,NCL”D,NG
ISOTOPE GEOSCIENCE
ELSEVIER
Chemical Geology
135 (1997) 213-231
Extreme Nd-isotope heterogeneity in the early Archaean - fact
or fiction? Case histories from northern Canada and West
Greenland
S. Moorbath
*, M.J. Whitehouse
‘, B.S. Kamber
Department of Earth Sciences, Oxford Uniuersiry, Parks Road, Oxford, OXI 3PR, UK
Received 30 June 1996; accepted 22 July 1996
Abstract
Sm-Nd data on rock suites from early Archaean provinces in northern Canada and West Greenland clearly demonstrate
that tectonothermal
(i.e. igneous, metamorphic, tectonic) processes which affected the rocks long after their formation
produced open-system
behaviour leading to effective resetting of the Sm-Nd system accompanied by complete, or
near-complete, Nd-isotope homogenisation. This means that extreme caution is required in interpreting highly variable initial
eNd values of ancient rocks in terms of long-standing regional mantle heterogeneity or of complex mantle-crust interaction
processes. In particular, calculated initial ??
Nd values based on high-precision zircon U-Pb dates may be of little or no
significance in terms of geochemical evolution of early mantle and crust source regions.
A striking example is provided by the Acasta gneisses of northern Canada, with published SHRIMP U-Pb zircon dates in
the range 3.6-4.0 Cia and apparent, initial eNd values in the range -4.8 to f3.6 (Bowring and Housh, 1995). A
combination of 34 published and new Sm-Nd whole-rock analyses for a wide range of rock types yields a regression
(error&on)
age of 3371 f 59 Ma (MSWD = 9.21, with initial ??
Nd = -5.6 k 0.7. Whilst the very negative initial Ed,,
provides strong, independent support for the extreme age of the Acasta gneiss protolith, resetting of the Sm-Nd system at
- 3.4 Ga renders calculation of initial Ed,, based on the zircon U-Pb dates geologically meaningless.
Analogous considl:rations for early Archaean Akilia enclaves and host Amitsoq gneisses of West Greenland suggest that
their published range of initial Ed,, values of - -4.5 to + 4.5 at - 3.73-3.87 Ga (Bennett et al., 1993) may be
unrealistically wide and, therefore, inappropriate for modelling upper-mantle heterogeneity.
In an attempt to determine a realistic initial eNd value, we have regressed 58 published and new Sm-Nd data for two
major rock units of the Isua supracrustal belt (felsites and mica-schists), regarded as having a short-term crustal history
combined with minimal Sm-Nd disturbance. The Sm-Nd regression yields an age of 3776 &-52 Ma (MSWD = 8.2), with
initial eNd = + 2.0 k 0.6. This value is much closer to conventional depleted-mantle models (e.g., DePaolo et al., 1991) than
to the value of - +4.5 proposed by Bennett et al. (1993) for this age range. Our observations suggest that there may have
been no major change in processes between early Archaean and more recent styles of depleted-mantle evolution.
Keywords: Archaean;
Mantle: Crust; Nd isotopes; Geochronology
* Corresponding author.
’Present address: Laboratory
0009-2541/97/$17.00
Copyright
PI1 SOOO9-2541(96)00117-9
for Isotope Geology,
Swedish Museum of Natural History,
0 1997 Elsevier Science B.V. All rights reserved.
Box 50007, S-104 05 Stockholm,
Sweden.
214
S. Moorbath et al. / Chemiwl
1. Introduction
Sm-Nd characteristics
of precisely dated early
Precambrian rocks show a wide range of initial ??
Nd
values, variously interpreted as evidence for transient, highly depleted mantle reservoirs during early
Earth history (e.g., Bennett et al., 19931, or as rock
genesis from mixtures between mantle-derived
and
crustal melts derived from extremely old ( N 4.3 Ga),
heterogeneous, depleted (high Sm/Nd) and enriched
(low Sm/Nd) reservoirs (e.g., Bowring and Housh,
1995). Such models are of great potential importance
for studying the geochemical evolution of the earliest
crust and mantle and could mean, for example, that
the isotopic evolution of the Earth’s mantle reflects
progressive eradication of primordial heterogeneities
related to early differentiation.
In addition, Bennett
et al. (1993) proposed a major change in processes
between early Archaean (> 3.7 Ga) and later styles
of depleted-mantle
evolution, based on a Nd-isotope
evolution curve with a pronounced
eNd peak of
f4.5 at 3.7 Ga. This contrasts sharply with the
conventional,
smooth depleted-mantle
Nd-isotope
evolution curve of DePaolo et al. (19911, which
reflects removal
of a light rare earth element
(LREE)-enriched
component through time, resulting
in a LREE-depleted
reservoir (see also Galer and
Goldstein, 199 1).
All these initial e,,-based models assume that the
analysed rocks remained closed to disturbance of the
Sm-Nd system since the measured age of the rock,
most commonly obtained from SHRIMP U-Pb dating
of zircons. Here we question this assumption, because published Sm-Nd data clearly suggest that
some critical sample suites from early Archaean
terrains have been open systems to Sm and/or Nd
during much later tectonothermal events, sometimes
accompanied
by nearly complete Nd-isotope
homogenisation.
Such open-system
behaviour would
negate the validity of using initial en,, values for
tracing early mantle/crust
evolution,
even when
based on precise and valid age data for the metamorphic precursor, because the calculated initial ??
Nd
values would be apparent rather than real. Numerous
recent studies have documented
open-system
behaviour in the Sm-Nd
system during subsequent
metamorphism
and metasomatism (e.g., McCulloch
and Black, 1984; Windrim et al., 1984; Black and
Geology 135 (1997) 213-231
McCulloch, 1987; Whitehouse, 1988; Bridgwater et
al., 1989; Li et al., 1990; Tourpin et al., 1991; Gruau
et al., 1992; Frost and Frost, 1995; Lahaye et al.,
1995; Poitrasson et al., 1995; Gruau et al., 1996).
Another line of evidence suggesting that the published range of initial eNd values for the early
Archaean is much too wide comes from a comparison of Hf- and Nd-isotope systematics in zircons
from ten early Archaean gneisses and supracrustal
rocks from West Greenland (Vervoort et al., 1996).
These authors conclude that
. . the range of Nd compositions measured in these
ancient gneisses do not faithfully represent primary
isotopic variations in their source regions but rather
have been produced, at least in part, by whole-rock
geochemical
disturbance.
Until these Nd data are
examined more critically, they should not be used to
constrain the details of early earth history.
Here we review published, and report new, SmNd data on a wide range of early Archaean rocks
from northern Canada and West Greenland. Close
analysis of their whole-rock Sm-Nd isotopic systematics demonstrates that, in some cases, open-system
disturbance of the Sm-Nd system has affected these
rock units long after their presumed age of rock
formation. We attach particular significance to wellcorrelated Sm-Nd regression lines (isochrons and
errorchrons),
interpreting
them as resulting from
nearly complete resetting of the Sm-Nd system during much later tectonothermal
events. In contrast,
some previous workers (e.g., Bennett et al., 1993;
Bowring and Housh, 1995) do not present their
Sm-Nd data on isotopic evolution diagrams because
they appear to assume, a priori, that petrologically
heterogeneous rock suites (with the same or different
zircon U-Pb ages) cannot possibly have homogeneous initial Nd-isotope ratios and that a linear array
on a Sm-Nd isotopic evolution diagram significantly
post-dating rock formation automatically represents a
mixing line with no age significance,
rather than
defining the age of a resetting event (e.g., Bowring
and Housh, 1995). We are very sceptical about this
highly constrained approach to the interpretation of
well-correlated Sm-Nd regressions and, particularly,
to the suggestion that minor statistical scatter about
well-correlated regression lines (but with MSWD >
1) necessarily results from initial end heterogeneity
S. Moorbath et al. / Chemical Geology 135 (1997) 213-231
of a rock unit, rather than from subsequent minor
open-system behaviour to Sm and Nd long after the
age of rock formation. Our overall philosophy to the
interpretation
of wihole-rock isochrons broadly follows that of Camleron et al. (1981) who clearly
distinguish between analytical and geological scatter
and demonstrate (what many geochronologists
intuitively realise and practise) that a realistic, though
variably precise, assessment of the primary igneous
or metamorphic
age, as well as the initial isotope
ratio, of a given rock suite is quite possible despite a
small degree of statistical, geological scatter in excess of analytical error on an isochron diagram. In
other words, it is not necessary to obtain perfect
isochrons
(MSW:D = 1) to yield
meaningful
geochronological
information. Whilst Cameron et al.
