-
The density structure of subcontinental lithosphere
throughtime
Yvette H. Poudjom Djomani a;*, Suzanne Y. O'Reilly a, W.L. Gri¤n
a;b,P. Morgan a
a GEMOC ARC National Key Centre, Department of Earth and
Planetary Sciences, Macquarie University, Sydney,N.S.W. 2109,
Australia
b CSIRO Exploration and Mining, P.O. Box 136, North Ryde, N.S.W.
1670, Australia
Received 5 October 2000; accepted 23 November 2000
Abstract
This study uses information on composition, thermal state and
petrological thickness to calculate the densities ofdifferent types
of subcontinental lithospheric mantle (SCLM). Data from
mantle-derived peridotite xenoliths andgarnet^xenocryst suites
document a secular evolution in the composition of SCLM: the mean
composition of newlyformed SCLM has become progressively less
depleted, in terms of Al, Ca, mg# and Fe/Al, from Archean,
throughProterozoic to Phanerozoic time. Thermobarometric analyses
of xenolith and xenocryst suites worldwide show that themean
lithospheric palaeogeotherms rise from low values (corresponding to
surface heat flows of 35^40 mW/m2) beneathArchean terranes, to
higher values (s 50 mW/m2) beneath regions with Phanerozoic crust.
The typical thickness of thelithosphere (defined as a chemical
boundary layer), ranges from about 250 to 180 km, 180^150 km and
140^60 km forArchean, Proterozoic and Phanerozoic terranes
respectively. The depth of this lithosphere^asthenosphere
boundarycorresponds to a temperature of 1250^1300³C. Using the
estimated compositions, average mineral compositions
andexperimental data on the densities of mineral end-members
(tables 1 and 2), we calculate mean densities at 20³C forPrimitive
Mantle (3.39 Mg m33) and for SCLM of Archean (3.31 þ .016 Mg m33),
Proterozoic (3.35 þ 0.02 Mg m33)and Phanerozoic (3.36 þ 0.02 Mg
m33) age. Curves of density and cumulative density versus depth,
which take intoaccount variations in geotherm with tectonothermal
age, have been constructed for each age type of lithospheric
sectionto assess the buoyancy of these columns relative to the
asthenosphere, modelled as a Primitive Mantle composition.
Thedensity curves show that Archean SCLM is significantly buoyant
relative to the asthenosphere at depths greater thanabout 60 km.
Proterozoic sections deeper than about 100 km thick also are
significantly buoyant. The buoyancy ofArchean and Proterozoic SCLM
sections, combined with their refractory composition, leads to high
viscosities andexplains the longevity and stability of old SCLM.
Replacement of Archean lithosphere, as beneath the
present-dayeastern Sino^Korean craton, probably involves mechanical
dispersal by rifting, accompanied by the rise of hot,
fertileasthenospheric material. Fertile Phanerozoic lithosphere is
buoyant when the geotherm is sufficiently high, as in manyCenozoic
volcanic provinces. However, as the geothermal gradient relaxes
toward a stable conductive profile,Phanerozoic SCLM sections
thinner than about 100 km become denser than the asthenosphere, and
hencegravitationally unstable. This could help to induce
delamination of the SCLM and upwelling of asthenosphericmaterial,
beginning a new cycle. The tectonic consequences of such
lithosphere replacement would include uplift and
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science
B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 3 6 2
- 9
* Corresponding author. Tel. : +61-2-9850-9673; Fax:
+61-2-9850-8943; E-mail: [email protected]
EPSL 5713 4-1-01
Earth and Planetary Science Letters 184 (2001) 605^621
www.elsevier.com/locate/epsl
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magmatism, and basin formation during subsequent thermal
relaxation. ß 2001 Elsevier Science B.V. All rightsreserved.
Keywords: lithosphere; composition; mantle; density; geothermal
gradient; xenoliths
1. Introduction
The subcontinental lithospheric mantle (SCLM)carries a
geochemical, thermal and chronologicalrecord of large-scale
tectonic events that haveshaped the Earth's crust. The SCLM is part
ofthe continental plate, and moves with the plateover the weak
asthenosphere. The idea that `old'(cratonic) lithosphere is
relatively thick, depletedand cold has been long accepted by
petrologists([1] and references therein). Recognition that`young'
lithosphere is relatively thin, fertile andhot is more recent;
Jordan [2] observed that thethermal boundary layer (TBL) beneath
late-Pro-terozoic and Phanerozoic mobile belts is moresimilar to
oceanic mantle than to cratonic mantlein its geophysical
characteristics.
Development of the 4-D Lithosphere Mappingmethodology [1] has
allowed the construction ofrealistic geological sections of the
SCLM in awide variety of tectonic settings and at di¡erenttimes.
Xenoliths and garnet and chromite xeno-crysts from mantle-derived
volcanic rocks (e.g.basalts, lamproites, kimberlites) provide
samplesof the lithospheric mantle at the time of eruption.Where
su¤cient xenoliths and/or xenocrysts ofappropriate composition are
available, determina-tion of the palaeogeotherm, the depth to
thecrust^mantle boundary, the detailed distributionof rock types
with depth within the SCLM, andthe depth to the petrological
lithosphere^astheno-sphere boundary (LAB) within the tectosphere
ispossible [1]. Mantle sections constructed in thisway provide a
basis for the calculation of physicalproperties (such as density)
of speci¢c lithospheredomains.
2. Palaeogeotherms and lithosphere thickness
2.1. Construction of palaeogeotherms
The distribution of temperature with depth atthe time of a given
volcanic eruption (the palaeo-geotherm) is one of the key
parameters that mustbe determined to derive the density of a
columnof SCLM. Empirical palaeogeotherms are derivedby
geothermobarometric calculations based onmineral chemistry, using
methods with experimen-tal or theoretical calibration relevant to
the min-eral assemblage, bulk composition and equilibra-tion
conditions of particular samples (e.g. [3,4]).Where xenoliths with
mineral assemblages appro-priate for calculation of both pressure
and tem-perature of equilibration are available,
severalgeothermobarometers commonly give concordantresults for a
xenolith suite, even though resultsfrom the application of di¡erent
techniques to asingle sample may show a high variance
(e.g.[5,6]).
The limited geographic and temporal distribu-tion of xenolith
suites is a major problem in thisapproach, and in many suites
(especially thosefrom basalts) only a small number of xenolithsin a
suite may have appropriate mineral assem-blages. The situation has
been improved by thedevelopment of single-element thermometers
(Niin garnet, Zn in chromite) based on partitioningbetween these
phases with mantle olivine, and amethodology for derivation of
geotherm parame-ters from suites of garnet and spinel
xenocrysts[6]. Where suites of both xenoliths and xenocrystsare
available, the two approaches give concordantresults ([6,7]).
