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Lithos 119 (2010) 457–466
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Lithos
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s
The growth of the continental crust: Constraints from zircon Hf-isotope data
E.A. Belousova a,⁎, Y.A. Kostitsyn b, W.L. Griffin a, G.C. Begg c, S.Y. O'Reilly a, N.J. Pearson a
a
b
c
GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW, 2109, Australia
Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, 119991, Russia
Minerals Targeting Intl., West Perth WA, 6005, Australia
a r t i c l e
i n f o
Article history:
Received 22 April 2010
Accepted 28 July 2010
Available online 3 August 2010
Keywords:
Continental growth
Crustal evolution
Hf-isotopes
Zircon
a b s t r a c t
A worldwide database of over 13,800 integrated U–Pb and Hf-isotope analyses of zircon, derived largely from
detrital sources, has been used to examine processes of crustal evolution on a global scale, and to test
existing models for the growth of continental crust through time. In this study we introduce a new approach
to quantitatively estimating the proportion of juvenile material added to the crust at any given time during
its evolution. This estimate is then used to model the crustal growth rate over the 4.56 Ga of Earth's history.
The modelling suggests that there was little episodicity in the production of new crust, as opposed to peaks
in magmatic ages. The distribution of age-Hf isotope data from zircons worldwide implies that at least 60% of
the existing continental crust separated from the mantle before 2.5 Ga. However, taking into consideration
new evidence coming from geophysical data, the formation of most continental crust early in Earth's history
(at least 70% before 2.5 Ga) is even more probable. Thus, crustal reworking has dominated over net juvenile
additions to the continental crust, at least since the end of the Archean. Moreover, the juvenile proportion of
newly formed crust decreases stepwise through time: it is about 70% in the 4.0–2.2 Ga time interval, about
50% in the 1.8–0.6 Ga time interval, and possibly less than 50% after 0.6 Ga. These changes may be related to
the formation of supercontinents.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The question of the rate at which the continental crust has grown
through time has divided geologists for many years (e.g., Hurley and
Rand, 1969; Stevenson and Patchett, 1990; Armstrong, 1991;
McCulloch and Bennett, 1994; Taylor and McLennan, 1995; Condie,
2000; Rino et al., 2004; Kemp et al., 2006; Rollinson, 2008; Hawkesworth et al., 2009). One view holds that most, or all, of the continental
crust was formed early in Earth's history and has been largely recycled
ever since (Armstrong, 1991). Other models suggest episodic growth
in which the volume of depleted mantle increases with time in a
stepwise manner, and is linked to major episodes of continental crust
formation at 3.6, 2.7 and 1.8 Ga (e.g. McCulloch and Bennett, 1994). A
more recent model for episodic continental growth (Condie, 1998;
2000; Rino et al., 2004) suggests that major peaks in zircon age
distributions at 2.7 and 1.9 Ga represent superplume events and the
generation of juvenile crust, related to the periodic collapse of
subducted slabs through the 660 km seismic discontinuity. However,
Hawkesworth et al. (2009) have argued that these pronounced peaks
in crustal ages reflect differences in the preservation potential of
crustal rocks rather than episodes of enhanced crustal generation.
⁎ Corresponding author. Tel.: +61 2 9850 6126.
E-mail address: [email protected] (E.A. Belousova).
0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2010.07.024
The continental crust can be regarded as an “end product” of the
chemical differentiation of the Earth's primitive mantle through time;
the question of crustal growth rate thus bears directly on the nature
and timing of this differentiation process. To understand the growth
of a crustal volume, we need to determine the sources of the
magmatic rocks added to that crust over its history: were these
magmas juvenile (i.e. derived from the convecting mantle) or recycled
(i.e. remelting of older crust), or do they represent mixing of magmas
derived from those two sources? Early crustal-evolution models made
extensive use of the whole-rock Sm–Nd isotopic system; age data
(typically from U–Pb dating of zircons) were combined with Ndisotope analysis of the host rocks to define the source(s) of crustal
material. The behaviour of the whole-rock Lu–Hf isotopic system in
magmatic rocks closely parallels that of the whole-rock Sm–Nd
system (Vervoort and Blichert-Toft, 1999, Vervoort et al., 1999), but
the former is strongly controlled by the mineral zircon. Once it
crystallises from a magma, zircon is stable up to high metamorphic
grades, whereas whole-rock isotopic systems may be disturbed by a
variety of processes. Well-crystallised (i.e. non-metamict) zircon is
resistant to diffusion and isotopic exchange (e.g. Cherniak et al.,
1997), and because of its very low Lu/Hf, it can preserve the 176Hf/
177
Hf of its parental magma at the time of crystallisation. Thus the link
between the age and the isotopic composition of the magma is more
likely to be preserved in zircon than in whole-rock isotopic systems.
High values of 176Hf/177Hf indicate a “juvenile” origin for the magma,
while low values imply the reworking of older crustal material. Recent
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studies of O isotopes and Hf isotopes in single zircon grains (eg Kemp
et al., 2006; Hawkesworth and Kemp, 2006) have confirmed this basic
interpretation, by showing that high 176Hf/177Hf commonly is linked
with mantle-like oxygen-isotope ratios. The Lu–Hf system in zircon
therefore is a powerful tool for studying processes of crustal and
mantle evolution (Amelin et al., 2000; Griffin et al., 2000; Condie et al.,
2005; Harrison et al., 2005; O'Reilly et al., 2008).