(1981) dealt entirely with the Rb-Sr method, we
believe that their general approach applies to the
Sm-Nd method with equal validity.
Applying these general principles to published
and new Sm-Nd data for rock units from northern
Canada and West Greenland
leads us to general
agreement with the recent Hf-isotope work of Vervoort et al. (1996), suggesting that the range of
initial eNd values for the early Archaean may be
considerably narrower than has been claimed in some
of the recent work already quoted above.
2. Acasta
Canada
gneisses,
Slave
Province,
N.W.T.,
2.1. Previous work
The most detailed geological description so far of
the Acasta gneisses has been given by Bowring et al.
(1990). They describe these rocks as a heterogeneous
assemblage composed mainly of strongly foliated
biotite-hornblende
tonalitic-to-granitic
orthogneisses
commonly interlayered on a small scale with amphibolitic and chlorifc schlieren, boudins and layers.
Large areas of amphibolites also occur, together with
less abundant litheologies of talc-silicate
gneisses,
quartzites, biotite schists and ultramafic schists, some
of which presumably
represent
metamorphosed
supracrustal rocks I(paragneisses). Metamorphic temperatures are regarded as somewhere between 400”
and 650°C. The above rocks are furthermore intruded
by weakly foliated gabbroic-to-dioritic
dykes and
215
pods, as well as weakly-to-strongly
foliated biotitebearing granitic rocks. In general, the post-formational tectonic and metamorphic history of the Acasta
gneisses is not well understood.
Zircon U-Pb dates for the Acasta gneisses have
been reported by Bowring et al. (1989a,b,1990) and
by Bowring and Housh (1995). Zircons from two
samples
indicated
that the tonalitic-to-granitic
gneisses crystallised at 3962 & 3 Ma (Bowring et al.,
1989a). Subsequently,
Bowring and Housh (1995)
reported further SHRIMP zircon U-Pb ages ranging
from 4.0 to 3.6 Ga for ten Acasta gneiss samples,
ranging in composition from amphibolitic to granitic.
Importance was also attached to a significantly later
event at 3.48 Ga (Bowring et al., 1990):
. . . consistent with periods of intrusion,
and metamorphism at ca. 3.4-3.6 Ga
deformation
A total of 13 Sm-Nd analyses has been reported
by Bowring et al. (1990) and Bowring and Housh
(1995). For reasons stated in these papers, zircon
U-W dates ranging from 3.6 to 4.0 Ga were used as
the basis for calculating initial ??
Nd values. Bowring
and Housh (1995) report that the Acasta gneisses
. . . exhibit a wide range of initial Ed,, ( + 3.5 to - 4
at 4.0 Ga, and + 4 to - 7 at 3.6 Ga).
This wide range was then used as the basis of
Bowring and Housh’s (1995) complex model of
heterogeneous mantle and crust evolution and interaction in the early Earth.
Our scepticism of the validity of the above approach was first aroused by plotting a Sm-Nd evolution diagram (143Nd/ IaNd vs. 14’Sm/ ‘44Nd) for 7
Acasta gneiss samples based on Bowring et al.‘s
(1990) original Sm-Nd analyses, combined with preliminary work on a small suite of compositionally
varied samples collected by M.J. Bickle and by S.
Maruyama, and analysed for Sm-Nd at Oxford. The
pooled 7-point regression age (unpublished) of 3464
_t 97 Ma (MSWD = 4.9) suggested that the Sm-Nd
system might have been reset at this time by an
igneous or metamorphic event. Furthermore, the negative initial eNd value of -4.8 + 1.0 obtained from
the Sm-Nd
regression suggested that the Acasta
gneisses had a low-Sm/Nd
(enriched) crustal protolith, probably as old as the age obtained from
zircon U-Pb analysis. (This is discussed in much
greater detail below.) Nevertheless, the apparent re-
216
S. Moorbnth et al./Chemical
setting of the Sm-Nd system at - 3.46 Ga precluded the use of samples with individually
determined zircon U-Pb ages in the range of 4.0-3.6 Ga
for calculating valid initial ??
Nd values.
2.2. Samples and results
In order to test and extend the above preliminary
findings, one of us (S.M.) made a collection of 20
samples of Acasta gneisses in summer 1995 (under
the general guidance of W. Padgham and S. Bowring)
from several of the type localities described previously (e.g., Bowring et al., 1990; Bowring and Housh,
1995). Sm-Nd analyses on these 20 samples, together with the previous 5 Oxford Sm-Nd analyses
Table 1
Sm-Nd analytical
Sample
SM/Acl
SM/Ac2
SM/Ac3
SM/Ac4
SM/AcS
SM/Ac6
SM/Ac7
SM/Ac8
SM/Ac9
SM/AclO
SM/Acll
SM/Acl2
SM/Acl3
SM/Acl4
SM/AclS
SM/Acl6
SM/Acl7
Geology 135 (19971 213-231
mentioned in the previous section, are reported in
Table 1. All 25 samples plot on a well-correlated
Sm-Nd regression (Fig. 11, yielding an age of 3348
k 65 Ma (MSWD = 8.8) with an initial eNd of
- 5.6 f 0.7. All regressions in this paper were calculated by the method of Ludwig (19911, with errors
quoted at 2 u.
The Acasta sample suite comprises a very wide
range of rock types from granitic, through intermediate, to amphibolitic gneisses, with ranges of SiO,
from 74 to 43 wt%, K,O from 5.3 to 0.9 wt%, CaO
from 0.9 to 10.7 wt%, MgO from 0.6 to 15.6 wt%,
and total Fe (as Fe,O,) from 0.6 to 19.6 wt%. The
range of mineralogical
compositions
varies accordingly. The rock samples were collected from several
data for Acasta gneisses
Mineralogy
PI, Hbl, Qtz, Chl
Pl, Hbl, Bt, Spn, Qtz
PI, Qtz, Ep, Chl, Grt, Bt
Hbl, PI, Bt, Qtz
Hbl, PI, Ilm, Spn, Bt, Qtz
Hbl, Pl, Ilm, Spn, Ep, Chl
Pl, Qtz, Kfs, Ep
Hbl, Chl, Pl, Ilm, Stp
Hbl, PI, Fe-ore, Chl, Qtz
Qtz, Pl, Kfs, Ep, Chl
Kfs, Qtz, Pl, Bt
PI, Qtz, Bt, Hbl, Grt
PI, Qtz, Hbl, Bt, Grt
Hbl, Pl, Bt, Qtz, Ilm
PI, Hbl, Qtz, Bt, Spn
Qtz, Kfs, Pl, Bt, Ep
PI, Qtz, Bt, Hhl, Kfs,
Ep, Spn
SM/Ac18
Bt, Grt, Hbl, Ilm, Qtz, Pl
SM/Acl9
Qtz, Kfs, PI, Bt
SM/Ac20
Hbl, Pl, Bt, Fe-ore
Maruyama Kfs, Qtz, PI, Bt
Bickle-A2 Qtz, Pl, Kfs, Bt
Bickle-A3a Kfs, Qtz, PI
Bickle-A3bP1, Hbl, Qtz, Bt, Chl, Spn
Bickle-A4 Qtz, PI, Kfs, Bt
‘43Nd/ ‘44Nd
14’Sm/ l”Nd
SiO,
(wt%)
Sm
(ppm)
Nd
@pm)
63.9
49.4
73.7
59.2
45.5
45.7
66.5
48.7
43.4
61.3
69.8
69.1
69.6
53.3
55.4
71.5
63.2
3.535
2.495
4.353
2.463
0.998
1.308
0.492
1.679
0.927
4.402
7.125
10.100
2.009
3.564
3.937
4.237
4.111
16.93
12.09
19.10
7.159
3.510
4.953
4.099
9.949
3.668
37.03
59.95
44.05
7.373
15.52
19.45
31.83
32.81
0.5 10842
0.510691
0.511090
0.512597
0.511809
0.511581
