Archean Lithospheric Mantle Origin: Petrological Constraints

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Origin of Archean subcontinental lithospheric mantle: Some petrological
constraints
ArticleinLithos · February 2009
DOI: 10.1016/j.lithos.2008.10.019
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Nicholas Arndt
University Joseph Fourier - Grenoble 1
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Nicolas Coltice
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Herwart H Helmstaedt
Queen's University
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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
abstractarticle info
Article history:
Received 9 June 2008
Accepted 17 October 2008
Available online 5 November 2008
Keywords:
Mantle
Lithosphere
Olivine
Archean
The longevity of the continental lithosphere mantle is explained by its unusual composition. This part of the
mantle is made up mainly of forsterite-rich olivine (Fo92–94), with or without orthopyroxene, and it is
essentially anhydrous. The former characteristic makes it buoyant, the latter makes it viscous, and the
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
Archean subcontinental lithospheric mantle include: (a) high-degree mantle melting in a plume or at an
Archean ocean ridge; (b) accretion of this material to older lithosphere and its reworking in a subduction
zone; (c) redistribution of material to eliminate high-density, low-viscosity lithologies. Following an
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
In most of the Archean subcontinental lithospheric mantle, the
dominant mineral is olivine that has an unusually magnesian compo-
sition, with forsterite contents (Fo=mole fraction MgO/(MgO +FeO))
in the range 92 to 94. In many regions, the magnesian olivine is
accompanied by orthopyroxene with about the same Mg/Fe ratios, to
produce a rock with harzburgitic bulk composition (Boyd and
Mertzman, 1987; Griffin et al., 1999); more rarely the rock consists
only of olivine and is a highly refractory dunite (Berstein et al., 1997).
Highly magnesian olivine and orthopyroxene, if anhydrous, have low
densities and high viscosity, features that enhance the chance that a
lithosphere composed mainly of these minerals survives as a layer
above the convecting mantle (Lenardic and Moresi, 1999). The long-
term stability of old subcontinental lithospheric mantle is therefore
directly linked to its particular composition.
It is not easy to explain how the Archean lithospheric mantle
acquired its peculiar composition. The problem is that olivine with a
forsterite content greater than 92 is not normally produced during
mantle melting. Highly magnesian olivine is restricted to the residues of
high-degree partial melting, and except under extreme conditions, this
type of olivine forms onlya small fraction of the total residue. To produce
the Archean subcontinental lithospheric mantle that survived for
billions of years after it initially formed therefore requires one or more
of the following conditions: (a) melting under highly unusual condi-
tions, (b) a petrological/tectonic process that transforms less-magnesian
olivine and other mantle minerals into forsterite-rich olivine, and/or (c)
a process that physically separates forsterite-rich olivine from less
magnesian olivine and other mantle minerals. In this contribution we
first investigate the models that have previously been proposed to
explain the composition of old subcontinental lithospheric mantle, then
we develop a modified version of these models thatbest accounts for the
features of the subcontinental lithospheric mantle.
2. Summary of the composition, structure, physical properties and
history of old subcontinental lithospheric mantle
Many recent papers (e.g. (Griffin et al., 1999; Gaul et al., 2000;
Poudjom Djomani et al., 2001; Gregoire et al., 2003; Griffin et al.,
2003; Gregoire et al., 2005; Lee, 2006; Simon et al., 2007)have
provided excellent summaries of the characteristics of old subconti-
nental lithospheric mantle. These papers make the following points.
a) Peridotite (ultramafic rock containing olivine, pyroxene and a
relatively small, b5–20%, proportion of an aluminous phase such as
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;
Lee, 2006). If the lithology of these suites accurately represents the
proportions of different rock types in the lithosphere itself, mafic
Lithos 109 (2009) 61–71
⁎Corresponding author.
E-mail address: [email protected] (N.T. Arndt).
