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PAGEOPH, Vol. 128, Nos. 3/4 (1988)
0033~553/88/040683M251.50 + 0.20/0
9 1988 Birkh/iuser Verlag, Basel
Recycling of the Continental Crust
SCOTT M . M C L E N N A N 1
Abstract--In order to understand the evolution of the crust-mantle system, it is important to
recognize the role played by the recycling of continental crust. Crustal recycling can be considered as two
fundamentally distinct processes: 1) intracrustal recycling and 2) crust-mantle recycling. Intracrustal
recycling is the turnover of crustal material by processes taking place wholly within the crust and
includes most sedimentary recycling, isotopic resetting (metamorphism), intracrustal melting and assimilation. Crust-mantle recycling is the transfer of crustal material to the mantle with possible subsequent
return to the crust. Intracrustal recycling is important in interpreting secular changes in sediment
composition through time. It also explains differences found in crustal area--age patterns measured by
different isotopic systems and may also play a role in modeling crustal growth curves based on Nd-model
ages. Crustal-mantle recycling, for the most part, is a subduction process and may be considered on three
levels. The first is recycling with only short periods of time in the mantle ( < 10 m.y.). This may be
important in explaining the origin of island-arc and related igneous rocks; there is growing agreement
that 1-3% recycled sediment is involved in their origin. Components of recycled crustal material, with
long-term storage (up to 2.5 b.y.) in the mantle as distinct entities, has been suggested for the origin of
ocean island and ultrapotassic volcanics but there is considerably less agreement on this interpretation.
A third proposal calls for the return of crustal material to the mantle with efficient remixing in order to
swamp the geochemical and isotopic signature of the recycled component by the mantle. This type of
recycling is required for steady-state models of crustal evolution where the mass of the continents
remains constant over geological time. It is unlikely if crust-mantle recycling has exceeded 0.75 km3/yr
over the past 1-2 Ga.
Good evidence exists that selective recycling is an important process. Sedimentary rocks preserved in
different tectonic settings are apparently recycled at different rates, resulting in a bias in the sediment
types preserved in the geologic record. Selective recycling has important implications for the interpretation of Nd model ages of old sedimentary rocks and in the analysis of accreted terranes. Although there
is evidence that continental crust was formed prior to 3.8 Ga, the oldest preserved rocks do not exceed
this age. It is likely that the intense meteorite bombardment, which affected the earth during the period
4.56-3.8 Ga, coupled with rapid mantle convection, which resulted from greater heat production, caused
the destruction and probable recycling into the mantle of any early formed crust.
Although crust-mantle recycling is seen as a viable process, it is concluded that crustal growth has
exceeded crust-mantle recycling since at least 3.8 Ga. Intracrustal recycling has not been given adequate
consideration in models of crustal growth based on isotopic data (particularly Nd model ages). It is
concluded that crustal growth curves based on Nd model ages, while vastly superior to those based on
K/Ar or Rb/Sr, tend to underestimate the volume of old crust, due to crust-mantle and/or intracrustal
recycling.
Research School of Earth Sciences, The Australian National University, Canberra, A.C.T., 2601,
Australia. Present address: Department of Earth and Space Sciences, State University of New York at
Stony Brook, Stony Brook NY 11794-2100, U.S.A.
684
Scott M. McLennan
PAGEOPH,
Key words: Continentalcrust, crustal evolution,isotopes, mantle, recycling,sediment,subduction.
1. Introduction
Recycling of continental material has played a central role in many of the
important debates in earth sciences. For example, during the great discussion over
the age of the earth during the second half of the nineteenth century, JOLY (1899)
estimated the age by calculating the time required to accumulate the net sodium
content of the oceans using estimates of the annual sodium flux from the continents
(a variation of this approach was first proposed by Edmund HALLEY, 1717).
FISHER (1900) pointed out the flaw in this argument by recognizing that much
sodium is recycled back onto the continents as sedimentary material, thus considerable extending the 80-90 million year age calculated by Joly. The calculation of
Joly, in fact, represents an estimate of the residence time of Na in sea water, which
now is known to approach 200 million years (BROECKER and PENG, 1982).
During the past 15-20 years, recycling of continental crust has become a major
issue in the debate over crustal and mantle evolution. Two landmark publications
are notable (ARMSTRONG, 1968; GARRELS and MACKENZIE, 1971). The suggestion, based on Pb isotopic data, that the continents have remained at near their
present mass over the past ca. 2.5 Ga (e.g., PATTERSON and TATSUMOTO, 1964)
was seen to be in conflict with Sr isotopic data that suggested continental growth
throughout earth history (e.g., HURLEY et al., 1962). ARMSTRONG (1968) attempted to reconcile these views by suggesting a steady-state model of crustal
evolution where considerable amounts of crustal material was recycled and remixed
into the mantle. With regard to a very different problem, GARRELS and MACKENZIE (1971) proposed a recycling model to explain the mass-age distribution of
sedimentary rocks. They argued that over Phanerozoic time, at least, the sedimentary mass remained constant and that the exponential mass-age relationship found
for sediments was due to recycling, rather than continual accumulation.
There are two very distinct types of continental recycling that are reflected in the
publications of ARMSTRONG (1968) and GARRELS and MACKENZIE (1971). The
first may be termed crust-mantle recycling. In this process, continental crust is
returned to the mantle and either stored as a separate entity or remixed with the
mantle. At a later time, this material may be incorporated into magmas erupted at
the earth's surface and ultimately reaccreted to the continental crust. Such a process
must be considered in questions dealing with island-arc and ocean island petrogenesis and with growth models of the continental crust. The second type of recycling
may be termed intracrustal recycling. This is concerned with crustal material which
loses its primary identity by processes taking place wholly within the crust.
Examples include sediment-sediment recycling, isotopic resetting (related to metamorphism), intracrustal melting and assimilation. This process is important in
Vol. 128, 1988
Recycling of the Continental Crust
685
questions of secular variations of sediment composition, area-age relationships of
sediments and basement provinces, as well as the origin and preservation of crustal
rock types.
In this paper, the role of continental recycling in crustal and mantle evolution
will be addressed. A clear distinction will be made between crust-mantle and
intracrustal recycling and some of the pitfalls associated with confusing these
processes will be considered.
1.1. Models of Crustal Evolution
It is not within the scope of this paper to review all the arguments associated
with the various models of crustal evolution. Such a discussion may be found in
TAYLOR and MCLENNAN (1985). It is important, however, to describe briefly the
importance which recycling plays in such models.
Models of crustal evolution may be divided into two distinct catagories: growth
models and steady-state models (Figure 1). The first takes what is considered a
conventional view of isotopic data, where crustal rocks with mantle-like initial
isotopic ratios (Sr, Pb, Nd, Hf) are taken to represent new additions to the crust
from the mantle. In such models, continental crust often is considered too buoyant
to ever be returned to the mantle in appreciable quantities (e.g., MOORBATH, 1978).
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Examples of various crustal growth models. They can be divided into two varieties. The first with all of
the growth early in earth history and later steady state (ARMSTRONG, 1968, 1981), and the second with
continuous (HURLEY and RAND, 1969) or episodic (McLENNAN and TAYLOR, 1982; TAYLOR and
McLENNAN, 1985; NELSON and DEPAOLO, 1985) growth throughout earth history. Steady state models
explicitly require significant crust-mantle recycling.
686
Scott M. McLennan
PAGEOPH,
Crustal growth may be either continual or episodic in such models although most
workers currently seem to favour episodic crustal growth with most of the continents ( > 50%) being in place by about 2.5 Ga (e.g., MOORBATH, 1978; VEIZER and
JANSEN, 1979; DEWEY and WINDLEY, 1981; TAYLOR and MCLENNAN, 1981; 1985;
MCLENNAN and TAYLOR, 1982; NELSON and DEPAOLO, 1985). In these models,
the well-known exponential area-age relationship (based mainly on K-Ar data with
some Rb-Sr data) for crustal provinces (HURLEY and RAND, 1969) may be
attributed to intracrustal recycling processes (VEIZER and JANSEN, 1979).
The steady-state models take a fundamentally different view. In these models,
crustal material is returned to the mantle in substantial quantities and remixed in an
efficient manner. Accordingly, their unique isotopic signature is swamped by the
mantle and is not recognizable when returned to the crust. To accommodate the
exponential basement age distribution (HURLEY and RAND, 1969), and the scarcity
of rocks >2.9 Ga old, such models rely on an exponentially declining rate of
crust-mantle recycling during earth history, in parallel with the declining heat
generation of radioactive elements (ARMSTRONG, 1981). It is axiomatic that such a
model predicts the isotopic data and consequently, the isotopes cannot be interpreted uniquely (ARMSTRONG, 1968; 1981; PATCHETT and CHAUVEL, 1984).
Accordingly, independent evidence for the occurrence and magnitude of crustmantle recycling must be sought.
A number of variations on these models has also been proposed. FYFE (1978)
has supported the Armstrong models but added the further suggestion that over the
more recent past the mass of the continents has actually diminished. REYMER and
SCHUBERT (1984) have argued that a substantial portion of the crust (ca. 50%) was
formed very early in earth history with the rest forming at a uniform rate over the
past four billion years.
It is commonly the case in scientific debate that divergent views become
polarized and middle ground is ignored. Thus it is important to emphasize that
crustal growth models are perfectly plausible even if recycling takes place on a large
scale. The important criteria is that generation of new crust exceeds removal of
crust to the mantle. The converse, however, is not the case and steady-state models
are untenable without substantial crust-mantle recycling.
2. Recycling Within the Continental Crust
Recycling of continental crust wholly within the continents (intracrustal recycling) occurs on many scales. For example, simple bioturbation can be considered
as intracrustal recycling on a very fine scale whereas the generation of batholiths
from intracrustal melting is an example of intracrustal recycling on a very large
scale. It is well beyond the scope of this paper to address all of these recycling
Vol. 128, 1988
Recycling of the Continental Crust
687
processes and only those which are particularly relevant to questions of crustmantle evolution will be addressed.
A first order pattern in the preservation of may geological features is the
exponential relationship between cumulative distribution and age (VEIZER and
JANSEN, 1979). Examples of this include the distribution of sedimentary rocks,
many ore deposits, oceanic crust and ages of crystalline basement of continents
measured by either K-Ar or Rb-Sr age determinations (Figure 2). Such features are
unlikely to be a consequence of simple accumulation. For example, the discrepancy
between the area-age relationship of crystalline basement measured by different
isotopic systems (Figure 2a) cannot be a primary feature and is related to secondary
processes resulting in the resetting of isotopic systems. The exponential area-age
relationship of oceanic crust (Figure 2b) is readily interpreted through recycling of
oceanic crust into the mantle at subduction zones.