0.509634
0.510331
0.511406
0.509577
0.509557
0.511026
0.511608
0.511047
0.510649
0.509789
0.50972 1
0.1262
0.1248
0.1378
0.2080
0.1719
0.1596
0.0725
0.1020
0.1527
0.07 18
0.0718
0.1386
0.1647
0.1388
0.1223
0.0804
0.0757
43.3
70.8
43.3
_
2.582
2.426
1.397
5.365
8.808
1.150
7.579
1.582
12.87
17.61
5.417
35.03
36.10
7.124
32.49
5.914
0.510721
0.509807
0.511446
0.510016
0.511280
0.510139
0.511125
0.511582
0.1212
-
0.1559
0.0925
0.1475
0.0976
0.1410
0.1617
fSm,Nd
ENd
<Nd
(present)
(I = 3.96 Gal
- 0.358
- 0.365
- 0.299
0.058
-0.126
-0.189
-0.631
-0.481
-0.223
- 0.635
- 0.635
- 0.295
-0.163
- 0.294
- 0.378
-0.591
-0.615
-35.0
-38.0
-31.2
-0.8
- 16.2
- 20.6
- 58.6
- 45.0
- 24.0
- 59.7
-60.1
-31.4
-20.1
-31.0
-38.8
-55.6
- 56.9
+ 1.03
- 1.23
-0.10
- 6.72
- 3.58
- 1.68
+4.98
+ 3.45
- 1.60
+4.20
+ 3.80
- 1.79
-3.77
- 1.45
- 0.79
+3.93
+ 5.02
- 0.383
- 0.577
- 0.207
- 0.530
- 0.250
-0.504
- 0.283
-0.178
- 37.4
-55.2
- 23.3
-51.1
- 26.5
-48.7
- 29.5
- 20.6
+ 1.19
+2.81
- 2.45
+2.15
- 1.38
+ 1.94
- 1.07
- 2.77
Analytical details refer only to previously unpublished data. Sm and Nd were separated from whole-rock powders by dissolution using
HF-HNO,
at 170°C in pressurised containers, followed by standard ion-exchange techniques. Sm and Nd concentrations determined by
isotope dilution; error on 14’Sm/ ‘44Nd ratios is _ *O.l%. Nd isotopic ratios were determined on a VG 54E mass spectrometer and
corrected for within-run mass fractionation by normalization to a ‘46Nd/ ‘44Nd ratio of 0.7219; replicate analyses of La Jolla standard yield
14’Nd/ ‘44Nd 0.511839 f 0.000023 (0.0044%, 2 cr; n = 20). Data from other sources have been normalised to this value. Decay constant
Nd parameters calculated relative to CHUR (‘43Nd/ l”Nd = 0.512638; 14’Sm/ ‘44Nd = 0.1966;
(A) for 14’Sm = 6.54 X lo- I2 a- ‘. ??
Hamilton et al., 1983); enrichment expressed as fsm,Nd relative to CHUR.
S. Moorbath et al./ Chemical Geology 135 (1997) 213-231
211
cogenetic or not in terms of ??
Nd heterogeneities. In
our model, significant geological scatter (MSWD >
1) about the regression line could imply that either
pre-3.37-Ga eNd heterogeneities were not totally
eradicated, or that post-metamorphic disturbance
(e.g., at N 1.9 Ga, see Hodges et al., 1995) caused
limited open-system behaviour in the Sm-Nd system, or both. In contrast, Bowring and Housh (1995)
assume that each of their analysed samples remained
a closed Sm-Nd system since time of rock formation, as estimated from the individually associated
zircon U-Pb ages. They state (Bowring and Housh,
1995, footnote 4) that
0.04
0.08
0.12
0.16
0.20
0.24
Fig. 1. Sm-Nd regression for Acasta gneiss samples from this
paper (filled circles), and Bowring et al. (1990) and Bowring and
Housh (1995) (open circles).
. . . the Acasta gneisses are compositionally and
temporally heterogeneous and thus were not all derived from a single homogeneous reservoir at the
same time
and further claim that
of the same localities for which Bowring and Housh
(1995) quote SHRIMP zircon U-Pb dates ranging from
3.6 to 4.0 Ga.
Most of the Acasta gneiss Sm-Nd analyses of
Bowring et al. (1990) and Bowring and Housh (1995)
fall on the same regression line defined by the
Oxford analyses, with no significant change in parameters. Thus a combined regression line based on
34 analyses (Fig. 1) yields an age of 3371 k 59 Ma
(MSWD = 9.21, with initial ??
Nd= - 5.6 f 0.7. Three
of the 3.6-Ga samples (granite 91-5, tonalitic gneiss
89-18, amphibolitic gneiss 91-38) of Bowring and
Housh (1995) and one sample (amphibolitic gneiss
BGXM) of Bowring et al. (1990) have been omitted
because of poor fit to the combined regression. Their
inclusion in the regression produces an age of 3329
f 110 Ma (MSWD = 1491, with initial ??
Nd= - 6.0
+ 1.7. However, i-n what follows, we rest our case
on the well-correlated regression of 34 (out of 37)
published and new Sm-Nd data points.
2.3. Interpretation
of the Sm-Nd
regression
Our main thesis is that the Acasta gneisses suffered a tectonothermal event at N 3.37 Ga (see
above) which caused open-system behaviour for
Sm-Nd and resulted in close approximation to Ndisotope homogenisation on the scale of sampling,
regardless of whether the samples were originally
. . . therefore, any linear array on an isochron diagram for these samples is a mixing line, and calculated ages and initial isotopic ratios have no geological significance
Their view reflects a general scepticism towards
whole-rock isochron dating. Indeed, several studies
have shown that linear arrays in isochron plots may
reflect mixing relationships and have no direct
geochronological significance. With respect to SmNd whole-rock dating, several attempts to date
mafic-ultramafic rock units have produced false
isochron ages (e.g., Cattell et al., 1984; Hegner et al.,
1984; Chauvel et al., 1985; Gruau et al., 1990) and
were shown to result from two-component mixing
(e.g., contamination of a mantle-derived melt by
continental crust). If the inverse of Nd concentration,
l/Nd, is plotted vs. 143Nd/ 144Nd ratio (either present-day, or corrected for 14’Sm decay since the
assumed geological age), simple two-component
mixing produces a straight line, usually with a positive slope since the radiogenic end-member tends to
have a lower Nd concentration. Although straight
lines in this diagram may also be obtained for true
isochrons, a lack of correlation reasonably argues
against mixing.
In Fig. 2a, we plot data for matic-ultramafic
whole-rock analyses by Cattell et al. (19841, which
define a provably false regression age. A broad
218
S. Moorbath et al. /Chemical
Geology 135 (19971213-231
0.5130
0.5125
0.5120
0.5115
0.5110
_ :
0.5105
0.5100
0.5120
~~,.~1~~~~1~~,~1~~~~1~~~~'~~~~
0.0
0.10
0.20
0.30
0.5095
0.40
0.50
0.60
.
.
*
‘.
.
- ??. .
.
.
0.050
.
,,
0.10
,
..,
. .._.
C
,
, ,,
0.20
, ,. , ,, ,I0.25
0.30
VW
,.
1.9 Ge
0.5096
,
0.15
VW
0.5100
.
.
_ :
-_. ‘; .
,.,., , I ,a,, , ,
0.0
. .
. . .
.
??
,.
,
I
. .
.*
.
??
.
.
0.
.
.
.
.
.
??
0.5078
"',
0.0
_._...
0.0
0.050
0.10
0.15
0.20
0.25
0.30
'3"".
0.050
0.10
"".""""I
0.15
”
0.20
0.25
”
0.30
Wdl
0.5077
0.5076
G.
??
*
.
.
.
.
'
.
??
.-
0.5075
t
f
4.0 Ge
.
.*.
0.5074
- *
:
.a'*'
.
.*
0.5073 1
.-
.
.
0.5072 :
0.5071 L
0.5070
.
.
.