0024-4937/$ –see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2008.10.019
Contents lists available at ScienceDirect
Lithos
journal homepage: www.elsevier.com/locate/lithos

rocks form only a very minor component (b1%) of the lithospheric
mantle beneath the Kaapvaal craton. Mafic rocks contain a higher
proportion of garnet and are present as eclogite or garnet pyrox-
enite under mantle conditions.
The peridotites are mainly harzburgites (olivine and orthopyroxene)
with rarer lherzolites (olivine, clinopyroxene and orthopyroxene) and
dunites (olivine alone). Until recently our knowledge of lithosphere
compositions was strongly influenced by information derived from
studies of copious suites of xenoliths from South African kimberlites.
These studies provided a picture of a lithosphere dominated by
orthopyroxene-rich harzburgite (Boyd and Mertzman, 1987; Boyd,
1989). Other authors have shown, however, that the lithosphere beneath
some other cratons (e.g. Greenland, (Berstein et al., 1997)contains
abundant refractory dunite, and that other segments of subcontinental
lithosphere contain a relatively high proportion (up to 40%) of
pyroxenite and eclogite (e.g. (Fung and Haggerty, 1995).
Olivine in peridotite xenoliths from the mantle beneath Archean
cratons has a relatively restricted range of forsterite contents, from a
minimum of around 89 to a maximum close to 95. In many compilations
there is a pronounced peak between 93 and 94 (e.g. (Boyd and
Mertzman, 1987; Gaul et al., 2000; Pearson et al., 2004)). This
distribution is in sharp contrast with that of olivine from younger
continental or oceanic lithosphere (e.g. (Sen, 1987; Griffin et al.,1998), or
with estimates of olivine compositions in peridotite from the convecting
mantle or asthenosphere (Lee, 2006), in which forsterite contents range
from about 88 to 93 with an abundance maximum at 89-90. In most
xenolith suites, the forsterite content of olivine correlates with the
modal abundance of olivine; i.e. the most common rocks are dunites
which are rich in Fo-rich olivine and contain little or no pyroxene or
garnet. The trend is broken, however, by the harzburgites from the
Kaapvaal craton, which contain high orthopyroxene contents and lower
olivine contents. In these rocks, the Mg/(Mg+ Fe) of both olivine and
orthopyroxene are mainly in the range 92–94 but they plot to the right of
the Fo vs. modal olivine trend because of their relatively low olivine
contents (Figure 4 of Lee, 2006).
Metasomatism resulting from the circulation within the upper
mantle of melts and fluids, including basaltic and kimberlitic melts, has
affected large portions of the lower lithosphere. (e.g. (Dawson, 1984;
Hawkesworth et al., 1984; Menzies and Erlank, 1987; Menzies et al.,
1987; van Achterbergh et al., 2001; Gregoire et al., 2003; Beyer et al.,
2006). This process transforms the dunites or harzburgites, the normal
components of the lithosphere mantle, into lherzolites, which are
richer in pyroxenes and hydrous minerals.
b) Radiometric dating, mainly using the Re-Os method, has shown that
the mantle portion of the lithosphere stabilized at about the same
time as the overlying crust, some 2–3 billion years ago in the case of
the oldest cratons (e.g. (Pearson et al., 1995; Riesberg and Lorand,
1995; Shirey et al., 2002; Carlson et al., 2005). In order that the
lithosphere survived for billions of years without being swept into
the convecting mantle , it must have been both buoyant and
relatively viscous (Jordan, 1978; Pollack, 1986; Jordan, 1988; Hirth
and Kohlstedt, 1996; Lenardic and Moresi, 1999; Kelly et al., 2003;
Lee, 2003; Sleep, 2003; Cooper et al., 2006; Lee, 2006). The buoyancy
of the lithosphere is related to its density and thus to its
mineralogical and chemical composition, as well as its temperature.
