Precambrian Research 127 (2003) 19–41
The origin and evolution of Archean lithospheric mantle
W.L. Griffin a,b,∗ , S.Y. O’Reilly a , N. Abe a , S. Aulbach a ,
R.M. Davies a , N.J. Pearson a , B.J. Doyle c , K. Kivi d
a
ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences,
Macquarie University, Sydney, NSW 2109, Australia
b CSIRO Exploration and Mining, North Ryde, Sydney, NSW 2113, Australia
c Kennecott Canada Exploration Inc., 354-200 Granville Street, Vancouver, BC, Canada, V6C 154
d Kennecott Canada Exploration Inc., 1300 Walsh Street, Thunder Bay, Ont., Canada P7E 4X4
Accepted 10 April 2003
Abstract
The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last
major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous
formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have
remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly
depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in
younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation
of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle (“lithospheric stacking”),
founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly
depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive
correlations between Fe, Cr and Al imply that no Cr–Al phase (i.e. spinel or garnet) was present on the liquidus during the
melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern
environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents
residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving
major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave
Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more
similar to modern plate tectonics. Preliminary data from in situ Re–Os dating of sulfide minerals in mantle-derived peridotites
suggest that much Archean SCLM may have formed in a small number of such major events >3.0 Ga ago. The survival of
Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a “life-raft model”
of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism,
and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive
modification of relict, buoyant Archean lithosphere.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Archean; Lithospheric mantle; Mantle xenoliths; Mantle overturns; Mantle plumes; Lithosphere secular evolution
∗
Corresponding author. Tel.: +61-2-9850-8954; fax: +61-2-9850-8943.
E-mail address:
[email protected] (W.L. Griffin).
0301-9268/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0301-9268(03)00180-3
20
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
1. Introduction
Uniformitarianism is a driving force in modern geology; linked with the plate-tectonics paradigm, it has
provided a powerful tool for the analysis of ancient tectonic regimes. However, many papers from this symposium reflect possible problems with extending the
Uniformitarian model into Deep Time. To what extent
can we assume that Archean tectonic processes were
analogous to Phanerozoic ones? How far back in time
can the plate-tectonic paradigm, derived from modern
observations, be extended before it must finally break
down?
In this discussion paper, we will argue that the
processes that formed most of the Archean subcontinental lithospheric mantle (SCLM), as sampled in
kimberlites and other volcanic rocks, have not operated in Phanerozoic time. We will argue that Archean
SCLM (with rare exceptions) was not generated by
subduction of oceanic/arc mantle, and discuss the implications of alternative models. We use new, in situ,
Re–Os data to suggest that most typically Archean
SCLM probably was generated more than 3 Ga ago,
and argue that the late Archean represents a fundamental “break-point” in Earth’s geodynamics, with
important implications for the formation and destruction of continents.
2. Secular evolution of SCLM composition
The concept of a linkage between crust-forming
and mantle-forming events has been the starting point
for our investigations. We have tested this concept by
a detailed examination of the composition of SCLM
beneath areas of different tectonothermal age (the
age of the last major thermal event) worldwide. If a
correlation exists between SCLM composition and
crustal tectonothermal age, the linkage must be a fact
and relate to the time of formation of the SCLM.
For convenience, we have adopted a modified version
of the Archon–Proton–Tecton subdivision of Janse
(1994). Archons have tectonothermal ages >2.5 Ga,
Protons have ages 1.0–2.5 Ga, and Tectons have ages
<1.0 Ga.
The major obstacle to a comprehensive study of
SCLM composition is the patchy distribution of good
xenolith suites, in both time and space. Xenolith data
from Archean cratons are heavily biased by samples
from a few kimberlite pipes in southern Africa and
one pipe (Udachnaya) in Siberia. The other large body
of data is on spinel peridotites from alkali basalts in
Tectons. These two suites are markedly different in
composition (Boyd, 1989; Griffin et al., 1999a), and
neither can represent the composition of the SCLM in
a meaningful way, though each has been used for that
purpose (e.g. McDonough, 1990; Maaloe and Aoki,
1977).
A more widespread and more easily accessible
source of data is available in the form of xenocryst
minerals, derived by disaggregation of mantle
wall-rocks, in volcanic rocks. While these mineral
grains have lost much of their petrological context,
they can be analysed in large numbers, to provide
broad and statistically meaningful information on
SCLM composition across a much wider range of
localities. Griffin et al. (1998, 1999d) showed that
there are strong correlations between the compositions of Cr-pyrope garnets and their peridotitic host
rocks, and that these correlations allow the calculation of mean SCLM composition from a garnet
(gnt) concentrate. The mantle compositions calculated in this way compare well with the mean
compositions of some well-studied xenolith suites
(Table 1).
This approach has been used to calculate the mean
composition of the SCLM beneath >30 areas worldwide (Griffin et al., 1999d; Table 2). The most striking feature is the consistent decrease in the degree of
depletion from Archon to Proton to Tecton (Fig. 1).
Archon SCLM is highly depleted in Ca, Al and other
magmaphile elements; Tecton SCLM (as represented
by garnet peridotites) is only moderately depleted relative to most estimates of the Primitive Mantle composition; Proton SCLM is intermediate between the
two.
This secular evolution in SCLM composition, correlated with the tectonothermal age of the overlying
crust, implies that the formation (or modification) of
crust and mantle are broadly contemporaneous, and
that crust and mantle have in general remained linked
through periods of eons. The differences in SCLM
composition also imply a secular change in the mechanisms that have produced the SCLM; this implies, in
turn, an evolution in the mechanisms by which continents have formed.