Detrital zircon grains from modern river systems (and ancient
sediments) may have been recycled many times, and during each
cycle the age distribution of the zircon population may change (Veizer
and Jansen, 1979). The importance of sedimentary recycling has been
quantitatively evaluated by Rahl et al. (2003) and Campbell et al.
(2005). However, these sediment samples also may provide zircons
from source rocks that no longer outcrop, or even exist as intact
lithologic units; these grains carry valuable information on crustal
history. Detrital zircons therefore have proven useful in crustalevolution studies focused on either single large river systems, or more
limited areas (Vervoort et al., 1999; Griffin et al., 2004; Rino et al.,
2004; Iizuka et al, 2005; Campbell and Allen, 2008; Belousova et al.,
2009; Kemp et al., 2009a).
2. Database description and treatment
A worldwide database of 12,375 TerraneChron® (integrated U–Pb,
Hf-isotope and trace-element analyses) analyses of zircon, largely
from detrital sources, has been generated at GEMOC since 2000. To
this dataset we have added U–Pb and Hf-isotope data available from a
number of recent publications (4070 analyses from rock samples and
sediments; a complete list of references is provided in the Supplementary Appendix A). This large volume of data (N = 16,445) makes
it feasible to examine processes of crustal evolution on a global scale,
and to test existing models for the growth of continental crust
through time. The geographical distribution of the samples in the
expanded dataset is shown in Fig. A1 (Supplementary Appendix).
An examination of the distribution of samples highlights potential
biases in the dataset. A larger proportion of the data is from Archean/
Proterozoic cratonic areas, generally with higher velocity, and
probably thicker, lithospheric-mantle roots. Mobile belts are represented mainly by samples from Mongolia, Scandinavia, central USA
and the Peruvian Andes. Continental rift zones and Phanerozoic
convergent-margin environments, where a larger juvenile input
would be expected, may be under-represented, but detrital zircons
collected from large rivers should help to cover this gap in sample
representation. The dataset also includes detrital zircons from ancient
sediments that cannot sample crust younger than the age of
deposition; in these samples there will be a possible underrepresentation of very young (b200 Ma) ages.
Fig. 1 shows the distribution of U–Pb ages in the GEMOC
TerraneChron® database (blue line) compared to the data from
other studies. Previous studies (e.g., Rino et al., 2004; Iizuka et al.,
2005) have shown that the distribution of zircon ages in a large
sample from the Mississippi River accurately reflects the relative areas
of the igneous provinces in the drainage area. The age data of
Campbell and Allen (2008; no Hf-isotope data) are shown separately
as a green line; these represent detrital zircons collected from the
mouths of 40 of the world's largest rivers and thus provide a broad
global picture of the age distribution. A good correlation between
the major peaks in the TerraneChron® (TC) and Campbell and Allen
(2008) (C/A) datasets thus indicates that the TerraneChron® dataset is
broadly representative of the world-wide distribution of crustal age
provinces. Distinct peaks in U–Pb age distribution patterns have been
attributed to differences in the preservation potential of crustal rocks
(Hawkesworth et al., 2009) or to super-mountain building during
supercontinent assembly (Campbell and Allen, 2008).
There are some discrepancies between these two datasets (TC +
C/A) and data reported from other studies, eg the absence of the ca
Fig. 1. Comparison of zircon U–Pb age distributions for data collected from different
sources (a complete list of references is provided in the Supplementary material). Each
point on the curves corresponds to the number of analyses in a 0.1 Ga interval. Red
curve shows distribution of ages in a worldwide data set including grains with ages but
no Hf-isotope data. TerraneChron® database (TC; blue curve) is from GEMOC; data of
Campbell and Allen (2008; green curve) are detrital zircons from the mouths of 40 of
large rivers.
0.5 Ga Pan-African, 1–1.2 Ga Grenville and ~2.7 peaks in the “other”
studies (Fig. 1). These are clearly due to bias in sample collection, where
specialised studies have focused on small areas. For example, the
Hadean population in the “other” data represents mainly zircons from
the intensively studied Jack Hills (Australia) quartzite and the Acasta
gneisses (Canada). The contribution of the data from each continent to
the total dataset is illustrated in Fig. A2 (Supplementary Appendix).
There is general congruence of major peaks from each continent.
The dataset that includes both U–Pb and Hf-isotopic data contains
16445 zircon analyses (Fig. 2a). It was filtered sequentially using the
following exclusion criteria:
– 1979 zircons with U–Pb age discordance over 10% were rejected;
– 304 zircons with analytical error (1σ) over 1.5 εHf were rejected;
– 76 zircons with 176Yb/177Hf N 0.2 or 176Lu/177Hf N 0.005 were
rejected because of the potential for uncorrected isobaric
interferences;
– 51 zircons with εHf N 2 εDM were rejected;
– 191 zircons with Th/U b 0.05 were rejected to avoid grains of
possible metamorphic origin (Rubatto et al., 2001; 2002).