Surface heat-£ow values and xenolith-derivedmantle geotherms are
commonly linked throughassumptions such as those made in the
downwardextrapolation of temperature from surface heat£ow by
Pollack and Chapman [8]. However, theseassumptions include poorly
constrained models of
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Letters 184 (2001) 605^621606
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the distributions of thermal conductivity and ra-diogenic heat
production with depth which arelaterally variable. In general,
surface heat £ow isa function of both the geotherm at depth
andupper crustal heat production, and the mantlegeotherm and
surface heat £ow may be onlyweakly linked. In the Pollack and
Chapman mod-el [8], the mantle geotherm is directly linked to
thesurface heat £ow by assuming that 60% of thesurface heat £ow is
derived from below the uppercrust. We use this model here to
illustrate a roughglobal average surface heat £ow equivalent to
themantle geotherms that we model, not an attemptto model any
particular local geotherm. In mostcratonic areas, and in
Phanerozoic regions with-out active Neogene volcanism,
palaeogeothermsderived from both xenoliths and xenocrysts tendto
parallel theoretically constructed model con-ductive geotherms such
as those of Pollack andChapman [8]. These geotherms are strongly
mod-el-dependent, involving assumptions that includethe
distribution of heat production and thermalconductivity with depth
and the variation of ther-
mal conductivity with temperature. The same sur-face heat £ow
can result from a range of heat-production distributions, and
similar lithosphericmantle geotherms may give rise to di¡erent
sur-face heat £ows, depending on the structure andcomposition of
the crust (e.g. [9,10]). However,the model geotherms of Pollack and
Chapman[8], which are parameterised in terms of the sur-face heat
£ow, provide a convenient referenceframe for comparison of the
xenolith and xeno-cryst data between di¡erent areas. We have
con-structed, or collected from the literature,
xenolith/xenocryst-based palaeogeotherms for more than300
localities worldwide, and classi¢ed these bytheir tectonothermal
age (i.e. the age of the lastmajor thermal event in the crust [1]).
These em-pirically determined palaeogeotherms are typicallylow
beneath Archean cratonic areas, higher be-neath Proterozoic
regions, and still higher beneathareas of Phanerozoic tectonic
activity, corre-sponding broadly to global variations in
surfaceheat £ow ([9] ; Fig. 1). Furthermore, the observeddi¡erences
in lithospheric mantle temperature are
Fig. 1. (A) The range of P^T conditions commonly derived from
xenolith and xenocryst suites in volcanic rocks that penetratecrust
of di¡erent tectonothermal age (crustal age domains modi¢ed from
Janse [15]: Archean, v 2.5 Ga; Proterozoic, 2.5^1.0Ga; Phanerozoic,
6 1 Ga). Also shown is the range of depths to the LAB typical of
each group, as de¢ned by the maximumdepth of Y-depleted garnets
beneath Archean and Proterozoic cratons (Ryan et al. [6]), and the
maximum depth of sampling inPhanerozoic regions. Reference
geotherms are conductive models of Pollack and Chapman [8],
labelled with corresponding sur-face heat £ow in mW/m2, and the
southeastern Australia advective geotherm (SEA) derived from
xenoliths in basalts [12].(B) Variation of surface heat £ow
measurements with tectonothermal age of the crust (after Morgan
[9]). The crosses at the centreof the boxes represent the mean heat
£ow for each age group; note the general correlation with the range
of geotherms in (A).
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Y.H. Poudjom Djomani et al. / Earth and Planetary Science
Letters 184 (2001) 605^621 607
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essentially those expected from a constant mantleheat £ow, but
variable crustal heat production([9,11]). In areas of basaltic
volcanism, such aseastern Australia and eastern China, xenolithsand
xenocrysts in basalts generally record high,strongly convex-upward
geotherms, consistentwith advective heat transport by magmas
andunderplating of basaltic magmas in the upperpart of the
lithospheric mantle ([12]).
2.2. Estimation of lithosphere thickness
Once the palaeogeotherm has been estimatedfrom
xenolith/xenocryst data or heat-£ow model-ling, temperature
estimates for individual samplescan be projected to this
palaeogeotherm to deter-mine their depth of origin. This approach
hasproved especially fruitful for use with xenocrystsuites, because
statistically meaningful numbersof samples can be gathered for each
section,something which often is di¤cult to achievewith xenoliths
alone. By determining the NickelTemperature (TNi) of individual
garnet grains in aheavy-mineral concentrate, and the Zinc
Temper-ature (TZn) of individual chromites, the geochem-
ical information contained in each grain can beplaced in
stratigraphic context, to map the verticaldistribution of rock
types and geochemical pro-cess signatures ([6,13]).
The resulting lithological/geochemical columns(Fig. 2) provide
1-D maps, equivalent to drill holelogs, through individual
lithospheric sections.Gri¤n and Ryan [13] and Ryan et al. [6]
haveshown that beneath many volcanic provincesthere is a sharply
de¢ned maximum depth atwhich garnets depleted in yttrium (a
lithospheresignature) are common (Fig. 3). They have arguedthat
this depth represents the base of the petro-logically de¢ned
lithosphere, and thus corre-sponds to the LAB. In most localities
worldwide,this petrologically de¢ned LAB corresponds totemperatures
of 1250^1300³C [6]. The depth ofthe LAB mapped in this way varies
broadlywith tectonic setting, being deepest (250^150 km)beneath
Archean terrains (e.g. [7,14]) and shallow-est beneath Phanerozoic
terrains (Fig. 1), aswould be expected from the observed
variationin the palaeogeotherms beneath terrains of di¡er-ent age
(Fig. 1a). Where high-quality deep seismicdata are available, this
LAB commonly corre-
Fig. 2. Lithologic sections through the lithospheric mantle of
several Archean cratons, constructed using the compositions
andcalculated temperatures of garnet xenocrysts ([47,7,14]). The
grey shading represents the asthenosphere. Note that for the
Kaap-vaal craton, the two time slices are shown representing mantle
xenoliths sampled by kimberlites intruded before and after 90
Ma.
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Y.H. Poudjom Djomani et al. / Earth and Planetary Science
Letters 184 (2001) 605^621608
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sponds to the L (Lehman) discontinuity in craton-ic areas (e.g.
[1]). Various geophysical and geo-chemical de¢nitions of the nature
of the LABhave been discussed in ([1] and references therein).These
di¡erent estimates converge at similardepths and may indicate the
interdependence ofthermal state, rheology and phase relationships
indetermining the location of this boundary in dif-ferent
sections.
3. Secular variation of lithosphere composition
Isotopic studies, especially of the Re^Os sys-tem, show that the
lithospheric mantle beneathArchean cratons ( = 2.5 Ga, as de¢ned by
theage of the last major tectonothermal event to af-fect the crust;
[15]) is largely Archean in age,while that beneath Proterozoic
crust is largelyProterozoic in age ([16^19]). These studies
providea priori evidence that the formation of the crustand the
SCLM are linked processes, and suggestthat a detailed analysis of
the tectonothermal his-tory of a crustal volume provides a basis
for esti-mating the formation age of the underlyingSCLM.
