Lithos 109 (2009) 61–71

Contents lists available at ScienceDirect

Lithos

journal homepage: www.elsevier.com/locate/lithos

Origin of Archean subcontinental lithospheric : 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 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 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 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 , 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 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 , 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. According to this idea, at every depth in the 2006). Other authors have proposed that reprocessing and possible lithosphere there is a balance between compositional buoyancy, remelting in a subduction environment introduced orthopyroxene which is related to the types and compositions of mineral phases, and increased the Mg/Fe ratio of the olivine. and the thermal buoyancy, which is related to the temperature difference between the colder lithosphere and hotter surrounding 3. Previous explanations of the origin of subcontinental asthenosphere. For this balance to hold, the compositional buoy- lithospheric mantle ancy must increase progressively from at the base, where the lithosphere has about the same temperature as adjacent convect- In this section we critically discuss previous explanations for the ing mantle, to the top, where it is far cooler. In practice this requires origin of low-density viscous subcontinental lithospheric mantle, then that the amount of garnet and/or the Fe content of olivine and add one or two of our own. Drawing from Lee (2006), we start with pyroxene must decrease with decreasing depth. three end-member models. d) The unusual mineralogy and composition (high Mg/Fe ratios, low garnet content, negligible water content) needed to assure the 3.1. Melting in a mantle plume longevity of old subcontinental lithosphere requires that it formed under unusual circumstances. Many authors (e.g., (Boyd, 1989; In this model, promoted, for example, by Boyd (1989), Pearson et al. Griffin et al., 1999, 2003) equate the presence of Fo-rich olivine and (1995), Arndt et al. (2002) and Griffin et al. (2003, 2004), the the paucity of other phases with that of a residue of high-degree subcontinental lithospheric mantle is said to have formed from the partial melting. Using simple mass balance or more sophisticated residue of melting one or more large and hot mantle plumes (Fig. 1a).

Fig. 2. Sketches of the melting zones beneath (a) modern and (b) Archean . The melting parameters and the compositions of residual ocean are calculated using the procedure described by Herzberg et al. (2006). In the case of a modern spreading centre, the mantle has a potential temperature of 1400 °C and this produces thin oceanic crust and a residual mantle in which the maximum Fo content is 91.5. Cooling as the plate migrates produces lithosphere with a maximum thickness from 60–90 km, comparable to the thickness of the melting column. Archean mantle with a potential temperature of 1600 °C would start to melt at greater depth and produces thicker oceanic crust and residual mantle containing olivine with Fo up to 93. Because of rapid spreading and higher mantle temperature, the lithosphere is thinner and its base passes through the upper part of the residual mantle layer. 64 N.T. Arndt et al. / Lithos 109 (2009) 61–71

The plume undergoes partial melting as it rises, the melt escapes to the markedly. This effect may explain the peak in olivine compositions surface, and the solid residue that remains in the plume becomes in the range Fo92–94. progressively depleted in easily fusible components. This process results in progressive change in the composition of the residue, from 3.2. Accretion and stacking of oceanic lithosphere fertile lherzolite at the first, high-pressure stage of melting, to highly refractory dunite at the final low-pressure stage. As a result of a process In this model, advocated originally by Helmstaedt and Schulze that is not well understood, the residues of melting then accumulate (1989), the subcontinental lithospheric mantle is proposed to have near the surface to form the subcontinental lithospheric mantle. grown through the accretion of slabs of oceanic lithosphere. The idea There are several obvious advantages to this model: (a) the is that portions of lithosphere that originally formed at a mid-ocean composition of the residue ranges from relatively Fe-rich garnet ridge were thrust one beneath another in a subduction zone at the lherzolite at the base of the melting column to highly refractory Fe- margin of the growing continent, as shown in Fig. 1b. poor dunite at the top. If incorporated into the lithosphere, the vertical The advantages of this model are: (a) it accounts for the presence distribution of lithologies, from relatively dense at the base to buoyant within suites of mantle xenoliths of eclogite and garnet pyroxenite, at the top, is isopycnic, at least qualitatively. (b) If the plume is hot which, in some cases, have geochemical and isotopic characteristics enough and the melting column long enough, the most refractory that point to their having formed as old oceanic crust (e.g. (Fung and residues, which are produced at the top of the column, will contain Haggerty, 1995; Rollinson, 1997; Barth et al., 2001). (b) It explains the very Fo-rich olivine (±orthopyroxene) whose composition is very like presence of dipping seismic reflectors at the edges of some cratons that in old subcontinental lithospheric mantle. (c) Because the (Bostock, 1998; Levander et al., 2006). (c) It is consistent with the extraction of melt removes volatiles, the residue is anhydrous. In inferred low-pressure origin of cratonic peridotites. Stacking of a other words, melting in a hot mantle plume is capable of producing series of slabs made up largely of low-pressure peridotite thereby the low-density, gravitationally stable, high viscosity material that provides a means of generating a large volume of subcontinental assures its long-term stability of the lithosphere. lithospheric mantle. Lee (2006) criticized two aspects of the model. First he notes that Lee (2006) discussed a major problem of the model, a problem that melting at depth in the lower part of the melting column leaves garnet centers on the wide dispersion of lithologies and compositions in in the residue. Through his quantitative modeling in which he oceanic lithosphere. The mantle portion of modern oceanic litho- assumed that fertile lherzolite underwent isobaric equilibrium partial sphere is made up of rocks ranging from fertile, Fe-rich garnet- or melting, he showed that the residues of high-pressure melting contain spinel lherzolite at the base, to harzburgite at the top (Fig. 2a). The high FeO, Al2O3 and Sc contents. In contrast, peridotites from old crustal portion is also stratified, from gabbros and Fe-rich olivine- subcontinental lithospheric mantle contain relatively low FeO, Al2O3 pyroxene cumulates in the lower part, to basalt in the upper part. The and Sc contents, features that correspond either to melting at shallow fraction of harzburgite and dunite is low (b10%) and material with the depths under conditions in which garnet is absent or to secondary composition of Fe-poor cratonic peridotite is absent. In modern processes, such as orthopyroxene addition, that decreased the lithosphere, the proportion of oceanic crust is about 10% (6–9 km thick contents of FeO and the other elements. Second, he notes that the crust overlies 60–100 km of lithospheric mantle), significantly higher generation of a large volume of refractory Fe-poor dunite requires the than the proportion of eclogite and garnet pyroxenite in most parts of extraction of a large volume of high-degree melt. This melt would the subcontinental lithospheric mantle. With such a high proportion have the composition of a komatiite, a type of magma that forms only of garnet-rich lithologies it is unlikely that lithosphere formed by a small fraction of the Archean volcanic sequences interpreted as the stacking of slabs of oceanic plates would have been sufficiently products of melting in mantle plumes. These aspects of the plume buoyant to have survived. model are discussed below. Lee mentions two possible solutions: (i) the more Fe-rich portions Bernstein et al. (1998) note that the Fo93 peak in abundance plots of the oceanic lithosphere could have been removed before or during from Greenland xenoliths coincides to the extent of melting required accretion; (ii) Archean oceanic lithosphere was derived from hotter, to eliminate orthopyroxene from the residue. At higher degrees of and perhaps more depleted Archean mantle (Davies, 1992) and it melting, the melt productivity drops drastically; i.e. the amount of would have had a different composition from modern oceanic melt produced for a given increase in temperature decreases lithosphere. It would have contained a high proportion of Fe-poor