* 1” I”““““““”
""'3"
0.0
0.050
0.10
0.15
0.20
0.25
0.30
l/IW
Fig. 2. Plots of inverse Nd concentration vs. 14’Nd/ ‘44Nd: (a) present-day data from Cattell et al. (1984) for late Archaean lavas from the
Abitibi Belt, Ontario, define a positive, linear slope, illustrating binary mixing, (b) present-day values from Acasta gneisses from this study,
Bowring et al. (1990), and Bowring and Housh (1995); same data back-corrected
to (c) 1.9 Ga, (d) 3.37 Ga, (e) 4.0 Ga. The data scatter
widely in (b), (c), and (e), indicating that binary mixing does not explain the tight correlation in the isochron plot (Fig. 1). With the
exception of three aberrant samples (squares), values back-corrected
to 3.37 Ga define a linear, horizontal array, supporting the
interpretation of a homogenisation event.
linear array is defined by these data, which supports
the view that the regression is a mixing line, so that
the resulting age and initial Nd isotope information
is meaningless.
(This plot of 143Nd/ ‘44Nd versus
l/Nd is not actually given in Cattell et al., 1984.) In
contrast, when the Acasta gneiss samples are plotted
in similar fashion (Fig. 2b) they scatter randomly so
that the tight linear regression plot is clearly not the
S. Moorbath et al. /Chemical
219
Geology 135 (1997) 213-231
supports our thesis of a 3.37-Ga event which severely
affected the Sm-Nd
systematics
of the Acasta
gneisses. A similar plot for ‘43Nd/ ‘44Nd ratios calculated back to 4.0 Ga shows significantly
greater
scatter and a negative slope (Fig. 2e).
It should be noted that Bowring and Housh’s
(1995) suggestion that the well-correlated
linear array in the isochron plot represents two-component
mixing actually conflicts with their postulate of a
range of preserved Nd-isotopic heterogeneities in the
Acasta gneisses. In other words, the wide range in
Nd ( - 4.8 to + 3.6) obtained by Bowring and
initial ??
Housh (1995) precludes binary mixing, if these val-
result of binary mixing, thus refuting Bowring and
Housh’s (1995) explanation. If 143Nd/ 144Nd ratios
are calculated for various times in the past, such as
the N 1.9-Ga metamorphic overprint (Hodges et al.,
1995) the postulated 3.37-Ga homogenisation
event,
the protolitb formation age of 4.0 Ga, and plotted
against l/Nd (Fig. 2c-e) the best-correlated
linear
fit is obtained at 3..37 Ga. With few exceptions, the
Acasta gneiss 143Nd,/ 144Nd ratios calculated back to
3.37 Ga broadly define a straight horizontal line
(Fig. 2d), showing that most analysed Acasta gneisses
had approximately the same Nd isotopic composition
regardless of their Nd concentration.
This further
i
5 -
MSWD=478
0.5110
B
:
i
P
1
I
age = 3661flW)
:
Ma
0.507818
Nd MaI=
-
.
.
0.5105
:
.
.
0.5100
-
I
8
.
I
o&ir~
33~;lien:
3
age = 37‘uMsu
hia
LM !..;li-l _ mll*r.
:
:
.
:
!
-
MsWD= 915
age = 3543%?50 Ma
Nd initial = .507583
___ _____ ..Nd initial = 507908
.
age i: 363W60
Ma
Nd initial = SO7618
.
:
.
.
.
.
.
I
.
gH?l~D~~3gliilX:
.
age = 37E4%kz?OMa
Nd initial = .507683
.
147Sm/144NC’
0.5090
’ ’ ’ ’’ i ’ ’’ ’ ’
-J
0.060
.
0.080
0.10
0.12
’’ ’ I ’ ’ ’
0.14
0.16
0.5090
t
0.060
*
I
’
0.060
1
0 10
1
0.12
‘4’Sml’4’Nd
’
1
1
0.14
0.15
0.18
Fig. 3. Model Sm-Nd isochron regressions for (a) data of Bowring and Housh (1995), assuming that each sample represents an entire rock
suite with 14’Sm/ ‘44Nd values ranging from 0.07 to 0.16, calculated for corresponding zircon U-Pb age; (b), (c), Cd) isochron plots for
three randomly selected data sets of 40 points from (a). Individual regressions were calculated firstly for all points, and secondly with the
omission of the three most aberrant samples (see text).
220
S. Moorbath et al./ Chemical Geology 135 (1997) 213-231
ues are interpreted as reflecting undisturbed primary
Nd-isotope systematics. Nevertheless, because of the
slow decay of 14’Sm, 4.0- and 3.6-Ga-old protoliths
with variable initial 143Nd/ ‘44Nd compositions will
tend to define a broadly linear array in an isochron
diagram. In order to explore the possible scatter
associated with such an array, we have performed a
simple numerical simulation. Following the model of
Bowring and Housh (1995) we have interpreted
each of their data points to represent one sample of a
cogenetic suite of rocks. For each rock suite we have
arbitrarily assumed a range of 14’Sm/ ‘44Nd ratios
from 0.07 to 0.16 (note that the true range is not
crucial to our calculations). Fig. 3a shows isochrons
for these suites, calculated for the appropriate U-Pb
zircon age, starting with an initial 143Nd/ 144Nd ratio
based on Bowring and Housh’s (1995) initial eNd
value. Here we treat this data array as a random
sampling pool, assuming that the Sm-Nd system
remained undisturbed by any later overprint, as argued by Bowring and Housh (1995). We have randomly selected 40 data points from this array and
calculated closeness of fit (indicated by MSWD), age
and initial 143Nd/ 144Nd ratio in isochron plots for
three simulations,
displayed in Fig. 3b-d. Whilst
computed ages and initial ‘43Nd/ 144Nd ratios do
depend on the true 14’Sm/ 144Nd ratios of individual
rock suites and should therefore not be regarded as
predictive, the respective MSWD values of 478, 915
and 763 are much less sensitive to choice of
14’Sm/ ‘44Nd values and can be regarded as the
expected closeness of fit of a linear data array. If we
regress all 37 measured Acasta gneiss Sm-Nd points
(see previous section), we obtain an MSWD of 149.
Even this is far better than predicted by the simulation. More importantly,
omitting the three worst
samples from the simulation regression only improves the fit partially (i.e. 369 instead of 478, 682
instead of 915, 480 instead of 763) whilst omitting 3
out of 37 real samples lowers the MSWD to 9.2 (Fig.
l), almost two orders of magnitude smaller than
predicted. We regard this as strong evidence that our
regression does not reflect random sampling from an
inherently linear data pool, and we conclude that the
obtained age and Nd-isotope information are of geological significance. Whilst the slow decay of 14’Sm
precludes using a whole-rock approach to resolve
Nd-isotope heterogeneities resulting from age differ-
ences of even several 100 Ma, the range of initial
Nd-isotope ratios postulated by Bowring and Housh
(1995) would, if genuine, provide sufficient dispersion to be easily detectable by whole-rock analysis.
In summary, inspection of the Acasta gneiss Sm-Nd
regression shows that the obtained errorchron is neither the result of binary mixing nor of random
sampling of an inherently linear original data pool.
The fact that the best correlation between l/Nd vs.
143Nd/ 144Nd is obtained for a horizontal line at
N 3.37 Ga further supports our model of a geological event which nearly eradicated any pre-existing
Nd-isotope heterogeneities.
There are several circumstantial lines of evidence
for limited open-system disturbance of the Sm-Nd
system in the Acasta gneisses post-dating the 3.37-Ga
resetting event. Hodges et al. (1995) report U-Pb
evidence for a Pb-loss event at 1.88 Ga, based on
fine-grained titanites and metamorphic overgrowths
on coarse titanite. Support for this metamorphic event
is provided by a 1.86-Ga 4o Ar/ 39Ar hornblende age.
Hodges et al. (1995) further state that
. . . Nd isotopic data for garnets from the same rocks
reveal complex systematics, probably in part related
to multiple episodes of garnet growth, and provide
no useful information
regarding the age of either
metamorphic event.