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
21
Table 1
Comparison of SCLM compositions, calculated from garnet concentrates, with xenolith medians
Kaapvaal Craton (Archon)
SiO2
TiO2
Al2 O3
Cr2 O3
FeO
MnO
MgO
CaO
Na2 O
NiO
Daldyn Field, Siberia (Archon)
Garnet
Lherzolite
calculated
from garnets
Garnet
Lherzolites
median
xenolith
Garnet
Harzburgite
calculated
from garnets
Garnet
Harzburgite
median
xenolith
Garnet
Lherzolite
calculated
from garnets
Garnet
Lherzolites
median
xenolith
Garnet
Harzburgite
calculated
from garnets
Garnet
Harzburgite
median
xenolith
46.0
0.07
1.7
0.40
6.8
0.12
43.5
1.0
0.12
0.27
46.6
0.06
1.4
0.35
6.6
0.11
43.5
1.0
0.10
0.28
45.7
0.04
0.9
0.26
6.3
0.11
45.8
0.5
0.06
0.30
45.9
0.05
1.2
0.27
6.4
0.09
45.2
0.5
0.09
0.27
45.8
0.05
1.2
0.31
6.5
0.11
44.9
0.7
0.08
0.29
44.3
0.04
1.0
0.37
7.6a
0.13
45.2
1.0
0.07
0.29
45.4
0.02
0.4
0.18
6.1
0.11
47.2
0.2
0.03
0.32
42.2
0.09
0.6
0.37
7.4a
0.10
47.8
1.0
0.07
0.31
Protons
SiO2
TiO2
Al2 O3
Cr2 O3
FeO
MnO
MgO
CaO
Na2 O
NiO
a
Tectons
S. Australia
calculated
from garnets
Mt. Gambier
(SA) median
xenolith
Obnazhennaya
calculated
from garnets
Obnazhennaya
median
xenolith
E. China
calculated
from garnets
E. China
median
xenolith
Vitim
calculated
from garnets
Vitim
median
xenolith
44.4
0.07
1.9
0.41
7.8
0.13
43.2
1.6
0.13
0.30
44.2
0.04
1.9
0.44
7.6
0.13
43.5
1.6
0.05
0.29
44.9
0.09
2.4
0.42
7.9
0.13
41.7
2.1
0.17
0.28
42.6
0.00
1.8
0.44
8.4
0.13
44.7
1.4
0.06
0.26
44.5
0.15
3.8
0.40
8.0
0.13
39.1
3.4
0.27
0.25
45.5
0.16
3.8
0.44
8.2
0.14
38.1
3.3
0.23
0.25
44.5
0.15
3.7
0.40
8.0
0.13
39.3
3.3
0.26
0.25
44.5
0.16
4.0
0.37
8.0
0.00
39.3
3.2
0.32
0.25
Affected by late Fe introduction on grain boundaries (Boyd et al., 1997).
3. Archon SCLM is unique
Archean SCLM is not simply more depleted than
younger SCLM. Boyd (1989) used a plot of olivine
(ol) composition against modal composition (Fig. 2)
to illustrate his observation that peridotite xenoliths
from the classical Archean localities in South Africa
and Siberia have higher Si/Mg (reflected in a higher
orthopyroxene/olivine ratio and lower olivine content)
than highly depleted rocks produced in oceanic and
arc-related settings; the few available suites of xenoliths from Proton settings lie between the “Archean”
and the “oceanic” fields on this plot. As well as higher
Si/Mg, Archean xenolith suites also show lower Cr#
(100 Cr/(Cr+Al)), Ca/Al and Fe/Al at any Mg#
(100 Mg/(Mg+Fe)) than Tecton or Proton xenolith
suites, or ocean and massif peridotites (Griffin et al.,
1999d; Figs. 3 and 4).
Archean SCLM is unique in other ways as well. One
distinctive difference, long recognised by diamond exploration companies (e.g. Gurney, 1984; Gurney and
Zweistra, 1995), is the presence of strongly subcalcic garnets in mineral concentrates from Archon settings, and their rarity (amounting to virtual absence)
from Proton and Tecton settings (Fig. 3). The depleted
harzburgites that contain these distinctive subcalcic
garnets are unique to Archon SCLM, and their uniqueness suggests the operation of a process that has not
produced this type of SCLM since the end of Archean
time.
Griffin et al. (1999c, 2002a) used a large database
(n ≥ 18,000) with major- and trace-element analyses
22
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
Table 2
Estimated mean SCLM compositions
Archons (mean
garnet SCLM)
Protons (mean garnet
SCLM + massifs
+ xenoliths)
Tectons (mean
garnet SCLM)
Tectons (mean
Spinel peridotite)
Primitive Mantle,
McDonough and
Sun (1995)
SiO2
TiO2
Al2 O3
Cr2 O3
FeO
MnO
MgO
CaO
Na2 O
NiO
Zn
V
Co
Sc
45.7
0.04
0.99
0.28
6.4
0.11
45.5
0.59
0.07
0.30
34
20
93
7
44.6
0.07
1.9
0.40
7.9
0.12
42.6
1.70
0.12
0.26
52
48
107
10
44.5
0.14
3.5
0.40
8.0
0.13
39.8
3.1
0.24
0.26
55
70
110
14
44.4
0.09
2.6
0.40
8.2
0.13
41.1
2.5
0.18
0.27
53
59
110
12
45.0
0.20
4.5
0.38
8.1
0.14
37.8
3.6
0.36
0.25
55
82
105
16
Mg#
Mg/Si
Ca/Al
Cr/Cr+Al
Fe/Al
92.7
1.49
0.55
0.16
4.66
90.6
1.43
0.80
0.12
3.02
89.9
1.33
0.82
0.07
1.66
89.9
1.38
0.85
0.09
2.23
89.3
1.25
0.73
0.05
1.30
Olivine/orthopyroxene/
clinopyroxene/garnet
Density, g/cc
Vp, km/s (room temperature)
Vp, 100 km, 700 ◦ C
Vs, Km/s (room temperature)
Vs, 100 km, 700 ◦ C
69/25/2/4
70/17/6/7
60/17/11/12
66/17/9/8
57/13/12/18
3.31
8.34
8.18
4.88
4.71
3.34
8.32
8.05
4.84
4.6
3.37
8.30
7.85
4.82
4.48
3.36
8.30
7.85
4.82
4.48
3.39
8.33
of mantle-derived Cr-pyrope garnets to evaluate approaches to the definition of populations using multivariate statistics. One of these techniques, Cluster
Analysis by Recursive Partitioning (CARP), recognised 15 individual populations, which can be correlated with specific petrological types of xenoliths
found in kimberlites and other volcanic rocks. These
CARP classes can be grouped into five major categories (Table 3). Depleted harzburgites contain subcalcic garnets depleted in Y, Ga, Zr, Ti and HREE;
depleted lherzolites have garnets with Ca–Cr relationships indicating equilibration with clinopyroxene
(cpx) (Griffin et al., 1999c), but depleted in HREE and
HFSE. The garnets of depleted/metasomatised lherzolites are depleted in Y and HREE, but enriched in
Zr and LREE, suggesting that they were subjected to
depletion and subsequent refertilisation; xenoliths of
this type commonly contain phlogopite ± amphibole.
The garnets of fertile lherzolites have high contents
4.81
of HREE and near-median contents of HFSE; they
retain no evidence of a depletion event. The garnets
of melt-metasomatised peridotites have a characteristic signature of enrichment in Zr, Ti, Y and Ga, and
correspond to the high-T sheared lherzolite xenoliths
found in many kimberlites.
The distribution of CARP classes in Archon, Tecton
and Proton SCLM (Table 4) again illustrates both the
secular evolution of the SCLM, and the uniqueness of
Archean SCLM. Some classes (mainly those related
to strong depletion) are restricted to Archon SCLM,
and/or are absent in Tecton SCLM, which is dominated
by a class of fertile peridotites (class L10B) that is
absent in Archon SCLM.