Fig. 3. Sketch of a subduction zone showing how material in the mantle wedge is drawn down through the melting zone to produce a Fo-rich low density residue at depth. This material is overlain by denser, more fertile peridotite and by still denser cumulates in sub-crustal magma chambers. Redistribution of lithologies is needed to produce a gravitationally stable configuration. N.T. Arndt et al. / Lithos 109 (2009) 61–71 65 peridotite and its inherent density would have been less than that of Oceanic lithosphere is well stratified because it is formed from modern oceanic lithosphere. layers that spread out laterally as newly solidified oceanic crust migrates away from the ridge (Fig. 2a). Flowage in the asthenosphere 3.3. Processes in subduction zones changes from vertical to horizontal beneath the ridge and this transformation is inherited by the residue of melting. The lowermost In this model, the cratonic mantle is said to have formed through layer in the stratified upper mantle has a composition close to that of processing of material in the mantle wedge above a subduction zone. the asthenosphere because it is the product of low-degree melting. (e.g. Jordan,1988; Herzberg,1999; Lee, 2006; Simon et al., 2007)(Fig.1c). This melting starts at 60–90 km at a modern oceanic ridge (Herzberg Relatively fertile peridotite is transformed into more refractory harzbur- et al., 2006). The shallowest layer, which forms beneath the crust at gite or dunite by melting triggered by fluid transfer from dehydrating about 10 km depth, is the most depleted in fusible components oceanic crust. The thickening called on to produce ~200-km thick because it is produced by high-degree melting. The base of the lithosphere is achieved by deformation associated with the accretion. lithosphere is the boundary between the rigid plate and deformable One way to look at the process is illustrated in Fig. 3. The material in the mantle, which depends on the temperature gradient, the composition mantle wedge — a mixture of older accreted oceanic slabs or plume the shear stress and other factors discussed by Michaut et al. (2009- residue — is drawn down through the melting zone by the drag of the this issue). As the plate cools during its migration away from the ridge, subducting plate. As this material is pulled downwards, it passes through the position of this boundary migrates from above the top of the azonewherefluids liberated from the dehydrating subducting oceanic melting column at the ridge to a depth of up to 100 km, near the base crust cause partial melting. The residue left after melt extraction is of the residual peridotite layer, in the oldest parts of the oceanic depleted in Fe- or Al-rich fusible components and this residue, which has basins. In Archean oceanic lithosphere the positions of these the composition of low-density Fe-poor harzburgite or dunite, under- boundaries would have been very different, as described below. plates the lithosphere. The residue left in the mantle after extraction of melt from a Problems with the model relate to the efficacy of the melting modern mantle plume, such as the one beneath Hawaii, originally has process. Can melting triggered by the input of fluids from subducting a cylindrical form but it becomes deformed as it accretes to the base of ocean crust leave a residue that (a) is anhydrous, as required for long- older oceanic lithosphere. The shape and form of the mantle sources of term stability of the lithosphere, and (b) lacks the geochemical continental or oceanic volcanic plateaus are very poorly understood. signature of the subduction process? Magmas derived from subduc- The well-known image of a large sphere atop a narrow stem, as tion zones are characterized by enrichment of incompatible elements illustrated in early models of starting mantle plumes by Griffiths and coupled with depletion of Nb, Ta and some other high-field-strength Campbell (1990), is most probably an oversimplification. Composi- elements (e.g. (McCulloch and Gamble, 1991; Hawkesworth et al., tional heterogeneities in mantle plumes strongly influence ascent 1994). The trace element contents in most mantle xenoliths do not dynamics and a variety of shapes and sizes can be obtained (Farnetani show the subduction signature (Hauri et al., 1993; Ionov et al., 1997; and Samuel, 2005). Coffin and Eldholm (1993) represent the source of van Achterbergh et al., 2001; Gregoire et al., 2003; Pearson et al., the largest volcanic plateaus as a sphere with a diameter between 700 2004; Gregoire et al., 2005). and 1000 km. In order that such a source undergoes high-degree Another problem with the model is that lithosphere generated or partial melting, it must pass within 200 km of the surface. Just how reprocessed in a subduction zone would be gravitationally unstable: material in the source flows to pass through the shallow melting zone low-density, Fe-poor residue underlies higher-density more fertile is an open question; just where and how the residue left after melt material yet to pass through the melting zone. extraction accumulates is even more uncertain. In brief, we know very little about the geometry of the mantle residues produced in large 3.4. Serpentinization of oceanic crust mantle plumes.

The idea here is that under some conditions olivine reacts to serpentine with higher Mg# than the original olivine. from the original olivine is present in magnetite. Dehydration of the serpentine then produces Fo-rich olivine. Li et al. (2004) have shown, for example, that when abyssal peridotite is subjected to ocean-floor hydration and later subduction-related high-pressure metamorphism, the olivine that results from prograde recrystallization has relatively high Fo contents. They report that the forsterite contents of olivine in recrystallized serpentinites of the Zermatt-Saas ophiolite, for example, ranges from Fo93 to Fo98. If this process operated during the accretion of oceanic lithosphere to growing continental lithosphere, it would boost the forsterite content of the accreted material.

