Download Origin of Archean subcontinental lithospheric mantle: Some

Lithos 109 (2009) 61–71
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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
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
b
a r t i c l e
i n f o
Article history:
Received 9 June 2008
Accepted 17 October 2008
Available online 5 November 2008
Keywords:
Mantle
Lithosphere
Olivine
Archean
a b s t r a c t
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 composition, 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 longterm 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 only a small fraction of the total residue. To produce
the Archean subcontinental lithospheric mantle that survived for
⁎ 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
billions of years after it initially formed therefore requires one or more
of the following conditions: (a) melting under highly unusual conditions, (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 that best 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 subcontinental 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 subcontinental 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
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N.T. Arndt et al. / Lithos 109 (2009) 61–71
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 pyroxenite 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 CO2, 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.
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 buoyancy must increase progressively from at the base, where the
lithosphere has about the same temperature as adjacent convecting 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
63
petrological modeling, it can be shown that the 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.
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N.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 Fepoor 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, Al2O3 and Sc contents. In contrast, peridotites from old
subcontinental lithospheric mantle contain relatively low FeO, Al2O3
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 lithosphere 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 olivinepyroxene 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.
N.T. Arndt et al. / Lithos 109 (2009) 61–71
peridotite and its inherent density would have been less than that of
modern oceanic lithosphere.
3.3. Processes in subduction zones
In this model, the cratonic mantle is said to have formed through
processing of material in the mantle wedge above a subduction zone.
(e.g. Jordan, 1988; Herzberg, 1999; Lee, 2006; Simon et al., 2007) (Fig. 1c).
Relatively fertile peridotite is transformed into more refractory harzburgite or dunite by melting triggered by fluid transfer from dehydrating
oceanic crust. The thickening called on to produce ~200-km thick
lithosphere is achieved by deformation associated with the accretion.
One way to look at the process is illustrated in Fig. 3. The material in the
mantle wedge — a mixture of older accreted oceanic slabs or plume
residue — is drawn down through the melting zone by the drag of the
subducting plate. As this material is pulled downwards, it passes through
a zone where fluids liberated from the dehydrating subducting oceanic
crust cause partial melting. The residue left after melt extraction is
depleted in Fe- or Al-rich fusible components and this residue, which has
the composition of low-density Fe-poor harzburgite or dunite, underplates the lithosphere.
Problems with the model relate to the efficacy of the melting
process. Can melting triggered by the input of fluids from subducting
ocean crust leave a residue that (a) is anhydrous, as required for longterm stability of the lithosphere, and (b) lacks the geochemical
signature of the subduction process? Magmas derived from subduction zones are characterized by enrichment of incompatible elements
coupled with depletion of Nb, Ta and some other high-field-strength
elements (e.g. (McCulloch and Gamble, 1991; Hawkesworth et al.,
1994). The trace element contents in most mantle xenoliths do not
show the subduction signature (Hauri et al., 1993; Ionov et al., 1997;
van Achterbergh et al., 2001; Gregoire et al., 2003; Pearson et al.,
2004; Gregoire et al., 2005).
Another problem with the model is that lithosphere generated or
reprocessed in a subduction zone would be gravitationally unstable:
low-density, Fe-poor residue underlies higher-density more fertile
material yet to pass through the melting zone.
3.4. Serpentinization of oceanic crust
65
Oceanic lithosphere is well stratified because it is formed from
layers that spread out laterally as newly solidified oceanic crust
migrates away from the ridge (Fig. 2a). Flowage in the asthenosphere
changes from vertical to horizontal beneath the ridge and this
transformation is inherited by the residue of melting. The lowermost
layer in the stratified upper mantle has a composition close to that of
the asthenosphere because it is the product of low-degree melting.