The garnet data can be plotted as a function of
depth; the equilibration temperature of each grain can
be calculated from its Ni content (Ryan et al., 1996),
and an estimate of its depth of origin is derived by
referring this temperature to a local paleogeotherm,
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
PRIMITIVE MANTLE
(= ASTHENOSPHERE)
4
E. China
R
E
NG
U
% CaO
YO
,,
3
,,
Phanerozoic
Spinel
Peridotite
Vitim
Zabargad
PHANEROZOIC
European
Massifs
PROTEROZOIC
2
S. Australia
Xenolith/ massif
averages
Kaapvaal
Gnt-SCLM
Archean
Proterozoic
Phanerozoic
1
ARCHEAN
0
23
1
2
3
4
5
% Al2O3
Fig. 1. CaO vs. Al2 O3 for calculated SCLM compositions, showing the secular evolution in the composition of the SCLM (after Griffin
et al., 1999d). Compositions are calculated from garnet concentrates (open symbols) or as means for xenolith suites and peridotite massifs
(crosses). Several published estimates for the composition of the Primitive Upper Mantle (PUM) are shown by asterisks. Garnet lherzolite
xenoliths from Tectons (E. China, Vitim) are only slightly depleted relative to PUM. All data from Griffin et al. (1999d).
derived either from xenolith data or from the garnet
concentrate itself (Ryan et al., 1996). When the relative abundances of CARP classes (Table 4) in typical
SCLM sections are plotted against depth (Fig. 4), they
provide a picture of the vertical distribution of different rock types and metasomatic styles.
These sections illustrate the dominance of depleted harzburgites and lherzolites in the Archean
SCLM, and their relative rarity in younger SCLM.
The pervasive effects of metasomatism over time in
the Archean SCLM are shown by the abundance of
depleted/metasomatised lherzolites, and the increase
in melt-related metasomatism downward in each section. Some sections, such as those from the Siberian
and Slave cratons, show pronounced stratification (cf.
Griffin et al., 1999a,e). The Slave section is unique in
having an ultradepleted upper layer, separated from
a more typically Archean lower layer by a sharp
boundary (145 ± 5 km). Proterozoic sections contain a high proportion of fertile lherzolites; some of
these lherzolites may represent metasomatic refertilisation of previously depleted rocks, while others
may never have been through a melt-extraction event.
Typical Tecton SCLM (in the garnet peridotite facies)
is compositionally simple, consisting of very fertile
lherzolites with no evidence of strong depletion, and
lherzolites showing a melt-metasomatism overprint.
4. Secular evolution by metasomatism?
As noted above, the garnet data illustrate extensive metasomatism of Archon and Proton SCLM, but
the metasomatic effects observed in xenolith suites
do not explain the high Si/Mg of Archon SCLM.
Griffin et al. (1999d) suggested that olivine addition
through melt infiltration has affected some Archean
peridotite suites and was accompanied by a decrease
in Mg#, as observed in abyssal peridotites (Niu, 1997).
Phlogopite-related metasomatism, which is common
in shallow garnet peridotites, has a similar effect, precipitating clinopyroxene and olivine at the expense of
garnet (±orthopyroxene), and lowering Mg# (Fig. 5;
Griffin et al., 1999f; van Achterbergh et al., 2001).
Numerous studies of mineral zoning show that the
compositions of high-T sheared peridotite xenoliths
24
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
Fig. 2. Plot of modal olivine content vs. Mg# in peridotite xenoliths and abyssal and ophiolite peridotites, showing the low olivine content
(related to high opx content) of Archean peridotite xenoliths from the Siberian and Kaapvaal cratons (after Boyd, 1987; field of Proterozoic
lherzolite xenoliths from Griffin et al., 1998). The “oceanic trend” of Boyd (1987) is modelled from shallow melting of a “pyrolite” or PUM
composition; most Phanerozoic lherzolite xenoliths, abyssal peridotites (Dick, 1989; Dick and Natland, 1996) and ophiolite peridotites lie
along this trend. Xenoliths from subduction zones also follow this trend. Data for Japanese localities from Abe (1997), Abe et al. (1998),
M. Makita, S. Arai (personal communication, 1997) and Tamura et al. (1999) (Parkinson and Pearce, 1998; Pearce et al., 2000).
found in many kimberlites reflect the infiltration of
asthenosphere-derived melts on a short time scale before kimberlite eruption (Smith and Boyd, 1987; Smith
et al., 1993; see review by Griffin et al., 1996). This
style of metasomatism decreases both Mg# and the orthopyroxene/olivine (opx/ol) ratio, and increases the
modal proportions of clinopyroxene and garnet. The
effect is to drive Archean peridotites toward the compositions of younger ones.
There are no documented examples of metasomatism driving “Tecton” compositions toward
“Archon” compositions. Instead, it would appear that
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
Shandong Province
Cr 2 O 3 (wt%)
15
25
Liaoning Province
Mengyin Kimberlites
450-500 Ma
15
10
10
5
5
Fuxian Kimberlites
450-500 Ma
1.5%
5
10
5
Taihang - Luliang
15
Yangtze Craton
15
Mesozoic - Tertiary
Low-Ca harz.
Ca harz.
Lherzolite
Low Cr
Wehrlite
10
10
Paleozoic
10
5
5
5
10
CaO (wt%)
5
10
Fig. 3. Ca–Cr plots for garnets from Chinese kimberlites and lamproites. Devonian kimberlites in Shandong and Liaoning Provinces
penetrate the Archean Sino–Korean Craton and contain high proportions of subcalcic pyropes derived from harzburgites (open squares and
diamonds). These garnets are essentially absent in Paleozoic lamproites erupted through the Proterozoic Yangtze Craton, although high-Cr
lherzolite garnets attest to a moderate degree of depletion. The Mesozoic-Tertiary Taihang-Luliang “kimberlites” have intruded rift zones
bisecting the Sino–Korean Craton; the low-Cr garnets are typical of fertile Phanerozoic peridotites. Data from Zhou et al. (1994).
metasomatism of the Archean SCLM over time will
tend to move it toward less depleted compositions
with lower Si/Mg (Fig. 5). This observation suggests
that metasomatic modification of Archean SCLM
could be at least part of the cause of the observed
secular evolution in the composition of the SCLM.
Secular evolution on a short time scale is demonstrated by a comparison of the SCLM sampled by
the spatially associated Group 2 (largely older than
110 Ma) and Group 1 (largely younger than 95 Ma)
kimberlites in the SW Kaapvaal Craton (Fig. 6). Over
the short time span separating these two eruptive
events, phlogopite-related metasomatism has substantially reduced the proportions of depleted harzburgites and lherzolites in the SCLM, and increased the
proportion of fertile lherzolites. Melt-related metasomatism has severely affected the lower part of the
section, effectively raising the base of the depleted
lithosphere by 30 km; at the same time the geotherm
has risen from near a 35 mW/m2 conductive model
geotherm to the 40 mW/m2 conductive model familiar
from many geothermobarometric studies of xenoliths
from the Group 1 kimberlites (Finnerty and Boyd,
1987; Griffin et al., 2003). The mean forsterite content
(% Fo, or Mg#) of olivine has decreased, at both the
top and bottom of the section (Fig. 6). The different
styles of metasomatism in the upper and lower parts
of the section probably reflect the activity of different
types of fluids, rising to different levels of the SCLM.