4. Evaluation of previous explanations

4.1. Compositions and geometries of residues of melting

In the residue produced during any melting process, the Fo content of olivine ranges from identical to that in the unmelted source at fringes of the melting zone, to a maximum value in the core of the melting zone. The maximum Fo content depends on the temperature and composition of the source and on the melting mechanism. High Fo contents between Fo 91 and 94 are only attained through melting of a very hot source. The spatial distribution of mineral proportions and Fig. 4. Phase diagram for mantle peridotite and the paths followed by ascending mineral compositions in the residue of melting depend on the material at modern and Archean mid-ocean ridges and mantle plumes from Herzberg geodynamic setting in which melting takes place. (1999); Herzberg and O'Hara, (2002). 66 N.T. Arndt et al. / Lithos 109 (2009) 61–71

With these complications in mind we will consider some harzburgitic layer, and it must founder through the harzburgite if it is examples, starting with the simplest: to be lost to the underlying convecting mantle. Alternatively, the accreted layers of oceanic lithosphere might be reworked in the a) Modern oceanic lithosphere formed by melting at a modern subduction zone (as described in a later section). mid-ocean ridge. In Herzberg et al.'s (2006) modeling of the The problem is compounded if Archean subduction were relatively formation of modern ocean crust, he assumed polybaric fractional flat, as has been suggested by several authors (see Van Kranendonk melting (Figs. 2a and 4) and showed that the degree of melting (2004) for a review) The basis for this idea are threefold. First, increases from 0% at 60–90 km depth to a maximum of 20% at the top Korenaga (2006) has argued that since melting begins at greater of the melting column. The process produces 6 km of basaltic crust depths in the Archean than at present, and since melt extraction above a 60–90 km thick layer of the residual mantle. In the mantle depletes the lithosphere in viscosity-decreasing volatile components, layer, the Fo content of olivine ranges from 89 at the base of the melting the total thickness of anhydrous, relatively viscous lithosphere should column to 91.5 at the top. None of this residue has a composition be greater than at present. Thick rigid lithosphere would have resisted comparable to that of Fe-poor Archean cratonic peridotites. bending at a subduction zone. The second argument depends on the b) Archean oceanic lithosphere. Advocates of slab-accretion models idea that plate movement was faster in the Archean because higher propose that Archean oceanic lithosphere had a more refractory mantle temperatures increased the vigor of convection. Oceanic composition more like that of Archean cratonic peridotite. lithosphere was hotter and more buoyant when it reached the Consider melting of a hotter mantle with a potential temperature subduction zones. The third argument is based on the composition of of 1600 °C. Using the phase diagrams from Herzberg and O'Hara Archean oceanic crust, which is produced by differentiation of picrite (2002), we estimate that under these conditions, melting starts at and contains a relatively thin proportion of basalt above a thicker layer about 135 km and reaches a maximum of 40% at about 20 km of olivine (±pyroxene) cumulates. The proportion of eclogite, the depth. This melting produces a layered lithosphere comprising a dense component that drags subducting crust down into the mantle, thick upper layer of oceanic crust about 20 km thick, a middle is therefore relatively low. All three arguments lead to the notion that portion (from the base of the crust at 20 km to about 60 km) made Archean oceanic crust, if it subducted, would have plunged at a up of harzburgite containing olivine with the composition Fo91–93 shallow angle into the mantle. and a lower portion (60 to 135 km) composed of peridotite with olivine compositions between Fo89–91. c) Residue of a modern mantle plume. The residue produced during melting of a modern mantle plume is zoned both horizontally and In this model, only about 30% of the residue in the melting column vertically. Because the temperature decreases from a maximum in has a composition like that of Fe-poor cratonic peridotites. If Archean the centre of the plume to ambient at its margins, the residue left oceanic lithosphere is to become part of the cratonic lithosphere, then after melt extraction consists of a refractory, low-density core both the lower layer of Fe-rich lherzolite and the upper crustal layer surrounded by a denser, less-depleted outer sheath (Arndt et al., must be removed. 2002); and because the extent of melting increases with decreas- The lower layer probably never forms part of the lithosphere ing depth, its composition changes from fertile peridotite at depth because it is too hot. Given a hotter Archean mantle and faster moving to refractory dunite close to the surface (Fig. 5). plates, the 800 °C isotherm will be located in the middle of the residual mantle, within the Fe-poor harzburgitic layer. If this isotherm defines Given that ambient temperatures in the Archean mantle were the base of the lithosphere, then Archean lithosphere is generally higher than in the present mantle, Archean mantle plumes probably thinner than modern oceanic lithosphere and its composition is were hotter than modern plumes. Constraints from Archean koma- indeed much more refractory. It is probable that a slab of oceanic tiites indicate potential temperatures greater than 1700 °C, compared lithosphere that accretes to a growing craton includes only the upper with about 1400 °C for the hottest modern plumes (Nisbet et al., 1993; harzburgitic layer and none of the underlying lherzolites. This resolves Herzberg et al., 2006; Arndt et al., 2008). Melting in such a plume starts the problem of getting rid of the dense Fe-rich peridotites, but what at about 7 GPa (~200 km) and leaves an initially cylindrical residue happens to the overlying 20 km-thick crust? This crust overlies the comprising a lower, ~60 km thick zone in which olivine compositions