Turning again to the preservation of the pre-3.37Ga record (other than zircon U-W ages) there is, of
course, no a priori requirement that all the different
Acasta gneiss rock types had an identical initial eNd
value at time(s) of rock formation. However, as
explained later, if the observed Sm-Nd regression
indeed represents a time of near-complete
Nd-isotope homogenisation,
then the record of any previous
isotopic heterogeneities in initial E’.,,, may have been
largely eliminated. Interpretation
of the regression
age as resetting of the Sm-Nd system is strongly
supported by the very negative initial end (3.37 Ga)
value of -5.6 + 0.7 (Fig. l), pointing to the existence of an enriched (low Sm/Nd) crustal protolith
with an age greatly exceeding 3.37 Ga. Indeed,
assuming an average continental crust Sm/Nd ratio
of 0.17 (Taylor and McLennan, 1985) for the Acasta
gneiss precursor, the observed initial eNd value of
-5.6 at 3.37 Ga (Fig. 1) extrapolates back to + 1.0
at 3.96 Ga, i.e. the oldest zircon U-Pb age obtained
S. Moorbath et al./Chemical
-12;
I
3.2
3.4
3.6
,
3.6
t(W
,
4.0
Fig. 4. Nd-isotope evolution diagram for all Acasta gneiss samples
plotted in Fig. 1. Individual evolution lines arc not shown, but all
fall within the shaded jield. The 2 u error polygon represents the
parameters
shown in the inset of Fig. 1. The open squares
represent the initial ??
Nd values based on individual zircon U-Pb
dates of Bowring et al. (1990) and Bowring and Housh (1995).
for the Acasta gnei:sses (Bowring et al., 1990). This
is much closer to uniformitarian models of early
Archaean Nd-isotope evolution in both the chondritic
uniform reservoir (CHUR) and depleted-mantle
models (e.g., DePaolo et al., 1991).
2.4. Interpretation
of individual, apparent initial eNd
values
If, as argued earlier, the Sm-Nd system in the
Acasta gneisses was largely reset at N 3.37 Ga, then
this would have destroyed all or most of the pre-3.37
Nd heterogeneities, so that calculation
Ga record of ??
of initial ??
Nd values based on individual zircon
U-W ages would be geologically meaningless. This
is illustrated in Fig. 4 in a Nd-isotope evolution
diagram. The 34 individual Nd-isotope evolution
lines are not plotted separately, but all fall within the
shaded field, focusing on the 2u error polygon
which represents the age and initial eNd value obtained from the Sm-Nd regression (Fig. 1). Also
shown (squares) are the zircon U-W ages of Bowring
et al. (1990) and IBowring and Housh (1995). Although there are no zircon U-Pb age measurements
on the samples analysed for Sm-Nd at Oxford, they
were collected frorn some of the same localities as
those of Bowring and coworkers and therefore presumably occupy the same age range. The corresponding values of apparent initial Ed,, can be read
off from the vertical1axis. The greater the age differ-
Geology 135 (1997) 213-231
221
ence between the measured zircon U-Pb age and the
Sm-Nd resetting age, the greater will be the range of
apparent initial ??
Nd values. For example, at the
maximum age of 4.0 Ga, the apparent range extends
from _ -5 to +7, but at 3.6 Ga only from u
- 6.5 to - 1. It is clear that the individual eNd
values based on associated zircon U-Pb ages have
no geological significance whatever. The plot in Fig.
4 rather simplistically assumes only Nd-isotope homogenisation in individual samples, so that individual whole-rock Nd-isotopic evolution lines pass
straight through the 2cr error polygon region. It is
probable, given their similar chemistry, that both Sm
and Nd would be mobile during a resetting event at
3.37 Ga, resulting in a change of Sm/Nd ratio for
individual samples in addition to Nd-isotope homogenisation. In that case, the overall field of Ndisotope evolution would be correspondingly less well
constrained and could yield an even wider range of
. ..
apparent pre-3.37 Ga mmal eNd values.
2.5. Interpretation
of the geological
nature
of the
resetting event
Earlier we saw that the observed MSWD of N 9
for the 34-point Sm-Nd regression (Fig. 1) was very
much smaller than can be accounted for by independent, undisturbed Nd-isotope evolution from different samples with different ages and initial ??
Nd values. The problem therefore is to propose a plausible
geological process that could cause near-complete
Nd-isotope homogenisation at 3.37 Ga in a petrologitally and geochemically wide variety of lithologies
over an area of w 15-20 km2 (for overall geological
and locality map, see Bowring and Housh, 1995). It
is unlikely that metamorphism and tectonism alone
were responsible for near-complete Nd-isotope homogenisation (e.g., Barovich and Patchett, 1992).
Over the past decade, attention has been drawn to the
fact that hydrothermal overprint, both at low metasom atic
tern perature
and
at
high
metamorphic/anatectic
temperature may severely
disturb the REE distribution of adjacent rocks (e.g.,
Windrim et al., 1984; Whitehouse, 1988; Frost and
Frost, 1995; Poitrasson et al., 1995; Bau, 1996).
Bowring and Housh (1995, footnote 4) state that
. . . the Acasta gneisses . . . are a sequence of layered me&igneous rocks that have been subjected to
222
S. Moorbath
et al. /Chemical
deformation.
This geological history resulted in a
series of layered rocks in which the layers reflect
differences in primary igneous compositions.
From our own field and microscopic observations we
interpret this layering to be the result of a powerful
tectonometamorphic
overprint accompanied by anatexis. In addition, the rocks suffered a much younger
low-grade
metamorphic
overprint
at N 1.9 Ga
(Hodges et al., 1995) which was clearly not responsible for the observed layering, but which may have
caused some of the scatter around the 3.37 Ga
regression line (Fig. l), as pointed out earlier. Thus
the geological history of the Acasta gneisses requires
a high-grade
event involving
melting, metamorphism, migmatisation
and deformation
which occurred between formation of the protolith (4.0 and
3.6 Ga) and the low-grade overprint at 1.9 Ga. Here
we postulate that this event may itself be recorded by
the 3.37 Ga Nd-isotope homogenisation
and that the
anatectic nature of the Acasta gneiss layering provides a framework for possible REE redistribution.
Under these circumstances,
preservation
of initial
??
Nd heterogeneity
corresponding
to rock type, as
postulated by Bowring and Housh (19951, would be
most improbable.
In our interpretation, therefore, it is probable that
the age of the Acasta gneisses is only N 3.37 Ga,
although both the zircon U-Pb ages (4.0 to 3.6 Ga)
and the initial eNd value of N -5.6 obtained from
the Sm-Nd regression (Fig. 1) provide incontrovertible evidence for the existence of a substantially
older precursor. Such a precursor - which is well
worth searching for in the field - may resemble the
kind of stored mafic/ultramafic
crust envisaged by
Chase and Patchett (1988) to account for early Archaean mantle depletion.
Such mafic/ultramafic
rocks alone, however, may be compositionally
inappropriate as the source of ancient, inherited zircons
in the Acasta gneisses. The protolith would also have
to comprise compositionally
evolved rocks as a
source for the zircons. The only early Archaean
terrain (_ 3.8 Ga) known to us which contains a
major, essentially bimodal assemblage of ultramaficto-mafic/felsic
volcanogenic rocks is that of the Isua
supracrustal belt of West Greenland (e.g., Nutman et
al., 1984), which is discussed later. Supracrustal and
infracrustal assemblages with this compositional polarity may have been common during the early Ar-
Geology
I35 (1997) 213-231
chaean and could have formed an appropriate protolith for the Acasta gneisses.
Finally we have to ask whether there is any
evidence
for contemporaneous
rock-forming
or
tectonothermal
events in the northern part of the
Slave Province. According to W. Padgham (pers.
commun.):
. . 3.4 Ga dates in the Slave Province are rare, and
none may have been published in refereed publications.
However, Yamashita et al. (1995) have reported the
existence of a N 3.4 Ga old tonalitic and gabbroic
basement in the Hanikahimajuk
Lake area at the
northwestern margin of the Slave Province,
N 425
km north of Yellowknife, N.W.T. This is N 200 km
northeast of the Acasta region, and we hope that
further age and isotope work in this region may
delineate the extent and intensity of the “3.4 Ga”
event, which we argue is so convincingly recorded in
the Acasta gneisses.
3. Early Archaean
rocks of West Greenland
3.1. Previous age work
An immense amount of geological, geochronological and related isotopic work has been published on
the early Archaean rocks of West Greenland over the
past 25 years and only the briefest summary is given
here. Most of the voluminous
Rb-Sr and Pb-Pb
whole-rock isochron work, together with conventional zircon U-Pb work, give ages in the range
N 3.65-3.75 Ga for both the earlier Isua supracrustal
rocks and later Amitsoq gneisses (Moorbath et al.,
1986, and references therein). Occasional
“excursions” from this range can extend down to N 3.6 Ga
(especially for the Amitsoq gneisses) and up to 3.8
Ga (especially for the Isua supracrustals).
