Gregoire et al., 2003; Griffin et al., It is not easy to explain how the Archean lithospheric mantle 2003; Gregoire et al., 2005; Lee, 2006; Simon et al., 2007)have acquired its peculiar composition. The problem is that olivine with a provided excellent summaries of the characteristics of old subconti- forsterite content greater than 92 is not normally produced during nental lithospheric mantle. These papers make the following points. mantle melting. Highly magnesian olivine is restricted to the residues of fi high-degree partial melting, and except under extreme conditions, this a) Peridotite (ultrama c rock containing olivine, pyroxene and a b – type of olivine forms only a small fraction of the total residue. To produce relatively small, 5 20%, proportion of an aluminous phase such as the Archean subcontinental lithospheric mantle that survived for spinel or garnet) is the most common lithology in suites of xenoliths brought to the surface in kimberlites from the sub- continental lithosphere, making up more than 99% of samples from the Kaapvaal craton in South Africa (Boyd and Mertzman, 1987; ⁎ Corresponding author. Lee, 2006). If the lithology of these suites accurately represents the E-mail address: [email protected] (N.T. Arndt). proportions of different rock types in the lithosphere itself, mafic 0024-4937/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2008.10.019 62 N.T. Arndt et al. / Lithos 109 (2009) 61–71 rocks form only a very minor component (b1%) of the lithospheric the Fo vs. modal olivine trend because of their relatively low olivine mantle beneath the Kaapvaal craton. Mafic rocks contain a higher contents (Figure 4 of Lee, 2006). proportion of garnet and are present as eclogite or garnet pyrox- Metasomatism resulting from the circulation within the upper enite under mantle conditions. mantle of melts and fluids, including basaltic and kimberlitic melts, has affected large portions of the lower lithosphere. (e.g. (Dawson, 1984; The peridotites are mainly harzburgites (olivine and orthopyroxene) Hawkesworth et al., 1984; Menzies and Erlank, 1987; Menzies et al., with rarer lherzolites (olivine, clinopyroxene and orthopyroxene) and 1987; van Achterbergh et al., 2001; Gregoire et al., 2003; Beyer et al., dunites (olivine alone). Until recently our knowledge of lithosphere 2006). This process transforms the dunites or harzburgites, the normal compositions was strongly influenced by information derived from components of the lithosphere mantle, into lherzolites, which are studies of copious suites of xenoliths from South African kimberlites. richer in pyroxenes and hydrous minerals. These studies provided a picture of a lithosphere dominated by orthopyroxene-rich harzburgite (Boyd and Mertzman, 1987; Boyd, b) Radiometric dating, mainly using the Re-Os method, has shown that 1989). Other authors have shown, however, that the lithosphere beneath the mantle portion of the lithosphere stabilized at about the same some other cratons (e.g. Greenland, (Berstein et al., 1997)contains time as the overlying crust, some 2–3 billion years ago in the case of abundant refractory dunite, and that other segments of subcontinental the oldest cratons (e.g. (Pearson et al., 1995; Riesberg and Lorand, lithosphere contain a relatively high proportion (up to 40%) of 1995; Shirey et al., 2002; Carlson et al., 2005). In order that the pyroxenite and eclogite (e.g. (Fung and Haggerty, 1995). lithosphere survived for billions of years without being swept into Olivine in peridotite xenoliths from the mantle beneath Archean the convecting mantle , it must have been both buoyant and cratons has a relatively restricted range of forsterite contents, from a relatively viscous (Jordan, 1978; Pollack, 1986; Jordan, 1988; Hirth minimum of around 89 to a maximum close to 95. In many compilations and Kohlstedt, 1996; Lenardic and Moresi, 1999; Kelly et al., 2003; there is a pronounced peak between 93 and 94 (e.g. (Boyd and Lee, 2003; Sleep, 2003; Cooper et al., 2006; Lee, 2006). The buoyancy Mertzman, 1987; Gaul et al., 2000; Pearson et al., 2004)). This of the lithosphere is related to its density and thus to its distribution is in sharp contrast with that of olivine from younger mineralogical and chemical composition, as well as its temperature. continental or oceanic lithosphere (e.g. (Sen,1987; Griffin et al.,1998), or The inherent density of mantle peridotite depends mainly on the with estimates of olivine compositions in peridotite from the convecting abundance of garnet, the densest of the four dominant mantle mantle or asthenosphere (Lee, 2006), in which forsterite contents range minerals, and on the Mg/Fe ratios of these minerals. The lithosphere from about 88 to 93 with an abundance maximum at 89-90. In most is cooler than underlying asthenosphere and so, in order to survive, it xenolith suites, the forsterite content of olivine correlates with the must contain a low proportion of garnet and/or its olivine and modal abundance of olivine; i.e. the most common rocks are dunites pyroxene must have high Mg/Fe ratios. As outlined above, this is which are rich in Fo-rich olivine and contain little or no pyroxene or indeed the case for old subcontinental lithospheric mantle. The garnet. The trend is broken, however, by the harzburgites from the viscosity of the lithosphere depends only weakly on its composition Kaapvaal craton, which contain high orthopyroxene contents and lower and mineralogy but strongly on the presence of volatiles, mainly olivine contents. In these rocks, the Mg/(Mg+Fe) of both olivine and water or CO2, which usually are present in hydrous minerals or orthopyroxene are mainly in the range 92–94 but they plot to the right of carbonates, or in nominally anhydrous minerals such as olivine (e.g. Fig. 1. Diagram, modified from Lee (2006), illustrating three models for the formation of subcontinental lithospheric mantle. N.T. Arndt et al. / Lithos 109 (2009) 61–71 63 (Kohlstedt et al.,1996; Mei and Kohlstedt, 2000). The longevity of the petrological modeling, it can be shown that the required composition lithosphere requires that it contained very low volatile contents. corresponds to that of the residue produced by 30 to 50% melting of c) Jordan (1975, 1978, 1988) introduced the notion of an isopycnic fertile mantle peridotite (Boyd et al.,1985; Bernstein et al.,1998; Lee, lithosphere.
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