Boyd ([20,21]) recognised a fundamental dis-tinction between
Archean cratonic SCLM, repre-sented by xenoliths of peridotitic
mantle from
African and Siberian kimberlites, and Phanero-zoic
circumcratonic mantle, represented by xeno-liths of peridotitic
mantle in intraplate basalts andby orogenic lherzolite massifs.
Compared to Pha-nerozoic mantle, the peridotite xenoliths from
on-craton kimberlites are not only more depleted onaverage, but
have higher Si/Mg (higher opx/oli-vine). This distinctive feature
is common to bothlherzolites and harzburgites, and to rocks of
bothgarnet and spinel facies [22].
Analysis of a large database of xenolith compo-sitions ([23,24])
also demonstrates that Archeanxenoliths have lower Ca/Al, Fe/Al and
Cr/Al ra-tios at equivalent mg# than xenoliths from Pro-terozoic
and Phanerozoic regions. ArcheanSCLM is distinctive in another
important respect:subcalcic (clinopyroxene-free) harzburgites
arewell represented in Archean xenolith and xeno-cryst suites, but
essentially absent in youngerones ([23,24]). These data clearly
indicate thatthe SCLM beneath Archean cratons and youngerterranes
is compositionally di¡erent. A larger da-taset, covering a larger
geographic range, is pro-vided by geochemical analyses of garnet
xeno-crysts (disaggregated from peridotitic mantlexenoliths) in
volcanic rocks and derived alluvialconcentrations. Analysis of s 13
000 Cr^pyropegarnet xenocrysts from volcanic rocks worldwideshows a
clear correlation of average garnet com-position with the
tectonothermal age of the crustpenetrated by the volcanic rocks
[14].
3.1. Calculating the composition of lithosphericmantle
If the composition of SCLM varies with tecto-nothermal age, the
interpretation of geophysicaldata and understanding of tectonic
processes re-quires that we consider di¡erences in the
meancomposition of the di¡erent types of SCLM be-neath the major
tectonic divisions noted above.The great bulk of the existing
analytical data forArchean xenoliths comes from a small area of
theKaapvaal craton in South Africa, and from a sin-gle kimberlite
pipe in Siberia (Udachnaya); thereare very few xenolith data for
Proterozoic ter-ranes. However, abundant data from garnet
con-centrates are available for many of the world's
Fig. 3. Y^TNi (Ni temperature; [6]) plot for garnets
fromkimberlites of Shandong Province, eastern China, showingthe LAB
de¢ned by the deepest occurrence of Y-depletedgarnets. After Gri¤n
et al. [47].
EPSL 5713 4-1-01
Y.H. Poudjom Djomani et al. / Earth and Planetary Science
Letters 184 (2001) 605^621 609
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cratonic areas and these expand our picture ofSCLM
variability.
In xenolith suites, the Cr2O3 content of garnetcorrelates well
with the Al2O3 content of the hostrock ([23,24]). These suites also
show good corre-lations between the content of Al2O3 and those
ofother major and minor elements. Such correla-tions make it
feasible to calculate the bulk com-position of a mantle section,
given the medianCr2O3 content of garnet xenocrysts from that
sec-tion; the technique gives good agreement with theaverage
compositions of xenolith suites, wheresuitably large data sets are
available (Table 1;see other examples in Gri¤n et al. [24]).
The mean composition of SCLM beneath ter-ranes of Archean,
Proterozoic and Phanerozoictectonothermal age, calculated in this
way, showsa secular evolution in all measures of depletion,such as
Al, Ca, mg#, and Fe/Al (Table 2). Theexisting database is not
su¤cient to establishwhether this secular evolution is continuous
ordiscontinuous; for the purposes of this work weadopt the broad
divisions recognised by Gri¤n etal. [25,26], who divide the
datasets into Archean(tectonothermal age = 2.5 Ga), Proterozoic
(2.5^1.0 Ga) and Phanerozoic ( = 1.0 Ga).
Unmodi¢ed Proterozoic SCLM is moderatelydepleted, and
intermediate in composition be-tween Archean and Phanerozoic SCLM.
Cenozoic
SCLM, exempli¢ed by the Zabargad peridotitesof the Red Sea and
by garnet peridotite xenolithsfrom young extensional areas of
China, Siberiaand Australia, is only mildly depleted relative
toPrimitive Upper Mantle (PUM: [3,23^25]).SCLM beneath some
Phanerozoic terrains, espe-cially in Europe, is more depleted and
may repre-sent reworked Proterozoic SCLM (Table 2: pre-ferred
values). The correlation of SCLMcomposition with crustal age is
strong evidencethat crustal volumes and their underlying
litho-spheric mantle formed quasi-simultaneously, andthat
syngenetic crust and mantle can, and in mostcases do, remain linked
for periods of aeons.
4. Calculating the density of SCLM
For each of the mean SCLM compositions de-¢ned above (Table 2),
we have estimated meanmineral compositions, based on broad
correla-tions between mineral and rock compositions inxenoliths
([23]). These mineral compositions havethen been used to calculate
the modal composi-tions of the average Archean, Proterozoic
andPhanerozoic SCLM (Table 2). We have approxi-mated the
composition of the asthenosphere bythe Primitive Mantle composition
of McDonoughand Sun [26], which is close to the pyrolite com-
Table 1Comparison of mean mantle compositions calculated from
garnets, with average compositions of xenolith suites (after Gri¤n
etal. [24])
Kaapvaal Kaapvaal Kaapvaal Kaapvaal Vitim Vitim6 90 MA 6 90
MAGarnet lherzolite Lherzolite
xenolithsGarnet harzburgite Harzburgite
xenolithsGarnet lherzolite Lherzolite
xenoliths(Calculated fromGarnets)
(Median) (Calculated fromGarnets)
(Median) (Calculated fromGarnets)
(Median)
Wt%SiO2 46.0 46.6 45.7 45.9 44.5 44.5TiO2 0.07 0.06 0.04 0.05
0.15 0.16Al2O3 1.7 1.4 0.9 1.2 3.7 4.0Cr2O3 0.40 0.35 0.26 0.27
0.40 0.37FeO 6.8 6.6 6.3 6.4 8.0 8.0MnO 0.12 0.11 0.11 0.09 0.13
0.10MgO 43.5 43.5 45.8 45.2 39.3 39.3CaO 1.0 1.0 0.5 0.5 3.3
3.2Na2O 0.12 0.10 0.06 0.09 0.26 0.32NiO 0.27 0.28 0.30 0.27 0.25
0.25
EPSL 5713 4-1-01
Y.H. Poudjom Djomani et al. / Earth and Planetary Science
Letters 184 (2001) 605^621610
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Tab
le2
Cal
cula
ted
mea
nco
mpo
siti
ons
for
Arc
hean
,P
rote
rozo
ican
dP
hane
rozo
icSC
LM
(aft
erG
ri¤
net
al.
