4.4 The crustal growth
problem
The question of whether the crust has grown
progressively over geological time, or maintained an approximately constant
volume, is one of the most fundamental in geology, but has proved hard to
answer conclusively. A review of the ‘crustal
growth’ model by its most persistent critic (Armstrong, 1991) shows that Nd isotope data provide critical tests for alternative
models. Hence, three of the most important lines of Nd
isotope evidence will be examined here.
4.4.1 Crustal accretion
ages
The ability of the Sm)Nd method to ‘see back’ through
younger thermal events and measure the crustal
formation ages of continental rocks makes the method ideally suited to chart
the present-day age distribution of crustal basement.
This yields an apparent profile of continental growth through time which does
not take into account crustal recycling into the
mantle. Nevertheless, it is an appropriate starting point for this subject.
Attempts
to map the age structure of the continents were begun using the Rb)Sr method (Hurley et al., 1962), before the development of Sm)Nd analysis. Hurley et al. compared Rb)Sr
isochron ages with Sr model
ages calculated assuming a mantle 87Sr/86Sr ratio of
0.708. On average the two values were correlated, leading them to suppose that
the isochron ages dated the time of crustal extraction from a basic source. This was a good
approach, although we now know that the Rb)Sr
system is too easily reset to yield reliable crustal
extraction ages for old terranes. (Also the mantle
growth curve is less radiogenic than 0.708). Hurley et al. applied the method to the North American continent in order
to calculate the approximate areas of crustal
basement attributable to geological provinces of various ages (Fig. 4.30).
Fig. 4.30. Estimated area of North American crustal basement attributable to different Rb)Sr age provinces. a)
Map showing provinces of different ages in Myr; b)
histogram of growth rate against time. After Hurley et al. (1962).
Hurley
and Rand (1969) extended this approach to include two-thirds of the land area
of the world (excluding the
A
study of comparable sweep to that of Hurley et
al. (1962) was performed by Nelson and DePaolo
(1985), who used Nd model ages to map the age
structure of the basement of the
As
more detailed model age mapping is performed in areas of old crustal basement it is likely that further increases in
average crustal extraction age will be found. The
Fig. 4.31. Estimated continental growth rates
on a cumulative basis. After Jacobsen (1988).
4.4.2 Sediment provenance ages
In response to proponents of the ‘continental
growth’ model (e.g. Moorbath, 1976),
Armstrong (1981) argued that the record of continental accretion documented by
various methods (as above) did not prove that the continental area had actually
grown over geological time. Armstrong argued that a model in which the
continental area was approximately the same 4.5 Byr
ago as it is today could also generate apparent
continental growth with time, provided that the rate of crustal
recycling into the mantle (by sediment subduction)
was equal to the rate of new crustal formation above subduction zones.
Undoubtedly,
recycling of crust back into the mantle does occur on a significant scale. Old crustal terranes may be shortened
by orogeny, then flattened
again by erosion and sediment subduction. However,
some sediment should be expected to escape the recycling process and provide a
record of the old, recycled terrane. Therefore, the
search for evidence of constancy or growth in the continental mass turned to
the sedimentary record. The ability of the Nd model
age method to ‘see back’ through erosion and sedimentation to an original crustal extraction event made it ideal for these studies.
To
study this problem, it is convenient to portray sediment data on a diagram of Nd model age (crustal residence
age) against the stratigraphic depositional age of
the sediment in question (Fig. 4.32). Sediments eroded from juvenile
mantle-derived sources will have TCR
= TSTRAT and lie on a ‘concordia’ line (Allegre and
Rousseau, 1984). In contrast, reworking of older sediments without any input of
juvenile material will displace compositions to the right along horizontal
vectors. A compilation of data from several sources is shown in Fig. 4.32,
including clastic sediments (Hamilton et al., 1983; O’Nions
et al., 1983; Taylor et al., 1983; Allegre
and Rousseau, 1984) and particulates from major river systems at the present
day (Goldstein et al., 1984).
Fig. 4.32. Model age versus stratigraphic
age diagram showing a compilation of early 1980s data from several clastic sediment studies. Crustal
growth models A, B and C are discussed in the text. After Goldstein et al. (1984).