2.1. Sedimentary Recycling
Sedimentary recycling can be viewed on two scales. The first is the direct
turnover of sedimentary material (sediment-sediment recycling). Direct evidence of
this process is seen in the petrographic examination of sedimentary rocks, where
sedimentary rock fragments are a regular occurrence (PETTIJOHN et al., 1973). It is
commonly held that, for Phanerozoic sedimentary rocks at least, the generation of
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Cumulative per cent area (or mass) v e r s u s age for a number of geological features. An exponential
relationship is commonly seen. In the cases shown, it is unlikely that simple accumulation is responsible
and some form of recycling is taking place (VEIZER and JANSEN, 1979, 1985). The best example is the
ocean crust (Figure 2b), where recycling into the mantle by plate tectonic processes is well established.
Data from compilations of VE]ZER and JANSEN (1979, 1985).
688
Scott M. McLennan
PAGEOPH,
extensive quartz-rich sand bodies can only be achieved through substantial sedimentary recycling (PETTI~OHN et al., 1973; however, see JOHNSSON et al., 1988). Such
petrographic measures of sediment recycling are useful, for example, in interpreting
the plate tectonic setting of sandstones (e.g., DICKINSON and SUCZEK, 1979).
It is this direct turnover of sedimentary debris that is considered in recycling
models that attempt to explain the mass-age distribution of preserved sedimentary
rocks (Figure 2c) (GARRELSand MACKENZIE,1971; VEIZER and JANSEN, 1979;
1985). The tendency for decreasing preservation with increasing age generally is not
considered to be a primary feature. One reason for this is that the pre-human
sediment flux to the ocean ( ~ 12.3 • 10 j5 g/yr; TAYLOR and MCLENNAN, 1985) is
sufficient to produce about 20 times the present sediment mass over earth history.
Thus, either some reworking of sediment must occur or the sedimentation rate
would have to be drastically lower earlier in earth history. The most likely
interpretations are that: 1) the sediment mass has remained essentially constant, at
least over Phanerozoic time where reasonable records are available, and continually
recycled or 2) there is a combination of accumulation and recycling. Arguments and
mathematical formulation of these models can be found in GARRELSand MACKENZIE (1971), LI (1972), VEIZER and JANSEN (1979, 1985) and DACEY and LERMAN
(1983).
The second variety of sedimentary recycling encompasses a much broader
definition. This is concerned with the recycling of continental crustal rocks significantly older than the stratigraphic age of the sediment, through sedimentary
processes. Thus, it includes the recycling of old metamorphic rocks, old basement,
as well as old sedimentary rocks. This view of sedimentary recycling has become
important with the recent study of Nd-isotopes in sedimentary rocks, pioneered by
MCCULLOCH and WASSERBURG (1978). The Nd-model age of a sediment dates the
average age of extraction from the mantle of the various provenance components
and thus gives the average provenance age of the sediment. As we will see below,
this age frequently exceeds the stratigraphic age of the sediment due to the
incorporation (recycling) of older material.
Nd-isotopes in sedimentary rocks. The Nd model age of terrigenous sediments is
a useful measure of the mean provenance age. A first-order observation is that for
most young sedimentary rocks, the model age generally exceeds the stratigraphic
age, whereas for sedimentary rocks older than about 2.5 Ga, the model age
approximates the stratigraphic age (Figure 3a). Two interpretations of these data
can be made. The first (ALLI~GRE and ROUSSEAU, 1984; O'NIONS et al., 1983)
assumes that the sediment provides a representative sample of the penecontemporaneous exposed continental crust and the Nd isotopic systematics record the growth
and recycling history of the continental crust. VEIZER and JANSEN (1979, 1985)
have pointed out, however, that intracrustal recycling due to metamorphism and
intracrustal melting is considerably less efficient than sediment--sediment recycling
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Plol of Nd model age (T~a4)r~ v e r s u s stratigraphic age (TsTgAT) for sedimentary rocks. The model ages use the depleted mantle
parameters of TAYLOR and MCLENNAN (1985). Each point represents one or more samples from a given formation, lithology, etc.
(from data compilation in TAYLOR and MCLENNAN, 1985; plus HENSEL et al., 1985; CLARKE and HALUDAY, 1985). 3(a) Nd model
ages are similar to stratigraphic ages for samples > 2.0-2.5 Ga but become increasingly greater than stratigraphic age in post-Arehean
sedimentary rocks. The favoured explanation is that during the Archean, the sedimentary mass was growing and preserved sedimentary
rocks a,e mainly first-cycle whereas during the post-Archean, the mass of sediments was near constant and preserved sedimentary rocks
were strongly influenced by sedimentary recycling processes. 3(b) Model showing the effects of recycling a sedimentary mass that has
remained constant for 4.5 Ga (recycling parameters derived from VEIZER and JANSEN, 1985). Evolution calculated for a system which
is closed to differing degrees; thus 90% closed represents a system which is recycled but in each cycling period (here taken at 250 Ma),
10% of the sediment is lost from the system (to the mantle, lower crust, etc.) and replaced by new mantle-derived material. In this
model, "0'-age sediments are best explained by a system which is > 80% closed. For a system in which sediment-sediment recycling
dominates, this represents an upper limit on crust-mantle recycling (VE~zER and JANSEN, 1985). 3(C) Model similar to that shown in
Figure 3b, but where the sediment mass grows from 4.5 2.5 Ga, with no recycling. From 2.5 Ga, recycling modeled as in Figure 3b. This
model fits the sediment Nd isotopic data best (VEIZER and JANSEN, 1985) and 'O'-age sediments require a system which is >85%
closed. Models which incorporate shorter time intervals for mixing and recycling, allow growth in the sediment mass during the
post-Archean or preferentially recycle young sediment, will increase the degree to which the system is closed.
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Scott M. McLennan
PAGEOPH,
and accordingly, the Nd-isotopic systematics of sedimentary rocks primarily record
the growth and sediment--sediment recycling history of the sedimentary mass
(while still recording any basement recycling which does occur) (also see
GOLDSTEIN et al., 1984). All workers agree that, in principle, the Nd model age
records the mean age of mantle derivation of the provenance rocks.
In Figure 3, the Nd-isotopic data are also modeled in terms of the efficiency of
sediment--sediment recycling (VEIZER and JANSEN, 1985). Two models are presented; one with the entire sedimentary mass present at 4.5 Ga and the other with
sediment accumulation (with no recycling) to 2.5 Ga, with the entire sedimentary
mass present by 2.5 Ga. As pointed out by VEIZER and JANSEN (1985), the latter
model fits the sedimentary data best. In either case, the data cannot be modeled by
a totally closed system but require the addition of young crustal material. The
degree to which this new young mantle-derived material may be incorporated (i.e.,
openness) is not highly sensitive to the exact model (see Figures 3b,c) and suggests
that the sedimentary system is about 10-20% open (i.e., 80-90% closed) to new
additions of young crust during the post-Archean (VEIZERand JANSEN, 1985).
In detail, the degree of openness is highly variable in sedimentary rocks.
MCLENNAN et al. (1985) have pointed out that there is some control by plate
tectonic setting on Nd isotopic composition of sediments, with high end values (low
Nd model ages) coming from active tectonic zones (island arcs, continental arcs, back
arcs). Similarly, HENSEL et al. (1985) found that Devonian-Carboniferous sediments
deposited in association with a volcanic chain had Nd model ages only slightly in
excess of stratigraphic age (TDM
Nd = 0.75 Ga; TSTRAT ~ 0.35 Ga), indicating a substantial mantle-derived component. MICHARD et al. (1985) also noted that during
the Phanerozoic, European sediments deposited during orogenic periods tended to
have higher end values which generally correlate with lower Nd model ages.
Much of the Nd isotopic data for young sediments, collected to date, have been
from rivers draining trailing edge margins or from ocean basins adjacent to trailing
edge margins (e.g., GOLDSTEIN et al., 1984). Thus it is possible that the present
'0'-age sediment data base is biased towards older Nd model ages. It is also notable
that in the geologic record, it is likely that there is a bias towards the preservation
of cratonic sedimentary rocks (see below). This is because active margin sediments
are recycled much faster than average sediment (VEIZERand JANSEN, 1985) and
become less abundant in the sedimentary record (see Section 4.1).
Sedimentary recycling and sediment composition. There is a long history of
examining secular variations in sediment compositions. Long-standing arguments
exist over the relative importance of compositional evolution of the upper crust
(e.g., RONOV and MIGDISOV, 1971) versus chemical changes resulting from recycling (GARRELS and MACKENZIE, 1971). Recently, most workers agree that both
processes may affect sediment composition (VEIZER, 1979; MCLENNAN and TAYLOR, 1980; TAYLOR and MCLENNAN, 1985).
Vol. 128, 1988
Recycling of the Continental Crust
691
The Th/U ratio of sedimentary rocks is particularly susceptible to the effects of
sedimentary recycling. During weathering, uranium may be oxidized from the
insoluble U +4 to the soluble U +0. The U may be lost (to ocean crust, ore deposits,
etc.) causing an increase in Th/U ratios. Much of the available high quality data on
Th/U ratios in shales is summarized in Figure 4 (McLEYNAN and TAYLOR, 1980;
TAYLOR and MCLENNAN, 1985). There is considerable spread in the data, but they
indicate an increase in Th/U ratio over post-Archean time, related to a gradual
decrease in U. This is consistent with successive periods of weathering and recycling.
The sharp break in U abundances (also seen for Th) has been related to a change
in upper crustal composition approximately at the Archean-Proterozoic boundary
(MCLENNAN and TAYLOR, 1980) (see below).
The Th/U ratio appears to remain near the planetary value of 3.8 during the
Archean. Two interpretations are possible. The first is that the atmosphere was
deficient in free oxygen prior to about 2.0-2.3 Ga (e.g., CLOUD, 1976), inhibiting
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Plot of Th/U and U against age for fine-grained terrigenous sedimentary rocks. Data from compilation
of TAYLOR and McLENNAN (1985). Uncertainties are great but the data are consistent with an increase
in Th/U, due to a decrease in U, over post-Archean time. This would be consistent with the effects of
successive weathering and recycling of the sedimentary mass. Archean U values, for sediments from
low-grade terrains, are much lower, consistent with a differing composition for the upper crust
(McLENNAN and TAYLOR, 1980). For Archean samples, Th/U ratios are near the planetary value of 3.8
(although, locally in high-grade terrains, they may rise to above 4). This could be due to a lack of
recycling during this period and/or to the paucity of free oxygen in the Archean atmosphere resulting in
little or no oxidation of U during weathering.
692
Scott M. McLennan
PAGEOPH,
the oxidation of uranium during weathering. Alternatively (or coincidentally),
sediment-sediment recycling was considerably less important in controlling overall
sediment composition during the Archean; with first cycle sediments dominating,
resulting in less opportunity for fractionation of Th from U. This latter explanation
is consistent with the Nd isotopic data which suggest that many preserved Archean
sediments did not have a long recycling history (either as sediment or pre-existing
crust). It is also consistent with the observation that in some Archean high-grade
terrains, Th/U rise to fairly high values ( > 6) which, if not a metamorphic effect,
would indicate considerable recycling at least on a local scale (TAYLOR et al., 1986).