The increase in SCLM density with depth (Fig. 7)
provides a natural density filter, as fluids of different
density rise to their depth of neutral buoyancy.
The section from northern Botswana (Fig. 6) represents SCLM beneath the Kheis Belt, which was
the passive margin of the Kalahari Craton from ca.
2.3 to 2.0 Ga; deformation, volcanism and granitoid
intrusion from 2.0 to 1.8 Ga define it as a typical
Proton setting. Its SCLM section is typical of many
Proton sections worldwide; it has few if any depleted
harzburgites, depleted lherzolites are rare, and fertile
26
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
Archean
Kaapvaal
>90 Ma
Projected Depth (km)
50
Siberia
Proterozoic
Phanerozoic
NE Siberia
E. China
Slave craton
Canada
100
LAB
Depleted
Harzburgite
Depleted
Lherzolite
150
LAB
Fertile
Lherzolite
Lherzolite (LREE
metasomatised)
200
LAB
Peridotite (Meltmetasomatised)
LAB
0
50
0
50
0
50
100 0
Cumulative %
50
100
0
50
100
Fig. 4. Vertical distribution of CARP garnet classes (Table 3) in representative Archon, Proton and Tecton sections, showing the abundance
and stratification of depleted rock types in Archon SCLM, and the more fertile nature of typical Proton SCLM. The Tecton sections in
the garnet facies are short, shallow and very fertile relative to older SCLM. Data from Griffin et al. (2002a).
Table 3
Classes and major groups recognised by CARP analysis of Cr-pyrope garnets
Mean % Fo in olivinea
Mean Y in garnetb
Mean Zr in garnetb
Mean TiO2 in garnetb
Depleted harzburgites
H2
93.7
2
21
345
Depleted lherzolites
L3
91.8
L5
92.9
2
3
6
40
250
293
Depleted/metasomatised peridotites
H3
93.1
L15
92.4
L18
92.8
L19
92.6
L21
92.0
24
13
15
9
47
126
40
67
54
123
250
650
930
880
980
Fertile lherzolites
L9
87.5
L10A
92.1
L10B
89.6
21
16
40
23
38
40
570
510
1300
Melt-metasomatised peridotites
L13
90.5
L25
91.7
L27
90.7
17
16
23
25
52
85
1265
2100
5300
Data from Griffin et al. (2001a).
a From classification of garnets in 200 peridotite xenoliths.
b Means from LAM-ICPMS database (n = 5403).
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
27
Table 4
Distribution of garnet populations identified by CARP analysis (Griffin et al., 2001a)
CARP
Archon
(n = 3082)
H2
L3
L5
9.0
11.1
10.8
0.8
9.2
6.2
0.0
0.0
0.0
10.4
12.9
12.5
0.9
10.4
7.0
0.0
0.0
0.0
Sum depleted peridotites
30.9
16.2
0.0
35.8
18.3
0.0
2.0
1.4
2.8
3.5
1.0
0.3
6.0
0.5
6.7
1.2
0.0
1.3
0.0
0.0
0.0
2.3
1.6
3.2
4.1
1.2
0.3
6.8
0.6
7.6
1.4
0.0
1.4
0.0
0.0
0.0
H3
L15
L18
L19
L21
Sum depleted/metasomatised
Proton
(n = 1698)
Tecton
(n = 623)
Archon
Proton, normalised
to 100% classified
Tecton
8.7
14.4
1.3
10.1
16.3
1.4
L9
L10A
L10B
2.7
15.0
2.9
4.6
12.7
6.2
5.8
16.2
44.5
3.1
17.4
3.4
5.2
14.4
7.0
6.3
17.7
48.5
Sum fertile lherzolites
20.6
23.5
66.5
23.9
26.6
72.5
L13
L25
L27
4.0
11.0
11.1
5.6
13.4
15.2
2.2
18.6
3.1
4.6
12.7
12.9
6.3
15.2
17.2
2.4
20.3
3.4
Sum melt-metasomatised
26.1
34.2
23.9
30.2
38.7
26.1
Total classified
86.3
88.3
91.7
100.0
100.0
100.0
Based on a database of 5403 Cr-pyrope garnets, analysed for major elements by electron microprobe and for trace elements by LAM-ICPMS
(Griffin et al., 2001a).
lherzolites dominate the section. However, this composition could simply reflect the further progress of
the type of metasomatic processes that modified the
SCLM beneath the SW part of the craton around
100 Ma ago.
Re–Os analyses of 10 xenoliths from the Letlhakane
kimberlite, which is included in the N. Botswana
SCLM section, give a mean model age (TRD ) of 2.6 ±
0.2 Ga (Irvine et al., 2001), indicating that this SCLM
was in fact originally generated in Archean time. It
may originally have resembled the Group 2 SCLM
(Fig. 6) but was modified to typical Proton composition by extensive metasomatism related to the Proterozoic rifting and compression (Griffin et al., 2003).
The density of typical Archean SCLM is significantly less than that of Proterozoic or Phanerozoic
SCLM (Table 2), and sections of Archean SCLM
more than ca. 60 km thick are significantly buoyant
relative to the underlying asthenosphere (Fig. 8). This
buoyancy persists even if the potential temperature
of the asthenosphere is much higher, as probable in
early Archean time (Poudjom Djomani et al., 2001).
This buoyant Archean SCLM cannot be removed by
gravitational forces alone, as proposed in many delamination models, and will tend to persist through
most tectonic events. However, it can be progressively
modified by addition of asthenosphere-derived material, especially in extensional settings (Fig. 7; O’Reilly
et al., 2001), and its lower parts might ultimately become unstable as they approach the composition of
the asthenosphere.
It therefore seems probable that many Proterozoic
SCLM sections, like the one from northern Botswana,
may be strongly modified Archean SCLM. However,
this probably does not mean that Tecton lithosphere is
simply more strongly modified Proterozoic or Archean
SCLM. The analysis of garnet populations (Table 4)
shows that Tecton SCLM is dominated by classes that
are absent or very rare in Archon or Proton sections.
If these compositions were the end result of metasomatic processes that have affected the older SCLM, we
would expect to see more examples of these classes in
28
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
Fig. 5. Boyd plot (cf. Fig. 1) showing the compositions of mean
Archon, Proton and Tecton SCLM (Table 2), and the mean composition of high-T sheared lherzolite xenoliths from kimberlites.