The inherent density of mantle peridotite depends mainly on the
abundance of garnet, the densest of the four dominant mantle
minerals, and on the Mg/Fe ratios of these minerals. The lithosphere
is cooler than underlying asthenosphere and so, in order to survive, it
must contain a low proportion of garnet and/or its olivine and
pyroxene must have high Mg/Fe ratios. As outlined above, this is
indeed the case for old subcontinental lithospheric mantle. The
viscosity of the lithosphere depends only weakly on its composition
and mineralogy but strongly on the presence of volatiles, mainly
water or CO
2
, which usually are present in hydrous minerals or
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.
62 N.T. Arndt et al. / Lithos 109 (2009) 61–71

(Kohlstedt et al.,1996; Mei and Kohlstedt, 2000). The longevity of the
lithosphere requires that it contained very low volatile contents.
c) Jordan (1975, 1978, 1988) introduced the notion of an isopycnic
lithosphere. According to this idea, at every depth in the
lithosphere there is a balance between compositional buoyancy,
which is related to the types and compositions of mineral phases,
and the thermal buoyancy, which is related to the temperature
difference between the colder lithosphere and hotter surrounding
asthenosphere. For this balance to hold, the compositional buoy-
ancy must increase progressively from at the base, where the
lithosphere has about the same temperature as adjacent convect-
ing mantle, to the top, where it is far cooler. In practice this requires
that the amount of garnet and/or the Fe content of olivine and
pyroxene must decrease with decreasing depth.
d) The unusual mineralogy and composition (high Mg/Fe ratios, low
garnet content, negligible water content) needed to assure the
longevity of old subcontinental lithosphere requires that it formed
under unusual circumstances. Many authors (e.g., (Boyd, 1989;
Griffin et al., 1999, 2003) equate the presence of Fo-rich olivine and
the paucity of other phases with that of a residue of high-degree
partial melting. Using simple mass balance or more sophisticated
petrologicalmodeling, it can beshown thatthe required composition
corresponds to that of the residue produced by 30 to 50% melting of
fertile mantle peridotite (Boyd et al.,1985; Bernstein et al.,1998; Lee,
2006). Other authors have proposed that reprocessing and possible
remelting in a subduction environment introduced orthopyroxene
and increased the Mg/Fe ratio of the olivine.
3. Previous explanations of the origin of subcontinental
lithospheric mantle
In this section we critically discuss previous explanations for the
origin of low-density viscous subcontinental lithospheric mantle, then
add one or two of our own. Drawing from Lee (2006), we start with
three end-member models.
3.1. Melting in a mantle plume
In this model, promoted, for example, by Boyd (1989),Pearson et al.
(1995),Arndt et al. (2002) and Griffin et al. (2003, 2004), the
subcontinental lithospheric mantle is said to have formed from the
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 oceanic crust. 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.
63N.T. Arndt et al. / Lithos 109 (2009) 61–71

The plume undergoes partial melting as it rises, the melt escapes to the
surface, and the solid residue that remains in the plume becomes
progressively depleted in easily fusible components. This process
results in progressive change in the composition of the residue, from
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
that is not well understood, the residues of melting then accumulate
near the surface to form the subcontinental lithospheric mantle.
There are several obvious advantages to this model: (a) the
composition of the residue ranges from relatively Fe-rich garnet
lherzolite at the base of the melting column to highly refractory Fe-
poor dunite at the top. If incorporated into the lithosphere, the vertical
distribution of lithologies, from relatively dense at the base to buoyant
at the top, is isopycnic, at least qualitatively. (b) If the plume is hot
enough and the melting column long enough, the most refractory
residues, which are produced at the top of the column, will contain
very Fo-rich olivine (±orthopyroxene) whose composition is very like
that in old subcontinental lithospheric mantle. (c) Because the
extraction of melt removes volatiles, the residue is anhydrous. In
other words, melting in a hot mantle plume is capable of producing
the low-density, gravitationally stable, high viscosity material that
assures its long-term stability of the lithosphere.