In all, 2601 zircons (16% of the database) were rejected and the
13,844 data remaining after filtering (Fig. 2) have been used for the
further modelling. Because of the large amount of the data with a high
proportion of overlapping analytical points, two-dimensional histograms as presented in Fig. A3 (Supplementary Appendix) help to
illustrate the distribution and density of the data in εHf vs age space.
Fig. 2b and A4 (Supplementary Appendix) show the filtered data
plotted by continent. The majority of the data come from three
continents (Asia 30%, Australia 26% and South America 20% of the
dataset). However, apart from a noticeable Hadean population
defined mainly by zircons from Australia (Jack Hills quartzite), most
continents are represented by a broad range of ages. More
importantly for this analysis, zircons with low values of εHf are not
limited to any single continent. The large proportion of data with
negative εHf values immediately suggests that reworking of ancient
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(1.867 × 10− 11 yr− 1) for 176Lu proposed by Scherer et al. (2001)
because it gives the best fit for terrestrial rocks (Amelin, 2005; Amelin
and Davis, 2005; Albarède et al., 2006).
Juvenile magmas are defined as those generated directly from the
depleted mantle, or by remelting of material recently extracted from
depleted mantle. These are defined here as having εHf ≥ 0.75 times the
εHf of the Depleted Mantle curve, which is equivalent to 75% of the
MORB range.
TDM model ages, which are calculated using the measured 176Lu/
177
Hf of the zircon, can only give a minimum age for the source
material of the magma from which the zircon crystallised. Therefore a
“crustal” model age (TCDM) also has been calculated, which assumes
that the zircon's parental magma was produced from a volume of
average continental crust (176Lu/177Hf = 0.015; Griffin et al., 2004)
that was originally derived from the depleted mantle.
4. Models of crustal growth
Fig. 2. Plot of εHf vs age showing data before [N = 16445; (a)] and after [N = 13844 (b)]
culling. (a) Rejected data and accepted ones are coloured differently. Numbers of
rejected analyses are given in the legend; and criteria for rejection are discussed in the
text. The curve for the median εHf value is smoothed in a 0.1 Ga window. The range of
MORB εHf values (Nowell et al., 1998; Kostitsyn, 2004) is given for comparison. (b) Data
left after culling, plotted by continent. Diagrams with the same data illustrated by twodimension histograms are provided in the Supplementary Appendix Fig. A3.
material is a significant and universal signature of the processes that
have formed the continental crust.
Different types of crustal growth curves proposed in the literature
are illustrated schematically in Fig. 3a. One group of end-member
models involves the rapid generation of continental crust in the
Hadean and Early Archean (eg Armstrong, 1981, 1991; similar to curve
1). At the other extreme (curves 4 and 5; Fig. 3a) are models that
involve an increasing rate of crustal growth through time (e.g., Hurley
and Rand, 1969). Armstrong (1981, 1991) argued for a balance
between rates of crustal accretion and rates of crustal loss (recycling)
by sediment subduction and tectonic erosion. Fig. 3b shows how
different ratios of crustal reworking versus recycling of crustal
material into the mantle impact on the crustal model age through time.
Numerous models of continental growth ranging between these
two extremes have been proposed, based on whole-rock Rb–Sr,
whole-rock Sm–Nd, zircon U–Pb ages and combinations of these. The
advantages of the combination of U–Pb ages and Lu–Hf isotope
systematics in zircon have been recognised over the past decade and
have been applied to the estimation of the continental growth rate in
several previous studies (eg Amelin et al., 2000; Condie et al., 2005;
Iizuka et al., 2005; Kemp et al., 2009a). Most of these studies are based
on the estimation of Hf model ages. However, as shown below, the
zircon model-age approach, taken on its own, significantly underestimates the mean age of the continental crust. A recent study by
Iizuka et al. (2010) suggests a quantitative way to estimate the
relative significance of juvenile magma addition (crustal generation)
and reworking of pre-existing crust. In this study we introduce a new
approach to quantitatively estimating the proportion of juvenile
contributions to the continental crust. This estimate is then used to
calculate the crustal growth rate through time. Application of this new
modelling approach to the current dataset of over 13,800 zircon
analyses allows us to produce a more reliable estimate of the growth
rate of the continental crust over the 4.56 Ga of Earth's history.
The resulting crustal growth curve is compared to the results of an
independent study known as GLAM (Global Lithospheric Architecture
Mapping; Begg et al., 2009) that has generated maps of the
composition and architecture of the upper lithosphere by integrating
geophysical, geological and geochemical data. The results are then
tested against a statistical simulation of the crustal formation process.
3. Hf model ages
5. Modelling the data
To calculate model ages (TDM) based on a depleted-mantle source,
we assume that the depleted mantle (DM) reservoir developed from
an initially chondritic mantle, and is complementary to the crust
extracted over time. In this model (Griffin et al., 2000) the DM has a
present-day 176Hf/177Hf = 0.28323, similar to that of average MORB;
the range of MORB εHf values (Nowell et al., 1998; Kostitsyn, 2004) is
shown on the vertical axis in Fig. 2a. Assuming an initial value of 176Hf/
177
Hf = 0.27982, this defines the DM as having 176Lu/177Hf = 0.0384.