A great
deal of additional Rb-Sr and Pb-Pb whole-rock data
on both rock units (Oxford, unpublished data) gives
essentially the same results. In view of the close
similarity, within limits of error, in Rb-Sr and Pb-Pb
ages for both rock units, it is probable that the
regional metamorphic heating event associated with
the emplacement of the main bulk of the magmatic
precursors of the Amitsoq gneisses partially or com-
S. Moorbath et al./ Chemical Geology 135 (19971213-231
pletely reset the Rb-Sr and Pb-Pb ages of the older
Isua supracrustals
to values characteristic
of the
Amitsoq gneisses, supposing that a significant age
difference actually exists between the two rock units.
Almost all whole-rock regressions exhibit a degree
of geological
scatter exceeding
analytical
scatter
(MSWD > 1) and this may be interpreted in the
normal way as either incomplete eradication of primary Sr and Pb isotope heterogeneities
or as opensystem isotopic disturbance
superimposed
by late
Archaean or Proterozoic metamorphic events which
have been documented in the region (e.g., Pankhurst
et al., 1973; Kalsbeek,
1981; Baadsgaard et al.,
1986a; Gruau et al., 1996). The regressions therefore
represent an “averaging”
of data points, in the sense
of Cameron et al. (1981). Nonetheless, much useful
information is provided by initial Sr- and Pb-isotope
ratios obtained from Rb-Sr and Pb-Pb regressions,
concerning the ultimate source region of the Isua
supracrustals and the Amitsoq gneisses (Moorbath et
al., 1986, and references therein). In particular, it
appears that the immediate protolith of each of these
rock units was itself derived from mantle-like source
regions not more than N 0.1-0.2 Ga before the age
recorded by Rb-Sr and Pb-Pb whole-rock isochrons
(e.g., Moorbath and Taylor, 1981).
The same general principles hold for the much
smaller published Sm-Nd data base on these rocks.
The first Sm-Nd age determinations
(Hamilton et
al., 1978) on a combined suite of ten felsic and mafic
Isua supracrustal rocks yielded a whole-rock regresNd value
sion age of 3770 + 42 Ma, with an initial ??
of _ +2. A mixed suite of 11 Isua supracrustal
rocks subsequently (Moorbath et al., 1986) yielded a
Sm-Nd whole-rock regression age of 37 16 + 73 Ma,
with an initial ??
Nd of + 2.0 f 0.8, whilst a suite of 7
Amitsoq gneiss sam,ples gave 3720 k 84 Ma, with an
initial ??
Nd of + 2.3 + 1.2. Some subsequent attempts
to date the Isua supracrustals were not very successful, leading Jacobsen and Dymek (1988) to conclude
that
. . . the stratigraphic age for the Isua rocks must lie
in the rather large a.ge range 3.64-3.83 Ga, which is
discomforting.
They further conclude
that
show complex
. . . the Isua elastic metasediments
Rb-Sr and Sm-Nd systematics. While much of the
223
complexities
in the Rb-Sr systematics can be attributed to disturbance(s)
during post-3.6 Ga metamorphic events (i.e. at 2.8 and 1.8 Ga), the heterogeneities observed in initial eNd values appear to
reflect real variability due to mixing of isotopically
diverse source materials at the time of deposition.
This is relevant to what follows later.
In order to resolve finer details of the geochronological record in West Greenland, much recent work
has been based on SHRIMP zircon U-Pb ages (e.g.,
Nutman et al., 1996 and references therein). These
workers obtain a spectrum of numerous precise dates,
extending from N 3870 to N 3600 Ma for the
Amitsoq gneisses and their enclaves (collectively
renamed the Itsaq Gneiss Complex), although most
of the ages appear to fall towards the lower end of
the range. For the Isua supracrustal
belt, zircon
U-Pb ages of 3807 f 2/3806 + 4 and 3708 + 3 Ma
reported
for
two
units
of
felsic
are
volcanic/volcanoclastic
rocks, suggesting that the
belt contains similar components differing in age by
N 100 Ma. No evidence was found for zircon derived from crust older than 3807 Ma.
Nd values
3.2. Initial ??
Bennett et al. (1993) report a wide range of initial
eNd values from -4.6
to f4.5
in samples of
Am?tsoq gneiss with zircon U-Pb dates in the range
3872 to 3729 Ma, as well as initial ??
Nd values from
I + 1.8 to 2 + 3.7 for mafic inclusions within the
Amitsoq gneisses (the so-called Akilia gabbros and
leucogabbros)
with ages constrained
to the range
2 3872 to 2 3784 Ma by zircon U-Pb dates from
the enveloping Amitsoq gneisses. This wide range of
initial eNd values was then used as a basis for
complex models of early mantle and crust evolution.
In fact, Bennett et al. (1993) postulate a LREE-depleted mantle reservoir with an initial eNd value of
* +4 prior to 3.8 Ga. They state that
. . if the source region for the oldest Greenland
gneisses was the prevalent upper mantle composition, it must have been an ephemeral feature later
modified either by mixing with less-depleted mantle,
or with recycled LREE-enriched,
negative-e,,
crust.
The generation of highly positive ??
Nd values by 3.8
Ga requires differentiation
of an extremely LREE
S. Moorbath
224
et al./ Chemical Geology 135 (1997) 213-231
fractionated reservoir very early in Earth’s history.
The Archaean Nd isotope data may record the isolation, depletion by crustal extraction and subsequent
partial rehomogenisation of Eimited portions of the
upper mantle, or alternatively may reflect transient
large-scale differentiation processes unrelated to
crustal extraction such as might occur in a terrestrial
magma ocean.
Intrigued by the difference between initial eNd
values of around +2 for the Isua supracrustal rocks
and for the Am?tsoq gneisses obtained by Moorbath
et al. (1986) from Sm-Nd regressions, and the wide
overall range from -4.6 to +4.5 reported by Bennett et al. (19931, we decided to examine their
Sm-Nd data more closely in an analogous manner to
that discussed earlier for the Acasta gneisses of
northern Canada.
We have plotted the seven Akilia enclave data
points of Bennett et al. (1993), which yield zircon
U-Pb dates in the range 2 3872 f 10 to 2 3784 f
. ..
22 Ma and mlhal Ed,, values in the range I + 1.8
to 2 +3.7, on a Sm-Nd diagram (Fig. 5). This
yields an almost perfect isochron with a regression
age of 3675 f 48 Ma and an initial eNd value of
+2.6 + 0.4 (MSWD = 2.1). We regard this as a
0.514
/
0.513
0.512
0.511
0.510
0.06
I
0.10
0.14
r47Sm/‘44Nd
I
1
0.16
0.22
Fig. 5. Sm-Nd regression for the Akilia suite of West Greenland
from Bennett et al. (1993). The inset shows Nd-isotope evolution
lines for representative samples extrapolated back to the overall
range of individual zircon U-Pb dates (shaded area). DM here
(and in Fig. 7) refers to the depleted-mantle model of DePaolo et
al. (1991). CHUR refers to the chondritic uniform reservoir (see
Table 1).
0.5106 -
‘47Sm/‘44Nd
0.5094
0.06
I
I
I
I
0.08
0.10
0.12
0.14
Fig. 6. Combined Sm-Nd regression (jilled circles) for Amitsoq
gneiss samples from Baadsgaard et al. (1986b), Moorbath et al.
(19861, and Shin-&u et al. (1988). The Amitsoq gneiss data of
Bennett et al. (1993) are shown for comparison (open squares),
and omitted from the regression calculation.
straightforward Nd-isotope homogenisation age,
probably reflecting the age of the tectonothermal
event associated with the emplacement of the magmatic precursors of the youngest of the nearby regional Amitsoq gneisses (e.g., Nutman et al., 1996).
The inset of Fig. 5 is a Nd-isotope evolution diagram
which shows a range of apparent initial eNd values
within the constraining age bracket imposed by the
Amitsoq gneiss zircon U-Pb dates, but without any
specific geological significance.