Arrows illustrate the effects of shallow phlogopite-related metasomatism, and the melt-related metasomatism responsible for the
composition of the sheared xenoliths (Smith et al., 1991). The
observed metasomatism in Archean xenoliths results in refertilisation, and may be responsible for at least some of the observed
secular change in SCLM composition between Archon and Proton
sections.
the older sections. It appears on present evidence that
Phanerozoic SCLM is produced by distinctly different
processes from those that operated in the Archean.
5. SCLM by subduction?
So how was the unique Archean SCLM produced?
One common model invokes the stacking of subducted slabs of oceanic or arc-related mantle beneath
the continents. This approach probably is meant to be
Uniformitarian; if so, it fails on two counts: (1) the
proposed modern analogues are not compositionally
similar to Archean SCLM; (2) there is little evidence
that Phanerozoic SCLM is formed by the subduction
of oceanic or arc-related mantle beneath continental
margins.
As noted above, modern abyssal peridotites, even
when highly depleted, do not have compositions similar to those of Archean SCLM. Uniformitarian models
for SCLM formation therefore have tended to concentrate on processes in subduction-zone environments,
where far fewer xenolith data are available. One common explanation for the high Si/Mg of Kaapvaal and
Siberian SCLM invokes the infiltration of slab-derived
silicic melts or fluids, usually in subduction settings
(e.g. Kelemen et al., 1992). This model requires that
while such processes occur beneath island arcs and
active continental margins today, the subduction of
such arc-related mantle to form SCLM was restricted
to Archean time. However, both experimental and observational evidence suggests that this process is not
a viable model for the generation of the unique composition of Archean SCLM.
Analyses of xenoliths from subduction-zone settings follow the same type of depletion trends observed
in oceanic peridotites and many xenolith suites from
Tectons (Fig. 2); although highly depleted rocks do
occur, these are olivine-rich, rather than opx-rich, and
have lower Mg# than most Archean suites (Fig. 2).
Metasomatism has been described in xenolith suites
from subduction settings (Kepezhinskas et al., 1995;
Abe, 1997; Abe et al., 1998, 1999; McInnes et al.,
2001; Gregoire et al., 2001), and in some cases this
metasomatism has led to enrichment in orthopyroxene.
However, this process does not produce an increase in
Mg#, and it is accompanied by the introduction of amphibole, phlogopite and clinopyroxene (McInnes et al.,
2001). It also involves strong enrichment in the LILE
(Sr, Ba, Rb, Th, U) and LREE (Gregoire et al., 2001),
and increases in Ca and Cr contents and Ca/Al, an effect opposite to those observed in Archean xenolith
suites.
These observations on xenoliths are consistent with
experimental studies (e.g. Prouteau et al., 1999, 2001),
which show that the production of slab melts in subduction settings probably involves wet melting, and
produces silicic, but strongly (per-)alkaline melts with
low Mg# (15–30). These melts can precipitate orthopyroxene by reaction with peridotites, but also will
deposit their alkali contents in the form of (sodic)
phlogopite ± amphibole. It therefore seems unlikely
that Archon SCLM attained its generally high Si/Mg
through metasomatic processes analogous to those in
modern subduction-zone environments.
Nor do the petrological data support the subduction
model for Phanerozoic SCLM. A detailed analysis of
most available data on Tecton xenolith suites (Griffin
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
29
Olivine
beneath the
Fig. 6. CARP sections showing the vertical distribution of ultramafic rock types (derived from garnet classes) and XMg
SW Kaapvaal Craton in two time slices, and beneath the Proterozoic craton margin in N. Botswana. Metasomatic activity ca. 100 Ma
Olivine
refertilised the SW Kaapvaal SCLM, leading to a decrease in the relative abundance of depleted rock types and mean XMg
, an increase
in the proportion of fertile lherzolites, and a thinning of the depleted lithosphere by ca. 30 km. Similar metasomatic processes during
Proterozoic rifting and compression may be responsible for the more fertile nature of the N. Botswana SCLM, which was originally formed
in Archean time (Carlson et al., 1999).
Fig. 7. (a) Change in density with depth for mean Archon and Proton SCLM; (b) cartoon illustrating typical depth distribution of different
types of metasomatism in Archon SCLM sections, related to ascent of metasomatic fluids of different densities.
30
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
0
crust
Depth (km)
50
upper
mantle
Phanerozoic
(50mW/m2)
100
150
*
*
Southeastern
Australia
Archean
200
(35mW/m2)
3.28
3.32
3.40 (g/cm3)
3.36
Cumulative density
Fig. 8. Curves showing the density of mean Archean and Phanerozoic SCLM as a function of lithosphere thickness, compared with the
density of the asthenosphere (taken as a Primitive Upper Mantle, or “pyrolite” composition). The difference between the SCLM curves and
the density of the asthenosphere at any depth is a measure of the buoyancy of that section. The Southeastern Australia section represents
young SCLM with a high advective geotherm; on cooling to a more typical conductive geotherm, it loses its buoyancy and becomes
inherently unstable. Archon SCLM more than ca. 60 km thick is buoyant relative to the asthenosphere (Poudjom Djomani et al., 2001).
12
%FeO
10
Phanerozoic
Arc
8
6
Kaapvaal
4
0.9
%Cr 2 O 3
Arc
0.7
Phanerozoic
0.5
0.3
0.1
0
1
2
3
4
5
6
%Al2O3
Fig. 9. FeO and Cr2 O3 vs. Al2 O3 for Phanerozoic xenolith suites and arc-related mantle (xenoliths, ophiolites), and Archean peridotite
xenoliths from the Kaapvaal Craton. The incompatible behaviour of Fe and Cr at high degrees of melting (low Al contents) in the Archean
suites is not obvious in the Phanerozoic suites, and implies significant differences in the style of depletion.
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
et al., 1999d) shows that these suites are dominated by
relatively fertile peridotites (especially in the garnet facies; Table 2), and depleted rocks analogous to abyssal
peridotites and arc-related harzburgites are rare. This
observation is consistent with seismic tomography images, which show the subducting slabs plunging to the
transition zone and lower mantle, rather than accumulating at lithospheric depths beneath the continents
(van der Hilst et al., 1997; Gurnis et al., 1998; Fukao
et al., 2001). If Phanerozoic SCLM is not formed by
“lithospheric stacking” of oceanic mantle slabs, the
modern plate-tectonic situation does not offer an analogue for the formation of Archon SCLM.