For the calculation of εHf values, we have adopted the decay constant
5.1. Distribution of U–Pb ages and model ages through time
Plots of relative probability (Fig. 4a) and cumulative (integral)
curves (Fig. 4b) for the U–Pb age data, showing that over 80% of
recorded events are post-Archean, clearly reflect the preservation of
younger crust relative to (destroyed or buried) older crust. It is also
notable that the curves for TCDM are much smoother than the U–Pb
curve and are shifted towards the older ages. Only a small proportion
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Fig. 3. (a) Model curves of crustal growth, where curve 1 models a very early formation of
the crust; this is close to the model of Armstrong (1981). The other extreme model (curve
5) represents an increasing rate of crustal growth through time (Hurley and Rand, 1969).
(b) Models showing different proportions of crustal reworking versus recycling of crustal
material into the mantle and corresponding model ages of the integrated crustal material.
Curve 1 on both plots models a very early formation of the crust, followed only by crustal
reworking. Curve 5 could reflect very efficient recycling of older crust into the mantle,
which produces an average crust with a very young present-day model age. Curve 2
reflects a decrease in crustal growth rates with time, so that 70% of the today's crustal mass
C
had formed by the end of Archean time; the average model age TDM
of this crust (curve 2)
is about 3 Ga. The straight line (3) represents continuous crustal growth at a constant rate.
The present-day average model age of such a crust is half of the Earth's age, i.e. 2.25 Ga, if
crustal reworking is balanced by crustal recycling.
Fig. 4. (a) Relative probability curves (left scale) of U–Pb zircon ages (blue line), TCDM model
ages (green line) and number of zircons with juvenile Hf-isotope compositions (red line;
defined as grains with εHf N 0.75×εDM
Hf ). Proportions of the continental lithosphere formed
during three major time intervals derived from GLAM mapping (see below) are shown by
the dashed line (right axis). (b) Cumulative/or integral curves of zircon U–Pb ages (blue)
and crustal model ages TCDM (green).
5.2. Calculation of juvenile proportion
each point during its evolution. The amount of juvenile material
produced at any given time is underestimated by the red curve in
Fig. 4a, because some of that material was later reworked, and the
record of the original juvenile material is lost during subsequent
crustal evolution. The approach proposed below attempts to offset
this effect, and “restore” an indication of the true juvenile input.
The approach used to calculate the proportion of juvenile addition
(XJUV) at any given time is illustrated in Fig. 5. For each individual time
slice (dark blue points) we can calculate both the number of zircon U–
Pb ages (NU–Pb Age) and the number of zircons with TCDM model ages
(NModel Age) corresponding to this time interval (green points). The
slope of the green band corresponds to the 176Lu/177Hf of the average
continental crust (0.015). The juvenile proportion is estimated as:
To understand the growth rate of the continental crust it is critical
to evaluate the proportion of juvenile material added to the crust at
XJUV = NModel Age =
(ca 10.5% of the data) of all zircons have Hf-isotopic compositions
close to that of DM (red curve in Fig. 4a). These observations again
emphasise the important role of crustal reworking through time. Even
taking a minimalist approach of using the integrated curve of crustal
model ages (Fig. 4b) to represent the growth of the continental crust,
about 45% of this crust must have been generated before 2.5 Ga.
NU−Pb Age + NModel Age
ð1Þ
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Fig. 5. Schematic illustration of the calculation of juvenile proportion. See text for
explanation. Data plotting above the dashed line are defined as “juvenile”; they have
εHf N 0.75 the εHf of the Depleted Mantle or 75% of the MORB range. Red curve in the
insert represents the integrated growth curve (e.g., the derived growth model). The
vertical array of black points illustrates possible cases of mixing between juvenile/
mantle (‘M’) and crustal (‘C’) material.
This approach assumes that the ages of zircons in the green bands
(Fig. 5) reflect later crustal magmatic events, while their model ages
reflect the timing of older mantle-derived inputs into the crust, which
were remelted to produce the host magmas of the younger zircons.
Integration of the obtained values of XJUV yields a crustal growth curve
(insert, Fig. 5). This approach does not address the possibility that the
Hf-isotope signatures of some younger zircons were produced by
mixing between melts with more radiogenic Hf (point ‘M’ in Fig. 5) and
older crustal material with low Hf-isotope ratios (point ‘C’ in Fig. 5). This
mixing process would produce a larger uncertainty in the estimate of
the juvenile proportion for this particular time-slice. In addition, some
juvenile addition during the reworking of pre-existing crust could result
in younger model ages; thus TCDM should be considered as minimum
estimates of crustal model ages. However, in general these processes
will not significantly bias the average crustal growth pattern, except
when considering the oldest and youngest zircon populations. This
effect may contribute to the sharp drops in the model age distribution at
agesb 0.5 Ga and N 3 Ga, as shown in Fig. 4a.