Allegre and Rousseau (1984) compared the sediment data with
various theoretical models for continental evolution involving different rates
of continental growth through time (Fig. 4.32). A ‘big bang’ model (A), whereby
the whole continental mass was extracted at ca. 4 Byr
or before, was ruled out. Allegre and Rousseau argued
that a model involving uniform growth of the continents from 3.8 Byr to the present (B) was a better fit to the data, but
that the best fit was produced by a curved line (C), representing decreasing
growth of the crust through time.
Unfortunately
this diagram is not as conclusive as it may appear, due to the great difficulty
of determining a global average sediment provenance age at any given time from
the very variable provenance ages in individual provinces. This makes the data
very susceptible to sampling bias. One source of such bias is preferential
recycling of old sediments relative to erosion of more juvenile cratonic material. This will exaggerate the slowing down of
continental growth with time, appearing to favour models of type C over type B.
Another source of bias is the neglect of young orogenic
belts such as the accreted terranes of the Canadian
Cordillera (Samson et al., 1989). The
inclusion of such data in Fig. 4.32 would favour linear evolution models (type
B), suggesting that crustal growth has not slowed significantly in the Phanerozoic.
The
interpretation of Fig. 4.32 is also heavily influenced by assumptions about the
degree of recycling of sediment into the mantle. The so called ‘big bang’ model
shown in Fig. 4.32 involves no recycling of crustal
material into the mantle. This does not correspond to Armstrong’s model, which
involves constant recycling of old crust into the mantle and its replacement by
an equal volume of juvenile crust. Armstrong (1991) claimed that his model gave
rise to a curve (shown in Fig. 4.33) which looks remarkably like the steady
growth model in Fig. 4.32. It is clear then that young sediments provide much
too loose a constraint on crustal growth models.
Therefore, the argument must focus on the provenance ages of the oldest
surviving sediments.
Fig. 4.33. Model age versus stratigraphic
age diagram showing data of Dia et al. (1990) for clastic sedimentary
rocks from
Isua supracrustals from western
Greenland, which are the oldest clastic sediments
analysed, yield identical stratigraphic and Nd model ages of 3750 Myr,
indicating that they did not incorporate a significant amount of older reworked
crust. However, the data of Dia et al. (1990) from South Africa show surprisingly old provenance
ages for Mid to Late Archean sediments. On balance,
the sediment data seem to favour a crustal growth
model, but ultimately the argument rests on a null hypothesis (no sediments
with very old provenance have yet been seen, therefore none exist). This is an
inherently weak argument upon which to base such an important conclusion. This
weakness comes from the need for representative sampling of old crust using a
sediment data set which is inherently very noisy.
4.4.3 Archean depleted
mantle
An alternative route to assessing the volume of
crust at a given time in Earth history is to measure the composition of the
depleted reservoir which balances the enriched crustal
reservoir, namely the upper mantle (section 6.2.1). Because the upper mantle is
stirred by convection, we can expect to sample this reservoir (in ancient
volcanism) in a much more representative fashion than the sampling of the
enriched reservoir by ancient sediments. Hence, the problem of crustal growth may be soluble if we can determine the
extent of mantle depletion in early Earth history. If there was a large volume
of continental crust in the early Earth, there should be evidence for strong
mantle depletion.
In
the mid 1980s, several studies revealed initial Nd
isotope data for Early and Mid Archean rocks which
lay well above the chondritic evolution line, and in
some cases above the depleted mantle evolution line of Goldstein et al. (1984). Smith and Ludden (1989) argued that some of the strongly positive , Nd values
calculated for early mafic rocks are in error due to
incorrect age assignments. The Kambalda example has
already been mentioned (section 4.1.2), and doubtless there are problems with
some of the other data. However, they concluded that there are enough depleted
mantle compositions in the Early Archean for the
phenomenon to be real.