This melting starts at 60–90 km at a modern oceanic ridge (Herzberg
et al., 2006). The shallowest layer, which forms beneath the crust at
about 10 km depth, is the most depleted in fusible components
because it is produced by high-degree melting. The base of the
lithosphere is the boundary between the rigid plate and deformable
mantle, which depends on the temperature gradient, the composition
the shear stress and other factors discussed by Michaut et al. (2009this issue). As the plate cools during its migration away from the ridge,
the position of this boundary migrates from above the top of the
melting column at the ridge to a depth of up to 100 km, near the base
of the residual peridotite layer, in the oldest parts of the oceanic
basins. In Archean oceanic lithosphere the positions of these
boundaries would have been very different, as described below.
The residue left in the mantle after extraction of melt from a
modern mantle plume, such as the one beneath Hawaii, originally has
a cylindrical form but it becomes deformed as it accretes to the base of
older oceanic lithosphere. The shape and form of the mantle sources of
continental or oceanic volcanic plateaus are very poorly understood.
The well-known image of a large sphere atop a narrow stem, as
illustrated in early models of starting mantle plumes by Griffiths and
Campbell (1990), is most probably an oversimplification. Compositional heterogeneities in mantle plumes strongly influence ascent
dynamics and a variety of shapes and sizes can be obtained (Farnetani
and Samuel, 2005). Coffin and Eldholm (1993) represent the source of
the largest volcanic plateaus as a sphere with a diameter between 700
and 1000 km. In order that such a source undergoes high-degree
partial melting, it must pass within 200 km of the surface. Just how
material in the source flows to pass through the shallow melting zone
is an open question; just where and how the residue left after melt
extraction accumulates is even more uncertain. In brief, we know very
little about the geometry of the mantle residues produced in large
mantle plumes.
The idea here is that under some conditions olivine reacts to
serpentine with higher Mg# than the original olivine. Iron 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
mineral compositions in the residue of melting depend on the
geodynamic setting in which melting takes place.
Fig. 4. Phase diagram for mantle peridotite and the paths followed by ascending
material at modern and Archean mid-ocean ridges and mantle plumes from Herzberg
(1999); Herzberg and O'Hara, (2002).
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N.T. Arndt et al. / Lithos 109 (2009) 61–71
With these complications in mind we will consider some
examples, starting with the simplest:
a) Modern oceanic lithosphere formed by melting at a modern
mid-ocean ridge. In Herzberg et al.'s (2006) modeling of the
formation of modern ocean crust, he assumed polybaric fractional
melting (Figs. 2a and 4) and showed that the degree of melting
increases from 0% at 60–90 km depth to a maximum of 20% at the top
of the melting column. The process produces 6 km of basaltic crust
above a 60–90 km thick layer of the residual mantle. In the mantle
layer, the Fo content of olivine ranges from 89 at the base of the melting
column to 91.5 at the top. None of this residue has a composition
comparable to that of Fe-poor Archean cratonic peridotites.
b) Archean oceanic lithosphere. Advocates of slab-accretion models
propose that Archean oceanic lithosphere had a more refractory
composition more like that of Archean cratonic peridotite.
Consider melting of a hotter mantle with a potential temperature
of 1600 °C. Using the phase diagrams from Herzberg and O'Hara
(2002), we estimate that under these conditions, melting starts at
about 135 km and reaches a maximum of 40% at about 20 km
depth. This melting produces a layered lithosphere comprising a
thick upper layer of oceanic crust about 20 km thick, a middle
portion (from the base of the crust at 20 km to about 60 km) made
up of harzburgite containing olivine with the composition Fo91–93
and a lower portion (60 to 135 km) composed of peridotite with
olivine compositions between Fo89–91.
In this model, only about 30% of the residue in the melting column
has a composition like that of Fe-poor cratonic peridotites. If Archean
oceanic lithosphere is to become part of the cratonic lithosphere, then
both the lower layer of Fe-rich lherzolite and the upper crustal layer
must be removed.