5.3. Integrated composition of continental crust
The combined worldwide dataset allows us to estimate the
integrated crustal history (as defined by the Hf-isotope composition)
at any point in time. The approach used to derive this estimate is
illustrated schematically in Fig. 6, where the average Hf-isotope
composition of the newly formed material at any time t is shown by
the red circle ‘S’. This average is calculated for all zircons crystallized at
time t. However, the integrated crustal Hf-isotope composition must
account for all pre-existing material, i.e. rocks generated before time t.
In Fig. 6, zircons 1–6 represent volumes of pre-existing crust and
the εCHf ðt Þ of these volumes at time t can be calculated using the
average 176Lu/177Hf of the continental crust (0.015). This modelling
approach is identical to the crustal model-age calculation but with
interpolation projected forward in time instead of backward. Thus, the
integrated crustal Hf-isotope composition εCHf ðt Þ at time t is represented by the green square. This composition is shown as a green
curve in Fig. 7, from 4.56 Ga to the present day.
461
Fig. 6. Calculations of the average and integral Hf-isotopic composition of the
Continental crust, where the red circle ‘S’ represents the average composition
calculated for all zircons crystallized at time t and the green point is the integrated
composition that account for all pre-existing material generated before time t. See text
for more explanation.
For any time t an integrated model age, shown as a green line in
C
Fig. 7b, can be also calculated from the zircon crustal model ages T DM ðt Þ,
using the average integrated Hf-isotope composition of the continental
crust εCDM ðt Þ. An integrated model age for the juvenile crust (red line,
Fig. 7b) was calculated using the Hf-isotope composition of the juvenile
input proportion at any given time t. Note that integrated εCDM ðt Þ and
C
integrated T DM ðt Þ decrease with age as does the proportion of juvenile
material in any time slice.
The proportion of juvenile versus crustal components is shown in
Fig. 7a (red curve), suggests that 50 to 80% of the melts generated at
almost any time were juvenile. A significant exception is the last
300 Myr, which shows a very low (b10%) juvenile component. There
are two distinct steps in the pattern (illustrated by the red blocks in
Fig. 7a) showing that the juvenile contribution to magmatic episodes
was on average about 70% before ca 2.2 Ga, dropped about 50% after
2.2, and dropped again after ca 0.6 Ga.
The average age of the continental crust, calculated using the
integrated composition of continental crust, is about 2.25 Ga (green
curve; Fig. 7b). The average age derived from the calculated juvenile
proportion (red curve; Fig. 7b, c) is about 2.75 Ga. Below, we use an
independent constraint (the “GLAM Model”) to explore this discrepancy.
5.4. Comparison with the GLAM model
Most of our understanding of the continental crust is derived from
upper-crustal exposures, whereas in some areas the lower crust
may be significantly older than the exposed crust (e.g., Zheng et al.,
2004a, b; 2006). The differences in the integral curves for U–Pb ages
and TCDM model ages in Fig. 4a suggest that the oldest crust probably is
grossly underrepresented at Earth's surface. For a complete model,
this aspect must be evaluated more quantitatively, including an
assessment of the lateral and vertical extent of the lower-crustal and
upper-crustal domains. Broad-scale information for such modelling
can be obtained from the global tectonic synthesis maps produced by
previous studies (e.g., Condie, 2005) or more recent ones constructed
as described by Begg et al. (2009).
Using Africa as an example, Begg et al. (2009) have shown
how geophysical (e.g., gravity, seismic tomography), geological,
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Archon-origin (N2.5 Ga), Proton-origin (2.5–1 Ga) and Tecton-origin
(b1 Ga) lithospheric domains have been calculated for North America,
South America and Africa. These relative areas are considered to be
representative enough for a global estimate, assuming that all upper
lithospheric domains have equivalent reliability as regards the determination of their tectonothermal history. This mapping indicates
that the existing continental upper lithosphere is 70.5% Archon, 19.3%
Proton and 10.2% Tecton; i.e. at least 70% of the existing crustal
volume originally was generated in Archean time (see Fig. 4a).
The curves shown in Fig. 7 could represent the time-integrated Hfisotope composition of the continental crust if the age distribution of the
zircons in the database is representative of the volumetric proportions
of different types of crust. Comparison with the GLAM model indicates
that this may not be the case; deviations are listed in Table 1. To explore
the impact of a possible discrepancy we have calculated weighting
factors, which are simply the ratios between the GLAM estimates and
the proportions of zircon data for different time intervals.
