4.3 Model ages and crustal
processes
As outlined above, one of the principal uses of
the Sm)Nd model age
method is to determine what are often called ‘crustal-formation’ or
‘crustal-extraction’ ages. However, the model age method is most often applied when
a long or complicated geological history precludes a more direct method of
determining crustal age. One of the strengths of the Sm)Nd model age method, as applied to
whole-rock systems, is that it provides the opportunity to see back through
erosion, sedimentation, high-grade metamorphism and even crustal melting
events, which usually re-set other dating tools. However, these processes may
cause complications in the interpretation of model ages. Hence, it is important
to examine Sm)Nd systematics in well-constrained examples in order to estimate the
reliability of model ages in more complex environments.
4.3.1 Sedimentary systems
The behaviour of the Sm)Nd system during erosion can be
examined by comparing the calculated model ages of river-borne particulates
with the average geological age of sediment sources in the watershed. Goldstein
and Jacobsen (1988) performed such a study on particulates in American rivers.
They found that rivers draining primary igneous rocks carried sediment which
accurately reflected the crustal residence age of the source (Fig. 4.21).
Rivers draining sedimentary watersheds were not properly testable, since the
crustal residence age of their sources had not been adequately quantified.
Fig. 4.21. Plot of Nd model ages for river
particulates against the area-weighted average crustal residence age of rocks
within the watershed. Data are shown for igneous)metamorphic drainage basins only.
After Goldstein and Jacobsen (1988).
The
behaviour of the Sm)Nd system during sedimentation can be further
tested by comparing Nd model ages on different size-fractions of sediment. In
an early study on bottom sediment from the Amazon River, Goldstein et al. (1984) found that different
size-fractions yield only a small range of crustal residence ages (1.54 ) 1.64 Byr), despite having a large
range of total Nd contents (17 ) 47 ppm). Similar agreements in model age were found by Awwiller and
Mack (1991) on mud- and sand-grade sediments from the
In
order to see whether a similar degree of homogeneity is displayed by deep-sea
turbidites, McLennan et al. (1989)
compared model ages on sand and mud pairs in turbidites from several different
tectonic environments (Fig. 4.22). Their findings were rather variable; some
pairs demonstrating good agreement in model age, whereas others gave poor
agreement. These variations probably reflect the petrological make-up of the
sediment. Both a mature passive margin sediment with less than 5% lithic
volcanic fragments and a very immature back arc sediment with ca. 90% lithic
volcanic fragments displayed good agreement of model ages agreement between
sand and mud fractions (square symbols). These uniform types of sediment may
therefore yield useful constraints on model age. In contrast, sediments with
intermediate fractions of volcaniclastic material gave inconsistent model ages
(8and o in Fig. 4.22). The latter type was prevalent in continental arcs,
and can be attributed to variable mixing between old continental detritus and
young volcanic detritus with different grain sizes. Continental arcs therefore
tend to generate widely scattered model age data.
Fig. 4.22. Plot of depleted mantle model ages
in mud versus sand grade fractions
from deep sea turbidites in different tectonic environments. After McLennan et al. (1989).
Nelson
and DePaolo (1988) tested the effects of mixed sediment provenance on Sm)Nd systematics in two small basinal
systems. In both cases, the different sediment sources were petrographically
and geochemically well characterised. In order to quantify the mixing process,
Nelson and DePaolo plotted , Nd against a petrographic index (percentage of lithic volcanic
fragments). The good correlation observed between the end-members and various
mixtures (Fig. 4.23) attests to the ‘immobile’ behaviour of Nd during erosion
and sedimentation. This does not avoid
the problem of mixed provenance, but it shows that coupled isotopic and
petrological analysis of a suite of samples can be used to detect and quantify
the mixing process.
Fig. 4.23. Plot of , Nd against modal % lithic volcanic
fragments to show petrographic dependence of the Sm)Nd system in sedimentary basins with
mixed provenance. ( € ) = Hagar basin; ( Q ) = Espanola basin. After Nelson and DePaolo (1988).