The lower layer probably never forms part of the lithosphere
because it is too hot. Given a hotter Archean mantle and faster moving
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
the base of the lithosphere, then Archean lithosphere is generally
thinner than modern oceanic lithosphere and its composition is
indeed much more refractory. It is probable that a slab of oceanic
lithosphere that accretes to a growing craton includes only the upper
harzburgitic layer and none of the underlying lherzolites. This resolves
the problem of getting rid of the dense Fe-rich peridotites, but what
happens to the overlying 20 km-thick crust? This crust overlies the
harzburgitic layer, and it must founder through the harzburgite if it is
to be lost to the underlying convecting mantle. Alternatively, the
accreted layers of oceanic lithosphere might be reworked in the
subduction zone (as described in a later section).
The problem is compounded if Archean subduction were relatively
flat, as has been suggested by several authors (see Van Kranendonk
(2004) for a review) The basis for this idea are threefold. First,
Korenaga (2006) has argued that since melting begins at greater
depths in the Archean than at present, and since melt extraction
depletes the lithosphere in viscosity-decreasing volatile components,
the total thickness of anhydrous, relatively viscous lithosphere should
be greater than at present. Thick rigid lithosphere would have resisted
bending at a subduction zone. The second argument depends on the
idea that plate movement was faster in the Archean because higher
mantle temperatures increased the vigor of convection. Oceanic
lithosphere was hotter and more buoyant when it reached the
subduction zones. The third argument is based on the composition of
Archean oceanic crust, which is produced by differentiation of picrite
and contains a relatively thin proportion of basalt above a thicker layer
of olivine (±pyroxene) cumulates. The proportion of eclogite, the
dense component that drags subducting crust down into the mantle,
is therefore relatively low. All three arguments lead to the notion that
Archean oceanic crust, if it subducted, would have plunged at a
shallow angle into the mantle.
c) Residue of a modern mantle plume. The residue produced during
melting of a modern mantle plume is zoned both horizontally and
vertically. Because the temperature decreases from a maximum in
the centre of the plume to ambient at its margins, the residue left
after melt extraction consists of a refractory, low-density core
surrounded by a denser, less-depleted outer sheath (Arndt et al.,
2002); and because the extent of melting increases with decreasing depth, its composition changes from fertile peridotite at depth
to refractory dunite close to the surface (Fig. 5).
Given that ambient temperatures in the Archean mantle were
higher than in the present mantle, Archean mantle plumes probably
were hotter than modern plumes. Constraints from Archean komatiites indicate potential temperatures greater than 1700 °C, compared
with about 1400 °C for the hottest modern plumes (Nisbet et al., 1993;
Herzberg et al., 2006; Arndt et al., 2008). Melting in such a plume starts
at about 7 GPa (~ 200 km) and leaves an initially cylindrical residue
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
range from 89 to 92, and a thicker (90 km) upper zone with olivine
compositions between Fo 92 and Fo 94. Again, a sheath of less depleted
peridotite surrounds the more refractory core (Arndt et al., 2002).
When this material reaches the base of oceanic or continental
lithosphere, the compositionally denser outer portions may cool and
sink back into the mantle leaving only the low-density refractory core.
The vertical change in composition — from buoyant refractory dunite
at shallow levels to denser more fertile peridotite at deeper levels — is
gravitationally stable. The upper part of the residue has a composition
like that of cratonic peridotite. When looked at this way, the Archean
plume model seems an attractive means of generating lithospheric
mantle.
But what of the objections posed by Lee (2006)? Consider first the
notion that a plume-generated residue should retain a geochemical
record of melting in the presence of garnet. As mentioned in an earlier
section, the dominant component of the continental lithospheric
mantle is refractory harzburgite or dunite, material that is produced
only at the highest degrees of melting. This material does not form in
the presence of garnet, for two reasons: first, it is produced at the top
of the melting column at pressures where garnet is not stable; second,
garnet will have been exhausted at an earlier stage of melting,
particularly if melting is fractional. Consider again the polybaric
fractional melting process. The initial melts, which form through lowdegree melting in equilibrium with garnet at 7 GPa, have peculiar
compositions enriched in both MgO and incompatible trace elements.