Archon SCLM also contains chemical evidence
that argues against its formation at shallow levels of
the Earth, and thus against subduction models. Fig. 9
shows a key difference between Tecton SCLM and
highly depleted oceanic or arc-related mantle on the
one hand, and Archon SCLM on the other (for more
detailed data, see Griffin et al., 1999d). Even in the
most depleted Phanerozoic peridotite suites, Fe and
Cr remain constant or increase as Al decreases during
progressive melt extraction, suggesting that melting
was buffered by the presence of spinel ± garnet even
at extreme degrees of depletion. The high degrees of
depletion seen in some arc-related peridotites proba-
31
bly require melting under “wet” conditions, because
of the rapid rise in liquidus temperature that occurs
after clinopyroxene (cpx) is eliminated by progressive melt extraction during dry melting (Dick and
Fisher, 1984; Kushiro, 2001). During wet melting,
spinel remains on the liquidus even to high degrees of
melt extraction (Kushiro, 2001), and the compatible
behaviour of Cr illustrated in Fig. 9 therefore is a
signature of melting under shallow, wet conditions.
In Archean peridotite suites, on the other hand, Fe
and Cr decrease together with Al; that is, they behaved
as incompatible elements during melt extraction. This
implies that neither spinel nor garnet remained in the
residue during the advanced stages of melting. These
peridotites therefore are unlikely to be the residues of
shallow melting processes and are unlikely to represent subducted Archean oceanic or arc mantle.
However, the Fe–Cr–Al relations of Archean peridotite xenoliths are consistent with high-degree melting under dry conditions at high pressure, where garnet
is eliminated from the residue, while complex magmatic pyroxenes contain garnet and clinopyroxene in
solid solution (Fig. 10). Mixtures of these residues
and cumulates can give most of the range of compositions observed in Archean mantle. Archon xenoliths
are extremely depleted in Sc (which resides primarily
Fig. 10. Al–Si relationships during melting of Primitive Mantle compositions at high pressures, after Herzberg (1999). At high degrees of
melting and pressures between 50 and 70 kb, garnet is removed from the residue, leaving only olivine ± orthopyroxene; magmatic opx has
high contents of cpx and gnt in solid solution. Mixtures of residues and magmatic opx mimic the compositions of Archean peridotites
(black dots).
32
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
Fig. 11. Ni, Co, Zn and Sc contents of xenolith suites and peridotite massifs, relative to Mg and Al contents. The low Ni/Mg, Co/Mg and
Zn/Mg of Archon xenolith suites are consistent with melting at high pressures, where the values of Di olivine/melt decrease. The extreme
depletion of Sc in Archon and Proton xenolith suites is consistent with melting under conditions where neither cpx nor gnt are present in
the residue.
in clinopyroxene and garnet), compared to samples of
Proton and Tecton SCLM (Fig. 11) and this is consistent with a lack of both phases in the residue during
the melting of Archean SCLM.
Melting at high pressure is also suggested by the
low Ni/Mg, Zn/Mg and Co/Mg of the Archon xenoliths, compared to those from Protons and Tectons
(Fig. 11). Bickle et al. (1977) showed that the parti(olivine/melt)
tion coefficient DNi
decreases with increasing pressure, so that high-P residues will be depleted
in Ni relative to MgO, compared with residual peridotites generated at shallow levels. Suzuki and Akaogi
(1995) confirmed this result for Ni and extended it to
other divalent elements such as Co and Zn. The low
Ni, Co and Zn contents of the Archon xenoliths are
consistent with melting at high P, leaving a residue
dominated by olivine ± orthopyroxene. Models based
on high-pressure melt extraction, rather than analogues
to modern subduction processes, are therefore more
likely to provide explanations for the unique nature of
Archean lithosphere.
Finally, it should be noted that Archean SCLM
compositions are highly buoyant relative to the underlying asthenosphere. If Archean SCLM initially was
formed at near-surface conditions (e.g. mid-ocean
ridges) this buoyancy would make it difficult to
subduct to depths of >200 km. Increasing the temperature of the Archean asthenosphere only aggravates
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
the situation, by raising the temperature at the base of
the (oceanic) lithosphere and further decreasing the
density of the depleted material.
6. Timing of SCLM formation
Most of our present information on the timing
of these mantle depletion events comes from the
33
whole-rock Re–Os dating of xenoliths from kimberlites; these data show that some of these rocks were
depleted of melt in the Archean, but (taken at face
value) they also suggest that the process may have
continued at least through the Proterozoic (Fig. 12a).
However, the recent development of in situ analytical
methods (Pearson et al., 2002) has shown that most
of these whole-rock ages represent mixtures, because
their Re–Os systematics are controlled by sulfide
Fig. 12. (a) Histograms of whole-rock Re–Os model ages for three Archon peridotite xenolith suites, (Kaapvaal, Wyoming, Siberia) after
Pearson (1999). (b) Cumulative-probability histograms of Re–Os model ages derived by in situ analysis of sulfide phases enclosed in
primary silicates. Kaapvaal Craton data from Griffin et al. (unpublished data); Slave Craton data from Aulbach et al. (unpublished data);
Siberian Craton data from Griffin et al. (2002a,b). The in situ data include only sulfides with 187 Re/188 Os ≤ 0.07, implying little disturbance
by Re addition and giving little difference between TRD and TMA model ages.
34
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
phases, and these rocks generally contain >1 generation of sulfides (Burton et al., 1999; Alard et al., 2000).
In situ analyses of the Re–Os isotopic compositions
of sulfides, enclosed in the primary silicate phases of
peridotite xenoliths, show a narrower range of depletion ages, and the mean age is pushed back significantly in time (Fig. 12b) compared to the whole-rock
data. These model ages probably represent minimum
ages for the stabilisation of Archean cratonic mantle;
the analysed sulfides may have been added to the
depleted mantle after its formation. While a great
deal of work remains to be done, the similarity of the
data sets suggests that much Archean SCLM formed
earlier than ca. 3 Ga. Griffin et al. (2002b) have suggested that beneath Siberia, SCLM formation ended
with a major event at ca. 2.9 Ga, which involved
the addition of crustal materials (eclogites) to the
SCLM.
7. When did subduction begin?
If Archean SCLM had been made by stacking slabs
of subducted oceanic crust and mantle, we would
expect a significant proportion of eclogites, representing the crustal component of the subduction package,
in the Archean SCLM. However, although they are
locally abundant, eclogites make up only 2–3% of
Archon xenolith suites (Schulze, 1989). It can be
argued that the eclogites, because of their high density relative to depleted peridotite, have sunk through
the SCLM and disappeared, but this is not a testable
hypothesis. There is still controversy about the origin of eclogites in the Archean lithosphere; many
are highly magnesian, and may represent the crystallisation of mafic to ultramafic magmas within the
lithosphere, rather than the metamorphism of basalts
(Milholland and Presnall, 1998; Snyder et al., 1997).
A combination of low δ18 O in eclogitic phases, low
δ13 C in diamonds of the eclogitic paragenesis, and
a range of δ32 S in low-Ni sulfides included in diamonds (Chaussidon et al., 1987; Eldridge et al., 1991)
commonly is regarded as evidence of the subduction of surficially altered components to lithospheric
depths (but see a dissenting view by Cartigny et al.,
1998).