Fig. 5. Sketch of the melting zone within a mantle plume showing variation in the composition of olivine in the residue of melting, calculated using the procedure described by Herzberg et al. (2006). The example shown is a modern plume; in a hotter Archean plume the proportion of Fo-rich olivine will be greater. N.T. Arndt et al. / Lithos 109 (2009) 61–71 67 range from 89 to 92, and a thicker (90 km) upper zone with olivine komatiite was far more abundant in the Archean than is commonly compositions between Fo 92 and Fo 94. Again, a sheath of less depleted thought. peridotite surrounds the more refractory core (Arndt et al., 2002). When this material reaches the base of oceanic or continental 4.2. Reworking of accreted slabs lithosphere, the compositionally denser outer portions may cool and sink back into the mantle leaving only the low-density refractory core. Fig. 3 is a sketch of a subduction zone. Drag of the subducting slab The vertical change in composition — from buoyant refractory dunite draws the mantle wedge first through the zone of hydration, then at shallow levels to denser more fertile peridotite at deeper levels — is through the zone of partial melting. Melt leaves the wedge to erupt as gravitationally stable. The upper part of the residue has a composition arc volcanics, leaving a residue stripped of low-temperature compo- like that of cratonic peridotite. When looked at this way, the Archean nents. What would happen if a previously accreted slab of Archean plume model seems an attractive means of generating lithospheric ocean lithosphere were drawn through the melting zone? This slab mantle. might consist of an upper ~20-km-thick crustal portion differentiated But what of the objections posed by Lee (2006)? Consider first the into a few kilometers of magnesian basalts and a thicker sequence of notion that a plume-generated residue should retain a geochemical pyroxene-olivine cumulates, underlain by maybe 20 km of refractory record of melting in the presence of garnet. As mentioned in an earlier harzburgite. Serpentinization may have resulted in the crystallization section, the dominant component of the continental lithospheric of relatively magnesian olivine. As the slab is drawn through the mantle is refractory harzburgite or dunite, material that is produced melting zone, the melts are extracted mainly from the crustal section — only at the highest degrees of melting. This material does not form in the refractory harzburgite contributes very little. Fluid-fluxed melting the presence of garnet, for two reasons: first, it is produced at the top extracts both the basaltic components and the Fe-rich fraction of mafic of the melting column at pressures where garnet is not stable; second, minerals from the source: the presence of water destabilizes garnet will have been exhausted at an earlier stage of melting, orthopyroxene producing siliceous arc magmas leaving a residue particularly if melting is fractional. Consider again the polybaric consisting almost entirely of Fo-rich olivine (Gaetani and Grove, 1998; fractional melting process. The initial melts, which form through low- Kelemen et al.,1998; Falloon and Danyushevsky, 2000). In other words, degree melting in equilibrium with garnet at 7 GPa, have peculiar such melting could, at least in theory, convert the ocean slab into Fo- compositions enriched in both MgO and incompatible trace elements. rich dunite of the type found in old continental lithospheric mantle. They have low Al/Ti, low Sc and high Gd/Yb and they leave a residue But what of the problems posed in the earlier section? Would the relatively rich in garnet, with high Al/Ti, high Sc and low Gd/Yb (Lee, residue retain the “subduction signature” (enrichment of incompatible 2006; Simon et al., 2007). As the plume material rises from 7 to 4 GPa, elements and deficits of elements like Nb-Ta and Zr) that is absent from melts are continually extracted leaving a residue that becomes most mantle xenoliths? The residue of high-degree melting consists progressively depleted in garnet as the pressure drops and as this only of olivine±orthopyroxene, two minerals that contain very low component is removed in departing melts. Through this process concentrations of incompatible elements. The small fraction of garnet is progressively eliminated. A hot plume passes through the incompatible elements retained in the residue would come almost garnet-out curve at very high pressures, between 5 and 6 GPa entirely from trapped liquid that had not escaped the source. If the depending on the temperature (Fig. 4), and from there on, melting has amount of trapped liquid is low and if the magma contained less than little effect on trace element characteristics of the residue, other than 10 ppm of elements such as Nb-Ta and the rare , as in primary arc to strip out what remains of the incompatible elements. The residue magmas (Kelemen,1990), then the residue would contain b0.01 ppm of that forms at pressures lower than 5 GPa consists of harzburgite to these elements. Higher levels of incompatible trace elements measured dunite, assemblages of olivine±orthopyroxene that retain little record in mantle xenoliths (1–20 ppm, Pearson et al., 2004)mostprobablyare of the earlier part of the melting process. introduced later through the influx of metasomatic fluids from the It is important to recall here that only the Fe-poor harzburgites and deeper mantle (Dawson, 1984; Hawkesworth et al., 1984; Menzies and dunites produced at shallow levels have compositions like that of the Erlank,1987; Menzies et al.,1987; van Achterbergh et al., 2001; Gregoire continental lithospheric mantle. The more Fe-rich portions — the only et al., 2003; Beyer et al., 2006). Simon et al. (2007) for example, conclude parts that retain the signature of melting in the presence of garnet — that xenoliths from the Kaavaal craton were subjected to three are not incorporated into the lithosphere. On this basis there is no metasomatic episodes, the first in a subduction setting and the latter reason to expect that the garnet signature, or in other words, the two as a result of influxes of kimberlitic magmas. These processes signature of high-pressure melting (Lee, 2006; Simon et al., 2007), profoundly change the composition of the lithosphere and it is possible should be preserved in rocks from the continental lithosphere. that the subduction signature was masked by later metasomatism. Lee's (2006) second objection to the plume-melting model The second question is whether fluid-fluxed melting can produce an concerns the abundance of komatiite. This is an intractable problem anhydrous residue; i.e., high viscosity material capable of surviving the because, in order to estimate the amount of komatiite that formed ravages of mantle convection. Olivine and other nominally anhydrous during Archean magmatism, we have to rely on the incomplete and minerals may contain a few hundred ppm of water (e.g. (Kohlstedt et al., probably biased record preserved in Archean greenstone belts. Some 1996; Bell et al., 2004; Grant et al., 2006), and any trapped interstitial of these volcanic sequences erupted on older , others melt will contribute larger amounts. If 1% of melt containing 6% H2O onto or adjacent to island arcs (Arndt et al., 2008). In both cases the (that of subduction-zone magmas, Wade et al. (2006)) remained presence of a layer of low-density crust would have hindered the trapped, the residue contains 600 ppm, in addition to that in the passage of high-density komatiite magma and facilitated the eruption minerals themselves. The total amount of water most probably exceeds of lower-density basalt. Under these conditions, the proportion of the threshold at which its presence significantly decreases the viscosity komatiite preserved in the volcanic successions will be lower than at of peridotite (Hirth and Kohlstedt, 1996). The persistence of water in the the site of melting. Large volumes of komatiite may have erupted as residue of hydrous melting poses a major problem for the hypothesis parts of volcanic plateaus in ocean basins, but such sequences are that cratonic lithospheric mantle formed in subduction zones. uncommon in greenstone belts; perhaps because they are dense and difficult to obduct. Archean oceanic crust may have contained a high 5. Isopycnicity and secular variation in the composition of proportion of komatiite but true Archean oceanic crust; i.e. crust cratonic lithosphere formed at a mid-ocean spreading centre, has never been convincingly documented. Given the probability that Archean greenstone belts Another problem arises from the distribution of lithologies in a represent a biased record of Archean volcanism, it is possible that subduction-processed mantle wedge. Reworking in the subduction 68 N.T. Arndt et al. / Lithos 109 (2009) 61–71 zone produces a low-density Fo-rich dunitic residue beneath higher- density Fe-rich (unmelted) peridotites. This distribution of lithologies is gravitationally unstable. In addition, when the hydrous basaltic magmas generated in the melting zone ascend to the base of the crust, they differentiate into two components — evolved melts that erupt as arc lavas, and dense, Fe-rich ol+px±plag or garnet residues. These residues overlie less dense peridotites, adding gravitational instability. To achieve a stable, isopycnic configuration, the material must be redistributed; the dense portions must be removed, leaving only the lower density material. During this redistribution, the low-viscosity hydrous portions of the lithosphere would also be removed. Added to this is the effect of secular cooling of the mantle. For the lithosphere to be gravitationally stable within hot Archean mantle requires that it contained a large proportion of chemically buoyant material; i.e. the Archean lithosphere must have had unusually high buoyancy derived from a very low Fe content and a complete lack of garnet. This characteristic of the Archean lithosphere is evident when one compares the composition of dunitic or harzburgitic Archean xenoliths with those of peridotitic Proterozoic and Phanerozoic xenoliths (e.g. (Boyd, 1989; Menzies, 1990; Griffin et al., 1998; Griffin et al., 1999). As the mantle cools, the Archean lithosphere, if it main- tained constant thickness and composition, would acquire a surfeit of buoyancy and this change should have resulted in progressive elevation of the land surface. The survival of Archean peneplains in ancient, low- lying continental shields suggests that this did not happen. Perhaps the thickness of the lithosphere decreased through time, implying that the original lithosphere was thicker than at present, or the composition of the lithosphere could gradually have changed, perhaps through the progressive introduction of a dense Fe-rich metasomatic component, or through progressive dilution of the least-dense materials. The differ- ences in the compositions of Archean, Proterozoic and Phanerozoic xenoliths support the latter interpretation. If we assume that the average mantle temperature decreased by 250 °C from the Archean to present (Abbott et al., 1994; Jaupart et al., 2007) this requires that the average density of an isopycnic lithosphere increased by about 40 kg m− 3 (Doin et al., 1996; Schutt and Lesher, 2006; Michaut et al., 2009-this issue). Through geological time, the Fig. 6. Results of modelling of the segregation of dense layers in the continental distribution of lithologies within the continental lithospheric mantle, lithosphere. The Ellipsis program of O'Neill et al. (2006) was used, with the following conditions. (a) Initial state with continental crust in red (ρ=2720 kg/m3, µ=1023 Pa s), and the density of the lithosphere as a whole, must have increased in eclogite in blue (ρ=3440 kg/m3, µ=1022 Pa s), harzburgite in light green composing the order to preserve isopycnicity. This readjustment probably takes place continental lithospheric mantle and a layer within the sandwich model (ρ=3200 kg/m3, through the rejection of portions with too-low densities or their µ=1022 Pa s), peridotite in dark green (r=3300 kg/ m3, m=1021 Pa s). (b) after reworking into higher-density lithologies. Mantle metasomatism, the 100 million years, (c) after 200 million years and (d) after 300 million years. The dense introduction of Fe- or garnet-rich lithologies in incoming fluids or slab of oceanic lithosphere rapidly founders but it takes with it the underlying layer of fi depleted harzburgite. Accretion and foundering of oceanic lithosphere is not a process melts, was probably responsible for this densi cation. that results it net addition of depleted harzburgite to the continental lithosphere. Details of the calculations are given in a forthcoming paper by Coltice and Arndt (in 6. Rejection of high-density components during reworking of the preparation). Archean lithosphere