They have low Al/Ti, low Sc and high Gd/Yb and they leave a residue
relatively rich in garnet, with high Al/Ti, high Sc and low Gd/Yb (Lee,
2006; Simon et al., 2007). As the plume material rises from 7 to 4 GPa,
melts are continually extracted leaving a residue that becomes
progressively depleted in garnet as the pressure drops and as this
component is removed in departing melts. Through this process
garnet is progressively eliminated. A hot plume passes through the
garnet-out curve at very high pressures, between 5 and 6 GPa
depending on the temperature (Fig. 4), and from there on, melting has
little effect on trace element characteristics of the residue, other than
to strip out what remains of the incompatible elements. The residue
that forms at pressures lower than 5 GPa consists of harzburgite to
dunite, assemblages of olivine ± orthopyroxene that retain little record
of the earlier part of the melting process.
It is important to recall here that only the Fe-poor harzburgites and
dunites produced at shallow levels have compositions like that of the
continental lithospheric mantle. The more Fe-rich portions — the only
parts that retain the signature of melting in the presence of garnet —
are not incorporated into the lithosphere. On this basis there is no
reason to expect that the garnet signature, or in other words, the
signature of high-pressure melting (Lee, 2006; Simon et al., 2007),
should be preserved in rocks from the continental lithosphere.
Lee's (2006) second objection to the plume-melting model
concerns the abundance of komatiite. This is an intractable problem
because, in order to estimate the amount of komatiite that formed
during Archean magmatism, we have to rely on the incomplete and
probably biased record preserved in Archean greenstone belts. Some
of these volcanic sequences erupted on older continental crust, others
onto or adjacent to island arcs (Arndt et al., 2008). In both cases the
presence of a layer of low-density crust would have hindered the
passage of high-density komatiite magma and facilitated the eruption
of lower-density basalt. Under these conditions, the proportion of
komatiite preserved in the volcanic successions will be lower than at
the site of melting. Large volumes of komatiite may have erupted as
parts of volcanic plateaus in ocean basins, but such sequences are
uncommon in greenstone belts; perhaps because they are dense and
difficult to obduct. Archean oceanic crust may have contained a high
proportion of komatiite but true Archean oceanic crust; i.e. crust
formed at a mid-ocean spreading centre, has never been convincingly
documented. Given the probability that Archean greenstone belts
represent a biased record of Archean volcanism, it is possible that
67
komatiite was far more abundant in the Archean than is commonly
thought.
4.2. Reworking of accreted slabs
Fig. 3 is a sketch of a subduction zone. Drag of the subducting slab
draws the mantle wedge first through the zone of hydration, then
through the zone of partial melting. Melt leaves the wedge to erupt as
arc volcanics, leaving a residue stripped of low-temperature components. What would happen if a previously accreted slab of Archean
ocean lithosphere were drawn through the melting zone? This slab
might consist of an upper ~20-km-thick crustal portion differentiated
into a few kilometers of magnesian basalts and a thicker sequence of
pyroxene-olivine cumulates, underlain by maybe 20 km of refractory
harzburgite. Serpentinization may have resulted in the crystallization
of relatively magnesian olivine. As the slab is drawn through the
melting zone, the melts are extracted mainly from the crustal section —
the refractory harzburgite contributes very little. Fluid-fluxed melting
extracts both the basaltic components and the Fe-rich fraction of mafic
minerals from the source: the presence of water destabilizes
orthopyroxene producing siliceous arc magmas leaving a residue
consisting almost entirely of Fo-rich olivine (Gaetani and Grove, 1998;
Kelemen et al., 1998; Falloon and Danyushevsky, 2000). In other words,
such melting could, at least in theory, convert the ocean slab into Forich dunite of the type found in old continental lithospheric mantle.