[24]
),m
odes
,m
iner
alco
mpo
siti
ons,
and
sele
cted
calc
ulat
edra
-ti
osan
dco
nsta
ntva
lues
Arc
hean
Pro
tero
zoic
Pro
tero
zoic
Pha
nero
zoic
Pha
nero
zoic
Pri
mit
ive
Man
tle
Gnt
-cal
cSC
LM
Gnt
-cal
cSC
LM
Xen
olit
hs,
mas
sifs
Gnt
-cal
cSC
LM
spin
elpe
rido
tite
(pre
ferr
ed)
[26]
Wt%
SiO
245
.744
.744
.644
.544
.445
.0T
iO2
0.04
0.09
0.07
0.14
0.09
0.2
Al 2
O3
0.99
2.1
1.9
3.5
2.6
4.5
Cr 2
O3
0.28
0.42
0.40
0.40
0.40
0.38
FeO
6.4
7.9
7.9
8.0
8.2
8.1
MnO
0.11
0.13
0.12
0.13
0.13
0.14
MgO
45.5
42.4
42.6
39.8
41.1
37.8
CaO
0.59
1.9
1.7
3.1
2.5
3.6
Na 2
O0.
070.
150.
120.
240.
180.
36N
iO0.
300.
290.
260.
260.
270.
25R
atio
sm
g#92
.790
.690
.689
.989
.989
.3M
g/Si
1.49
1.42
1.42
1.33
1.38
1.25
Ca/
Al
0.55
0.80
0.80
0.82
0.85
0.73
Cr/
Cr+
Al
0.43
0.30
0.30
0.17
0.18
0.05
Fe/
Al
4.66
2.64
2.64
1.66
2.23
1.30
Mod
esol
/opx
/cpx
/gnt
69/2
5/2/
470
/15/
7/8
70/1
7/6/
760
/17/
11/1
266
/17/
9/8
57/1
3/12
/18
Min
eral
com
posi
tion
sO
livin
eF
o 93
Fo 9
1F
o 91
Fo 9
0F
o 90
Fo 9
0
Ort
hopy
roxe
neE
n 93
En 9
1E
n 91
En 9
0E
n 90
En 9
0
Clin
opyr
oxen
eaD
i 70
Hd 1
0D
i 70
Hd 1
0D
i 70
Hd 1
0D
i 70
Hd 1
0D
i 70
Hd 1
0D
i 70
Hd 1
0
Jd10
Cc 2
En 8
Jd10
Cc 2
En 8
Jd10
Cc 2
En 8
Jd10
Cc 2
En 8
Jd10
Cc 2
En 8
Jd10
Cc 2
En 8
Gar
netb
Py 7
0A
lm15
Py 7
0A
lm25
Py 7
0A
lm25
Py 7
0A
lm25
Py 7
0A
lm25
Uv 1
5U
v 5U
v 5U
v 5U
v 5D
ensi
ty(M
g/m
3)
3.31
3.34
3.34
3.37
3.36
3.39
Con
stan
tsa 0U
103
40.
2716
50.
2701
4^
0.26
970.
2776
8^
a 1U
103
81.
0497
11.
0594
5^
1.01
920.
9545
1^
a 23
0.15
031
30.
1243
^3
0.12
823
0.12
404
^k
129
130
^13
012
813
4aD
i=di
opsi
de,
Hd
=he
denb
ergi
te,
Jd=
jade
ite,
Cc
=co
smoc
hlor
e,E
n=
enst
atit
e.bP
y=
pyro
pe,
Alm
=al
man
dine
,U
v=
uvar
ovit
e.
EPSL 5713 4-1-01
Y.H. Poudjom Djomani et al. / Earth and Planetary Science
Letters 184 (2001) 605^621 611
-
position of Ringwood [27]. The assumption thatthe major-element
composition of the modern as-thenosphere is only slightly depleted
relative toPUM is supported by the common occurrenceof Phanerozoic
xenoliths with near-PUM compo-sitions, as noted above.
The calculated modes (Table 2) illustrate theincrease in modal
garnet+clinopyroxene and cli-nopyroxene/garnet from Archean to
Proterozoicto Phanerozoic SCLM, and the relatively smalldegree of
depletion of Phanerozoic SCLM relativeto Primitive Mantle
compositions. The mean den-sity of each SCLM composition at surface
tem-perature (T) and pressure (P) has then been cal-culated by
combining the modes and mineralcompositions with the end-member
mineral den-sities of Smyth and McCormick [28].
The major controls on density are mg# and theproportion of
olivine to clinopyroxene+garnet,and these are the parameters that
probably arebest constrained in these calculations. A changeof 1%
(s 1c) in the mg# of olivine results in achange in density of 0.008
Mg m33. In Archeanrocks, a variation of 50% (1c ; [23]) in the
abun-dance of (cpx+gnt) also produces a variation indensity of þ
0.008 Mg m33. These uncertaintieswill be correlated, because a more
iron-rich oli-vine is likely to be associated with a higher
fertil-ity, as expressed in higher (cpx+gnt). The total
1cuncertainty in density therefore is on the order ofþ 0.016 Mg
m33. In the average Proterozoic orPhanerozoic compositions, where
the mean(cpx+gnt) is higher, a þ 50% variation in (cpx+gnt) has a
larger e¡ect, producing a 1c uncer-tainty of 0.02 Mg m33.
Our calculations show a signi¢cant increase inmean standard
temperature and pressure (STP)density from Archean (3.31 þ 0.016 Mg
m33) toProterozoic (3.34 þ 0.02 Mg m33) to Phanerozoic(3.36 þ 0.02
Mg m33) SCLM. The most depletedProterozoic SCLM overlaps only the
most fertileArchean SCLM at the 1c level, while the di¡er-ence
between Proterozoic and Phanerozoic SCLMis much less. The average
Archean SCLM is 2.5%less dense than Primitive Mantle (3.39 Mg m33
;Table 2) at the same temperature and pressure;for the
less-depleted Phanerozoic mantle the dif-ference is about 1%. If
Cenozoic lithosphere is
assumed to be similar to the more fertile Zabar-gad peridotites,
or the Phanerozoic garnet perido-tites (Garnet SCLM of Table 2),
the di¡erencedrops to about 0.6%.
Jordan [2] used a suite of kimberlite-bornexenoliths to derive a
linear relation between nor-mative (at STP) density and mg#, given
byb= 5.093^0.019 mg#. This equation reproducesour derived densities
to within 0.01 Mg m33 ; weare encouraged by the agreement.
Boyd and McAllister [29] used modal estimatesand cell-volume
data on separated minerals tocalculate room-T densities for a
strongly depletedArchean garnet lherzolite xenolith (samplePHN1569;
b= 3.30 þ 0.02 Mg m33) and a high-T-sheared peridotite xenolith
(sample PHN1611; density = 3.39 þ 0.02 Mg m33). The densityestimate
for PHN1569 agrees well with our calcu-lated average for Archean
SCLM (Table 1). Thedensity of the sheared xenolith PHN1611 is
higherthan that of our average Archean lherzolite. How-ever, the
garnet and cpx content of PHN1611 hasbeen greatly increased (and
its bulk mg# de-creased) by high-T melt-related metasomatismshortly
prior to its entrainment in the host kim-berlite [30]. This
`refertilisation' has produced abulk composition similar to some
`pyrolite' com-positions, and the density of PHN1611 is
identicalwithin error of our estimated density for PrimitiveMantle,
and that of Jordan [31]. Jordan's [31]higher estimated density
(3.35 Mg m33) for `aver-age continental garnet lherzolite' included
manyhigh-T-sheared lherzolites like PHN1611. How-ever, this class
of xenolith appears to be signi¢-cant only in the deepest parts of
the SCLM, andis excluded from the initial analysis given below.