Insoluble trace elements (REE, Th, Sc) in fine-gained sedimentary rocks may be
used as an index to monitor changes in upper continental crustal composition (see
recent discussion in TAYLOR and MCLENNAN, 1985). A distinct break in sedimentary trace element characteristics, notable REE abundances and Th/Sc ratios, has
been observed at the Arcbean-Proterozoic boundary (Figure 5) which is consistent
with a change to more felsic upper crustal composition (MCLENNAN et al., 1980;
TAYLOR and MCLENNAN, 1985). The data show no secular trend during postArchean time and this has been taken as evidence that there has been no change in
upper crustal composition during that time (TAYLOR and MCLENNAN, 1985).
VEIZER and JANSEN (1979) pointed out that intracrustal sedimentary recycling
would inhibit or buffer the detection of changes in upper crustal composition in
sedimentary rocks. This effect can be seen in Figure 5 where the expected evolution
of sedimentary Th/Sc ratios are modeled for a given evolution in upper crustal
composition (c~mparable to the magnitude of change seen across the ArcheanProterozoic boundary) and differing degrees of openness during the sedimentary
recycling. Even when recycling is taken into account, for reasonable degrees of
openness of about 10-20% (see Figure 3), the available sedimentary data would
appear to argue against changes of the magnitude shown in Figure 5.
Two important points can be made from the above discussion. First, sedimentary recycling may greatly buffer changes in sediment composition resulting from a
change in upper crustal composition. Consequently, only elements or elemental
ratios in sedimentary rocks which are extremely sensitive to bulk compositional
differences (Th/Sc is the best example) provide reasonable indexes to changes in
upper crustal composition. This latter point is important in relation to recent
suggestions by GOLDSTEIN et al. (1984) and MILLER and O'NIONS (1985) that
Archean and post-Archean upper crusts were of similar composition because
average Sm/Nd ratios are indistinguishable in Archean and post-Archean sedimentary rocks. Sm/Nd ratios are not highly sensitive to changes in bulk composition
and models which suggest changes in upper crustal composition (TAYLOR and
MCLENNAN, 1985) predict only < 20% differences in Sm/Nd ratios between
Archean and post-Archean upper crusts. This would be difficult to detect in
sedimentary rocks without a large data base and recognizing such a change would
be further hampered by any effects of sedimentary recycling.
Vol. 128, 1988
Recycling of the Continental Crust
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Plot of Th/Sc against age for fine-grained terrigenous sedimentary rocks. Data from compilation of
TAYLORand MCLENNAN(1985). Since Th is strongly concentrated in acidic, and Sc in basic rocks, this
ratio is particularly sensitive to the composition of the upper continental crust. The sharp increase in
Th/Sc ratio (factor 2.5) at the Archean-Proterozoic boundary is taken as evidence for a change in upper
crustal composition. Th/Sc values remain constant throughout the post-Archean. A model is shown to
illustrate how recycling, with the system being closed to differing degrees, may buffer the sedimentary
response to changes in upper crustal composition. The solid line shows a model of upper crustal
evolution from Th/Sc = 1.0 to 3.0 over 2.5 Ga. The dashed lines indicate the predicted response in
sedimentary rocks for a recycling system which is closed to varying degrees. In spite of this buffering,
changes in upper crustal composition, of magnitude similar to the change at the Archean-Proterozoic
boundary (factor of 3 for Th/Sc) appear to be excluded by the available sedimentary data. Preferential
recycling of young sediment would increase the expected response in the sedimentary system. Concentration data and modeling parameters of TAYLOR and MCLENNAN (1985) (see their Figure 5.10) are
updated to be consistent with recycling constraints of VEIZER and JANSEN (1985).
2.2. Metamorphism and Isotope Resetting
The possible resetting o f most isotop!c systems d u r i n g m e t a m o r p h i s m is well
established, a l t h o u g h the c o n d i t i o n s required vary with the i n d i v i d u a l isotopes. It
is generally recognized that K - A r systems m a y be reset at fairly low t e m p e r a t u r e s
(ca. 200~
with R b - S r a n d U / T h - P b whole rock systems reset at m u c h higher
temperatures a n d generally o n a smaller scale. Recent work (DEPAOLO et al., 1982;
694
Scott M. McLennan
PAGEOPH,
MCCULLOCH and BLACK, 1984; BLACKand MCCULLOCH, 1987) indicates that the
Sm-Nd system can also be reset, at least on a local scale, at high temperatures
(>__700~ where deformation has occurred. This observation is particularly relevant for Nd model age studies which use individual hand samples of commonly
heterogeneous rock units.
VEIZER and JANSEN (1985) have quantified these effects on a large scale by
examining the area-age distribution of basement provinces using the K-Ar and
Rb-Sr (plus some U-Pb) data (Figure 2a). The differences in the rates of recycling
required to explain the discrepancies in the curves depend on the exact model of
crustal growth which is adopted. For exposed rocks and for growth models similar
to those of TAYLOR and MCLENNAN (1985) or VEIZER and JANSEN (1985) (see
Figure 1), K-Ar has been reset about 7 times more efficiently and Rb-Sr has been
reset about 3 times more efficiently than Sm-Nd systems in exposed rocks. The
discrepancy between the area-age relationships for the various isotopic systems and
the actual growth curve for the continental crust is due to recycling. For the Sm-Nd
system, any difference may be due either to crust-mantle recycling or to intracrustal
recycling (intracrustal melting, isotopic resetting, assimilation). The discrepancies
between the Sm-Nd distribution and the Rb-Sr and K-Ar distributions are almost
certainly due exclusively to intracrustal recycling.
2.3. Intracrustal Melting
The role of intracrustal melting in the development of the continental crust is
well documented. For example, the Andesite model for crustal growth during the
Phanerozoic (TAYLOR, 1967, 1977) relies on intracrustal melting as the primary
mechanism for the development of a granodioritic upper continental crust from
accreted island-arcs and continental-arcs.
The most exhaustive geochemical studies of granitic rocks have been conducted
in south-east Australia (WHITE and CHAPPELL, 1983; MCCULLOCH and CHAPPEEL, 1982; CHAPPELL, 1984). Most granites here can be divided into I-type
(igneous precursor) and S-type (sedimentary precursor), both of which are derived
from melting within the crust. I-type granites represent a first stage recycling
episode with the precursors probably having a fairly direct mantle origin, with only
a short crustal residence prior to final intrusion. S-type granites are examples of
second or third stage recycling with the precursor sediments having gone through at
least one weathering event and possibly being derived from continental crust with
a recycling history.
The available isotopic data further support an intracrustal origin for many
granitic rocks, at least during the Phanerozoic. The Nd data are particularly
significant, and are summarized in Figure 6 (ALLt~GREand BEN OTHMAN, 1980;
HAMILTONet al., 1980; MCCULLOCHand CHAPPELL,1982; DEPAOLO and FARMER,
1984). Granitic rocks older than about 2.0Ga have Nd-isotopic compositions
/~
++++++++++L+§
~Nd -I0 9++~ ++- t++++++++++++++++++~+~+++++++++++++++*+++++++++++++++++++++§+++ /
-15
-20
-2~
++++++++++++++++++++~.~
+
++
++
++
++
++
++
++
++
++
++
++
+ ~+ ++ ++ ++ ++ ++ +~ ~ . O ~
+++++++++++++++++5//
~+++++++++++++++4
++++++++++++++~
+++++++++§
++++++++++~++
+++++++++++~
~
r
~++++++++++u
+++++++++++
+ + + + + §+ + + +
++++++++'
+++++++++++++~
+++++
++++
+++~
+++
;Y
695
Recycling of the Continental Crust
Vol. 128, 1988
l
I
I,O
f.~
fr~ v
/~_~ ~
~
i,,
I
I
2.0
I
I
B.O
I
I
4.0
Age (Ga)
Figure 6
Plot of end against crystallization age for granitic rocks. Data defining the shaded area are listed in
ALLEGREand BENOTHMAN(1980) and HAMILTONet aL (1980); McCULLOCHand CHAPPELL(1982);
DEPAOLOand FARMER(1984). Also shown are the evolution paths for typical mantle and typical crust
of varying ages. Most younger granitic rocks have low end values inconsistent with a direct origin from
the mantle. The data indicate an increasing importance of intracrustal melting after a significant crustal
residence time, for the origin of granitic rocks during the post-Archean.
consistent with derivation from mantle-like sources with or without a short period
( < 1 0 0 - 2 0 0 m . y . ) of crustal residence. Younger granites appear to be derived
through remelting of older crustal sources with highly variable contributions from
young mantle sources. TAYLOR and MCLENNAN (1981, 1985) have pointed out
that Sm/Nd ratios usually decrease during intracrustal melting processes. This
results in Nd model ages being minimum estimates of the mantle extraction ages of
the crustal precursors (also see MCCULLOCH, 1988).
Rare earth data for sedimentary rocks also provide important insight into the
role of intracrustal melting through earth history. Sedimentary rocks of postArchean age are typified by negative europium anomalies in the REE patterns
(Figure 7) with Eu/Eu* -- 0.65 on average. On the other hand, Archean sedimentary
rocks rarely display anomalous Eu behavior (TAYLOR and MCLENNAN, 1985).
Only locally, in sone high grade terrains are negative Eu anomalies found in
Archean sedimentary rocks indicative of small cratonic environments (TAYLOR et
al., 1986).
696
Scott M. McLennan
I
I
I
I
I
I
I
PAGEOPH
I
I
,
I
I
I
I
I
I
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I
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u')
<I)
%
tM o.9
"~ 0.8
I00 0c-
(D
"-..
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ArcheQn
~
0_
rm
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rm
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IC-
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Lo Ce Pr Nd
I
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Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Figure 7
Chondrite-normalized REE diagram of average compositions of Archean and post-Archean sedimentary
rocks. Inset shows a plot of Eu/Eu* against time for fine-grained terrigenous sedimentary rocks. Data
from compilation in TAYLOR and MCLENNAN (1985). The presence of the Eu-anomaly in post-Archean
sedimentary rocks is considered to be representative of the upper continental crust. Negative Euanomalies are most likely the product of intracrustal melting with residual Eu-enriched plagioclase being
held in the lower crust. The sedimentary REE data indicate that intracrustal melting processes were of
limited importance during the Archean but have been dominant in differentiating the continental crust
during the post-Archean.