We now return to the problem of generating the peculiar modern asthenosphere as a result of greater internal heat production composition of Archean subcontinental lithospheric mantle, particu- and secular cooling. Less widely appreciated is the likelihood that the larly the paucity of garnet and clinopyroxene and the high Mg/Fe overlying continental crust was also much hotter, because of greater ratios of the olivine and orthopyroxene. We have established that this heat production from radioactive elements. Isotopes such as 40K, 235U composition cannot be explained by any reasonable combination of and 232Th would have been about 3 times more abundant 3 billion melting or fluid-influx processes. What is required is a subsequent years ago and their presence in granitoid crustal rocks would have process that rids the lithosphere of high-density, low viscosity kept temperatures near the melting point (e.g. (Sandiford and materials (Fe-rich olivine, garnet, oceanic crust, etc) and leaves low McLaren, 2006). In addition, Michaut et al. (2009-this issue) have density, high viscosity Fo-rich olivine (±orthopyroxene). We must also calculated that internal heating from material with the composition of explain why this process appears to have acted efficiently in the peridotite xenoliths from Archean cratons would raise temperatures Archean but not during later times. within the lithosphere by nearly 200 °C; the heat contribution from We propose that the sorting took place in the accumulated more fertile material, which is richer in Fe, garnet and in heat- residues of plume or oceanic crust melting, and that the sorting was producing elements as well, would have been greater still. facilitated by strong heating due to heat input from both the In their paper, Michaut et al. (2008) demonstrate that internal underlying hot Archean asthenosphere and overlying hot Archean heating causes a negative temperature gradient to develop in the continental crust, and by internal heating within the lithospheric lower part of the lithosphere–i.e. temperatures at the base of the mantle itself. Most scientists (e.g. (Abbott et al.,1994; Vlaar et al.,1994) lithosphere exceed those in the underlying convecting mantle. The hot accept that the Archean asthenosphere was some 250 °C hotter than lower layer may partially melt, producing magmas that would ascend N.T. Arndt et al. / Lithos 109 (2009) 61–71 69 to the surface (perhaps to form lamprophyres and other peculiar post- heat loss during melting) and are richer in weak minerals like garnet and orogenic intrusions that irrupt in many Archean cratons) and a more pyroxene. They are also relatively dense. These layers at the base of the refractory residue; or the hot, weakened dense layer could founder oceanic lithosphere will be pushed aside by the impinging plume, which and be removed from the lithosphere. The lower Fe- and garnet-rich will rise until it encounters a layer of harzburgite or dunite that is suffi- lower layers of the stratified plume (Figs. 1a and 5) would thereby be ciently cool, rigid and buoyant to stop its ascent. The new plume material ejected from the lithosphere. But what of any Fe-rich material that will therefore underplate beneath a layer consisting of Fo-rich olivine± might be trapped at higher levels in the lithosphere? How could the orthopyroxene. Each subsequent plume adds another layer, building up a lithosphere rid itself of segments of oceanic crust that became thick pile of depleted lithosphere. Subduction zones may form at the stranded within the lithosphere during accretion of slabs of oceanic margins of the growing craton and the influx of fluids from dehydrating lithosphere (Fig. 1c)? crust causes the melting in the mantle wedges that initiates the complex In principle the dense components of the accreted oceanic series of processes that lead to the formation of continental crust. lithosphere (eclogitic crust and Fe-rich peridotite) could be removed The distribution of U-Pb ages of zircons from crustal rocks provide by gravitational segregation. The buoyancy differences driving the a record of semi-continuous growth through the mid-Archean (4 to downward motion are counterbalanced by the strength and viscosity 2.7 Ga) followed by a period of episodic growth from 2.7 to about of the lithospheric material. Numerical modeling by Vlaar et al. (1994) 1.8 Ga (Condie, 1994; Stein and Hofmann, 1994). The global event at shows that a thick layer of uniformly high-density material such as 2.7 Ga was followed by a period of about 200 million years during ecologitised oceanic crust would segregate in a relatively short time, which very little new continental crust formed. We suggest that the of the order of 10 million years. What, however, is the fate of a slab of continental lithosphere accumulated progressively through the stratified oceanic lithosphere, which consists of a crustal portion that Archean and that final processing — the rejection of high-density, has differentiated into an eclogitised upper layer of basaltic crust, a low-viscosity components — took place as the lithosphere evolved in lower layer of Fe-rich olivine and pyroxene cumulates, and an the period following the 2.7 Ga crustal growth peak. The broad underlying layer of residual harzburgite (Fig. 2)? A numerical coincidence between the ages of overlying crust and those recorded in experiment involving a “slab” sandwich model (Fig. 6) shows that the lithospheric mantle is explained in this way. although an isolated eclogite layer is dense enough to sink to the asthenosphere, the foundering crustal layer drags down with it the 8. Conclusions layer of depleted harzburgite. To decouple the low-density layer from the other denser layers requires the presence of a weak internal The peculiar composition of subcontinental lithospheric mantle lubricating layer between the harzburgite and the surrounding denser results from two separate processes. Melting at ocean ridges or in layers (van Keken et al., 1996). The presence of such a layer at what subduction zones does not produce material of appropriate composi- was once the Moho seems rather unlikely. We conclude, therefore, tion and we propose that the main source of the Fo-rich olivine and that although layers of accreted oceanic crust will be removed from magnesian orthopyroxene was the residue of high-degree mantle the lithosphere by gravity-driven segregation, this process cannot melting in unusually hot mantle plumes. This residue was stratified result in the net addition of the high-Fo olivine that constitutes the from fertile peridotite at the margins and towards the base of the major component of the sub-continental lithosphere. Accretion of melting zone to refractory Fo-rich olivine±orthopyroxene in upper slabs of oceanic lithosphere is therefore not a viable process to form parts of the core of the melting zone. Only the latter material has a subcontinental lithsopheric mantle. composition appropriate to form stable and durable lithospheric The same type of argument applies to the model of reprocessing of mantle and only this material could have accumulated; the denser, peridotite within a subduction zone. The density difference between more fertile parts must have been rejected. The sorting of Fo-rich unprocessed peridotite above the zone of partial melting and the olivine and magnesian orthopyroxene from the denser and less underlying refractory residue is too small for the unprocessed material viscous components of fertile peridotite took place during the to be rejected within a reasonable period of time. impingement of subsequent mantle plumes and during reworking of accumulated peridotites. 7. The preferred model: accumulation and reworking of plume residues Acknowledgements