4.1. Variation of density with depth
Density variations in the Earth depend on thecomposition of the
mantle section, and are alsoa¡ected by temperature variations,
which in turna¡ect the elasticity of the minerals ; the
calculateddensity at a given depth is a function of the
tem-perature, the bulk thermal expansion coe¤cientand the bulk
compressibility of rock, as deter-mined by the relative proportions
of mineralend-members. In this section, we calculate the
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variation in density with depth for each averagemantle type,
assuming a constant compositionwith depth and using the range of
geothermscharacteristic of Archean, Proterozoic and Pha-nerozoic
SCLM (Fig. 1).
As noted above, the palaeogeotherms (exceptfor the southeastern
Australian `SEA' geotherm(Fig. 1a)) derived from xenolith and
xenocrystdata tend to parallel the model conductive geo-therms of
Pollack and Chapman [8]. In this sec-tion of the paper, we
therefore have used Pollackand Chapman's conductive model geotherms
forcalculating T-depth relations because these geo-therms are
easily parameterised.
For each lithosphere type, we assumed a two-layer model with an
average crustal thickness of35 km and a total lithosphere thickness
of up to280 km. For the crust, we assume a constant ther-mal
conductivity of 2.5 W m31 K31. FollowingPollack and Chapman [8], we
assumed that about40% of the mean £ux arises from surface
radio-genic sources, and the rest comes from greaterdepths. We used
this assumption to de¢ne an em-pirical equation relating the
reduced heat £ow andthe mean heat £ow at depth, which then
de¢nesthe distribution of heat production with depth. Inour models,
we assumed a uniform characteristicdepth of heat source
distribution, equal to 8 kmas de¢ned by Pollack and Chapman [8]. In
thecase of the mantle, we assumed that there wasno heat production,
and we used Schatz and Sim-mons' method [32] to calculate the
thermal con-ductivity of olivine as a function of the temper-ature
and depth. We have parameterised the SEAadvective geotherm ([12])
using a spline function.The resulting parameterised geotherms were
usedto calculate the pressure and temperature-depen-dent density
variations for each mantle type.
To calculate the temperature-dependent densityvariations, we
used the relation:
bT b 203b 20TK 1
where b20 is the density of the section estimated ata room
temperature of 20³C (Table 2), bT is thedensity of the section at a
given temperature, andK, is the bulk thermal expansion coe¤cient
calcu-lated from the relevant mineral end-members. A
polynomial expression of the thermal expansioncoe¤cient is given
by [33] :
K a0 a1T a2T32 2
where a0, a1 and a2 are constants which are esti-mated from the
mineralogical composition ofeach mantle type.
The density variations with depth/pressure arealso a function of
the bulk compressibility of therocks. A change in pressure will
result in a frac-tional change in the volume of a given mass
ofmaterial, and this is given by the thermal dilata-tion or the
compressibility L of the material,which can be related to the
density by the rela-tion:
Nb bT LP 3
where bT is the density at a given temperature Tand pressure P
(GPa).
Table 2 lists the mean composition, the con-stants of the
thermal expansion coe¤cients (a0,a1, a2) and the bulk moduli (k =
1/L), calculatedfrom the mineral end-members (Table 2), of
theaverage Archean, Proterozoic, Phanerozoic andPrimitive Mantle
compositions used in our den-sity calculations.
As temperature changes with depth in theEarth, the competing
e¡ects of the compressibilityand thermal expansion of the solid
phases havesigni¢cant e¡ects on the density variations withdepth
[34]. Therefore, we used Eq. 1 to calculatethe
temperature-dependent density variation withdepth, and Eq. 3 to
compute the compressibilitye¡ect on the density variations with
depth. Thesetwo e¡ects are then combined to express the
tem-perature and pressure-dependent density variationwith depth for
Archean, Proterozoic and Phaner-ozoic mantle sections. In the
Phanerozoic section,we have shown the e¡ect on density of the
spinelperidotite^garnet peridotite transition at ca 55km. In the
more depleted Proterozoic and Arche-an sections, the e¡ect of this
phase transition isvery small (and a¡ects a narrower depth
intervalof the section as the transition depth decreaseswith
decreasing temperature) and has beenignored; the e¡ect is to give a
maximum density
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for these sections. The density of the astheno-sphere at the LAB
has been calculated using aPrimitive Mantle composition (Table 2)
(densityat 20³C and surface pressure = 3.288 Mg m33).We represent
the LAB by an adiabat with a po-tential temperature of 1300³C,
independent of thedi¡erent thicknesses of the lithospheric
columnfor SCLM of di¡erent types. The density of theasthenospheric
(Primitive Mantle) composition asa function of depth is shown in
Fig. 4 as a solidheavy line. The density of the asthenosphere at220
km depth, derived from two standard Earthmodels based on seismic
data (PEM and PREM;B.L.N. Kennett, pers. commun., 1998) showsgood
agreement with our calculated values (Fig.4).
The results show that the density of typical Ar-chean
lithospheric mantle, within a typical rangeof cratonic geotherms
(35^40 mW m32 ; Fig. 1a,increases with depth at a slower rate than
that ofthe asthenosphere (Fig. 4), becoming less densethan
asthenosphere at about 80^100 km. TypicalProterozoic mantle is
denser than the astheno-sphere at depths shallower than 145^175 km
(de-
pending on the geotherm), but buoyant relative toasthenosphere
at greater depths (Fig. 4). In areasof active volcanism, where
advective heat trans-port by magmas raises the geotherm (e.g.,
theSEA geotherm; [12]), Phanerozoic mantle actuallydecreases in
density with depth due to the over-riding e¡ects of thermal
expansion. Where thegeotherm approximates a steady-state
conductivemodel, and within the range typically observed inolder,
tectonically inactive Phanerozoic terrains,the density of
Phanerozoic mantle increases withdepth in the shallow spinel
lherzolite stability¢eld, rises sharply at the spinel^garnet
lherzolitetransition (ca 55^65 km) and then rises again withdepth
in the garnet lherzolite stability ¢eld (belowca 55 km). The
density increase due to the phasechange is about 3%. At depths of
160^185 km inareas with a typical Phanerozoic conductive geo-therm,
the density of Phanerozoic SCLM ap-proaches that of the
asthenosphere; in thinnersections the lithosphere is denser than
the subja-cent asthenosphere.