Because of indeterminate complexities of the geochemical processes affecting the Sm-Nd system before and during the N 3.675 Ga Nd-isotope homogenisation event, it is not possible at this stage to
give a specific interpretation for the initial eNd value
of + 2.6 f 0.4 obtained from the Sm-Nd regression
(Fig. 5). However, it may not have evolved far in
terms of time and Sm/Nd ratio from its starting
value(s). This needs much more work.
Turning now to the Amitsoq gneisses, in Fig. 6
we plot the Sm-Nd data of Bennett et al. (1993) for
nine Amitsoq gneiss samples, which have U-Pb
zircon ages ranging from 3872 f 10 to 3729 f 3 Ma,
. ..
and initial ??
Nd values ranging from -4.6 to +4.5.
These data are far too scattered to fall on a meaningful Sm-Nd regression line, whilst most of them also
scatter widely about a regression line (Fig. 6) based
S. Moorbarh et al. / Chemical Geology 135 (1997) 213-231
on published Sm-Nd data for varied suites of
Amitsoq gneisses without individual zircon U-W
age control (Baadsgaard et al., 1986b; Moorbath et
al., 1986; Shimizu et al., 1988). This 26-point regression yields an age of 3640 + 120 Ma (MSWD = lo>,
with an initial en,, value of + 0.9 * 1.4. Clearly,
from this evidence alone, we cannot counter the
claims of Bennett et al. (1993) for gross heterogeneities in initial eNd values in their rocks. However, it is at least possible that some of their samples
were even more subject to later Sm-Nd open-system
behaviour than the regressed samples in Fig. 6,
which themselves show significant geological scatter
about the regression line. The comparatively low
initial ??
nd value of _ +0.9 for the combined
Amitsoq gneiss regression (even allowing for the
large error of + 1.4.) suggests the possibility of limited participation of a significantly older, enriched
(i.e. low Sm/Ncl) crustal precursor, such as
supracrustal or infracrustal rocks of the Isua/Akilia
type, in the genesis of the magmatic precursors of
the Amitsoq gneisses. This problem is currently under investigation.
Whilst the Sm-Nd regression age of - 3640 Ma
for the Amitsoq gneisses is considered to be a reasonable estimate for the age of rock formation of the
bulk of the younger Amitsoq gneisses (in broad
agreement with the numerous published whole-rock
Rb-Sr, Pb-Pb and zircon U-Pb dates, e.g., Moorbath et al., 1986; Nutman et al., 1993, 19961, the
scatter about the Sm-Nd regression line (Fig. 6) is,
in our view, most likely due to limited open-system
behaviour during late Archaean and mid-Proterozoic
tectonothermal events, which are well documented in
this region (for references, see later).
In view of the above uncertainties, is it possible to
Nd value for mantledetermine a plausible initial ??
like source region:3 of crustal rocks at N 3.7-3.8
Ga? In principle, this can be done only by using rock
suites which have a short-term crustal history and
which have remained a closed Sm-Nd system since
time of deposition, We consider that some members
of the Isua supracrustal belt offer this possibility.
Our interest was stimulated by finding that many
published Sm-Nd data for the Isua supracrustal rocks
(e.g., Hamilton et al., 1978, 1983; Miller and
O’Nions, 1985; Moorbath et al., 1986; Jacobsen and
Dymek, 1988; this paper) fell on a combined, well-
225
correlated Sm-Nd regression line (not presented in
full here) yielding an age of N 3.7-3.8 Ga, and an
initial ??
Nd value of N -I-1.5 to + 2.0. The best-correlated regressions were given by felsic, volcanogenic metasediments, and by mica-schists possible derived from (Nutman et al., 1984):
. . . either weathered basic rocks or from basic tuffs
that interacted with water at the time of deposition
. ..
These comprise two of the major rock units within
tbe Isua supracrustals. The worst-con-elated regression points were given by mafic and ultramafic
metavolcanic rocks and by chemical sediments such
as carbonates and banded iron-formation, many of
which have very low Sm and Nd contents and are
thus particularly sensitive to post-depositional chemical alteration and metasomatic processes, which are
well documented in this area (e.g., Rosing, 1990;
Gruau et al., 1996).
In this context, we disagree with the recent re-interpretation (Rosing et al., 1996) of the Isua
supracrustal sequence, which regards the felsic rock
units of Nutman et al. (1984) as totally metasomatised, discordant Amitsoq gneisses, and the
mica( + garnet)-schists as metasomatised amphibolites, which could, at face value, raise doubts about
the validity of our approach. We have seen no field
or petrographic evidence as yet for such thorough
and pervasive regional metasomatism, although both
rock types sometimes exhibit secondary calcite infiltration, which is stronger in some areas than in
others. We regard metasomatism in the Isua
supracrustal rocks as a localised, variably intense
phenomenon which has not obliterated the integrity
of the individual rock types, nor the original lithological and petrographic record of the provenance of
any of the major rock units described by Nutman et
al. (1984). All Isua supracrustal rocks show
medium-grade metamorphism (Boak and Dymek,
1982; Dymek and Klein, 19881, as well as locally
highly variable degrees of deformation, frequently
leading to local preservation of primary sedimentary
and volcanic structures (e.g., graded bedding, slump
structures, true conglomerates, pillow-lava& without
evidence for pervasive metasomatism.
In the Isua supracrustal rocks analysed for Sm-Nd
at Oxford, we have concentrated on felsic vol-
S. Moorbrrth et al. / Chemicul Geology 135 C19971 213-231
226
canogenic
metasediments
and on the mica
($gamet)-schists
because they have the highest Sm
and Nd contents and generally appear to be least
affected by later alteration and metasomatism.
This
is also evident from their relatively coherent wholerock Rb-Sr
and Pb-Pb
isotope systematics (see
previous section) which mostly give regression ages
Ga. We have avoided
in the range N 3.65-3.75
analysing any samples with significant amounts of
Table 2
Sm-Nd data for lsua supracrustal
Sample
secondary calcite infiltration. Not unexpectedly,
we
have not been able to obtain any easily interpretable
whole-rock Sm-Nd systematics from low-Sm, lowNd mafic/ultramafic
meta-igneous
rocks or from
chemical
metasediments,
such as carbonates
or
banded iron-formation.
A suite of 32 samples (collected by S.M. in 1978
and 1993) from along 12 km of strike in the eastern
sector of the Isua supracrustal belt was analysed for
rocks
Sm
Nd
@pm)
(wm)
6.345
5.142
6.448
5.102
4.438
5.343
6.473
5.760
3.666
4.030
4.659
3.750
2.794
2.175
3.963
2.290
2.179
33.91
28.48
34.89
27.36
23.89
28.20
35.37
30.77
24.57
26.54
29.06
22.92
17.51
13.46
25.61
14.18
13.72
0.510663
0.510581
0.5 10663
0.5 10643
0.510657
0.510678
0.510605
0.5 10662
0.5 10073
0.5 10086
0.5 10234
0.510255
0.510189
0.5 10206
0.510186
0.510215
0.5 10220
0.1131
0.1091
0.1131
0.1127
0.1123
0.1145
0.1106
0.1131
0.0902
0.0917
0.0969
0.0989
0.0964
0.0977
0.0935
0.0976
0.0964
2.939
2.769
1.136
3.05 1
4.333
4.607
5.181
5.169
5.554
8.025
7.593
4.57 1
9.726
20.45
20.26
27.05
26.41
25.27
0.5 13376
0.513338
0.511592
0.512550
0.511034
0.51 1284
0.5 10789
0.510815
0.511148
10.92
18.63
10.72
31.33
4.717
27.83
0.510717
0.510407
0.5 10652
0.5 10654
0.510918
0.5 10577
‘43Nd/ 14jNd
‘“‘Sm/
‘44Nd
fsm,Nd
ENd
ENd
(present)
(t = 3.776 Ca)
- 0.425
- 0.445
- 0.425
- 0.427
- 0.429
-0.418
- 0.437
- 0.425
- 0.541
- 0.534
- 0.507
- 0.497
-0.510
- 0.503
- 0.524
-0.504
-0.510
- 38.5
-40.1
- 38.5
- 38.9
-38.6
- 38.2
- 39.7
- 38.5
- 50.0
-49.8
-46.9
-46.5
-47.8
-47.4
-47.8
-47.3
- 47.2
2.22
2.57
2.22
2.02
2.50
1.83
2.31
2.20
1.88
1.39
1.75
1.18
1.11
0.80
2.48
1.03
1.72
0.2227
0.2217
0.1502
0.1897
0.1280
0.1374
0.1163
0.1188
0.1335
0.133
0.128
- 0.236
- 0.035
- 0.349
- 0.301
- 0.408
- 0.396
-0.321
+ 14.4
+ 13.7
- 20.4
- 1.70
-31.3
- 26.4
-36.1
- 35.6
-29.1
1.68
1.43
2.25
1.66
2.19
2.48
3.13
2.40
1.73
0.1128
0.1056
0.1127
0.1122
0.1250
0.1086
-
- 37.5
-43.5
- 38.7
- 38.7
- 33.6
- 40.2
3.43
0.87
2.20
2.49
1.38
2.74
Felsic unit:
24848 la
248481b
24848 le
24848 lg
24848 Ij
24848 1k
24848 11
24848 1m
2484 14
248416
248418
248419
248422
248429
24843 1
248433
248430
Schist unit:
248443(i)
248443Cii)
SM/GR/13
SM/GR/33
SM/GR/57
SM/GR/75
248484A
248484F
248484L
Touwwline
248483E- 1
248483E-2
248483F
248483J- 1
2484833-2
248483X
boulder unit
2.028
3.240
1.988
5.791
,971
4.979
See Table 1 for analytical
details.