In general, unfractionated or primary igneous rocks which are derived from the
mantle do not have Eu-anomalies and this is taken as evidence that the continental
crust as a whole is not anomalous with respect to Eu (TAYLOR and MCLENNAN,
1985). The most reasonable explanation for the presence of negative Eu-anomalies
in post-Archean sedimentary rocks and hence the upper continental crust is
widespread intracrustal melting to produce K-rich granitic rocks which typically
yield negative Eu-anomalies. The Eu-anomalies in granitic rocks are the result of
the fractionation of plagioclase, a mineral stable only to about 40 km depth, thus
confirming an intracrustal origin. Both fractional crystallization and intracrustal
melting may produce this effect but melting is judged as the dominant process
within the crust. The fact that Eu-anomalies are rare in Archean sedimentary rocks
is taken as evidence that intracrustal melting processes were of only local importance in the formation of the early Archean crust. This contrasts with the postArchean crust where the sedimentary REE data indicate that intracrustal melting
has been a dominant process.
Vol. 128, 1988
Recyclingof the ContinentalCrust
697
2.4. Assimilation
The final process of intracrustal recycling under consideration is that of assimilation. This is where mantle derived magma interacts with the continental crust and
is contaminated with it to a lesser or greater degree. DEPAOLO (1981) has provided
the appropriate equations of mixing, which include the complicating factor of
combined assimilation and fractional crystallization.
There are many examples where crustal assimilation has been demonstrated or
suggested. Examples include:
1) assimilation of lower continental crust by island-arc volcanics to explain the
incompatible element and isotopic character of island arc volcanics (ARCULUS
and JOHNSON, 1981).
2) assimilation of continental crust to explain the isotopic characteristics of some
continental flood basalts (CARLSON et al., 1981; Cox and HAWKESWORTH,
1984) and some Archean volcanic rocks (e.g., COMPSTON et al., 1986).
3) contamination of granitic rocks with relatively young ages of mantle extraction
by much older continental crust (PATCHETTand BRIDGWATER,1984; DEPAOLO
and FARMER, 1984; DICKIN et al., 1984; PATCHETT and Kouvo, 1986).
4) mixing and reworking of old continental crust to explain the isotopic character
of some Archean gneisses (TAYLOR et al., 1983; COLLERSONand MCCULLOCH,
1982).
In some cases, the degree of crustal contamination can be very large. For
example, CARLSONet al. (1981) proposed that as much as 25% crustal contaminant
was present in the Columbia River basalts. COLLERSONand MCCULLOCH (1982)
suggested that 3.0 Ga gneisses from Labrador were derived by mixing a mantle
source with between 4~100% pre-existing 3.65 Ga Uivak-Nulliak gneisses (Figure
8). SPARKS (1986) has recently pointed out that eruption temperatures of magmas
have likely decreased through geological time and accordingly, the process of
assimilation may have been considerably more efficient during the Archean.
2.5. lntracrustal Recycling and Crustal Evolution
It is important to identify and quantify the role of intracrustal recycling in crustal
rocks. Recent attempts to generate crustal growth curves rely greatly on Nd model
ages of upper crustal rocks to date the time of extraction from the mantle and,
accordingly, give a 'true' crustal growth curve (McCuLLOCH and WASSERBURG,
1978; NELSON and DEPAOLO, 1985; MCCULLOCH, 1986; PATCHETT and ARNDT,
1986). However, if the role of intracrustal recycling (notably assimilation, isotopic
resetting and decreases in Sm/Nd ratio during intracrustal melting) is not assessed
(most efficiently by detailed zircon U-Pb data), the possibility of deriving mixed ages
is always present (e.g., see PATCHETTand BRIDGWATER,1984). Thus, it is important
to stress that crustal growth curves based on Nd model ages, while decidedly better
than those based upon K-Ar and Rb-Sr age determinations, will only approach the
698
Scott M. McLennan
I
I0
-
~
O
I
I
r
I
~0-
PAGEOPH,
I
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"..'"untie evm,..~..,..~
~ =V~OlUfio
-
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Uiv~ksseNU~iaOgke
0
~Nd
-5
o7
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-I0
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-15
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.,/ / to
~
/
-
/
I
I
l.O
F
I
I
2.0
I
I
30
I
I
4.0
Age (Ga)
Figure 8
Plot of end against time for Archean gneisses from Labrador (after COLLERSONand McCuLLOCH, 1982).
Also shown is mantle evolution for typical mantle and that suggested by Sm-Nd data for the early Archean
Uivak-Nulliak assemblages from Labrador, as well as the expected evolution of end for Uivak-Nulliak
crust and new crust formed at about 3.0 Ga (the age of the late Archean gneisses). Late Archean gneisses
have initial 143Nd/J44Nd ratios which are inconsistent with direct mantle derivation. The favoured
interpretation is an origin by mixing 40-100% old crust with new mantle derived crust at 3.0 Ga.
true g r o w t h
components
components
because the
pre-existing
curve. Such m o d e l s will always tend to u n d e r e s t i m a t e the age o f s o m e
o f a n y crustal segment, or give a mixed age. The difficulty in identifying
o f o l d e r crust using N d m o d e l ages increases c o n s i d e r a b l y in older rocks
difference in isotopic c o m p o s i t i o n between the m a n t l e a n d average
crust is greatly d i m i n i s h e d (for example, see Figures 6 a n d 8).
3, Crust-Mantle Recycling
C r u s t - m a n t l e recycling is the transfer o f c o n t i n e n t a l crust to the mantle. This m a y
take place in at least four ways ( F i g u r e 9). The first a n d p r o b a b l y m o s t i m p o r t a n t
is the s u b d u c t i o n o f sediments derived either from the u p p e r c o n t i n e n t a l crust or from
Vol. 128, 1988
Recycling of the Continental Crust
699
Figure 9
Schematic diagram to illustrate the various mechanisms of crust-mantle recycling. Upper. Mechanisms
for return of crustal material into the mantle, include: 1) subduction of sediment derived either from the
upper continental crust or from volcanic arcs; 2) enrichment of continental crust-derived elements in the
oceanic crust and subduction with the oceanic crust; 3) direct tectonic erosion of continental crust at
subduction zones; 4) assimilation of continental crust by the mantle at the base of the crust. Lower.
Mechanisms for the recycling of continental material from the mantle back into the continental crust,
including: 1) direct return at volcanic arcs, with short residence times ( < 10 m.y.) in the mantle; 2) return,
mainly in oceanic islands and continental alkali volcanics, after a long residence time (> 1 b.y.) in the
mantle as a distinct entity; 3) efficient remixing with geochemical and isotopic signatures swamped by the
mantle signatures, and return in all mantle-derived igneous rocks.
the exposed volcanic arc (which here is considered as a less evolved part o f the
continental crust). It has long been argued that such a transfer o f sediment is not
plausible because o f simple density considerations (e.g., MOORBATH, 1978). Such
objections are no longer tenable. It has been established that p r o n o u n c e d horst and
graben structures are developed as the oceanic crust bends and descends into the
mantle at subduction zones (e.g., HILDE, 1983; UYEDA, 1983). Such structures
provide adequate traps which m a y ' d r a g ' sediment to mantle depths. Questions
concerning the efficiency o f this process and the fate o f the subducted sediment are
matters o f debate, but the physical plausibility o f sediment subduction appears
established (also see KURTZ et al., 1986).
A second m e t h o d o f recycling continental crust into the mantle is t h r o u g h the
enrichment o f certain elements in oceanic crust by h y d r o t h e r m a l alteration and
submarine weathering processes and subsequent subduction. Such processes have
been reviewed by a n u m b e r o f authors (HOLLAND, 1978, 1984; FYFE, 1978, 1979,
1980; HART and STAUDIGEL,1982; ALBAREDE and MICHARD, 1986). In terms o f
overall mass balance, FYFE (1978, 1979, 1980) has argued that a significant mass o f
potassium m a y be added to the oceanic crust during submarine spilitization and
700
Scott M. McLennan
PAGEOPH,
may be transferred to the mantle this way. It has also been established, however,
that substantial amounts of K (and other elements) are transferred from the lower
ocean crust to the oceans via hydrothermal activity at mid-ocean ridges (HART and
SXAUDmEL, 1982). The exact balance of these competing processes is a matter of
debate (compare HART and STAUDICEL, 1982 and HOLLAND, 1984) and the
magnitude of the net mass transfer of continental potassium into the oceanic crust
must be considered as uncertain. Other elements of continental origin which may be
subducted in significant amounts by this process include Mg, S and U (e.g.,
ALBAREDE and MICHARD, 1986).
The two final transport mechanisms are less well established. The first is the
direct tectonic erosion of continental crust at subduction zones (e.g., KARIG and
KAY, 1981; ZmGLER et al., 1981; UYEDA, 1983). The most likely place that this is
occurring is in the central part of the Andean arc (KULM et al., 1977). Whether or
not this represents a significant flux remains in doubt (KARIG and KAY, 1981). The
final transport mechanism is purely speculative and involves the assimilation of
continental material directly into the mantle at the base of the crust. There is no
direct information to indicate whether this mechanism occurs at all, let alone
estimates of flux rates.
The overall conclusion to be drawn at this point is that recycling of continental
crust to mantle is essentially a subduction process. The possible fate of continental
material which is recycled is also shown schematically in Figure 9. Return of
recycled material to the continents can be considered on three different scales. The
first is direct return of subducted material to arcs after only a short residence time
at mantle depths ( < 10 m.y.). The second is return of subducted continental crust
as ocean island volcanics (which ultimately may be accreted) or ultrapotassic
continental lavas after long residence times in the mantle (up to ca. 2500 m.y.). The
final mechanism is the return and efficient remixing of continental crust with the
mantle so that the unique chemical and isotopic composition of the continental
crust (regardless of its age) is swamped by the mantle signature. This final process
is crucial for proposals of steady-state crustal evolution models.
3.1. Island-Arc Volcanics
A recycled component of continental crust commonly is invoked to explain the
chemical and isotopic composition of island-arc volcanic rocks (see review of GILL,
1981). It has been noted for some time that island-arc basalts are enriched in a
number of incompatible elements, notably K, Ba, Sr and Pb, relative to MORB and
oceanic island basalts. In addition, island arc volcanics have radiogenic Sr and Pb
and unradiogenic Nd and Hf isotopic characteristics, compared to MORB. The
interpretation of these characteristics has been debated actively for many years.
Various proponents have argued for an upper continental crust sediment component (of varying magnitude) mixed with MORB sources (e.g., SUN, 1980; KAY,
Vol. 128, 1988
Recycling of the Continental Crust
701
1980), contamination of MORB sources by lower continental crust during ascent
(ARCULUS and JOHNSON, 1981) and ocean-island basalt components (as blobs in
the mantle) mixed with MORB sources (MORRIS and HART, 1983).
Of these three possible processes, the mixing of sediment is the only one which
would indisputably represent crust-mantle recycling. The suggestion of contamination by lower continental crust would be an example of intracrustal recycling, since
the lower crustal component remains within the crust. The suggestion of an OIB
component may or may not represent an example of crust-mantle recycling,
depending on one's views of OIB petrogenesis (see Section 3.2).