To evaluate the stability of di¡erent lithospher-ic sections
relative to the asthenosphere, it is use-
Fig. 4. Plots of density versus depth for lithospheric mantle
sections of di¡erent age, calculated using the preferred
compositionsshown in Table 2 and the range of geotherms and LAB
depth shown in Fig. 1. SEA, southeastern Australia xenolith-based
geo-therm, representing advective heat transport in active volcanic
areas. This geotherm will decay toward the high conductive
geo-therms typical of Phanerozoic areas with a time constant of W10
Ma ([8]). PEM and PREM are the values for the density ofthe
asthenosphere at 220 km depth, based on seismic data and derived
from these Earth models (B.L.N. Kennett, pers. commun.,1998). The
density of the asthenosphere as a function of depth has been
calculated using a Primitive Mantle composition and anadiabat of
0.5³C/km. The shaded areas for each mantle type and age correspond
to the sections of the lithosphere that are unsta-ble at any depth
relative to the underlying asthenosphere. The rapid increase in
density around 55 km on the Phanerozoic curvescorresponds to a
change in composition from spinel lherzolite (for depths 6 55 km)
to garnet lherzolite (for depths s 55 km).The 1300³C point marking
the asthenosphere temperature is shown with grey circles for each
geotherm. The horizontal stripedarea tracks the locus of thermal
relaxation from the high advective geotherm.
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ful to calculate the cumulative density of eachcolumn as a
function of its thickness, and to com-pare this density with that
of a column of asthe-nospheric mantle whose temperature follows
the1300³C adiabat (Fig. 5). These curves demon-strate that sections
of Archean lithospheric mantlethicker than ca 60 km are buoyant
relative to theunderlying asthenosphere. Typical Archean
litho-sphere is 150^250 km thick (Fig. 1) and thereforethese
sections are signi¢cantly buoyant; a 200-kmsection is 2.5% less
dense than the asthenosphereat the LAB. A Proterozoic SCLM section
thickerthan ca 125 km is buoyant relative to the astheno-sphere,
but less so than Archean lithosphere; typ-ical sections 160 km
thick are ca 1% less densethan the asthenosphere at the LAB.
PhanerozoicSCLM sections with advective geotherms decreasein
density with depth, and are strongly buoyantrelative to the
asthenosphere. However, older sec-tions that have cooled to
steady-state conductivegeotherms are buoyant relative to the
astheno-sphere only if they are more than 110^120 kmthick.
Fig. 6 summarises lithospheric density pro¢lesfor each SCLM age
type and the upper astheno-sphere. The geotherm used for each SCLM
sec-tion is within the typical range shown in Figs. 4and 5: the
asthenosphere thermal state assumes a
potential temperature of 1300³C and an adiabatof 0.5³C/km. This
¢gure emphasises the densitycontrasts between lithospheric sections
of di¡erentages and the relationship between the density ofeach
lithospheric section and that of the under-lying asthenosphere.
5. Discussion
5.1. Comparison with other models
Jordan [2] showed that the elevation of the con-tinents, and
particularly of the cratonic areas re-quires a chemical boundary
layer beneath the con-tinents that is buoyant relative to oceanic
mantle.He also gave a formulation for the condition formarginal
stability in a layer with such a lateralvariation in density, which
gives a minimum valuefor the compositional density contrast across
thelayer. One pro¢le that satis¢es this condition forArchean
lithosphere is the isopycnic curve, inwhich the density of the
lithospheric column ateach depth is the same beneath oceans and
con-tinents [2]. The negative buoyancy produced bythe lower
temperature gradient of continentallithosphere is balanced by a
positive buoyancydue to a more depleted composition. This
formu-
Fig. 5. Plots of cumulative density versus lithosphere thickness
for lithospheric mantle sections of di¡erent age, calculated
usingthe preferred compositions shown in Table 2 and the range of
geotherms and LAB depth shown in Fig. 1. The shaded areas foreach
mantle type and age correspond to the integrated sections of the
lithosphere that are unstable relative to the underlying
as-thenosphere. The thick line indicating the density of the
asthenosphere as a function of depth assumes a Primitive Mantle
compo-sition (Table 2) and an adiabat of 0.5³C/km for the LAB. The
striped line on the Archean plot shows the density of a hotter
as-thenosphere (1500³C) calculated with an adiabat of 0.5³C/km.
Same abbreviations as in Fig. 4.
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lation requires that the normative density (atSTP) of the
continental section increases withdepth. The isopycnic curve,
corrected for the ef-fects of T and P, would necessarily
approximatethe density of the asthenosphere as shown on Fig.5. This
represents a minimum buoyancy; a con-tinent with a lithospheric
density pro¢le corre-sponding to the isopycnic curve would have
littleor no freeboard relative to the oceans.
Jordan [2] used xenolith data to argue that thenormative density
of the Archean lithosphere in-creases with depth as required by the
isopycnichypothesis. A more detailed analysis based onthe study of
garnet xenocrysts in volcanic rocks
from cratonic areas worldwide ([35]) con¢rms ageneral decrease
in mg# with depth, implyingthat the density of the lower parts of
Archeanand Proterozoic sections can approach that ofPhanerozoic
lithosphere. However, this e¡ect ismost pronounced in the deeper
parts of the litho-sphere (s 180 km depth) and represents a
rapidincrease in density over a short vertical distance;a similar
distribution of density with temperatureis seen in the data used by
Jordan ([2], Fig. 3).
The cumulative density curves shown in Fig. 5,which assume a
constant normative density, prob-ably give the maximum buoyancy
contrast to beexpected for each type of lithospheric section.
TheArchean lithosphere approximated in this wayshows signi¢cant
buoyancy for sections with typ-ical Archean lithosphere thicknesses
of 180^220 km, while Proterozoic sections with typicalthicknesses
of 150^180 km are only moderatelybuoyant, and Phanerozoic sections
(100^130 kmthick) are neutrally buoyant. The low to
moderatebuoyancy of Proterozoic and Phanerozoic litho-sphere,
relative to the asthenospheric mantle, sat-is¢es the minimum
requirements for marginalstability in the plate. However, in
contrast tothe isopycnic situation, the density^depth curvesderived
here show lithospheric densities greaterthan that of the
asthenosphere at shallow depths,and less than that of the
asthenosphere at greaterdepths. The very low normative densities at
shal-low depths implied by the isopycnic relationwould require
Archean-type densities at Mohodepths beneath all sections, and
there is no evi-dence in the xenolith record for this except insome
Archean sections. The overall buoyancy ofArchean sections (Fig. 5)
probably is a maximumestimate, if the rapid decrease in mg# at
depths of180^220 km observed by Gaul et al. [35] is a gen-eral
feature of Archean lithosphere, but even withthis caveat, it
appears that Archean lithosphere issigni¢cantly positively buoyant
with respect to theunderlying asthenosphere. Is such buoyancy
com-patible with observation?