Given these difficulties with oceanic-crust and subduction-zone We acknowledge support received from the French Agence models, we prefer the hypothesis that the subcontinental lithospheric National de Recherche, (BEGDy project) and from the “Archean mantle is made up primarily of the residues of one or more mantle Environment” research networking program of the European Science plumes. We see formation of the Archean continental lithosphere as a Foundation. We thank Cin-Ty Lee, Sally Gibson and an anonymous complex process in which different components are produced in reviewer for helpful reviews. different ways: the refractory, Fe-poor composition of the mantle portion can be explained only by melting in hot mantle plumes; the References crust, on the other hand, forms during subduction that proceeds at the margins of the growing continent. We emphasize that even the Abbott, D., Burgess, L., Longhi, J., 1994. An empirical thermal history of the 's upper mantle. Journal of Geophysical Research 99, 13835–13850. residue of plume melting must undergo further reworking after it Arndt, N.T., Bruzak, G., Reischmann, T., 2001. The oldest continental and oceanic initially accumulated. plateaus: geochemistry of basalts and komatiites of the Pilbara Craton, Australia. In: We note that the oldest well-preserved Archean cratons such as the Ernst, R.E., Buchan, K.L. (Eds.), Mantle Plumes: Their Identification Through Time. – Pilbara and Kaapvaal record multiple episodes of plume volcanism Geological Society of America, Special Paper, Boulder, pp. 359 388. Arndt, N.T., Albarède, F., Lewin, E., 2002. Strange partners: formation and survival of (de Wit et al., 1992; Arndt et al., 2001) and we suggest that each episode continental crust and lithospheric mantle. Journal of the Geological Society adds its contribution of refractory Fo-rich olivine±orthopyroxene to the (London), Special Publication 199, 91–103. lithospheric mantle. The process may have been as follows. Consider a Arndt, N.T., Barnes, S.J., Lesher, C.M., 2008. Komatiite. Cambridge University Press, Cambridge. 487 pp. new plume which arrives at the base of an or normal Barth, M.G., et al., 2001. Geochemistry of xenolithic ecolgites from West Africa: part 1, a oceanic lithosphere. The mantle portion of the oceanic lithosphere is link between low MgO ecologites and Archean crust formation. Geochimica et stratified, from Fe-rich fertile peridotite at the base to refractory dunite Cosmochimica Acta 65, 1499–1527. Bell, D.R., Rossman, G.R., Moore, R.O., 2004. Abundance and partitioning of OH in a high at the top (Fig. 2). The lower parts are readily deformable, being hotter pressure magmatic system: megacrysts from the Monastery Kimberlite, South than the overlying dunite (due to cooling during adiabatic ascent and Africa. Journal of Petrology 45, 1539–1564. 70 N.T. Arndt et al. / Lithos 109 (2009) 61–71