To calculate the integrated weighted average εC;W
ðt Þ value of the
Hf
continental crust at any given time t all zircons with U–Pb ages older
than t (T ≥ t) were used:
C
C;W
εHf ðt Þ =
∑εHf ðt Þ⋅W ðt Þ
ð2Þ
∑W ðt Þ
where weights W(T) are taken from the last column of Table 1. For any
time t an integrated model age can be calculated from the crustal model
C
ages T DM ðt Þ using the average integrated Hf isotopic composition of the
C;W
continental crust εCDM ðt Þ. The integrated weighted Hf model age T DM ðt Þ
uses GLAM weights as given in Table 1. The calculation for all zircons
with U–Pb age older than time t (T ≥ t) is given by:
C;W
T DM ðt Þ =
Fig. 7. (a) εHf versus zircon U–Pb age for all zircons in the database (N = 13,844), where
the integral εHf of the crust (green line) is calculated using the approach explained in
Fig. 6 and the juvenile proportion (red line; right scale) is calculated using the
algorithm illustrated in Fig. 5. (b) Crustal model age versus U–Pb age for database
zircons, with some generalised curves. Integral crustal model age (green line) is
calculated as the average model age of all zircons with U–Pb age older than a given time
t. Red curve represents model ages of a hypothetical crust that grew according to the
integrated growth curve shown in Figs. 5 and 7c. (c) The red curve represents the
derived growth model.
geochronological and geochemical data on the crust and lithospheric
mantle can be integrated to generate maps of lithospheric composition and architecture. The same GLAM (Global Lithospheric Architecture Mapping) approach has been applied to the mapping of North
and South America (Begg et al., in prep.). While exposed Archean
crust covers only about 6% of Earth's land surface (about 10 M km2),
the GLAM mapping suggests that about 70% of the sub-continental
lithospheric mantle (SCLM) has an Archean parentage. Most preserved Proterozoic crust overlies Archean SCLM that has been variably
refertilised and metasomatised by mantle melts associated with
convergent margin, post-collisional, and mantle plume processes.
Detailed studies of specific terrains using zircon U–Pb and Hf-isotope
analysis suggest that most of these “Proterozoic cratons” have
Archean crust at depth, which has contributed to Proterozoic magmas
(eg Zheng et al., 2006; Murgulov et al., 2007; Belousova et al., 2009).
The GLAM mapping classifies individual upper lithospheric (crust
and upper SCLM) domains on the basis of their tectonothermal age
(the time since the last major tectonothermal event) and their
original age, as defined from isotopic data. The areal percentages of
C
∑TDM
⋅W ðt Þ
∑W ðt Þ
ð3Þ
The result of this calculation is shown in Fig. 8 by the blue curve.
This line deviates from the green line of the simple average crustal
composition only in post-Archean time because between 4.56 and
2.5 Ga all data points are given the same weighting. The crustal
growth curve (red line) calculated using the Hf-isotope composition
of the juvenile input (Figs. 7, 8) is shown for comparison. Note that no
GLAM normalization (weighting) was applied for this growth curve.
Thus, two approaches (Fig. 8) using (1) the GLAM weighted integrated
crustal composition (blue line) and (2) the integrated proportions of
juvenile inputs (red line) indicate that the present-day average Hf model
age of the crust is about 2.8 Ga. The mean TCDM according to the GLAM
model (grey band) shows a similar pattern, and is given for comparison.
These curves are most similar to the model curve 2 in Fig. 3, suggesting a
decreasing rate of crustal growth through time.
The results of the GLAM analysis (Begg et al., 2009; in prep) suggest
that about 70% of the continental crust was formed by the end of the
Archean time. The modelling based on the zircon database (red curve
in Fig. 9) suggests that about 62% of the present crustal volume existed
by the end of the Archean. The lower value derived from the zircon
dataset may reflect either (1) the preservation of ancient crustal
material in the lower crust of some cratonic areas, where it has not
Table 1
Proportions of data for different time intervals in our database compared to the GLAM
estimates. The discrepancy between the database and GLAM proportions is used to
calculate ratios for further weighting of the calculated integral crustal curve (blue
curve; Fig. 8).
Time interval
GLAM portion, %
Database, %
Ratios (weights)
b 1 Ga
1–2.5 Ga
N 2.5 Ga
10.2
19.3
70.5
20.6
47.4
32.0
0.49
0.41
2.21
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E.A. Belousova et al. / Lithos 119 (2010) 457–466
463
crustal growth, to investigate the nature of crustal-growth processes.
This modelling is based on an artificially created set of numbers that
simulates 30,000 magmatic events distributed over a time range of
4.56 Ga. Thus, this simulation does not claim to model a natural
distribution of crystallization and model ages of crustal material, but it
can be used to qualitatively illustrate the distortion of crystallizationage and model-age distribution curves that might be produced by
reworking of older rocks. The usefulness of the modelling illustrated
in Fig. 10 is that it allows comparison between a simulated growth
curve and the integrated growth curve calculated using the
approaches developed above.
This modelling consists of two steps:
Fig. 8. Crustal model age versus U–Pb age for database zircons, and some calculated
curves. The GLAM TCDM model (grey band) is calculated as the weighted average age for
any time t using the time-normalized GLAM proportions from Table 1 (column 2); these
weights also are shown by the dashed line in Fig. 4. The green curve is the integral
crustal model age for the integral crust composition (green line in Fig. 7). The blue curve
represents the integral crustal model age weighted using GLAM proportions. The red
curve is the model ages of a crust that grew according to the integrated growth curve
(shown as insert in Fig. 5 and in Fig. 7c).
been available for re-sampling/re-working/re-melting during later
magmatic events, or (2) the complete removal and destruction (via
recycling into the mantle) of old crust, but preservation of underlying
lithospheric mantle, which is later resurfaced by younger crust.