It
should not be inferred from these results that the Sm–Nd system is completely
immune to disturbance during erosion and sedimentation. Any situation where
chemical weathering is involved can potentially mobilise the REE, and if this
mobilisation occurs a very long time after crustal formation, it can have a
significant effect on model ages (e.g. Ohlander et al., 2000). This means that Nd model ages should always be based
on the sampling of fresh, unweathered material. However, the above studies show
that in the absence of chemical weathering, Sm–Nd analysis of sedimentary rocks
can often give accurate provenance information.
4.3.2 Meta-sedimentary systems
Many studies have been undertaken to assess the
mobility of REE, and specifically Sm–Nd, under various diagenetic and
metamorphic conditions. Paradoxically, the evidence suggests that the REE may
be more mobile during diagenesis and low-grade metamorphism than during
high-grade metamorphism and partial melting. This may be because of a paucity
of mineral phases growing under low grade metamorphic conditions into which REE
are strongly partitioned. In contrast, there are several igneous and high grade
metamorphic minerals into which REE are strongly partitioned.
Stille
and Clauer (1986) and Bros et al.
(1992) demonstrated that in carbonaceous (black) shales, Sm)Nd systematics in the microscopic
clay-mineral fraction can be re-set by diagenesis. They showed that in some
cases, sub-micron sized particles could yield Sm)Nd isochrons, which they interpreted
as dating diagenesis. The accuracy of such ages remains to be proven, given the
evidence that Rb)Sr dating of clay minerals can be upset by detrital inheritance (section
3.5.1).
Diagenetic
mobilisation of REE on a mineralogical scale does not necessarily imply the
existence of open Sm)Nd systems on a whole-rock scale. A suggestion
that such a scenario could occur was
made by Awwiller and Mack (1991) on the basis of Sm)Nd analysis of borehole samples from
Texas. Weak positive correlations were observed between depth in the bore hole
and depleted mantle model age, which these authors attributed to diagenetic
loss of radiogenic Nd, as well as minor increases of Sm/Nd ratio with depth.
However, the study was based on very small ‘whole-rock’ samples (less than 10
g), and variations in sediment provenance could not be ruled out, so the
evidence was equivocal.
Additional
evidence for diagenetic disturbance of Sm-Nd systems was obtained by Bock et al. (1994), based on the sampling
of turbiditic sandstones and shales from
eastern New York State, deposited during the Taconic orogeny (ca. 470 Myr ago).
Nd isotope analysis was undertaken to determine sediment provenance, but the
model age results displayed more scatter than could explained by variations in
provenance alone. This can be seen in
Fig. 4.24, where two shales gave impossibly old ages, while two other samples
gave ages somewhat older than the remainder of the suite. Furthermore, these
four samples were found to be moderately or severely depleted in light REE
relative to the other samples. Since Nd isotope evolution lines converged at
around 500 Myr, it was suggested that the isotope system in some samples had
been disturbed, probably during early diagenesis. Unfortunately, the
size of samples analysed in this work was not reported.
Fig. 4.24. Nd isotope evolution diagram for
Middle Ordovician turbidites from eastern New York State showing an average
provenance age of 1.8 Byr, along with disturbed samples with anomalously old
model ages. ( ––– ) = sandstone; ( - - - ) = shale. After Bock et al. (1994).