Berstein, S., Kelemen, P.B., Brook, C.K., 1997. Highly depleted spinel harzburgite from basalts, picrites and komatiites. Geochemistry, Geophysics and Geosystems 8, xenoliths in Tertiary dykes from East Greenland. Earth and Planetary Science Letters Q02006. doi:10.1029/2006GC001390. 154, 221–235. Hirth, G., Kohlstedt, D.L., 1996. Water in the oceanic upper mantle: implications for Bernstein, S., Kelemen, P.B., Brooks, C.K., 1998. Depleted spinel harzburgite xenoliths in rheology, melt extraction and the evolution of the lithosphere. Earth and Planetary Tertiary dykes from East Greenland: restites from high degree melting. Earth and Science Letters 144, 93–108. Planetary Science Letters 154, 221–235. Ionov, D.A., Griffin, W.L., O'Reilly, S.Y., 1997. Volatile-bearing minerals and lithophile Beyer, E., Griffin, W.L., O'Reilly, S.Y., 2006. Transformation of Archean lithospheric trace elements in the upper mantle. Chemical Geology 141, 153–184. mantle by refertilisation: evidence from exposed peridotites in the Western Gneiss Jaupart, C., Labrosse, S., Mareschal, J.C., 2007. Mantle Dynamics. Treatise of Geophysics, Region, Norway. Journal of Petrology 47, 1611–1636. vol. 7, pp. 253–303. Chapt. 6. Bostock, M., 1998. Mantle stratigraphy and evolution of the Slave Province. Journal of Jordan, T.H., 1975. The continental tectosphere. Geophysics and Space Physics 13, 1–12. Geophysical Research 103, 21183–21200. Jordan, T.H., 1978. Composition and development of the continental tectosphere. Nature Boyd, F.R., 1989. Compositional distinction between oceanic and cratonic lithosphere. 274, 544–548. Earth and Planetary Science Letters 96, 15–26. Jordan, T.H., 1988. Structure and formation of the continental tectosphere. Journal of Boyd, F.R., Mertzman, S.A., 1987. Composition and structure of the Kaapvaal lithosphere, Petrology, Special Lithosphere Issue 11–13. southern Africa. In: Mysen, B.O. (Ed.), Magmatic Processes: Physicochemical Kelemen, P.B.,1990. Reaction between ultramafic rock and fractionating basaltic magma Principles. Geochem. Soc. Spec. Publ., vol. 1, pp. 13–24. I: phase relations, the origin of calk-alkaline magma series, and the formation of Boyd, F.R., Gurney, J.J., Richardson, S.H., 1985. Evidence for a 150–200 km thick Archaean discordant dunite. Journal of Petrology 31, 51–98. lithosphere from diamond inclusion thermobarometry. Nature 315, 387–389. Kelemen, P.B., S.R., H., Berstein, S., 1998. Silica enrichment in the continental upper Carlson, R.W., Pearson, D.G., James, D.E., 2005. Physical, chemical and chronological mantle via melt/rock reaction. Earth and Planetary Science Letters 164, 387–406. characteristics of continental mantle. Reviews in Geophysics 43, RG1001. Kelly, R.K., Kelemen, P.B., Jull, M., 2003. Buoyancy of the continental upper mantle. Coffin, M.F., Eldholm, O., 1993. Scratching the surface: estimating dimensions of large Geochemistry, Geophysics and Geosystems 4, 1017. doi:10.1029/2002GC000399. igneous provinces. Geology 21, 515–518. Kohlstedt, D.L., Keppler, H., Rubie, D.C.,1996. Solubility of water in the a-phase, b-phase and Condie, K.C., 1994. Archean Crustal Evolution. Elsevier, Amsterdam. 420 pp. c-phase of (Mg,Fe)2SiO4. Contributions to Mineralogy and Petrology 123, 345–357. Cooper, C.M., Lenardic, A., Levander, A., Moresi, L., 2006. Creation and preservation of Korenaga, J., 2006. Archean geodynamics and the thermal evolution of earth. In: Benn, K., cratonic lithosphere: seismic constraints and geodynamic models. In: Benn, K., Mareschal, J.-C., Condie, K.C. (Eds.), Archean Geodynamics and Environments. Mareschal, J.C., Condie, K.C. (Eds.), Archean Geodynamics and Environments. Americal Geophysical Union Monograph, Washington, pp. 7–32. American Geophysical Union Monograph, Washington, pp. 75–88. Lee, C.-T.A., 2003. Compositional variation of density and seismic velocities in natural Davies, G.F., 1992. On the emergence of . Geology 20, 963–966. peridotites at STP conditions: implications for seismic imaging of compositional Dawson, J.B., 1984. Contrasting types of upper mantle metasomatism? 3rd International heterogeneities in the upper mantle. Journal of Geophysical Research 108, 2441. Kimberlite Conference Proceedings, 1984, pp. 289–295. doi:10.1029/2003JB002413. de Wit, M.J., et al., 1992. Formation of an Archaean continent. Nature 357, 553–562. Lee, C.-T.A., 2006. Geochemical/petrologic constraints on the origin of cratonic mantle. Doin, M.P.,Fleitout, L., Mckenzie, D.,1996. Geoid anomalies and the structure of continental In: Benn, K., Mareschal, J.-C., Condie, K.C. (Eds.), Archean Geodynamics and and oceanic. . Journal of Geophysical Research 101, 16119–16136. Environments. Americal Geophysical Union Monograph, Washington, pp. 89–114. Falloon, T.J., Danyushevsky, L.V., 2000. Melting of refractory mantle at 1.5, 2 and 2.5 GPa Lenardic, A., Moresi, L.,1999. Some thoughts on the stability of cratonic lithosphere: effects under anhydrous and H2O-undersaturated conditions: implications for the of buoyancy and viscosity. Journal of Geophysical Research 104, 12747–12759. petrogenesis of high-Ca boninites and the influence of subduction components Levander, A., Niu, F., Lee, C.-T.A., Cheng, X., 2006. Imag(in)ing the continental lithosphere. on mantle melting. Journal of Petrology 41 (2), 257–283. Tectonophysics 416, 167–185. Farnetani, C.G., Samuel, H., 2005. Beyond the thermal plume paradigm. Geophysical Li, X.-P., Rahn, M., Bucher, K., 2004. Metamorphic processes in rodingites of the Research Letters 32, L07311. doi:10.1029/2005GL022360. Zermatt–Saas ophiolites. International Geology Review 46, 28–51. Fung, A.T., Haggerty, S.E., 1995. Petrography and mineral composition of ecolgites from McCulloch, M.T., Gamble, J.A., 1991. Geochemical and geodynamical constraints on the Koidu kimberlite complex, Sierra Leone. Journal of Geophysical Research 100, subduction zone magmatism. Earth and Planetary Science Letters 102, 358–374. 20451–20473. Mei,S.,Kohlstedt,D.L.,2000.Influence of water on plastic deformation od olivine aggregates; 1. Gaetani, G.A., Grove, T.L., 1998. The influence of water on melting of mantle peridotite. diffusion creep regime. Journal of Geophysical Research 105, 21471–21481. Contributions to Mineralogy and Petrology 131, 323–346. Menzies, M.A., 1990. Archaean, Proterozoic and Phanerozoic lithospheres. In: Menzies, Gaul, O.F., Griffin, W.L., O'Reilly, S.Y., Pearson, D.G., 2000. Mapping olivine compositions M.A. (Ed.), Continental Mantle. Clarendon, Oxford, pp. 67–86. in the lithospheric mantle. Earth and Planetary Science Letters 182, 223–235. Menzies, M.A., Erlank, A.J., 1987. Mantle Metasomatism. Academic Press, London. Grant, K.J., Kohn, S.C., Brooker, R.A., 2006. Solubility and partitioning of water in Menzies, M.A., Rogers, N., Tindle, A., Hawkesworth, C.J., 1987. Metasomatic and