6. Statistical modelling
The models derived above suggest there has been little episodicity
in the production of new crust, as opposed to peaks in magmatic ages.
Here we present a straightforward statistical model of continuous
1. 30,000 crustal-formation events in the time interval from 4.56 to
0 Ga are simulated with uniform (Fig. 10a) or skewed (Fig. 10b)
statistical distribution. These events have produced 30,000 objects
of corresponding ages.
2. Pair-wise interaction of randomly selected objects. As a result of
every interaction, a new object is produced having the age of the
youngest object from the interacting pair and a model age that is
the average of the two interacting objects. This step is an
oversimplified model of an interaction between a younger
magma and an older rock.
The statistical model (Fig. 10) simulates interaction (mixing) of
randomly selected pairs of all the crustal volumes; each volume
participates in an interaction event only once. The ages of the newly
formed volumes (after interaction) are shown by the blue line and
their resulting model ages are shown by the green line. The modelled
volumetric growth (black dotted line) is calculated by incrementally
adding all the crust-formation episodes (pink line), while the
calculated growth curve obtained by the algorithm described in
Section 5.2 (red band) represents our preferred model of crustal
growth rates through time.
The statistical modelling of uniform crustal growth (Fig. 10a)
assumes that the ages of crustal generation events are uniformly
distributed through the time (pink line); the incrementally calculated
crustal growth is shown by the black dotted line. In this model, there
is poor agreement between the simulated growth curve (dotted line)
and the one calculated using the algorithm proposed in this study (red
band). The modelling shown in Fig. 10b assumes a predominance of
early crust formation and thus a decreasing crustal growth rate
through time (dotted back curve). Crustal formation events (pink
line) are more frequent in the Archean time, thus producing larger
volumes of older crustal material. The relative abundances of Archean
and post-Archean crustal generation events are taken from the GLAM
model (Table 1). In this case there is a satisfactory agreement between
the two curves (dotted and red band). These simple exercises show
that the zircon data can best be modelled in terms of quasi-continuous
crustal growth, with the rate of growth dropping steadily after the end
of the Archean.
7. Discussion and conclusions
Fig. 9. The integrated crustal growth curve (red line) derived in this study, compared
with the GLAM model (grey band; see Fig. 4a). Integrated curves for the zircon U–Pb age
data and crustal model age are shown for comparison.
Because the zircon dataset is biased toward recording younger
events (Table 1; Fig. 4a), whereas older crust has been lost into the
mantle or buried, the growth rate of continental crust during the
Archean and Hadean is probably significantly underestimated, even
using the approach demonstrated here. Furthermore, there is a
tectonic bias toward the preferential recycling of young juvenile crust
into the mantle, because its underlying lithospheric mantle is prone to
delamination, while the Archean crust is generally underlain by the
more buoyant Archean lithospheric mantle, and thus is less prone to
recycling (Poudjom Djomani et al., 2001). Taking into account new
evidence coming from other sources (e.g., geophysical data, Begg et
al., 2009; crustal gold endowment, Frimmel, 2008), the formation of
most continental crust early in Earth's history is even more probable.
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E.A. Belousova et al. / Lithos 119 (2010) 457–466
Fig. 10. Results of statistical modelling assuming (a) Uniform crustal growth and
(b) Relative abundances of Archean and post-Archean crustal formation events similar
to that in the GLAM model. Pink lines represent the distribution of the crustal formation
episodes; blue line – crystallisation ages; green line – crustal model ages; dotted black
line — simulated growth calculated by incrementally adding all the crust formation
episodes; red band — calculated growth curve obtained by the algorithm proposed in
this study (Figs. 5 and 9).
This is also in agreement with the recent study by Hawkesworth et al.
(2010), which concluded that the present volume of continental crust
was established 2–3 Ga ago. From this perspective, the high
proportion of new crust (e.g., over 50% of juvenile component;
Fig. 7a) being produced in magmatic events in post-Archean time,
suggested by the zircon record, probably has been balanced by the
relatively rapid recycling of new crust into the mantle.
The crustal growth curve derived by incremental/integral addition
of juvenile additions defined through time (Fig. 9) implies that the
continental crust was generated quasi-continuously. However, there
are two distinct drops in the juvenile contribution to magmatic
episodes (Fig. 7a): from about 70% to about 50% after ca 2.2, and much
less than 50% after ca 0.6 Ga. The reason for the lower juvenile
component of each successive step is unclear, but possibly is a
reflection of a cooling Earth, and thus changes in processes of plate
tectonics through time. The first distinct drop at 2.2–2.1 Ga coincides
with the transition from breakup of one of the earliest proposed
supercontinents Kenorland (rifting began at ca 2.4 Ga; Williams et al.,
1991; Bleeker, 2003) to the building of the next supercontinent,
Columbia/Nuna (which existed from ~ 1.8–1.5 Ga; Zhao et al., 2002,
2004). The second distinctive drop at ca 0.6 Ga coincides with the
transition from dispersal of the Rodinia supercontinent (which existed
from ~ 1.1–0.75 Ga; Torsvik, 2003) to the building of Gondwana and
ultimately the next supercontinent, Pangea (~300–180 Ma; Cawood
and Buchan, 2007).