Shirey et al. (2001) summarise Re–Os isotopic data
for eclogites and eclogitic diamonds from the Kaap-
vaal Craton, and conclude that many were formed
around 3 Ga, whereas a subsidiary group formed
around 0.9–1 Ga. Similarly, Pearson et al. (1995) presented data suggesting that eclogites formed in the
Siberian SCLM around 2.9 Ga.
However, eclogites and eclogite-paragenesis diamonds are much more abundant in Proton settings
than in Archons (Fig. 13). The great majority of
eclogitic diamonds with δ13 C < −10 (Fig. 13b) are
from Protons, or Archon-margin situations such as
the Koffiefontein mine in South Africa or the Slave
Craton, where Proterozoic subduction beneath the craton is suggested by seismic reflection studies (Cook
et al., 1999). Eclogitic garnets and clinopyroxenes
included in diamonds from Protons also show a narrower range of Ca/(Mg + Fe) than those in diamonds
from Archons (Fig. 13c), suggesting that they formed
over a narrower range of temperature. This might
be evidence of metamorphic recrystallisation during
subduction, rather than igneous crystallisation.
Shirey et al. (2001) have noted that the Re–Os ages
of eclogites from the Kaapvaal Craton are significantly older than most Re–Os ages on the associated
peridotites. They suggested that this anomaly may indicate problems with the models used to calculate depletion ages from the peridotites, or that lithospheric
stabilisation actually occurred later than subduction.
However, as noted above, in situ Re–Os analysis of
sulfide inclusions suggests that the whole-rock Re–Os
ages on peridotite xenoliths generally represent mixtures of several sulfide generations. In this case, as
in the Siberian example (Fig. 12), the true depletion
ages of the peridotites are significantly older than
indicated by the whole-rock ages, and the eclogites
become younger than most of the peridotites. The
isotopic data and the change in the nature of eclogitic
diamonds from Archon to Proton may indicate that
subduction became important only near the end of the
Archean, and perhaps contributed more to the construction of the SCLM during Proterozoic time than it
did earlier.
8. A preferred model
The data reviewed here lead us to suggest that
most Archean SCLM was generated in rising diapirs,
with massive melting at 150–250 km depth (Fig. 10;
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
35
Fig. 13. Data on eclogitic diamonds. (a) Distribution of δ13 C in eclogite- and ultramafic-paragenesis diamonds; (b) distribution of δ13 C
in eclogite-paragenesis diamonds from Archons and Protons; (c) Ca# vs. Cr# for eclogitic cpx inclusions in diamonds from Archons and
Protons. Most eclogitic diamonds with low δ13 C are from Proton settings, and cpx inclusions in these diamonds reflect a narrower range
of temperatures (Ca#) than eclogitic diamonds from Archons.
Herzberg, 1990, 1999; Herzberg and Zhang, 1996).
The residues, mixed to some degree with cumulates,
would be highly buoyant, and would rise further
through the upper mantle to form potential continental
nuclei. Although the available Re–Os data (including the in situ data) provide only minimum ages for
SCLM formation, it may be significant that SCLM as
old as the oldest crust has not yet been found. If major
36
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
SCLM formation took place from 3 to 3.5 Ga, as suggested by preliminary in situ Re–Os data (Fig. 12b),
the scattered relics of pre-3.5 Ga crust may represent
the fortunate bits that were picked up by rising plugs
of younger SCLM before they could be recycled: a
“life-raft” model of craton formation.
Archean-type SCLM apparently has not been produced since ca. 2.5 Ga; that observation alone suggests
that its generation involved a process that no longer
operates on any large scale in the Earth. This implies in
turn a tectonic regime that was unique to the Archean.
The major unidirectional factor in Earth’s evolution is
its secular cooling, as a consequence of the decay of
short-lived radionuclides and the loss of accretional
heat, and it is natural to examine the implications of
this cooling in the search for behaviour unique to the
Archean. Davies (1995) has modelled the behaviour
of a two-layered Earth during secular cooling. Models based on a moderate level of heat transfer between
lower and upper mantle predict periodic mantle overturns in the Archean, leading to massive melting as
adiabatically rising lower mantle passed through the
peridotite solidus (Fig. 14).
As Earth cools, this regime is superseded by
one with lesser overturns, sparked by plate penetration into the lower mantle, and grades into present
plate-tectonic regime with further cooling. The
smaller overturns do not result in massive melting.
These models provide a mechanism for generating
early Archean SCLM through large-scale melting at
depth, and predict that this mechanism would cease to
operate as Earth cooled. They therefore are consistent
with the observations reviewed here, which require
Age (Ga)
3.6
2.7
1.8
0.9
E. Greenland
After Davies (1995)
2000
94
1800
Archean harz + lherz
93
Dry Peridotite Solidus
at 150 km
1600
Mg # of olivine
Temperature (°C)
Upper mantle
1400
Whole mantle
1200
92
ARCHONS
PROTONS
TECTONS
90
Convective overturns
Slab-induced
overturns
Wholemantle
convection
89
Massive
Melting
High Degree
Melting
Shallow
Melting
Residues:
Residues:
Residues:
harzburgite
v. depleted
lherzolite
depleted
lherzolite
mildly
depleted
lherzolite
Fig. 14. Model for the thermal evolution of a two-layered Earth,
after Davies (1995). This model is based on a moderate level of
heat transfer between upper mantle and lower mantle. In an early
hot Earth, heat builds up in the lower mantle faster than it can be
lost from the upper mantle, leading to periodic convective overturns
in which the rising lower mantle passes through the dry peridotite
solidus and undergoes large-scale melting at depths ≥150 km. This
provides a mechanism for the generation of Archean-type SCLM
(cf. Fig. 10), and this mechanism would cease to operate after
Archean time due to the secular cooling of Earth.
Proter ozoic lherz
91
Ph
an
ero
zo
ic
lhe
rz
oceanic trend
90
80
70
Modal olivine, %
60
50
Slave Craton Xenoliths
Deep layer harzburgite, Lac de Gras
Deep layer lherzolite, Lac de Gras
Shallow layer lherzolite, Lac de Gras
Shallow layer lherz. (Boyd and Canil, 1997)
Fig. 15. “Boyd plot” (cf. Fig. 2) showing compositions of Archean
xenoliths from the upper and lower layers of the lithosphere beneath the central Slave Craton (Boyd and Canil, 1997; Aulbach
et al., unpublished data) and from East Greenland (Bernstein et al.,
1998; Hanghøj et al., 2001). Most of these xenoliths are small
and the modal analyses must be treated with caution, but it seems
clear that the shallow levels of the SCLM in both localities contain
rocks similar to the depleted members of some arc-related suites.
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
a uniquely Archean process involving high-degree
melting at depth.