But what of the problems posed in the earlier section? Would the
residue retain the “subduction signature” (enrichment of incompatible
elements and deficits of elements like Nb-Ta and Zr) that is absent from
most mantle xenoliths? The residue of high-degree melting consists
only of olivine± orthopyroxene, two minerals that contain very low
concentrations of incompatible elements. The small fraction of
incompatible elements retained in the residue would come almost
entirely from trapped liquid that had not escaped the source. If the
amount of trapped liquid is low and if the magma contained less than
10 ppm of elements such as Nb-Ta and the rare earths, as in primary arc
magmas (Kelemen, 1990), then the residue would contain b0.01 ppm of
these elements. Higher levels of incompatible trace elements measured
in mantle xenoliths (1–20 ppm, Pearson et al., 2004) most probably are
introduced later through the influx of metasomatic fluids from the
deeper mantle (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). Simon et al. (2007) for example, conclude
that xenoliths from the Kaavaal craton were subjected to three
metasomatic episodes, the first in a subduction setting and the latter
two as a result of influxes of kimberlitic magmas. These processes
profoundly change the composition of the lithosphere and it is possible
that the subduction signature was masked by later metasomatism.
The second question is whether fluid-fluxed melting can produce an
anhydrous residue; i.e., high viscosity material capable of surviving the
ravages of mantle convection. Olivine and other nominally anhydrous
minerals may contain a few hundred ppm of water (e.g. (Kohlstedt et al.,
1996; Bell et al., 2004; Grant et al., 2006), and any trapped interstitial
melt will contribute larger amounts. If 1% of melt containing 6% H2O
(that of subduction-zone magmas, Wade et al. (2006)) remained
trapped, the residue contains 600 ppm, in addition to that in the
minerals themselves. The total amount of water most probably exceeds
the threshold at which its presence significantly decreases the viscosity
of peridotite (Hirth and Kohlstedt, 1996). The persistence of water in the
residue of hydrous melting poses a major problem for the hypothesis
that cratonic lithospheric mantle formed in subduction zones.
5. Isopycnicity and secular variation in the composition of
cratonic lithosphere
Another problem arises from the distribution of lithologies in a
subduction-processed mantle wedge. Reworking in the subduction
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N.T. Arndt et al. / Lithos 109 (2009) 61–71
zone produces a low-density Fo-rich dunitic residue beneath higherdensity 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 maintained 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, lowlying 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 differences 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
distribution of lithologies within the continental lithospheric mantle,
and the density of the lithosphere as a whole, must have increased in
order to preserve isopycnicity. This readjustment probably takes place
through the rejection of portions with too-low densities or their
reworking into higher-density lithologies. Mantle metasomatism, the
introduction of Fe- or garnet-rich lithologies in incoming fluids or
melts, was probably responsible for this densification.
6. Rejection of high-density components during reworking of the
Archean lithosphere
We now return to the problem of generating the peculiar
composition of Archean subcontinental lithospheric mantle, particularly the paucity of garnet and clinopyroxene and the high Mg/Fe
ratios of the olivine and orthopyroxene. We have established that this
composition cannot be explained by any reasonable combination of
melting or fluid-influx processes. What is required is a subsequent
process that rids the lithosphere of high-density, low viscosity
materials (Fe-rich olivine, garnet, oceanic crust, etc) and leaves low
density, high viscosity Fo-rich olivine (±orthopyroxene). We must also
explain why this process appears to have acted efficiently in the
Archean but not during later times.