If the arguments favouring an upper crustal sediment component are accepted
(see MORRIS and HART, 1983; 1986; PERFIT and KAY, 1986 for a recent debate of
the sediment vs OIB arguments), then Pb isotopes are most sensitive for providing
quantitative constraints, because the ratio of Pb in upper crustal sediments to that
i n the mantle is > 250, whereas the ratio is < 50 for Sr, Nd and Hf. The lead
isotope data are consistent with a very small component of sediment in the range of
< 1 3% (SUN, 1980).
Be isotopes. The most persuasive evidence of sediment involvement in island-arc
volcanism comes from the recent work involving l~ (BROWN et al., 1982). 1~ is
produced primarily by spallation reactions on N and O in the upper atmosphere.
The half-life is about 1.5 m.y., and 1~ is concentrated only in surficial sediments.
Thus l~
is a powerful tracer of sedimentary material during the subduction
process (BROWN et al., 1982).
Table 1 lists much of the presently available data for island-arc, other volcanic
and sedimentary rocks (BROWN et aI., 1981; TERA et al., 1986). It is notable that
most non-arc volcanic rocks have very low l~ concentrations whereas marine
sediments are enriched by a factor of > 10 4. Volcanic rocks from island arcs and
continental arcs have highly variable l~
concentrations. The most important
observation is that about one-half of the arcs so far studied have l~ concentrations considerably above those of other volcanics (a cut-off of about 1 x 10 6
atoms/g has been adopted for defining anomalously high values (TERA et al.,
1986)), strongly suggesting the presence of a sediment component. Low values of
l~ do not of course necessarily preclude the presence of a sediment component.
The short half-life, combined with old sediment being involved (due to combinations of slow sedimentation rates, slow subduction rates and large trench-arc
distances) can result in the decay of ~~ before the sediment interacts with the
island-arc volcanic source (TERA et al., 1986). The exact amount of sediment
indicated by the l~ data depends on the model for sediment subduction (e.g.,
graben fill only, mixture of l~ poor and rich layers during subduction, etc.), but
generally is consistent with 1-10%. This is in agreement with the 1 3% sediment
component indicated by the Pb isotopic data.
An important point is that only the upper 50-100 m of pelagic sediment will
Scott M. McLennan
702
PAGEOPH,
Table 1
I~
in island arc volcanic rocks (in 106 g/atoms _4-_1s.d.).
n
l~
_ 1 s.d.
Island arc voleanies
Central America
New Britain
Aleutians
Peru
Japan
Philippines
Taiwan
Mexico
Sunda
Marianas
Cascades
Halmahera
17
1
17
6
9
1
1
4
12
12
1
12
6.5 + 6.5
4.1
3.1 _+ 1.3 l
2.1 _ 3.0
1.1 + 1.42
0.8
0.6
0.5 + 0.3
0.4 + 0.2
0.3 _+ 0.3
0.3
0.2 __+0.1
Other volcanics
Oceanic island and rift volcanics
MORB
Flood basalts
14
4
4
0.3 __+0.2
0.2 ___0.13
0.4 + 0.2
Marine sediments
Non-marine sediments
16
10
5600 _ 1800
362 _ 67
Volcanic data from TERA et al. ( 1986); sedimentary data from BROWN et al. ( 1981).
excluc~es one value of 15.3.
2 exclu/des one value of 13.5.
3 excludes one value of 0.9.
contribute ~~ due to the short half life and slow sedimentation rate. Such
sediment is most likely to be accreted or underplated (subcreted) to the base of the
arc (KARIG and K a y , 1981), whereas the buried sediments with no ~~ are most
likely to be subducted (KAY, 1984). The presence of l~ thus suggests that the
entire sediment column is available for subduction, the sediment column is well
mixed prior to or during subduction (a model favoured by TERA et al., 1986) or
that only graben-fill sediment is involved.
Role of arc-derived sediment. Most estimates of sediment involvement based on
radiogenic isotope data are concerned only with sediments derived ultimately from
the upper continental crust. Sediment derived directly from the volcanic arc will
have isotopic characteristics identical to arc lavas (MCLENNAN et al., 1985), and
insufficient time is available for significant isotopic evolution (Figure 10). Trace
element characteristics may approach upper crustal values if intra-crustal differentiation has occurred within the arc and the arc is dissected to expose such material
(e.g., Aleutians; MCLENNAN et al., 1985), however isotopic compositions will
remain identical to the arc itself.
-20 -
-15 -
-I0 -
-,5 --
B
I
0
A S I ~ ' ~ 2%
' ~ v ' ~~
2~176
I
I00
I
I
200
Arc sediment (AS)
Upper crust sediment (UCS)
Depleted mantle (DM)
I
400
200
13
Sr
I
500
I
.300 - I 0
2.31:560
.046 -30
86Sr ESr
87n-
I
400
16
32
1.0
Nd
I
.140
.106
.217
6
-15
I0
t44Nd ~:Nd
147c,
Figure 10
Plot of eNa against ~s, to show the effect of subducting upper continental crust-derived and island arc-derived sediment. The modeling shows
that sediment derived from the upper crust has a significant isotopic effect when mixed with depleted mantle (mixing shown as thin lines),
whereas arc-derived sediment must evolve (evolution shown as heavy lines with arrows) for a considerable period of time before an effect
will be seen. For subduction occurring 1.1~2.0 billion years ago, the effects of subducting upper crust sediment would have been
considerably less because intracrustal recycling effects were less and the isotopic composition of sediments would have been closer to the
contemporaneous mantle (see Figure 3).
~Nd
0
I0
o/o~
I
Lo
E
0
C3
(3
~0
oo
<
o
704
Scott M. McLennan
PAGEOPH,
It is possible that anomalous l~ concentrations would be produced in arc lavas
by the subduction of arc-derived material. Sediments which are deposited rapidly,
such as many non-marine sediments, have highly elevated 1~ concentrations,
although about 10 times lower than pelagic sediments (Table 1). Thus, if such
sediment were to be deposited in the trench and transferred rapidly (say < 35 m.y.) to depths required for involvement in island arc volcanism, then it could
play a significant role. In any case, attempts to correlate anomalous 1~ abundances with other isotopic characteristics must be done with caution.
The main point here is that it is not possible to distinguish the role of
arc-derived sediment in island-arc volcanics. Considerable amounts of such sediment could be subducted without detection. This is important when considering
oceanic island volcanic petrogenesis (see below).
3.2. Ocean Island Basalts and Ultrapotassic Lavas
The differences in geochemistry between MORB and ocean island basalts (OIB)
has long been recognized (e.g., GAST, 1968), with OIB being considerably enriched
in most incompatible elements in comparison to MORB or the primitive mantle.
This was first interpreted as resulting from OIB being derived by much smaller
degrees of partial melting of sources similar to MORB (GAST, 1968). Nd and Sr
isotopic data suggest a long-term depletion of incompatible elements in the source
of many OIB, similar to MORB sources (e.g., CARLSON et al., 1978; HOFMANN
and WroTE, 1982), indicating the incompatible element enrichment is a recent event.
However, such a model is at odds with the Pb isotopic data which indicate that the
MORB and OIB sources have been distinct for the past ca. 2.0 billion years (e.g.,
SUN, 1980). More recent workers have suggested a number of alternative models
including derivation of OIB from:
1. primitive mantle (e.g., SCHILLING, 1973).
2. primitive mantle contaminated with MORB source material (e.g., WASSERBURG
and DEPAOLO, 1979).
3. metasomatized mantle OIB source regions (e.g., MENZIES and MURTHu 1980).
4. A recent hypothesis has received considerable attention. This suggests ocean
island basalts are derived from partial melting of ancient subducted oceanic crust
mixed with depleted mantle (MORB) sources (CHASE, 1981; HOFMANN and
WHITE, 1982). In this model, oceanic crust is subducted and stored in the mantle
as a separate entity. After sufficient time (ca. 1-2 b.y.) internal heating causes
instability and partial melts separate and rise to the surface, interacting with
surrounding upper mantle. Such a model does not require a sediment component, and in fact altered MORB crust and sediment may be geochemically and
isotopically distinct (see ZINDLER and HART, 1986). In any case, a sediment
component in the subducted oceanic crust would represent a significant reservoir
of incompatible elements (see WEAVER et al., 1986).
Vol. 128, 1988
Recycling of the Continental Crust
705
H f isotopes and O I B genesis. As with island-arc petrogenesis, interpretation of
Sr, Nd and Pb isotopic data is somewhat equivocal with respect to the role of a
sedimentary component. The short half-life of l~
excludes it as a tracer for
processes operating over such long time scales (1-2 b.y.). PATCHETT et al. (1984)
have pointed out that Lu is fractionated from H f during sedimentary processes
(related to zircon fractionation during sorting) and that shales and pelagic sediments have considerably higher L u / H f ratios than sandstones, which are mainly
preserved on the continents. This fractionation allows for the monitoring of a
sedimentary component over long-time scales.
PATCHETT et al. (1984) analysed a number of sediments for L u / H f and Sm/Nd
ratios and modeled the results to determine if the OIB Nd and H f isotopic data are
consistent with a sediment component (Figure 11). The modeling indicates that the
OIB data would be consistent with about a 2% component of mixed turbidite and
pelagic sediment which had evolved in the mantle for about 2 b.y. PATCHETT et al.
E
I
I
I
i
i
I
I
I
I0
I
20
3O
2_010
-20 -
~ M O R B
Pelogic
-:50 Mix 2
-40-
-50 -
Turbidite
1
'- 0
~
2 Ga
I
-50
I
-20
I
-I0
I
0
~Nd
Figure 11
Plot of ~ur against end to show the evolution (heavy lines with arrows) of typical pelagic and deep sea
turbidite sediments. Mixing calculations (thin lines) indicate that ocean island basalts can be explained
by mixing depleted mantle with about 2% of sediment comprising a 1.2/I blend of turbidite/pelagic. It
is considered unlikely that turbidite and pelagic sediment would be mixed at such a constant ratio and,
accordingly, the sediment explanation has not been favoured (PATCHETT e l al., 1984). Adapted from
PATCHETTet al. (1984).
706
Scott M. McLennan
PAGEOPH,
(1984) have pointed out, however, that the ratio of pelagics to turbidite sediment
would have to be constant (at about 1 : 1.2) and well mixed for most OIB's. This was
considered unlikely and PATCHETT et al. (1984) concluded that recycled sediment
probably was not the primary cause of the Hf-Nd isotopic characteristics of OIB.
Role of arc-derived sediments. In contrast with the origin of island-arc volcanics,
the subduction of arc-derived sediment may be relevant to the question of OIB
genesis. All workers agree that if subducted oceanic crust is the source of OIB, then
it would have to reside in the mantle for considerable periods of time, of the order
of 1.0-2.5 b.y. (CHASE, 1981; HOFMANN and WHITE, 1982; PATCHETT et al., 1984).