The
lack of sample information in the previous study was rectified in later work by
Cullers et al. (1997) on Silurian
pelitic schists from western Maine. Whole-rock samples averaging more than 1 kg
in size were collected from pelitic units within lithologically variable
formations consisting of finely interbedded quartzite and pelitic schist. Most
samples gave very consistent depleted mantle model ages of 1.8 " 0.1 Byr, but a few samples from the
Perry Mountain Formation were found to give abnormally old ages from 2.5 to 5.3
Byr. The samples which yielded these old ages were again found to be light REE
depleted, yielding abnormally large 147Sm/144Nd ratios
from 0.15 - 0.19. These disturbances were attributed to leaching of light REE
from shales during diagenesis, and the lack of suitable minerals locally to
take up the released REEs. Based on comparison with an earlier study of REE
mobility in similar carbonaceous shales from central Wales (Mildowski and
Zalasiewicz, 1991), it was suggested that the REEs released from shale layers
may have been incorporated into phosphates which grew in more arenaceous
layers.
Collectively,
these studies show that caution must be exercised when using Nd isotope data to
determine sediment provenance ages, especially on carbonaceous shales. One way
of dealing with this kind of material is to use a ‘two-stage’ model age
calculation (e.g. Keto and Jacobsen, 1987). In this approach the measured Sm/Nd
ratio of the sample is used to make an age correction to the estimated time of
disturbance, beyond which an average crustal Sm/Nd ratio is used to estimate
the provenance age. This approach may have some validity, but there is no
substitute for the analysis of a large sample suite containing a variety of
rock types. It is then possible to detect and screen out samples that have been
subjected to diagenetic disturbance, allowing accurate provenance ages to be
determined for the formation as a whole.
4.3.3 Meta-igneous systems
Mafic and ultramafic rocks cannot be used to
determine accurate crustal formation ages because they have Sm/Nd evolution
lines sub-parallel to the chondritic evolution line. However, this property
allows the determination of precise initial Nd isotope ratios, which have been
widely used to determine the degree of mantle depletion in early Earth history
(section 4.4.3). A study by Lahaye et al.
(1995) has important implications for this type of data because it implies that
the initial Nd isotope signatures of many komatiites may have been disturbed by
subsequent alteration. Lahaye et al.
compared calculated initial isotope compositions (, Nd[t]) for whole-rock samples and
separated pyroxenes in five komatiite flows from the Abitibi and Barberton
belts. Many whole-rocks showed small (1 – 2 , unit) deviations from the
pyroxenes, but a few show much larger deviations, up to +5 and –10 units (Fig.
4.25). In view of this evidence, Nd isotope data on komatiites should be based
on a combination of whole-rock and mineral analyses in order to determine
reliable initial Nd isotope ratios.
Fig. 4.25.
Calculated initial Nd isotope ratios for whole-rock samples of
komatiites compared with separated pyroxenes (dashed lines). Data are plotted
against Yb concentration. After Lahaye et
al. (1995).
In
contrast to the evidence for disturbance of Sm-Nd systems in meta-basic rocks,
most granitoid rocks show much greater resistance to re-setting. For example,
Barovich and Patchett (1992) demonstrated that whole-rock Sm)Nd systems in granitic rocks can
remain undisturbed even during severe metamorphic deformation. They studied a
60 m-wide Mesozoic ductile shear zone cutting the Mid Proterozoic Harquahala
granite. Samples of increasingly deformed granite were found to yield a narrow
range of TCHUR model ages
around 1.58 Byr in two different traverses to within 1 m of the thrust plane
(Fig. 4.26). Closed-system behaviour was preserved even in samples showing
widespread sericitisation of plagioclase and significant epidote growth. Only
in ultra-mylonites less than 1m from the main thrust was a reduction in model
age of up to 150 Myr observed, possibly due to a high fluid flux which caused
calcite veining and intense alteration in the immediate vicinity of the thrust.