We propose that the sorting took place in the accumulated
residues of plume or oceanic crust melting, and that the sorting was
facilitated by strong heating due to heat input from both the
underlying hot Archean asthenosphere and overlying hot Archean
continental crust, and by internal heating within the lithospheric
mantle itself. Most scientists (e.g. (Abbott et al., 1994; Vlaar et al., 1994)
accept that the Archean asthenosphere was some 250 °C hotter than
Fig. 6. Results of modelling of the segregation of dense layers in the continental
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),
eclogite in blue (ρ = 3440 kg/m3, µ = 1022 Pa s), harzburgite in light green composing the
continental lithospheric mantle and a layer within the sandwich model (ρ = 3200 kg/m3,
µ = 1022 Pa s), peridotite in dark green (r = 3300 kg/ m3, m = 1021 Pa s). (b) after
100 million years, (c) after 200 million years and (d) after 300 million years. The dense
slab of oceanic lithosphere rapidly founders but it takes with it the underlying layer of
depleted harzburgite. Accretion and foundering of oceanic lithosphere is not a process
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
preparation).
modern asthenosphere as a result of greater internal heat production
and secular cooling. Less widely appreciated is the likelihood that the
overlying continental crust was also much hotter, because of greater
heat production from radioactive elements. Isotopes such as 40K, 235U
and 232Th would have been about 3 times more abundant 3 billion
years ago and their presence in granitoid crustal rocks would have
kept temperatures near the melting point (e.g. (Sandiford and
McLaren, 2006). In addition, Michaut et al. (2009-this issue) have
calculated that internal heating from material with the composition of
peridotite xenoliths from Archean cratons would raise temperatures
within the lithosphere by nearly 200 °C; the heat contribution from
more fertile material, which is richer in Fe, garnet and in heatproducing elements as well, would have been greater still.
In their paper, Michaut et al. (2008) demonstrate that internal
heating causes a negative temperature gradient to develop in the
lower part of the lithosphere–i.e. temperatures at the base of the
lithosphere exceed those in the underlying convecting mantle. The hot
lower layer may partially melt, producing magmas that would ascend
N.T. Arndt et al. / Lithos 109 (2009) 61–71
to the surface (perhaps to form lamprophyres and other peculiar postorogenic intrusions that irrupt in many Archean cratons) and a more
refractory residue; or the hot, weakened dense layer could founder
and be removed from the lithosphere. The lower Fe- and garnet-rich
lower layers of the stratified plume (Figs. 1a and 5) would thereby be
ejected from the lithosphere. But what of any Fe-rich material that
might be trapped at higher levels in the lithosphere? How could the
lithosphere rid itself of segments of oceanic crust that became
stranded within the lithosphere during accretion of slabs of oceanic
lithosphere (Fig. 1c)?
In principle the dense components of the accreted oceanic
lithosphere (eclogitic crust and Fe-rich peridotite) could be removed
by gravitational segregation. The buoyancy differences driving the
downward motion are counterbalanced by the strength and viscosity
of the lithospheric material. Numerical modeling by Vlaar et al. (1994)
shows that a thick layer of uniformly high-density material such as
ecologitised oceanic crust would segregate in a relatively short time,
of the order of 10 million years. What, however, is the fate of a slab of
stratified oceanic lithosphere, which consists of a crustal portion that
has differentiated into an eclogitised upper layer of basaltic crust, a
lower layer of Fe-rich olivine and pyroxene cumulates, and an
underlying layer of residual harzburgite (Fig. 2)? A numerical
experiment involving a “slab” sandwich model (Fig. 6) shows that
although an isolated eclogite layer is dense enough to sink to the
asthenosphere, the foundering crustal layer drags down with it the
layer of depleted harzburgite. To decouple the low-density layer from
the other denser layers requires the presence of a weak internal
lubricating layer between the harzburgite and the surrounding denser
layers (van Keken et al., 1996). The presence of such a layer at what
was once the Moho seems rather unlikely. We conclude, therefore,
that although layers of accreted oceanic crust will be removed from
the lithosphere by gravity-driven segregation, this process cannot
result in the net addition of the high-Fo olivine that constitutes the
major component of the sub-continental lithosphere. Accretion of
slabs of oceanic lithosphere is therefore not a viable process to form
subcontinental lithsopheric mantle.