Most arc-derived sediments have trace element characteristics which would result in
considerable evolution, on this time scale, of their isotopic characteristics (high
Rb/Sr, U/Pb; low Sm/Nd, Lu/Hf), and high enough abundances to provide a
significant effect on mass balances (Figure 10).
Subduction of Precambrian sediment. Models suggesting a role of subducted
oceanic crust (with or without sediment) require a 1.0-2.5 b.y. residence in the
mantle. Modeling of sediment subduction and storage has only considered modern
or comparatively young sediment compositions (PATCHETTe t al., 1984; WEAVER et
al., 1986). An obvious question is whether Precambrian deep sea sedimentation
(pelagic and turbidites) was different, and whether these differences were sufficient
to affect the models? Trace element and isotopic data on Precambrian deep sea
sediments are meagre but some qualitative observations can be made which are
relevant to this question.
It is unlikely that the composition of the upper continental crust has changed
appreciably over the past 2.5 b.y. Thus, elements which are transferred nearly
quantitatively into clastic sediments will similarly be unchanged over this period
(TAYLOR and MCLENNAN, 1985). Accordingly, it is unlikely if Sm/Nd or Lu/Hf
ratios have changed significantly in the terrigenous fraction of deep sea sediments
over this time. On the other hand, ancient sediments were less affected by intracrustal recycling processes and, accordingly, their isotopic composition was
closer to the contemporaneous mantle (Figure 3). In addition, a number of elements
of isotopic significance, including Rb, Sr, U and Pb are affected by many secondary
processes.
Other possible differences in ancient sediments which could significantly affect
any isotopic modeling include the following:
l) Evolution of calcareous microplankton during the Mesozoic, resulting in a
change in the sites of carbonate deposition from the shelves to the deep oceans
(FISCHER, 1984).
2) Introduction of significant amounts of free oxygen at about 2.3-2.0 Ga (e.g.,
CLOUD, 1976) resulting in a change in behavior of uranium at surficial conditions (see Figure 4).
Vol. 128, 1988
Recycling of the Continental Crust
707
3) Differing effects of biogenic processes (and debris) in sedimentary geochemistry.
4) Importance of iron-formations as a deep-sea chemical sediment prior to about
1.8 Ga (CLouD, 1976; ERIKSSON, 1983).
5) Increased heat flow resulting in ocean sediments being considerably more
influenced by hydrothermal processes and less influenced by upper continental
crust (FRYER et al., 1982; VErZER et al., 1982).
From this discussion, it is concluded that it is likely that sediment subducted
> 1.0 b.y. ago could have had a very different composition than that of today. It is
equally possible that differences could have been significant enough to seriously
affect isotopic evolution, although there are so few relevant data that a judgement
cannot be made as to whether such differences would diminish or enhance suggestions of a sediment component in OIB. However, the possible differences indicate
that considerable caution is warranted in the use of modern sediment compositions
to model subduction of ancient sediment.
Some inferences regarding crust-mantle recycling early in earth history may be
obtained from studies of diamonds. There is general agreement that diamonds are
mantle derived and recent He, Nd and Sr isotopic data for garnet inclusions raises
the possibility that diamonds may have ages in excess of 3.0 Ga (OZIMAe t al., 1983;
RICHARDSON et al., 1984). Suggestions of a recycled crustal component in diamonds have been made on the basis of carbon isotopic data (e.g., M~LLEDGE et al.,
1983) and noble gas data (HONDA et al., 1987).
Ultrapotassic lavas. The ultrapotassic lava suite possesses a number of geochemical and isotopic characteristics broadly similar to alkali basalts found in ocean
islands. Such rocks comprise only a trivial volume on any global balance, however,
the identification of a sediment component in their source could be important in
understanding quite fundamental processes of mantle and crust evolution. The
ultrapotassic suite is notable for its variable Pb isotopic composition, often with high
2~176
and low 2~176
(NELSON et al., 1986). Accordingly, the possibility
of an ancient sediment component also has been suggested in the origin of these rocks
(NELSON et al., 1986). Many of the uncertainties associated with this interpretation,
outlined for the origin of OIB, equally apply for the ultrapotassic suite.
3.3. Limits on C r u s t - M a n t l e Recycling
The overall limits on the mass of continental crust which may be returned to the
mantle are not well constrained. Whether or not crust may be returned by direct
assimilation at the base of the crust is not known and no judgement can be made.
Tectonic erosion at subduction zones is a more likely phenomenon. DEWEY and
WINDLEY (1981) suggest an upper limit of 0.48 x 1015 g/yr or 0.17 km3/yr (for a
crustal density of 2.8 g/cc), based on the apparent erosion of western South
America.
708
Scott M. McLennan
PAGEOPH,
It is most likely that direct subduction of sedimentary rocks is the most efficient
mechanism for transferring continental crust to the mantle. If we accept the
evidence from island-arc volcanics for a sediment component, this provides a
minimum estimate. The most recent estimate of Mesozoic-Cenozoic arc accretion
rates is 1.1 km3/yr (REYMER and SCHUBERT, 1984) or 3.1 • 1015 g/yr. If we adopt
a 2% by mass sediment component, this represents 0.06 x l015 g/yr or 0.02 km3/yr
of crustal material.
An extreme value on sediment subduction rates over the past 100-200 m.y. may
be derived from the total amount of continental crustal material which is delivered
to the ocean floor annually. The most recent review of the DSDP and seismic records
is that of HOWELL and MURRAY (1986). They conclude (p. 448) that the sedimentary
flux from the continents to the oceans is 1.65 km3/yr. Allowing a correction for
density, this is equivalent to about 1.5 km3/yr of continental crust. The actual amount
available for subduction must be considerably less than this figure since much
sediment is recycled back onto the continent during accretionary processes associated with subduction and continental collisions (HOWELL and MURRAY, 1986).
Perhaps the best constraints can be obtained by examining long-term global
sediment recycling rates (see Section 2.1). This approach was first proposed by
VEIZER and JANSEN (1985). For the mass-age relationship of the entire sedimentary
mass (Figure 2c), the calculated recycling rate will depend on the model adopted for
the accumulation history. An upper limit on recycling can be determined by
assuming the entire sedimentary mass has been in place for the past 4.5 b.y. Such
a model implies a recycling rate of 11.6 x 10 Is g/yr or 4.1 km3/yr of crustal material
(VEIZER and JANSEN, 1985). Because the Nd model age of sediments greatly
exceeds their stratigraphic age over the post-Archean (Figure 3a), it is clear that
much of this recycling is intracrustal in nature. In Figure 3b, the sediment Nd model
age data are modeled for a sediment mass which is closed to differing degrees. It is
apparent that young mantle-derived material is required to explain the 1.5 Ga
average Nd model age of young sediments. The sediment which these new additions
replace represents an upper limit on sediment subduction for a sediment mass which
is constant or growing (see Figure 3c) through earth history (because most of the
earth's degassing occurred earlier in its history, it is unlikely if the sediment mass
has diminished through time; GARRELS and MACKENZIE, 1971). From Figure 3b,
the sediment mass is at least 80% closed. This provides an upper limit of sediment
subduction of 2.3 x 1015 g/yr (0.82 km3/yr of crust).
A lower value is imposed for a model of sediment evolution where the sediment
mass grows during the Archean and reaches its present mass at about 2.5 Ga, with
recycling being the dominant process since then. This explains the Nd model age
data best (VEIZER and JANSEN, 1985; Figure 3c). This modeling suggests a recycling
rate of 10.8 • 1015 g/yr (VEIZER and JANSEN, 1985) with the system being at least
85% closed, implying an upper limit on sediment subduction of 1.6 x 1015g/yr or
0.57 km3/yr of crust.
Vol. 128, 1988
Recycling of the Continental Crust
709
Table 2
Some estimates of recycling of continental crust into the mantle.
Mass
( 10~5g)
Volume
( km3)
2.5 7.5
1-3
1.22
0.44
7.0 _+Z8
2.5 _+ 1.0
MCLENNAN and TAYLOR, 1983
REYMER and SCHUBERT, 1984
<8.4
1.65
<3,0
0.59
VEIZER and JANSEN, 1985
TAYLOR and McLENNAN, 1985
This paper
<-4,1.1 + 0.5
<2.8
<2.1
--<0.4_+0.2
< 1.0
<0.75
Reference
ARMSTRONG, 1981
DEWEY and WINDLEY, 1981
DEPAOLO, 1983
Method of estimate
Isotope mixing model
for steady-state system
Sediment accumulation
on ocean floor plus
tectonic erosion
Hf, Nd isotope analysis
of mantle derived rocks
Sedimentation rates
Estimates of sediment
subduction, tectonic
erosion and K-fixation
in oceanic crust
Sediment recycling rates
Sediment recycling rates
Sediment recycling rates
and tectonic erosion
Mass and Volume conversions assume 2.8 g/cm3 for continental crust for 2.5 g/cm3 for sediment.
It s h o u l d b ~ e m p h a s i z e d t h a t these values also represent u p p e r limits on
/
.
s e d i m e n t s u b d t r c n o n for the recycling m o d e l s being considered. It is n o t r e q u i r e d
t h a t the lost sediment be r e m o v e d to the mantle, b u t only isolated f r o m the
s e d i m e n t a r y mass. Thus, sediment which is stored, for example, in the lower
c o n t i n e n t a l crust is included, a n d this region m a y be i m p o r t a n t . M e t a s e d i m e n t a r y
lithologies are c o m m o n in lower crustal x e n o l i t h suites from a r o u n d the w o r l d (e.g.,
D u P u v et al., 1979; PADOVANI a n d CARTER, 1977; LEYRELOUp et al., 1982). Also,
if there has been some g r o w t h in the sediment m a s s over the p a s t 2.5 G a , the
a m o u n t available for c r u s t - m a n t l e recycling w o u l d also be lower.
It can be safely c o n c l u d e d t h a t the present rate o f c r u s t - m a n t l e recycling is n o t
well constrained. A lower limit o f negligible mass transfer can be argued. A
r e a s o n a b l e u p p e r limit o f sediment s u b d u c t i o n plus a n u p p e r limit on tectonic
e r o s i o n w o u l d suggest that no m o r e t h a n 1.0 km3/yr a n d p r o b a b l y significantly less
than 0.75 km3/yr o f crustal m a t e r i a l has been recycled into the m a n t l e o v e r the
recent past. This is c o m p a r e d with o t h e r estimates in T a b l e 2.
4. Selective R e c y c l i n g
It is possible that certain types o f c o n t i n e n t a l crust ( n o t a b l y sediments) m a y be
preferentially recycled either i n t r a c r u s t a l l y o r between crust a n d mantle. This has
710
Scott M. McLennan
PAGEOPH,
been proposed by a number of workers to explain various characteristics of
sediment and basement distribution and composition. Several examples of selective
recycling will be examined below.