Such
evidence for very early depletion of the upper mantle presents a problem for the
model in which continental crust grew progressively over Earth history. On the
other hand, Armstrong (1991) argued that these data supported his model of no crustal growth. In order to examine this claim, the data
compilation of Armstrong (1991) is shown in Fig. 4.34, together with an
evolution line for the MORB source which he claimed was a product of his 1981
model. However, most of the available Nd data can be
satisfied by a less extreme evolution line in Fig. 4.34 (solid line), which is sub-parallel
to DePaolo’s curve since 4 Byr
ago. The solid evolution line represents the composition of the most depleted
mantle sources, whereas DePaolo’s line represents the
source of arc magmatism, which is generally less
depleted.
Fig. 4.34. Initial , Nd for
terrestrial rocks, compiled by Armstrong (1991), compared with his ‘big bang’
MORB evolution line. Solid curve is an alternative MORB depletion line for a crustal growth model. Note that this is not expected to
agree with the dashed arc-source model of DePaolo
(1981).
The
gradual depletion of the upper mantle which is portrayed by the solid line in
Fig. 4.34 can only be reconciled with a constant-crustal-volume
model if the average composition of
the crust changes over geological time. In principle this requirement is satisfied
in a model where the Earth begins its evolution with a thick basaltic
(‘oceanic’) crust, which is gradually replaced by continental crust over
geological time. This involves a non-plate tectonic model for Archean crustal evolution (e.g.
West, 1980). A similar model was also supported by Galer
and Goldstein (1991), who proposed that a thick, long-lived alkali basalt crust
was built up in the Archean by small-degree melting of
the deep mantle. However, as evidence mounts for earlier and earlier operation
of plate tectonic processes in Earth history (e.g. Williams et al., 1992), there seems little reason
to invoke a prolonged pre-plate tectonic era.
Chase
and Patchett (1988) proposed that accelerated early
mantle depletion is in fact consistent with plate tectonic processes. They
postulated that the storage of subducted oceanic
crust in the mantle, before re-homogenisation with the depleted mantle (by
convection), would give rise to a hidden enriched reservoir in the deep mantle
to balance early depleted mantle. According to this model, the amount of
‘stored’ subducted oceanic crust has grown over Earth
history, although gradual cooling of the
earth prevents the system from reaching a steady state by increasing the
lifetime of subducted crust over geological time.
Taking the cooling process into account, a duration of several hundred Myr to establish Early Archean
mantle depletion is consistent with evidence for a 1 ) 2 Byr
present-day lifetime of subducted oceanic crust, as
deduced from ocean island basalts (section 6.3.1).
4.4.4 Early Archean crustal provinces
Evidence
for open system behaviour of Sm-Nd in komatiites (section 4.3.3) casts doubt on some of the
evidence for strong mantle depletion during the Earth’s early history. However,
new evidence for early depletion of the mantle was provided by Bennett et al. (1993) and Bowring and Housh (1995) based on the analysis of granitoid
orthogneisses from Western Greenland and the Slave
Province of northern Canada. Since these rock types are generally resistant to
metamorphic Nd disturbance, the new evidence for
Early Archean depleted mantle appeared much stronger.
However, the new evidence was itself challenged by Moorbath
and Whitehouse (1996) and Moorbath et al. (1997). Since this discussion has
critical implications for crust–mantle evolution it will be examined in some
detail, beginning with the data from the Acasta
gneisses of the
Bowring
and Housh (1995) used SHRIMP U–Pb
ages to calculate Nd initial ratios for a variety of
rock types from Early Archean Acasta
gneisses, from 3.6 to 4.0 Byr age, yielding , Nd values
as high as +4 and as low as –5 (Fig. 4.35). However, Moorbath
and Whitehouse (1996) observed that most
of the suite analysed by Bowring and Housh lay on an
‘errorchron’ (section 2.6.3) with an age of ca. 3.3 Byr, which they attributed to an intense metamorphic event
which partially homogenised whole-rock Sm-Nd systems
at that time. This result was later confirmed (Moorbath
et al., 1997) by the analysis of 20 new samples, yielding a combined errorchron
age of 3370 " 60 Myr and initial ratio (, Nd)
of -5.6 (Fig. 4.35). Hence, they argued
that , Nd values calculated at the U–Pb
crystallisation ages of 3.6 to 4.0 Byr were not
accurate measures of the magma source compositions at those times.