The same type of argument applies to the model of reprocessing of
peridotite within a subduction zone. The density difference between
unprocessed peridotite above the zone of partial melting and the
underlying refractory residue is too small for the unprocessed material
to be rejected within a reasonable period of time.
7. The preferred model: accumulation and reworking
of plume residues
Given these difficulties with oceanic-crust and subduction-zone
models, we prefer the hypothesis that the subcontinental lithospheric
mantle is made up primarily of the residues of one or more mantle
plumes. We see formation of the Archean continental lithosphere as a
complex process in which different components are produced in
different ways: the refractory, Fe-poor composition of the mantle
portion can be explained only by melting in hot mantle plumes; the
crust, on the other hand, forms during subduction that proceeds at the
margins of the growing continent. We emphasize that even the
residue of plume melting must undergo further reworking after it
initially accumulated.
We note that the oldest well-preserved Archean cratons such as the
Pilbara and Kaapvaal record multiple episodes of plume volcanism
(de Wit et al., 1992; Arndt et al., 2001) and we suggest that each episode
adds its contribution of refractory Fo-rich olivine ± orthopyroxene to the
lithospheric mantle. The process may have been as follows. Consider a
new plume which arrives at the base of an oceanic plateau or normal
oceanic lithosphere. The mantle portion of the oceanic lithosphere is
stratified, from Fe-rich fertile peridotite at the base to refractory dunite
at the top (Fig. 2). The lower parts are readily deformable, being hotter
than the overlying dunite (due to cooling during adiabatic ascent and
69
heat loss during melting) and are richer in weak minerals like garnet and
pyroxene. They are also relatively dense. These layers at the base of the
oceanic lithosphere will be pushed aside by the impinging plume, which
will rise until it encounters a layer of harzburgite or dunite that is sufficiently cool, rigid and buoyant to stop its ascent. The new plume material
will therefore underplate beneath a layer consisting of Fo-rich olivine±
orthopyroxene. Each subsequent plume adds another layer, building up a
thick pile of depleted lithosphere. Subduction zones may form at the
margins of the growing craton and the influx of fluids from dehydrating
crust causes the melting in the mantle wedges that initiates the complex
series of processes that lead to the formation of continental crust.
The distribution of U-Pb ages of zircons from crustal rocks provide
a record of semi-continuous growth through the mid-Archean (4 to
2.7 Ga) followed by a period of episodic growth from 2.7 to about
1.8 Ga (Condie, 1994; Stein and Hofmann, 1994). The global event at
2.7 Ga was followed by a period of about 200 million years during
which very little new continental crust formed. We suggest that the
continental lithosphere accumulated progressively through the
Archean and that final processing — the rejection of high-density,
low-viscosity components — took place as the lithosphere evolved in
the period following the 2.7 Ga crustal growth peak. The broad
coincidence between the ages of overlying crust and those recorded in
the lithospheric mantle is explained in this way.
8. Conclusions
The peculiar composition of subcontinental lithospheric mantle
results from two separate processes. Melting at ocean ridges or in
subduction zones does not produce material of appropriate composition and we propose that the main source of the Fo-rich olivine and
magnesian orthopyroxene was the residue of high-degree mantle
melting in unusually hot mantle plumes. This residue was stratified
from fertile peridotite at the margins and towards the base of the
melting zone to refractory Fo-rich olivine ± orthopyroxene in upper
parts of the core of the melting zone. Only the latter material has a
composition appropriate to form stable and durable lithospheric
mantle and only this material could have accumulated; the denser,
more fertile parts must have been rejected. The sorting of Fo-rich
olivine and magnesian orthopyroxene from the denser and less
viscous components of fertile peridotite took place during the
impingement of subsequent mantle plumes and during reworking of
accumulated peridotites.
Acknowledgements
We acknowledge support received from the French Agence
National de Recherche, (BEGDy project) and from the “Archean
Environment” research networking program of the European Science
Foundation. We thank Cin-Ty Lee, Sally Gibson and an anonymous
reviewer for helpful reviews.
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