Jordan [36] argued that cratons are neutrallybuoyant relative to
oceanic mantle because theyare not observable in the
long-wavelength gravity¢eld. Richards and Hager [37] and Forte et
al.[38] found that in fact there is a weak association
Fig. 6. Density pro¢les for each lithospheric mantle age typeand
for the upper asthenosphere: Primitive Mantle composi-tion from
Table 2 is used for the asthenosphere compositionwith an adiabat of
0.5³C/km. The representative average (la-belled) geotherm for each
lithosphere section is within thetypical geotherm range shown in
Figs. 3 and 5. The starsshow where the geotherms for each
lithosphere section crossthe LAB. The average LABs taken from Figs.
4 and 5 are200, 150 and 100 km for the Archean, Proterozoic and
Pha-nerozoic lithosphere, respectively.
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between geoid perturbations and the continentalshields, while
Shapiro et al. [39] argued that thisdi¡erence is not signi¢cant at
the 2c level. How-ever, the wavelengths used (of necessity) in
thesestudies may be too long to recognise the signi¢-cant buoyancy
of Archean domains. The param-eterisations have grouped Archean and
Protero-zoic shield areas, which will tend to combine largeareas of
neutrally to slightly buoyant Proterozoicshields with smaller areas
underlain by morebuoyant Archean keels, leading to a small
totalsignal at such long wavelengths. On a smaller (re-gional)
scale, analysis of upward-continued grav-ity data indicates a
signi¢cant density de¢cit be-neath Archean cratons, but not beneath
thesurrounding Proterozoic mobile belts (PoudjomDjomani et al., in
prep.).
5.2. Can continental lithosphere `delaminate'?
The cumulative density curves in Fig. 5 providesome constraints
on tectonic and geochemicalmodels that invoke the gravity-driven
detachment(`delamination') of SCLM and its recycling intothe deep
mantle. Our results show that a typicalsection of Archean
lithospheric mantle is signi¢-cantly buoyant relative to the
asthenosphere.Lowering the geotherm below the 35 mW/m2 con-ductive
model could increase the density of thecolumn, but even if the
geotherm were greatlydepressed, such a section of Archean SCLM
isunlikely to become negatively buoyant if it isthicker than 100
km. Tectonic stacking, often in-voked as a mechanism for
delamination, will sim-ply increase the buoyancy of the Archean
SCLMsection relative to the asthenosphere, as the den-sity of the
asthenosphere at the depressed LABincreases faster than that of the
thickened litho-sphere, assuming constant composition. This typeof
lithospheric mantle therefore cannot be delami-nated through
gravitational forces alone.
The compositional buoyancy of ArcheanSCLM also acts to stabilise
the TBL against con-vective disruption by reducing the stress,
whichleads in turn to a reduction in viscosity and anincrease in
strength. Such compositional buoy-ancy appears to be required by
£ow models thatinvoke activation energies less than those
required
for the £ow of olivine [40]. Finally, the refractorynature and
low heat production of the Archeanlithosphere reduces the
probability of its beingweakened by partial melting, even when
subjectedto relatively large temperature perturbations. Thecombined
e¡ects of their highly depleted compo-sitions on buoyancy, strength
and resistance tomelting provide a simple explanation for
thethickness and apparent long-term stability of Ar-chean
lithospheric roots.
Proterozoic SCLM, while denser than ArcheanSCLM at the same
temperature because of its lessdepleted composition, also typically
has some-what higher geotherms (Fig. 1). Any Proterozoicsection
more than about 150^180 km thick ismoderately buoyant (Fig. 5),
consistent with theobserved preservation of lithosphere with
Prote-rozoic Re^Os ages beneath Proterozoic cratons(e.g. [16^19])
and beneath some areas where Pha-nerozoic tectonic (but not
magmatic) activity hasreworked Proterozoic crust, such as the
Caledo-nides of western Norway ([41]) and parts of west-ern Europe
([23]). Because the density contrastbetween asthenosphere and
Proterozoic SCLMat the LAB is less than for Archean SCLM, it
ispossible that a decrease in the geotherm belowthose modelled here
could destabilise a sectionas thick as 150 km.
Phanerozoic terranes have a wide range of geo-therms (Fig. 1).
With the high advective geo-therms, typical of areas of active or
recent basalticvolcanism (e.g. [12,42]), Phanerozoic SCLM is
ex-tremely buoyant. Some Phanerozoic SCLM sec-tions may sustain
relatively high conductive geo-therms because they have high
internal heatproduction. However, with lower geotherms ob-served in
older Phanerozoic areas (45^50 mW/m2
conductive models ; Figs. 1, 4 and 5), a section ofPhanerozoic
SCLM thinner than about 110 km isdenser than the asthenosphere at
the LAB, andhence is gravitationally unstable. This is a mini-mum
estimate: if the denser Cenozoic lithosphericmantle found as garnet
peridotites in some re-gions (e.g. eastern China; [3]) is
considered, sec-tions up to about 150 km thick are unstable evenon
the higher conductive geotherms characteristicof many Phanerozoic
terranes.
Lithosphere delamination, accompanied by the
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upwelling of the asthenosphere to shallow depths,is widely
invoked to explain tectonic uplift andcrustal magmatism and
metamorphism. The datapresented here suggest that this mechanism is
un-likely to have been relevant during the growth ofArchean
cratons, unless the lithospheric roots ofthese cratons formed after
the major magmatic/metamorphic episodes. However, available Re^Os
data suggest that such roots are at least asold as the overlying
crust ([16] and referencestherein).
On the other hand, the density^depth system-atics of Phanerozoic
SCLM suggest that such de-lamination is highly likely as a result
of coolingfollowing crustal formation.
5.3. `Lithosphere erosion' ^ mechanisms and e¡ects
The data presented here suggest that tectonic ormagmatic events
that lead to the replacement ofold SCLM by younger material will
cause changesin the density and geothermal pro¢le of the
litho-spheric column, with major e¡ects at the surface.These e¡ects
will include regional uplift due to thelower density of material on
an elevated geotherm[43], followed by subsidence as the elevated
geo-therm relaxes [44] and the overall density of thesection
increases due to the replacement of olderbuoyant mantle with
younger, denser mantle.
Several studies have documented situations inwhich thick,
depleted (Archean) lithospheric man-tle has been wholly or
partially removed and re-placed by thinner, hotter and more fertile
SCLM.Eggler et al. [45] showed that beneath the Wyom-ing craton, an
original Archean lithosphere about200 km thick, with a low
conductive geotherm,has been largely replaced by fertile
CenozoicSCLM with a thickness of about 120 km. In theKaapvaal
Craton, Brown et al. [46] used ¢ssiontrack dating and xenolithic
material from kimber-lites of di¡erent ages to show that the
removal ofca 40 km of old, depleted lithospheric mantle andits
replacement by hotter and chemically re-charged (metasomatised)
lithosphere around 90Ma ago, is correlated with signi¢cant uplift
anderosion of the craton. In the eastern Sino^Koreancraton, China,
the removal of about 100 km ofArchean lithosphere during the late
Mesozoic was
accompanied by uplift, basin formation and wide-spread magmatism
[47].