Fig. 4.35. Nd isotope
evolution diagram showing initial , Nd values calculated
at the various U/Pb ages of the Acasta
gneisses ( ! ) compared with the initial ratio of a 3.3 Byr
old best-fit errorchron. The outer envelope of Nd isotope growth curves is shown for reference. After Moorbath et al.
(1997).
Bowring
and Housh (1996) argued in reply that the 3.3 Byr errorchron age could itself
be a mixing line with no age significance. However, Moorbath
et al. (1997) showed that there was
no correlation between Nd isotope ratios and Nd concentrations in the Acasta
gneisses, as would be expected from a mixing line. Such a mixing line was seen
in Archean lavas from the Abitibi
Belt of Ontario which had been contaminated with crustal
material (Fig. 4.7). Moorbath et al. also argued that the relatively low MSWD value for their own
Acasta samples (8.8) could not be explained by a
fortuitous combination of short segments of 3.8 Byr isochrons, since this would yield a much higher MSWD value
of several hundred. From this evidence, it appears that the 3.3 Byr old errorchron may date a real geological event (or series of
events) which caused homogenisation of Sm-Nd systems
in the Acasta gneisses. Since this event postdates
the oldest zircon ages by up to 600 Myr, it is
concluded that reliable initial , values for the mantle source cannot be
calculated.
Early
Archean rocks from western Greenland represent the
other principal source of evidence about
the composition of the Early Archean mantle. Evidence
for strongly depleted mantle sources was first found in the Isua
supracrustal sequence (e.g. Hamilton et al., 1983), and was supported by
analysis of Amitsoq gneisses, and mafic
enclaves in these gneisses named the Akilia suite
(Bennett et al., 1993). The upper
envelope of these three suites defines an evolution line for highly depleted
mantle in the Early Archean (Fig. 4.36). This led
Bennett et al. to propose a model of
two stage evolution in the early Earth, in which early intense mantle depletion
was followed by a period of mixing with deeper less depleted mantle, causing an
inflection in the depleted mantle evolution line.
Fig. 4.36. Nd isotope
evolution plot showing , Nd values of Bennett et al. (1993) at the ages determined by U–Pb
analysis. ( ! ) = Amitsoq gneiss; ( " ) = Akilia
enclaves in Amitsoq gneisses; shaded zone = Isua supracrustals. Large
diamonds indicate ages and initial ratios for three Sm–Nd errorchrons of Moorbath et al. (1997).
However,
Moorbath et al.
(1997) showed that Akilia, Amitsoq
and Isua suites all yield Sm-Nd
errorchrons with ages significantly younger than
SHRIMP U–Pb zircon ages. This suggested to Moorbath et al.
that the Sm–Nd systems in
many of these rocks had been reset in a manner similar to the Acasta gneisses. However, this critique was itself the
subject of a scientific discussion (Bennett and Nutman,
1998; Kamber et
al., 1998), after which further debate was continued by Kamber
and Moorbath (1998), Whitehouse et al. (1999) and Nutman et al. (2000). Since space here is
limited, the present author will give only a brief overview of the debate.
The
belt of Early Archean rocks in west Greenland runs in
a northeasterly direction parallel to Godhabsfjord, from Amitsoq on the
coast, to Isua at the edge of the inland ice field.
Based on detailed SHRIMP U–Pb analysis (see section
5.2.3 for the method), it now appears that this belt (termed the Itsaq gneiss complex) was created in two main events. Near Isua in the north, most U–Pb ages
cluster around 3.8 Byr, which appears to be the age of
the earliest crust-forming event in the area (Nutman et al., 2000). However, near Amitsoq in the south, most U–Pb
ages cluster round 3.65 Byr, but zircons sometimes
have cores up to 3.8 Byr in age. Furthermore, in the
latter area, whole-rock Rb–Sr,
Pb–Pb and Sm–Nd errorchrons
all give ages around 3.65 Byr (Whitehouse et al. 1999). This suggests that most of
the crust in the south is 3.65 Byr old, but contains
inherited fragments of 3.8 Byr old material. We can
infer from this that any fragments of 3.8 Byr old
rocks in the south probably had their Nd isotope
systems reset 3.65 Byr ago, but the extent to which
the rocks in the north preserve an accurate 3.8 Byr
old initial ratio is unclear.