9. A second tectonic regime?
Although our preferred model involves a tectonic
regime that was unique to the Archean, a form of plate
tectonics may have operated in the Archean as well.
Most of our data on Archean SCLM has come from
the Kaapvaal and Siberian cratons. However, new data
from the Slave Craton and East Greenland suggest that
very depleted mantle with high Mg# but low orthopyroxene/olivine, similar to that found in modern arc settings, is present in the shallow parts of some cratons
(Fig. 15). In the Slave Craton, ultradepleted garnet and
spinel peridotites make up a distinct shallow layer,
separated from the underlying more normal Archean
SCLM by a sharp boundary at 145 ± 5 km. This more
“modern” type of SCLM forms a body of limited areal
extent that has been mapped by EM techniques and
mantle-petrology studies (Griffin et al., 1999b; Jones
et al., 2001). In East Greenland, T estimates (mean
T = 850 ◦ C) combined with spinel–lherzolite mineralogy indicate that the depleted material, which has
some Re–Os depletion ages as old as 2.6–3.7 Ga, re-
37
sides at even shallower depths (<65 km; Bernstein
et al., 1998; Hanghøj et al., 2001).
Griffin et al. (1999a) have suggested that the shallow
ultradepleted layer of the Slave mantle was generated
in a collisional setting, and that the deeper layer was
added from below as a rising diapir of lower mantle
material. The lower mantle origin of the deeper layer
is supported by the high proportion of diamonds with
lower mantle parageneses in Slave Craton kimberlites
(Davies et al., 1999) and the minor-element systematics of sulfide inclusions in olivine and diamonds
(Aulbach et al., 2002). The timing of its emplacement
may be constrained by a 3.3 Ga Re–Os isochron derived from the sulfide inclusions (Aulbach et al., 2002).
If this scenario is correct, depleted mantle similar
to that formed at modern mid-ocean ridges or collisional settings was being generated in Archean time.
However, this material is unlikely to be preserved,
except in rare cases. The problem is illustrated by
a cumulative-density section for typical Phanerozoic
mantle (Fig. 8). This sort of mantle, in which Fe is
not depleted during melting, is buoyant while hot, but
once it cools to a stable conductive geotherm, it is
neutrally, or even negatively, buoyant relative to the
asthenosphere. It can be delaminated through stress
(Houseman and Molnar, 1996; Neil and Houseman,
3.35
Fo 90
3.34
Fo 89.5
Oceanic Trend
Density (STP) g/cc
Fo 93
3.33
3.32
Archean (Fo
Arche
3.31
an (F
o
92 )
94 )
3.30
40
50
60
70
80
90
100
% Olivine
Fig. 16. Density as a function of % olivine, for two Archean SCLM compositions, and Phanerozoic compositions lying along the “oceanic
trend” of Boyd (1987) (cf. Fig. 2). In each case the ratios of opx/cpx/gnt are held constant as % olivine increases.
38
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
1999) and will be replaced by upwelling of relatively
fertile mantle, which will go through the same cycle of cooling, delamination and replacement. More
extremely depleted mantle, like that described from
East Greenland, will have a slightly lower density
than typical Phanerozoic mantle (3.332 g/cm3 versus
3.342 g/cm3 ), but will not achieve the low density of
Archean mantle of similar Mg# (3.301 g/cm3 ) because
olivine is denser than opx of similar Mg# (Fig. 16).
This higher density suggests that Archean mantle
that resembled modern oceanic or arc-related mantle
rarely would survive to be picked up in the xenolith
record.
Therefore it is probable that two tectonic regimes
operated to produce SCLM in the Archean. One, involving high-degree melting in mantle overturns or
massive plumes (Stein and Hofmann, 1994; Davies,
1995), produced most of what we see today as Archean
SCLM and probably has not operated since at least
2.5 Ga ago. The other tectonic regime was more similar to the one we recognise from modern tectonic settings, but its products were only preserved where they
were extremely depleted, or were picked up on buoyant “life-rafts” of more typically Archean mantle. The
rest has been recycled and lost.
10. Conclusions
The unique nature of Archean SCLM implies that
the Archean tectonic regime was very different from
the modern one; SCLM was being generated by processes that have not operated for at least the last 2.5 Ga
of Earth’s history. We suggest that these processes involved high-degree melting at sub-lithospheric depths,
and that their products were preserved by their distinctive refractory compositions, buoyancy and rheology.
This Archean SCLM has been progressively modified
since its formation, and few mantle samples today,
even beneath Archean cratons, may represent pristine
samples of Archean material.
The observed secular evolution in the composition
of the SCLM has been interpreted as reflecting a secular evolution in the processes that generate SCLM
(Griffin et al., 1998, 1999d). However, if much Proterozoic SCLM is strongly modified Archean SCLM,
preserved by its buoyancy despite metasomatic refertilisation, then Earth may only ever have had two tec-
tonic regimes. One, which we recognise as uniquely
“Archean” produced thick, highly depleted volumes of
buoyant harzburgites, rising to form the roots of continents. The production of this kind of Archean SCLM
probably ended as a result of the secular cooling of
Earth. The other, operating concurrently, would be
similar to the modern regime, with moderate depletion
at spreading centres, subduction, and cyclic delamination and replacement. Evidence of the latter regime
during Archean and Proterozoic time would rarely be
preserved, as its products would not contribute to the
construction of long-lived continental roots except in
rare circumstances.
If this two-regime model is correct, the late Archean
marks an even bigger change in Earth’s geodynamics
than generally thought; it may be when the formation
of stable SCLM ended and modern plate tectonics and
lithosphere recycling became the dominant regime.
The problem for Archean tectonics is to define the
uniquely Archean process, to understand its implications for crustal tectonics, and to understand when
and why it ceased to operate. It will require more
detailed knowledge of the age structure and compositional evolution of SCLM beneath terrains of different
tectonothermal age to test this model. New in situ
methods of Re–Os analysis (Pearson et al., 2002;
Griffin et al., 2002b) will play an important role in
this work.
Acknowledgements
We are grateful to Tin Tin Win, Ashwini Sharma,
Carol Lawson and Oliver Gaul for assistance with
analytical work, to Oleg Belousov for designing
the GeoSpeed software used to analyse the data,
and to Oliver Gaul and Sally Ann Hodgekiss for
graphics. Funding has been provided by ARC grants
to WLG/SYO’R, Macquarie University internal research funds and GEMOC. The conclusions presented
here have benefited from discussions with many
colleagues, especially Joe Boyd, John Gurney, Jon
Hronsky, Bram Janse, Simon Shee, Craig Smith and
Bruce Wyatt. Thoughtful and constructive reviews
were provided by Kent Condie, Geoff Davies and
an anonymous referee. This is publication no 303
from the ARC National Key Centre for Geochemical
Evolution and Metallogeny of Continents (GEMOC).
W.L. Griffin et al. / Precambrian Research 127 (2003) 19–41
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