4.1. Selective Recycling of Sediments
GARRELS and MACKENZIE (1971) recognized that some sediments were more
readily weathered and eroded and concluded that differential recycling rates may
affect different sediments. Thus, evaporites are eroded and recycled more rapidly
than carbonates which in turn are eroded more rapidly than shales and sandstones.
From this observation, GARRELS and MACKENZIE (1971) speculated that a bias
may exist in the relative proportions of differing lithologies preserved through time.
According to this model, the relative lithological proportions being deposited
during the past have always been maintained and the apparent differing proportions
of lithologies seen over geological time (e.g., RONOV, 1983) is "a function of
differential recycling. VEIZER (1988) has recently argued that erosion of different
sediment types cannot be considered in isolation since they are generally interlayered. Consequently, Veizer argues that any differential recyling of sedimentary
lithologies is probably related to a preferential association of sediment type with
tectonic setting (see below).
VEIZER and JANSEN (1985) have further refined this concept of differential
recycling of sediments. They examined the area-age relationships of sediments
9 deposited in differing tectonic environments and calculated recycling rates (Table 3).
These rates differ by more than a factor of ten with the half life of the different
Table 3
Recycling rates for sediments from differing tectonic settings (VEzzER and
JANSEN, 1985).
Tectonic setting
All sediments
Abyssal and pelagic
Active margin basins (e.g., back-arc
basins, Mediterranean)
Deep sea fans
Passive margin basins
Immature orogenic belts
Mature orogenic belts
(within continents)
Platforms
Preferred
recycling
constant
( • 10 l~
Half-life
(Ma)
40 + 3
126
235
40
144
15
337
-
223 _+ 51
230 _+ 55
88 _+ 8
85 _+ 35
30
25
80
100
< 1350
540
18.4 _+ 0.5
18.6 _+ 0.5
380
380
Mass
(1021 g)
2701
217
Vol. 128, 1988
Recycling of the Continental Crust
711
sediments ranging from 25 m.y. to about 380 m.y. This compares with a half life for
the entire sedimentary mass of about 235 m.y. One consequence of this is that if
significant amounts of sediment are subducted to the mantle, sediment deposited in
certain tectonic environments is likely to be preferentially affected. Thus, the
conclusion arrived at above, that it may be inappropriate to consider average
sediment in modeling is reinforced. This observation also has important implications for understanding accretionary processes at continental margins and for the
study of allochthonous terranes. These will be addressed below.
4.2. Preferential Recycling of Young Continental Crust
It has long been recognized that crust formation ages tend to cluster at given
times. MOORBATH (1976) recognized five major periods of rock-forming events
based mainly on Rb-Sr dating, at 3.8-3.5, 2.8-2.5, 1.9-1.6, 1.2q3.9, 0.5~0.0 Ga. Nd
model age data are less abundant but groupings at about 3.8-3.6, 2.7-2.6, 2.3-2.1
(Australia), 2.0-1.7 (North America), 1.5-1.3, 1.1 Ga have so far been recognized
(e.g., MCCULLOCH and WASSERBURG, 1978; NELSON and DEPAOLO, 1985;
MCCULLOCH, 1986; PATCHETT and ARNDT, 1986).
GURNIS and DAVIES (1985, 1986) have examined some of the possible implications of selectively recycling young continental crust, on the area-age distribution of
continental basement rocks. This modeling indicated that it is possible to generate
apparent peaks in crust formation ages for a continental crust that had a regular
growth rate but in which young crust was preferentially recycled (GuRNIS and
DAVIES, 1986). These authors assumed that only crust-mantle recycling was involved (in spite of widespread evidence for intracrustal recycling; see Section 2),
however, their conclusions seem equally valid if the preferential recycling is intracrustal or a combination of intracrustal and crust-mantle.
The suggestion of preferential recycling of young crust would appear to have
merit in sedimentary systems. Young crust tends to be elevated in response to young
tectonic activity and thus is prone to erosion and recycling. For example, TAYLOR
et al. (1983) found that Nd isotopic data indicated that Pleistocene loess deposits
preferentially sampled comparatively young orogenic regions. The results from
VEIZER and JANSEN (1985; see Table 3) also support the suggestion that sediments
in young orogenic belts and active continental margin basins are recycled at a
greater rate than those from older tectonic provinces.
One implication of preferential intracrustal recycling of young crust is that the
average Nd model age of young sediments would tend to give an age younger than
the mean age of the upper continental crust (see Figure 3). In addition, it would
also allow for the sedimentary system to be considerably more closed (Figures 3b,c)
thus further restricting the mass of sediment presently available for crust-mantle
recycling (see Section 3.3).
712
Scott M. McLennan
PAGEOPH,
4.3. Preferential Recycling o f High Heat Production Crust
A number of lines of evidence suggests that the Archean continental crust
contained lower concentrations of the heat producing elements (e.g., TAYLOR and
MCLENNAN, 1981, 1985; MORGAN, 1984). TAYLOR and MCLENNAN (1985) have
interpreted these data as indicating differing compositions for the Archean and
post-Archean crusts. MORGAN (1985) has provided an alternative model in which
during the Archean, when radioactive heat production was greater, crust with high
concentrations of heat producing elements (K, Th, U) was preferentially recycled
during orogenic episodes and low heat producing crust survived preferentially. In
this model, final stabilization of the crust with high concentrations of K, Th and
U did not occur until the Proterozoic.
There are two problems with the recycling model of MORGAN (1985). Clastic
sedimentation is closely related to orogenic activity (POTTER, 1976; READING,
1982). Most Archean sedimentary rocks are synorogenic in character (e.g., LOWE,
1980, 1982; MCLENNAN, 1984; OJAKANGAS, 1985). It would be expected, accordingly, that these sediments should record in their abundances of K, U and Th the
signature of this high heat production crust which is being recycled. In Table 4,
the concentrations of K, Th and U are compared for average Archean mudstone
(derived mainly from turbidite-shale sequences) and average post-Archean mudstones (TAYLOR and MCLENNAN, 1985). The Archean sediments are depleted by
a factor of two in heat producing elements (and other incompatible elements), in
conflict with the preferential recycling model. The Nd model age data for sedimentary rocks equally lend no support to this recycling model. Archean sediments
have Nd model ages indistinguishable from their stratigraphic ages (Figure 3a),
thus greatly restricting the role of recycling. Early Proterozoic sedimentary rocks
show some indication of recycling but not on the scale suggested by the preferential recycling model, where model ages well in excess of 2.7-2.8 Ga might be
expected.
The evidence for preferential recycling of high heat production crust throughout
the Archean (MORGAN, 1985) is thus not persuasive and an interpretation of
differing bulk compositions of Archean and post-Archean crusts is still favoured.
Table 4
Heat producing elements in Archean and post-Archean mudstones.
K (%)
Th (ppm)
U (ppm)
Heat production (~W m-3)
Archean
Post-Archean
1.9
6.3
1.6
1.08
3.1
14,6
3.1
2.22
Data from TAYLORand MCLENNAN(1985).
Vol. 128, 1988
Recycling of the Continental Crust
713
5. Recycling, Accretion and Terrane Analysis
The fate of sediments that reach subduction zones, either transported on the
oceanic crust (pelagic and terrigenous sediments) or derived from the continental
margin (terrigenous sediments) is a question of great concern. Major debate centers
on the relative role of sediment accretion versus underplating (subcretion) versus
subduction (e.g., KARIG and KAY, 1981; MOORE et al., 1982; HILDE, 1983). A
common approach (e.g., GILLULY, 1971; FYFE, 1978, 1979) is to consider a simple
equation:
Sediment reaching subduction zone - Sediment preserved in
accretion wedge -- Sediment subducted
Such an equation is inadequate without also considering the effects of sedimentary recycling. Once accretionary prisms are exposed to erosion, it is likely that
sediment will be removed and recycled either directly back onto the incoming
oceanic plate or indirectly via other sedimentary basins. The magnitude of the
effects will depend on the recycling rate and the time scale over which the
subduction zone has evolved. The speed at which the recycling can occur is
remarkably rapid (Table 3).
Another, somewhat related, area of recent interest is the concept of terrane
analysis (e.g., SCHERMER et al., 1984). It is now recognized that many areas of the
continents are comprised of exotic tectonic blocks which have been transported
greater or lesser distances; their present tectonic setting may be unrelated to the
original setting. In order to understand, in detail, the history of continental growth
and development, it is necessary to unravel the history of individual terranes and
understand the relationships of one to another. The prime tool in such investigations is detailed stratigraphic study of sedimentary and volcanic rocks which
provides evidence of their origin.
The present understanding of crustal recycling provides an important caveat to
this field of research. Notably, it is to be expected that such terrane analyses become
less reliable as older rocks are considered because of the effects of crustal recycling.
It can be seen from Table 3 that tectonic environments, particularly those at
continental margins, are recycled at vastly different rates. Over time scales of 50-100
million years, the geologic record would be expected to produce a severe bias in the
preservation of crustal rocks originating from differing tectonic settings.
6. Recycling on the Early Earth
Current views of earth accretion (see review of TAYLOR, 1982) appear to favour
the planetesimal model (WETHERILL, 1980) According to this hypothesis, approximately 100 million years after To (4.56 Ga) are required for the accumulation of the
714
Scott M. McLennan
PAGEOPH,
earth to near its present mass. On the other hand, present data indicate no rocks
greater than about 3.8 Ga in age are present on the earth. A major outstanding
question is whether continental crust formed during this 800 million year 'gap' and
if it did, what happened to it?
There is evidence suggestive of the presence of continental crust prior to 3.8 Ga.
Initial 143Nd/144Ndratios in early Archean volcanic rocks between 3.6-3.8 Ga from
Greenland (HAMILTON et al., 1978, 1983), West Australia (HAMILTON et al., 1981),
India (BASU et al., 1981) and South Africa (HAMILTON et al., 1979; CARLSON et
al., 1983) indicate derivation form a mantle already depleted in Nd relative to Sin.
This depletion could be interpreted to be the result of an earlier extraction of
continental crust from the mantle. Some care must be taken with this interpretation.
Sm-Nd isochrons for early Archean volcanics almost invariably include both felsic
and mafic lithologies in order to obtain a spread in Sm/Nd ratio for precise dates.
However, the genetic relationship between such samples often is uncertain and it is
possible that the data represent mixing lines of populations with different initial
143Nd/144Nd ratios. Evidence from Kambalda (MCCULLOCHand COMPSTON,1981;
CLAOUE-LONG et al., 1984; COMPSTON et al., 1986) indicate that extremely large
errors can be generated from such procedures. In spite of this complication, the
Nd-isotopic data do suggest the likelihood of depleted mantle (due to crust
extraction?) prior to 3.8 Ga but the magnitude of the depletion and its extent are
uncertain. More direct evidence for early continental crust comes from U-Pb dating
of individual zircon grains (FROUDE et al., 1983). Data from zircons in Archean
metaquartzites from Western Australia indicate ages as old as 4.2 Ga.