The
Amitsoq gneisses analysed by Bennett et al. (1993) came from both ends and
the middle of the Itsaq gneiss complex. On the Sm–Nd isochron
diagram (Moorbath et
al., 1997) these samples had much more scatter than 26 Amitsoq
gneisses from the southern end of the belt, which defined an errorchron age of 3640
" 120 Myr (MSWD = 10) with an initial , Nd value
of + 0.9 " 1.4 (Fig.
4.37). Unfortunately there were not enough Nd isotope
analyses from the northern area of the gneiss complex, where old U–Pb ages predominate, to see whether this part of the
complex had consistently different Nd isotope
signatures from the southern part. Therefore, Moorbath
et al. suggested that the generally
increased scatter in the suite analysed by Bennett et al. was more likely due to later partial metamorphic disturbance
during Late Archean or Mid Proterozoic
events. They were not able to prove that resetting had occurred, although Hf isotope analysis (section 9.2.3) suggested that this
might be the case, because the extremely positive , Nd values
determined by Bennett et al. were not
matched by similarly positive , Hf values.
Fig. 4.37. Sm–Nd isochron diagram showing Amitsoq gneisses of the Itsaq
gneiss complex. ( ! ) = southern suite of gneisses which define a 3640 Myr
errorchron. ( " ) = more scattered data of Bennett et al. (1993). After Moorbath
et al. (1997).
When
Moorbath et al.
(1997) examined Nd data for five samples of Akilia mafic enclaves analysed by
Bennett et al. (1993), they
discovered a very strong linear array with an age of 3675 " 48 Myr
and an initial ratio of + 2.6 " 0.4 , units (Fig. 4.38). The low MSWD of 2.1 for this regression makes it
statistically an isochron, but since U–Pb ages for the gneissic host rocks range from 3784 to 3872
Myr, Moorbath et al. interpreted the age as an
isotopic homogenisation event associated with the engulfing of the enclaves by
the Amitsoq magmas. Bennett and Nutman
(1998) countered that these samples came from too wide an area to be attributed
to metamorphic homogenisation, and in response Kamber
et al. (1998) reinterpreted the Sm–Nd age for the enclaves as
intrusive, and attributed the older U–Pb ages in the
host gneisses to inherited zircons.
Unfortunately,
these interpretations are also equivocal, due to the small size of the sample
suite. Four of the five samples analysed are from the southern area of the
gneiss complex where the host gneisses are generally of 3.65 Byr age. Hence, in this area, both the isotope
homogenisation model of Moorbath et al. (1997) and the young intrusive age proposed by Kamber et al.(1998)
can explain the 3675 Myr array. Only one enclave from
the northern area was analysed, although three duplicates were determined. This
sample appears to be more than 3.8 Byr old, but since
it has the same Nd isotope signature as the host
rocks, it cannot be proven that the sample has not been isotopically
homogenised with the host rocks at some time. Hence, this sample might provide
evidence for very depleted Early Archean mantle, but
it is very risky to base such a model on a single sample.
Fig. 4.38. Sm-Nd isochron for Akilia enclaves in
the Amitsoq gneisses. Inset shows Nd
isotope evolution lines for four samples. Shaded box shows range of U-Pb ages. After Moorbath et al. (1997).
It
is concluded from the above discussion that the re-setting model of Moorbath et al.
(1997) does not apply to most of the western Greenland rocks, but nevertheless,
the extremely depleted , Nd values proposed by Bennett et al. (1993) remain unproven in the
face of the geologically complex
evolution of these rocks, and in the face of Hf
isotope evidence (section 9.2.3). A more conclusive determination of Early Archean Nd isotope signatures
must depend on the analysis of a larger number of samples from the northern
part of the Itsaq gneiss complex, where 3.8 Byr old rocks seem to dominate. This shows that geochemical
deductions are only as good as the geological sampling, even in one of the
world’s most inaccessible field localities.
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