Fig. 4.26. Plot of TCHUR model ages for samples
of the Harquahala granite (
The
resistance of whole-rock Sm)Nd model ages to significant re-setting, even
during granulite-facies metamorphism, is demonstrated by the Lewisian
granulites from NW Scotland (Whitehouse, 1988). A ten-point Sm)Nd isochron for tonalitic gneisses
(section 4.1.3) yields an age of 2.60 Byr, and an initial ratio (, [t]) of !2.4 relative to CHUR (Fig. 4.27a). This isochron is argued to date the
metamorphic event. However, the TDM
model ages of these gneisses fall in the range 2.84 ) 3.04 Byr, with an average
value of 2.93 Byr (Fig. 4.27b). These ages have been scattered slightly by
metamorphism, but still yield an average value very close to the undisturbed
isochron age of the Drumbeg mafic complex (2.91 Byr). The effects of
metamorphism on Sm–Nd systematics in granitoid rocks will be discussed further
in section 4.4.4.
Fig. 4.27. Nd isotope evolution diagrams for
the Lewisian complex of NW Scotland. a) Showing initial ratios for layered
mafic bodies and a suite of granulite-facies tonalitic gneisses; b) showing Sm)Nd evolution lines for individual
tonalitic gneisses. After Whitehouse (1988).
4.3.4 Partially melted systems
Nelson and DePaolo (1985) attempted to place
upper limits on the disturbance of model ages under conditions of intra-crustal
reworking by considering the limiting case of crustal anatexis. From crustal
melting models (Hanson, 1978), they estimated that the maximum amount of Sm/Nd
fractionation likely to arise by intra-crustal melting processes ()) was 20% of the pre-existing
fractionation between sample Sm/Nd and CHUR Sm/Nd. This fractionation factor f was defined by Nelson and DePaolo
(1985):
147Sm/144Ndsample
fSm/Nd = ))))))))))
! 1 [4.6]
0.1967
Using this notation, the error in depleted
mantle model age (TDM) introduced
by an intra-crustal fractionation event is given by:
Err
TDM = ) fSm/Nd
. (TCF ! Tm) [4.7]
where TCF
is the true crustal formation age and Tm
is the age of the partial melting event. This error propagation is illustrated
schematically in Fig. 4.28. The problem can be minimised by analysing samples
with melting ages fairly close (< 300 Myr ?) to their formation age.
Fig. 4.28. Schematic diagram of Nd isotope
systematics to show possible errors in model age arising from Sm/Nd
fractionation during intra-crustal melting. After Nelson and DePaolo, (1985).
Evidence
that intra-crustal melting causes relatively minor perturbations in model age
has encouraged the use of granitic plutons to determine crustal formation ages
on associated country-rocks (assuming that the granites are the products of
anatexis of those country-rocks). The approach has the advantage of allowing
basement mapping of large areas with a minimal number of analyses, since each
pluton can be expected to have averaged the composition of a large volume of
crust. It was used to great effect by Nelson and DePaolo (1985) to map out the
crustal extraction ages of huge belts in the central United States. The method
is appropriate for this application because Phanerozoic cover obscures most of
the central
Weaknesses
in this approach are revealed, however, when model age results do not
correspond to known events represented by igneous crystallisation ages. The
model ages of 2.0 )
2.3 Byr in the ‘Penokean’ and ‘Mojavia’ terranes proposed by Bennett and
DePaolo (1987) exemplify this problem. It is likely that they represent
Proterozoic mantle-derived magmas which mixed with large quantities of
re-melted Archean crust to generate mixed model ages (Fig. 4.29) which have no
meaning as crustal extraction ages (Arndt and Goldstein, 1987).
Fig. 4.29. Schematic illustration of magma
mixing as a mechanism capable of generating mixed provenance ages which do not
date any real geological event. After Arndt and Goldstein (1987).
It
is concluded that model age mapping of gneiss terranes is a powerful method to
delimit the geographical extent of crustal provinces, but that geochronological
confirmation of the resulting model age provinces must be provided by other
methods. Typically, Nd model ages define the oldest apparent age for a terrane,
while a minimum age is defined by U–Pb dating of cross-cutting plutons. The
smaller the difference between the average Nd model age and U–Pb igneous
crystallisation ages, the tighter the constraints on the Nd model age as a true
crustal extraction age.
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