If continental crust were present on the earth prior to 3.8 Ga, there is little doubt
that it would have been affected profoundly by recycling processes. The amount of
heat produced by K, Th, and U would have been at least three times as great as at
present. In addition, considerable heat was probably available from gravitational
energy released during accretion. It is not clear if there was an equivalent to the
lunar 'magma ocean', which may be unique to the moon, but extensive early
melting seems likely (TAYLOR, 1982). In any case, convection on the early earth was
almost certainly considerably more vigorous, with resulting increased plate production rates, to accommodate the loss of heat (e.g., BICKLE, 1978). This would have
facilitated any recycling associated with such processes (SMITH, 1981, 1982).
Perhaps more important to early recycling was the influence of meteorite
bombardment. It is now well established that intense meteorite bombardment was
endemic within the solar system during its early history and that the earth could not
have escaped such activity (TAYLOR, 1982). The cratering record is best documented on the lunar surface where tectonic acitvity has been minimal since
accretion and observations may be made comparatively easily. In Figure 12 a model
for the lunar cratering record is shown in which a smoothly declining rate is
assumed (HARTMANN, 1980). The cratering density on the Earth would have been
as much as twice as large as the Moon, due to increased gravitational attraction
Vol. 128, 1988
Recycling of the Continental Crust
I
I
I
I
I
715
I
I
I
2O
Lunar accretion
E
I j M o d e l (for smooth
decline)
0
.+-
\
0
\
c-
\
__1
0
._J
LO
I
4,0
~
I
Lunor surface
record
I
I
I
15.0
2.0
Age (Go)
I
I
1.0
Figure 12
Plot of lunar cratering rate against time for a model which assumes a smoothly declining rate through
time. A similar or even more intense cratering flux would have affected the earth. The greatly increased
cratering rate during the period 4.5-3.8 Ga would have had a profound effect on any crust forming on
the earth at that time. Adapted from HARTMANN(1980).
(SMITH, 1981).
The effects of this b o m b a r d m e n t on any terrestrial crust would have
been profound, as testified by the lunar highland regolith, and efficient recycling
both within the crust and into the mantle are likely.
Conservative estimates of the lunar cratering record in the period 4.4-3.8 G a
(WILHELMS, 1985) indicate that 80 ringed basins, with diameters > 300 km were
formed. In addition, 104 craters with diameters in the range 30-300 km were also
formed. Extrapolation of these rates to the early Earth in the same time interval
indicate a minimum of 200 ringed basins with diameters > 300 km and > 104
craters in the range 30-300 km diameter. The effect of one such ringed basin
(Orientale; 900 km diameter) on the m o o n (TAYLOR and MCLENNAN, 1985; see
their Figure 12.7) indicates rather clearly that the preservation of significant
amounts of crust formed on the early earth prior to 3.8 G a is not to be expected.
There is no clear evidence to indicate the extent of early continental crust,
although the limited isotopic data would support at least local continental crust
extraction prior to 3.8 Ga. The combination of high heat production and intense
meteorite b o m b a r d m e n t during the period 4.56-3.8 G a would certainly have been of
importance for recycling into the mantle of any continental crust which may have
formed during this period.
716
Scott M. McLennan
PAGEOPH,
7. Crustal Recycling and Crustal Growth
A final matter is to try to asses the overall effects that crustal recycling has had
on the evolution of the continental crust and make a judgement on the overall rate
of crustal growth. This writer is of the opinion that much of the divergence in views
of crustal growth models has arisen from the failure to clearly distinguish between
intracrustal recycling and crust-mantle recycling. Two examples will suffice. The
first, stated previously, is the practice of equating to crust-mantle recycling, the
difference between long term continental margin sediment accretion and sediment
delivery to subduction zones. This however, must represent an upper limit on
sediment subduction because of the process of sediment-sediment recycling. In
another example, GURNIS and DAVIES (1986) stated that "Crustal growth curves
are the same as crustal age distributions only in the absence of crustal recycling into
the m a n t l e . . . " . This statement similarly ignores the effects of intracrustal recycling,
which can have many of the same effects on crustal age distributions (but with
differing effects for different isotope systems, as discussed above) as does crustmantle recycling.
Models which have favoured a steady-state system for the continental crust
require a present day crust-mantle recycling rate of 1-3 km3/yr (ARMSTRONG,1981;
DEPAOLO, 1983). In the modeling of ARMSTRONG (1981), an exponentially increasing rate of recycling with increasing age was required to reproduce the HURLEY and
RAND (1969) basement area-age distribution. However, the area-age relation for
Nd-model ages differs considerably from the K-Ar distribution (Figure 13), and the
quantitative effects on how the recycling rate varies over time are yet to be assessed
for the Armstrong model. DEPAOLO (1983) used a somewhat different approach by
modeling the difference between measured initial Nd and Hf isotopic compositions
of mantle derived rocks of various ages and those expected from their Sm/Nd and
Lu/Hf ratios; the difference was attributed to crust-mantle recycling (see PATCHETT
and CHAUVEL, 1984 for criticism of this approach). This modeling implied
2.5 ___1.0 km3/yr for crust-mantle recycling which may or may not have increased
back through time.
In this paper, the upper limit on the present crust-mantle recycling rate was
determined to be 1.0 km3/yr, with a favoured rate considerably less than 0.75 km3/
yr. It would appear that the steady-state model is only valid if the lower limit of
required recycling and the upper limit of potential recycling are adopted. Accordingly, a growth model seems more plausible, although some recycling can by no
means be excluded. Most recent attempts to balance crustal growth and recycling
have reached similar conclusions (DEWEY and WINDLEY, 1981; REYMER and
SCHUBERT, 1984; HOWELL and MURRAY, 1986).
A second factor which must be considered in formulating a crustal growth
model is the possibility of preferential recycling of young crust (either intracrustal
or crust-mantle) to give apparent peaks in radiometric age distributions (GURNIS
Vol. 128, 1988
Recycling of the Continental Crust
I
I
I
I
I
I
I
717
I
i
I
r
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Taylor&
Mciennan t,~'"~~''
-
(m ~deI)//c,~-','~
.\~'~~~'o
[,,~
"9" |'/
_
//
-
T
/ /Nelson & I
/,~ / DePaolo '
h
-o.6o ~S
-
:,:_.
o
/
/Hurley 8~
"[
-~
,o o
I
4.0
:5.0
2.0
Age (Ga)
1.0
Figure 13
Plot of cumulative per cent area against age for K-Ar basement ages (HURLEYand RAND, 1969) and
Nd model ages for North America (adapted from NELSON and DEPAOLO, 1985). Also shown is a model
for crustal growth adapted from TAYLOR and MCLENNAN (1985). The difference between the K-Ar
distribution and Nd model age distribution is almost certainly a function of intracrustal recycling.
Assuming the measured Nd model ages are representative of the entire crust, differences between Nd
model age distribution and the true crustal growth curve could be due to intracrustal and/or crust-mantle
recycling. In either case, the Nd model age distribution, while approaching the true growth curve, will
tend to minimize the amounts of old crust.
a n d DAVIES, 1985, 1986). This is not c o n s i d e r d to be an a d e q u a t e e x p l a n a t i o n for
the a p p a r e n t g r o w t h o f c o n t i n e n t a l crust at the end o f the A r c h e a n because the p e a k
in r a d i o m e t r i c ages correlates with o t h e r changes, including greater crustal heat
g e n e r a t i o n (MORGAN, 1984), changes in u p p e r crustal c o m p o s i t i o n (e.g., MCLENNAN et al., 1980) a n d changes in S7Sr/S6Sr ratios in seawater (e.g., VEIZER a n d
JANSEN, 1979). O n the o t h e r hand, such a process c a n n o t be excluded as an
e x p l a n a t i o n for o t h e r p e a k s in isotopic ages, such as those d u r i n g the P r o t e r o z o i c
(NELSON a n d DEPAOLO, 1985; MCCULLOCH, 1986), where a d d i t i o n a l evidence for
crustal g r o w t h is n o t present.
Because the S m - N d isotopic system is least susceptible to resetting, it is clear
t h a t the c o n t i n e n t a l a r e a - a g e r e l a t i o n s h i p based on this system w o u l d m o s t closely
represent the ' t r u e ' crustal age d i s t r i b u t i o n curve (e.g., MCCULLOCH a n d WASSERBURG, 1978; PATCHETT a n d BRIDGWATER, 1984; NELSON a n d DEPAOLO, 1985;
718
Scott M. McLennan
PAGEOPH,
MCCULLOCH, 1986; PATCHETTand ARNDT, 1986). An important assumption is that
the isotopic data, providing a measure of surface area, are representative of the entire
continental crust.
In Figure 13, the growth curve derived from the Nd model age data for North
America (NELSON and DEPAOLO, 1985) is compared to the K-At curve of HURLEY
and RAND (1969). Also shown is a model for crustal growth proposed by TAYLOR
and MCLENNAN (1985). The difference between the Nd-model age curve and the
K-Ar curve (or Rb-Sr curve) is best ascribed to recycling processes taking place
wholly within the crust (see above, VEIZER and JANSEN, 1979, 1985). There is an
additional discrepancy between the model of TAYLOR and MCLENNAN (1985) (or
that of VEIZER and JANSEN, 1979; ARMSTRONG, 1981; DEWEY and WINDLEY, 1981
and others) and the Nd model age curve. Two factors are considered to be important.
First, it has been argued above that intracrustal recycling processes, notably
intracrustal melting (causing changes in Sm/Nd) and assimilation, can affect Nd
model ages. In addition, some degree of crust-mantle recycling probably takes place.
Using the models of VE~ZER and JANSEN (1985), on the order of < 1 kmS/yr of total
recycling (intracrustal plus crust-mantle) would be required to explain the discrepancy. Secondly, a time of major crust generation during the period of about
2.3-2.1 Ga (Australia) and 2.0-1.7 Ga (North America) has been emphasized
recently (PATCHETT and BRIDGWATER, 1984; NELSON and DEPAOLO, 1985; PATCHETT and ARNDT, 1986; PATCHETT and Kouvo, 1986; MCCULLOCH, 1986. The
magnitude of this event has not been satisfactorily determined because the role of
the older crust (intracrustal recycling) and the possibility of preferential recycling of
young crust (GuRNIS and DAVIES, 1985, 1986) have not been fully assessed. In any
case, if the amount of crust generated during this time is < 15-20%, it would be
consistent with the crustal growth model of TAYLOR and MCLENNAN (1985). If it
is greater, the amount of crust estimated to be present at 2.5 Ga in the TAYLOR and
MCLENNAN (1985) model (70-90%; 75% favoured) would have to be revised
slightly downward, to perhaps about 50-70% of the crust present by 2.5 Ga.
Acknowledgements
I am grateful to Malcolm McCulloch, Roberta Rudnick, Ross Taylor and Jan
Veizer for commenting on an earlier version of the manuscript. Reviewers comments
were particularly thorough and helpful.
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