The more detailed pattern of a rising and falling juvenile component
on timescales of the order of 100–200 Ma more likely records changing
geodynamic conditions, from extension-dominant to compressiondominant, respectively (e.g., Kemp et al., 2009b). Each of these drops in
the proportion of juvenile melts may reflect a change in tectonic
behaviour accompanying the onset of the accretionary and collisional
conditions that accompany supercontinent assembly. These are likely to
be periods of increased consumption of juvenile crust via delamination
and/or uplift and erosion. The relatively compressional tectonic
conditions will have inhibited the access of juvenile mantle melts to
the upper crust. This will have a major influence on the expression of
magmatism in the upper crust.
It has been recognized that juvenile crust addition (as well as
magmatic activity in general) is globally minor at 2.4–2.2 Ga (Condie
et al., 2009). However, the juvenile proportion estimated in this study
for this period is markedly high. First of all it should be pointed out
that this particular time interval shows a significant drop in the
number of zircons in our database (only 442 analyses, in contrast to
1164 analyses available for the next time slice 2.4–2.6 Ga). This gap is
clearly reflected by a trough on the Relative Probability curve for the
U–Pb zircon ages (Fig. 1) and this minimum is also well defined by
previous studies (eg Condie et al., 2009 and references there in). On
the εHf and TCDM vs age plots, the data available for the 2.4–2.2 Ga time
interval concentrate around the median curve, with an obvious lack of
zircons with both highly-radiogenic and non-radiogenic Hf-isotope
signatures. The markedly high juvenile proportion estimated for this
time interval (Fig. 7a) could be due to the model age uncertainty of
about 100 Ma, so the number of zircons with model ages of 2.4–2.2 Ga
may be over-estimated. Thus, the juvenile proportion for this time
interval might be over-estimated. However, the 2.4–2.2 Ga time
interval coincides with the beginning of the breakup of Kenorland
(rifting began at ca 2.4 Ga; eg Bleeker, 2003). The markedly high
juvenile proportion estimated for this time interval corresponds to
magma-plume rifting processes suggested for that time. The scarcity
of geological record (gap in the samples available) may be due to the
lack of zircon in mafic rocks and/or reflect the preferential recycling/
reworking of this juvenile crust, while younger rocks carry the record
of this 2.4–2.2 Ga juvenile event in their model ages.
The conclusions of this study can be summarised as follows:
1. Previous model approaches, that are based on Hf model ages only,
do not take into consideration the influence of mixing of juvenile
and crustal sources. As a consequence they underestimate the
mean age of the continental crust.
2. Two different approaches to modelling the distribution of age-Hf
isotope data from zircons worldwide suggest that at least 60%, and
probably ≥70%, of the existing continental crust originally formed
before 2.5 Ga.
3. In any given magmatic episode, the juvenile proportion of newly
formed crust fluctuates between 30% and 80% and decreases
stepwise through time:
– about 70% in the 4.0–2.2 Ga time interval
– about 50% in the 1.8–0.6 Ga time interval, and
– possibly less than 50% after 0.6 Ga.
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E.A. Belousova et al. / Lithos 119 (2010) 457–466
The changes at 2.2 and 0.6 Ga may be related to onset of accretionary
and collisional events that culminate in the formation of supercontinents.
4. Crustal reworking has dominated over net juvenile additions to the
continental crust, especially since the end of the Archean. This
explains the increasing number of zircons with non-radiogenic Hfisotope compositions (with εHf lower than−20) throughout postArchean time.
5. The present-day average Hf model age of the continental crust is
about 2.8 Ga.
6. Statistical modelling based on the random mixing of older and
younger crustal volumes, and assuming that the Archean crust is
more abundant than the younger crust (i.e. using the GLAM proportions), produces results consistent with the calculated growth
curves and thus supports the proposed approach for the estimation
of the crustal growth rate.
Supplementary materials related to this article can be found online
at doi: 10.1016/j.lithos.2010.07.024.
Acknowledgements
We are grateful to Tom Andersen for the constructive discussions
and two anonymous reviewers for their useful comments that helped to
improve this manuscript. Funding for this work came from collaborative
projects with industry (including WMC, BHP Billiton, DeBeers, Newmont, Gold Fields), Macquarie University External Collaborative
Research Grants and ARC Discovery and Linkage grants. The analytical
data were obtained using instrumentation funded by ARC LIEF, and
DEST Systemic Infrastructure Grants, industry partners and Macquarie
University. The research was supported by ARC and Macquarie
University grants to S.Y. O'Reilly, W.L. Griffin and E. A. Belousova, and
collaborative research with industry partners, especially Western
Mining Resources and BHP-Billiton. Analytical data were obtained at
GEMOC using instrumentation funded by ARC LIEF, and DEST Systemic
Infrastructure Grants and Macquarie University. This is contribution
no. 671 from the ARC National Key Centre for the Geochemical Evolution
and Metallogeny of Continents (www.es.mq.edu.au/GEMOC/).
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