Lee (2006) criticized two aspects of the model. First he notes that
melting at depth in the lower part of the melting column leaves garnet
in the residue. Through his quantitative modeling in which he
assumed that fertile lherzolite underwent isobaric equilibrium partial
melting, he showed that the residues of high-pressure melting contain
high FeO, Al
2
O
3
and Sc contents. In contrast, peridotites from old
subcontinental lithospheric mantle contain relatively low FeO, Al
2
O
3
and Sc contents, features that correspond either to melting at shallow
depths under conditions in which garnet is absent or to secondary
processes, such as orthopyroxene addition, that decreased the
contents of FeO and the other elements. Second, he notes that the
generation of a large volume of refractory Fe-poor dunite requires the
extraction of a large volume of high-degree melt. This melt would
have the composition of a komatiite, a type of magma that forms only
a small fraction of the Archean volcanic sequences interpreted as the
products of melting in mantle plumes. These aspects of the plume
model are discussed below.
Bernstein et al. (1998) note that the Fo93 peak in abundance plots
from Greenland xenoliths coincides to the extent of melting required
to eliminate orthopyroxene from the residue. At higher degrees of
melting, the melt productivity drops drastically; i.e. the amount of
melt produced for a given increase in temperature decreases
markedly. This effect may explain the peak in olivine compositions
in the range Fo92–94.
3.2. Accretion and stacking of oceanic lithosphere
In this model, advocated originally by Helmstaedt and Schulze
(1989), the subcontinental lithospheric mantle is proposed to have
grown through the accretion of slabs of oceanic lithosphere. The idea
is that portions of lithosphere that originally formed at a mid-ocean
ridge were thrust one beneath another in a subduction zone at the
margin of the growing continent, as shown in Fig. 1b.
The advantages of this model are: (a) it accounts for the presence
within suites of mantle xenoliths of eclogite and garnet pyroxenite,
which, in some cases, have geochemical and isotopic characteristics
that point to their having formed as old oceanic crust (e.g. (Fung and
Haggerty, 1995; Rollinson, 1997; Barth et al., 2001). (b) It explains the
presence of dipping seismic reflectors at the edges of some cratons
(Bostock, 1998; Levander et al., 2006). (c) It is consistent with the
inferred low-pressure origin of cratonic peridotites. Stacking of a
series of slabs made up largely of low-pressure peridotite thereby
provides a means of generating a large volume of subcontinental
lithospheric mantle.
Lee (2006) discussed a major problem of the model, a problem that
centers on the wide dispersion of lithologies and compositions in
oceanic lithosphere. The mantle portion of modern oceanic litho-
sphere is made up of rocks ranging from fertile, Fe-rich garnet- or
spinel lherzolite at the base, to harzburgite at the top (Fig. 2a). The
crustal portion is also stratified, from gabbros and Fe-rich olivine-
pyroxene cumulates in the lower part, to basalt in the upper part. The
fraction of harzburgite and dunite is low (b10%) and material with the
composition of Fe-poor cratonic peridotite is absent. In modern
lithosphere, the proportion of oceanic crust is about 10% (6–9 km thick
crust overlies 60–100 km of lithospheric mantle), significantly higher
than the proportion of eclogite and garnet pyroxenite in most parts of
the subcontinental lithospheric mantle. With such a high proportion
of garnet-rich lithologies it is unlikely that lithosphere formed by
stacking of slabs of oceanic plates would have been sufficiently
buoyant to have survived.
Lee mentions two possible solutions: (i) the more Fe-rich portions
of the oceanic lithosphere could have been removed before or during
accretion; (ii) Archean oceanic lithosphere was derived from hotter,
and perhaps more depleted Archean mantle (Davies, 1992) and it
would have had a different composition from modern oceanic
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.
64 N.T. Arndt et al. / Lithos 109 (2009) 61–71