synthetic forsterite and enstatite in the system MgO-SiO2-H2O. Contributions to enrichment processes in lithospheric peridotites, an effect of asthenosphere- Mineralogy and Petrology 151, 651–664. lithosphere interaction. In: Menzies, M.A., Hawkesworth, C.J. (Eds.), Mantle Gregoire, M., Bell, D.R., Le Roex, A.P., 2003. Garnet lherzolites from the Kaapvaal Craton Metasomatism. Academic Press, London, pp. 313–361. (South Africa): trace element evidence for a metasomatic history. Journal of Michaut, C., Jaupar, C., 2009. Thermal evolution of cratonic roots. Lithos. Lithos 109, Petrology 44 (4), 629–657. 47–60 (this issue). Gregoire, M., Tinguely, C., Bell, D.R., Le Roex, A.P., 2005. Spinel lherzolite xenoliths from Nisbet, E.G., Cheadle, M.J., Arndt, N.T., Bickle, M.J., 1993. Constraining the potential the premier kimberlite (Kaapvaal Craton, South Africa): nature and evolution of the temperature of the Archaean mantle: a review of the evidence from komatiites. shallow upper mantle beneath the Bushveld Complex. Lithos 84, 185–205. Lithos 30, 291–307. Griffin, W.L., O'Reilly, S.Y., Ryan, C.G., Gaul, I., Ionov, D., 1998. Secular variation in the O'Neill, C., Moresi, L., Müller, R.D., Albert, R., Dufour, F., 2006. Ellipsis 3D: a particle-in-cell composition of the subcontinental lithospheric mantle. In: Brann, J., Dooley, J.C., finite element hybrid code for modelling mantle convection and lithospheric Goleby, B.R., van der Hilst, R.D., Klootwijk, C.T. (Eds.), Structure and Evolution of the deformation. Journal of Computers and Geosciences 32, 1769–1799. Australian Continent. Americal Geophysical Union, Washington, Washington, pp.1–26. Pearson, D.G., et al., 1995. Re-Os, Sm-Nd, and Rb-Sr isotopic evidence for thick Archaean Griffin, W.L., O'Reilly, S.Y., Ryan, C.G., 1999. The composition and origin of sub- lithospheric mantle beneath the Siberian craton modified by multistage metaso- continental lithospheric mantle. In: Fei, Y., Bertka, C.M., Mysen, B.O. (Eds.), Mantle matism. Geochimica et Cosmochimica Acta 59, 959–977. Petrology: Field Observations and High-Pressure Experimentation. The Geochem- Pearson, D.G., Canil, D., Shirey, S.B., 2004. Mantle samples included in volcanic rocks: ical Society, Houston, pp. 13–46. xenoliths and diamonds. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Griffin, W.L., et al., 2003. The origin and evolution of Archean lithospheric mantle. Geochemistry. Elsevier, Amsterdam, pp. 171–275. Precambrian Research 127, 19–41. Pollack, H.N., 1986. Cratonization and thermal evolution of the mantle. Earth and Griffin, W.L., O, 'Reilly, S.Y., Abe, N., Aulbach, S., Davies, R.M., Pearson, N.J., Doyle, B.J., Kivi, K., Planetary Science Letters 80, 175–182. 2004. Lithospheric mapping beneath the North American plate. Lithos 77, 873–922. Poudjom Djomani, Y.H., O'Reilly, S.Y., Griffin, W.L., Morgan, P., 2001. The density Griffiths, R.W., Campbell, I.H., 1990. Stirring and structure in mantle starting plumes. structure of the subcontinental lithospheric mantle through time. Earth and Earth and Planetary Science Letters 99, 66–78. Planetary Science Letters 184, 605–621. Hauri, E.H., Shimizu, N., Dieu, J.J., Hart, S.R., 1993. Evidence of hotspot-related Riesberg, L., Lorand, J.P., 1995. Longevity of subcontinental mantle lithosphere from carbonatite metasomatism in the oceanic upper manlte. Nature 365, 221–227. osmium isotope systematics in orogenic peridotite massifs. Nature 376, 159–162. Hawkesworth, C.J., Rogers, N.W., Van Calsteren, P.W.C., Menzies, M.A., 1984. Mantle Rollinson, H.R., 1997. Eclogite xenoliths in West African kimberlites as residues from enrichment processes. Nature 311, 331–336. Archaean granitoid crust formation. Nature 389, 173–176. Hawkesworth, C.J., Gallagher, K., Hergt, J.M., McDermott, F., 1994. Destructive plate Sandiford, M., McLaren, S., 2006. Thermo-mechanical controls on heat production margin magmatism: geochemistry and melt generation. Lithos 33, 169–188. distributions and the long-term evolution of the continents. In: Brown, M., Helmstaedt, H., Schulze, D.J., 1989. Southern African kimberlites and their mantle Rushmer, T. (Eds.), Evolution and Differentiation of the Continental Crust. sample; implications for Archean tectonics and lithosphere evolution. Geological Cambridge University Press, Cambridge, pp. 67–91. Society of Australia, Special Publication 14, 358–368. Schutt, D.L., Lesher, C.E., 2006. Effects of melt depletion on the density and seismic Herzberg, C., 1999. Phase equilibrium constraints on the formation of cratonic velocity of garnet and spinel lherzolite. Journal of Geophysical Research 111. mantle. In: Fei, Y., Bertka, C.M., Mysen, B.O. (Eds.), Mantle Petrology: Field doi:10.1029/2003JB002,950. Observations and High-Pressure Experimentation. The Geochemical Society, Houston, Sen, G., 1987. Xenoliths associated with the Hawaiian hot spot. In: Nixon, P.H. (Ed.), pp. 13–46. Mantle Xenoliths. John Wiley, Chichester, pp. 359–375. Herzberg, C., O'Hara, M.J., 2002. Plume-associated ultramafic magmas of Phanerozoic Shirey, S.B., et al., 2002. Diamond genesis, seismic structure and evolution of the age. Journal of Petrology 43, 1857–1883. Kaapvaal-Zimbabwe craton. Science 297, 1683–1686. Herzberg, C., Asimow, P.D., Arndt, N.T., Nui, Y., Lesher, C.M., Fitton, J.G., Cheadle, M.J., Simon, N.S.C., Carlson, R.W., Pearson, D.G., Davies, G.R., 2007. The origin and evolution of Saunders, A.D., 2006. Temperatures in ambient mantle and plumes: constraints the Kaapvaal Cratonic lithospheric mantle. Journal of Petrology 48, 589–625. N.T. Arndt et al. / Lithos 109 (2009) 61–71 71

Sleep, N., 2003. Survival of Archean cratonal lithosphere. Journal of Geophysical Van Kranendonk, M.J., 2004. Preface Archaean tectonics 2004: a review. Precambrian Research 108. doi:10.1029/2001JB000169. Research 131, 143–151. Stein, M., Hofmann, A.W., 1994. Mantle plumes and episodic crustal growth. Nature 372, Vlaar, N.J., van Keken, P.E., van den Berg, A.P., 1994. Cooling of the earth in the Archaean. 63–68. Earth and Planetary Science Letters 121, 1–18. van Achterbergh, E., Griffin, W.L., Stiefenhofer, J., 2001. Metasomatism in mantle Wade, J.A., Plank, T., Melson, W.G., Soto, G.J., Hauri, E.H., 2006. The volatile content of xenoliths from the Lethakane kimberlites: estimation of element fluxes. Contribu- magmas from Arenal volcano, Costa Rica. Journal of Volcanology and Geothermal tions to Mineralogy and Petrology 141, 397–414. Research 157, 94–120. van Keken, P.E., Karato, S., Yuen, D.A., 1996. Rheological control of oceanic crust separation in the transition zone. Geophys. Res. Lett. 23, 1821–1824.