If, as argued above, Archean SCLM cannot be`delaminated' and is
too refractory to be meltedsigni¢cantly, how does such lithosphere
replace-ment occur? A detailed discussion of possible sce-narios is
outside the scope of this paper. However,at least one mechanism is
suggested by geologicaland geophysical studies of the Sino^Korean
cra-ton. Detailed seismic tomography of part of theeastern
Sino^Korean craton, an area of very highheat £ow, shows a
lithospheric mantle made up ofvertically and laterally extensive
blocks of high-velocity (probably Archean) mantle embedded ina
matrix of lower-velocity (presumably Cenozoic)mantle [48]. Yuan
[48] suggests that this heteroge-neity re£ects the disruption of
the Archean litho-sphere due to rifting processes that allowed
up-welling of young fertile mantle along breaks in theArchean root.
This mechanism has resulted inmechanical dispersal, heating and
`dilution' ofthe Archean root, rather than its removal. Wesuggest
that the prolongation of this processwould involve heating and
metasomatism of therelict Archean blocks by asthenospheric
material,leading to a progressive increase in normativedensity and
decrease in their buoyancy and vis-cosity, to the point where they
would be di¤cultto distinguish from young lithosphere, except
pos-sibly by isotopic techniques.
5.4. Implications for lithosphere generationprocesses
The material of Archean SCLM is so buoyantrelative to
asthenosphere that even relatively smallvolumes would rise and
accumulate, especially ifthey were generated in high-temperature
events[49] so that their density would be lower thanmodelled in
Figs. 4 and 5. Fig. 5 shows the e¡ectof a hotter asthenosphere in
the Archean, mod-elled as an adiabat with a potential temperatureof
1500³C. Even in this situation, Archean litho-sphere thicker than
ca 75 km would be signi¢-cantly buoyant. This buoyancy o¡ers a
mecha-nism for generating Archean protocontinentsfrom the depleted
residues of small-scale events.Indeed, it would be di¤cult not to
accumulate
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such material, and the current lack of Re^Os de-pletion ages
greater than ca 3.5 Ga for mantlematerials may imply that these
lithosphere-form-ing processes did not begin earlier.
The relatively low density of a hotter astheno-sphere could have
led to the preferential preserva-tion of only the least dense
lithosphere sections,thus exaggerating the apparent contrast in
norma-tive density between Archean and younger litho-sphere.
However, if this density-sorting mecha-nism was the primary cause,
we would expect to¢nd Archean-type material in younger
lithospheresections, and this has not been identi¢ed to date([24]).
Thus even if only the most buoyant parts ofthe Archean lithosphere
have survived, the di¡er-ences between Archean and younger SCLM
arguefor a secular change in the processes that havegenerated SCLM
through time. Discussions ofthe nature of these processes are given
elsewhere[25,26,49].
Proterozoic lithospheric sections are typically150^180 km thick,
but are only gravitationallystable if they are 130 km thick. These
thick sec-tions would have to have accumulated either rap-idly, or
at high enough temperatures to remainbuoyant until they have
thickened to the pointwhere thermal relaxation would not make
themunstable. This accumulation would be assistedby secular cooling
of the Earth, as the cooler as-thenosphere became more dense. The
processesthat produced these relatively thick stable rootsappear to
be largely con¢ned Archean and Prote-rozoic time, though
present-day analogues mayexist under regions such as the Sierra
Nevada ofCalifornia.
A section of Phanerozoic SCLM cooling from ahigh advective
geotherm (such as the SEA geo-therm (Figs. 4 and 5) toward a
typical high con-ductive geotherm undergoes a dramatic change
indensity (s 2% for a section 100 km deep). Whilethis section is
very buoyant on the high advectivegeotherm it is gravitationally
unstable and subjectto delamination (Rayleigh^Taylor instability ;
[50]and references therein) on lower geotherms. Theremoval of such
a section, and its replacement byupwelling asthenospheric material,
would producea new advective geotherm, starting the cycle
overagain. This instability may be a common feature
of Phanerozoic orogenic belts, and could explainthe apparent
scarcity of oceanic or island^arc de-pleted mantle components in
xenolith suites fromyoung mobile belts such as eastern China
andeastern Australia, as documented by Gri¤n etal. [23,24].
6. Conclusions
Di¡erences in SCLM composition and geo-therms between Archean,
Proterozoic and Pha-nerozoic regions result in marked, age-related
dif-ferences in mean lithospheric density and in thedensity^depth
relationship within the SCLM.
Archean SCLM sections (depth to base oflithosphere 180^240 km)
are signi¢cantly buoyantrelative to asthenosphere under any
reasonablegeological scenario, and this buoyancy reducesboth stress
and viscosity. Therefore ArcheanSCLM cannot be delaminated by
gravitationalprocesses alone, and will tend to be
preserved.Tectonic processes such as rifting may allow
itsdisruption and replacement by upwelling, morefertile
asthenospheric (or plume) material.
Proterozoic SCLM is less depleted and there-fore denser (at the
same T) than ArcheanSCLM. Thin Proterozoic SCLM (less than about120
km thick) is denser than asthenosphere andwould be gravitationally
unstable. However, Pro-terozoic lithospheric sections are typically
150^180 km thick; such SCLM columns are moder-ately buoyant and,
like Archean SCLM, are un-likely to be delaminated.
Phanerozoic lithospheric sections are com-monly less than about
100 km thick. These sec-tions are stable under the elevated
geotherms as-sociated with volcanic activity, but
gravitationallyunstable once they have cooled to typical
steady-state conductive geotherms. This instability sug-gests that
lithospheric delamination may be acommon feature of the history of
Phanerozoicmobile belts. Replacement of such a section byupwelling
asthenospheric material will start an-other cycle of cooling,
instability and delamina-tion. This process may explain the common
oc-currence of highly fertile SCLM beneathPhanerozoic mobile belts,
where we might instead
EPSL 5713 4-1-01
Y.H. Poudjom Djomani et al. / Earth and Planetary Science
Letters 184 (2001) 605^621 619
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expect to ¢nd depleted oceanic and arc-relatedperidotites.
Acknowledgements
The concepts discussed here owe much to dis-cussions with many
colleagues, including JoeBoyd, Rod Brown, Geo¡ Davies, Buddy
Doyle,Kevin Kivi, Lev Natapov, Norm Pearson, ChrisRyan, Yuan
Xuecheng and Ming Zhang. Wethank Brian Kennett for providing his
indepen-dent estimates of asthenosphere density at 220km and A.
Lenardic for helpful comments onan earlier version of this
manuscript. Commentsby I. Artemieva and an anonymous reviewer
haveclari¢ed issues in the manuscript. Funding for thiswork has
come from several sources, includingARC Large Grants (S.Y.O'R. and
W.L.G.), Mac-quarie University Industry Collaborative Grants,MURG
(Y.H.P.D., S.Y.O'R. and W.L.G.), Aus-AID grants and the mineral
exploration industry.This is contribution no 228 from the ARC
Na-tional Key Centre for Geochemical Evolution andMetallogeny of
Continents (www.es.mq.edu.au/gemoc/).[AC]
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