0.427
0.463
0.427
0.430
0.365
0.448
221
S. Moorbath et al./ Chemical Geology 135 (1997) 213-231
Sm-Nd (Table 2). Data for nine of the samples were
previously reported by Moorbath et al. (1986), but
are included here for convenience.
Seventeen of
these samples are from the two felsic formations A6
and Bl described by Nutman et al. (19841, and
consist of a fine-grained assemblage of plagioclase
+ quartz + muscovite + alkali-feldspar
+ biotite.
Nine samples are from the mica-schist formation B2
of Nutman et al. (1!984), and consist predominantly
of biotite + plagioclase + quartz + garnet + chlorite
f hornblende. We also include six samples from a
single, large tourmaline-rich
conglomerate inclusion
found in a sequence of finely-layered
garnet-mica
schists of formation B2 (see above) in the northeastem part of the belt. This unique rock and its locality
have been fully described by Appel(1984).
The rock
is medium-grained
and consists of alternating darkand light-coloured layers which contain w 60% black
tourmaline
+ quartz + Ca-poor
plagioclase
+
muscovite + biotite (dark layers), and quartz + Capoor plagioclase + muscovite + biotite + apatite, allanite, garnet (light layers). Whilst the tourmalinerich boulder clearly predates the host garnet-mica
schist, there is no (evidence as yet that it is measurably older.
On a Sm-Nd diagram, these 32 samples yield a
well-correlated
regression which gives an age of
3771 * 54 Ma (MSWD = 6.41, with an initial eNd
value of 2.1 f 0.8. When this regression is combined
with a further 26 samples of similar rock types
(except for the tourmaline-rich
rock) taken from the
literature (Hamilton et al., 1978, 1983; Jacobsen and
Dymek, 19881, the results change only very slightly
to 3776 ? 52 Ma (MSWD = 8.2) and +2.0 & 0.6
(Fig. 7).
In the absence cd any geological or geochronological evidence for a significantly older crustal source
region for the Isua supracrustal rocks (e.g., Nutman
et al., 1996) we mterpret the regression age as a
close approximation to the average age of deposition.
This does not negate the evidence for significantly
different zircon U-Pb ages of N 3700 and 3800 Ma
from different stratigraphic horizons within the belt
(Nutman et al., 1996). The geological scatter about
the regression line (MSWD = 8.2) is best interpreted
as minor open-system behaviour resulting from one
or more later regional tectonothermal events, such as
emplacement
of -the magmatic precursors of the
Amitsoq gneisses at N 3.6-3.7 Ga (Moorbath et al.,
1972; Baadsgaard et al., 1986b; Nutman et al., 1993,
19961, as well as late Archaean and mid-Proterozoic
events, which are known to have partly or wholly
reset mineral dates in several decay schemes (e.g.,
Pankhurst et al., 1973; Kalsbeek, 1981; Baadsgaard
et al., 1986a; Gruau et al., 1996).
Individually computed initial ??
Nd values at 3.776
Ga for each sample range from +0.80 to + 3.43
(Table 2). We regard this as statistical geological
scatter induced by one or more of the post-depositional events already mentioned,
and we do not
attribute any part of this variation to initial, pre-3.776
Ga, isotopic heterogeneities,
as was done by Jacobsen and Dymek (1988). Consequently, the initial eNd
value of + 2.0 f 0.6 is regarded as a reliable approximation to the mantle(?) source region of the Isua
supracrustal belt, implying a short-term, crustal multistage history.
The initial eNd value of N +2.0 obtained for
the Isua supracrustals is very different from the value
of * t-4.5 proposed by Bennett et al. (1993) for
mantle source regions in this region at N 3.7 Ga,
and implies a much smaller degree of LREE depletion. It is much closer to a value of N + 1.5
0.06
0.10
0.14
0.18
0.22
0.26
Fig. 7. Combined Sm-Nd regression for Isua supracmstal rocks
from this paper, Hamilton et al. (1978, 19831, and Jacobsen and
Dymek (1988). The inset shows a comparison of the initial c~,,
value of +2.0 from the regression with the proposed depletedmantle evolution curve of Bennett et al. (1993).
228
S. Moorbath et al./Chemical
obtained from the latest depleted-mantle
model of
DePaolo et al. (199 1). The difference between the
two models is shown in the inset to Fig. 7. However,
we are well aware that this may not be the last word
on the initial eNd value of mantle source regions at
Isua in early Archaean times. It is relevant to note
that, in their Hf-isotope study of zircons from nine
samples
of Amitsoq
gneisses
and one Isua
supracrustal rock, Vervoort et al. (1996) point out
that
. . . if the general relationship of cur = 2~~~ approximates Hf-Nd
isotope compositions
of the early
Archaean mantle, then initial cur values of + 2 to
+5 would appear to be consistent with initial ??
Nd
values in the + 1 to + 1.5 (misprint for +2.5?)
range but not with ??
Nd values greater than + 3.
This followed on from a much earlier study (Pettingill and Patchett, 1981) in which a Lu-Hf regression
on a suite of Amitsoq gneisses yielded a nearchondritic initial 176Hf/ 177Hf value of 0.280482 k
33 at 3.55 f 0.22 Ga.
4. Conclusions
The principal recommendation
of this paper is
that in order to use initial Nd-isotope ratios for
modelling the evolution of early Archaean mantle
and crust, as well as the interaction between them, it
is essential that the Sm-Nd system in the analysed
rock suites has not been subject to open-system
behaviour or Nd-isotope homogenisation
(resetting)
at a time significantly
post-dating rock formation.
Partial or complete disturbance
of this sort may
partly or completely destroy the record of initial
Nd-isotope heterogeneities.
Here we propose that
well-correlated
regressions (i.e. isochrons or smallMSWD errorchrons) in a Sm-Nd diagram provide
an effective way of demonstrating
resetting in rock
units with significantly
older protoliths. In those
cases where Sm-Nd regressions are demonstrably
not mixing lines, the regressions may yield a geologically meaningful
age related to regional igneous
and/or metamorphic events. Statistical scatter about
well-correlated
regressions in excess of analytical
error (i.e. MSWD > 1) may reflect either incomplete
eradication of initial Nd-isotopic heterogeneities
or,
Geology 135 (1997) 213-231
probably more often, minor disturbance of Sm-Nd
systematics resulting from open-system
behaviour
during subsequent tectonothermal
events, for which
independent evidence may be available from Sm-Nd
and other mineral dates.
Numerous attempts have been made to define the
shape of the Nd-isotope evolution curve through
geological time from Sm-Nd measurements on precisely dated rock units. The best known summaries
of the available data resulted in the so-called depleted-mantle model (e.g., DePaolo, 1988; DePaolo
et al., 1991) which postulated that even the oldest
terrestrial rocks evolved from a mantle source with
small, but significant, LREE depletion compared to
CHUR. The Nd-isotope evolution curve is least well
defined in the early Archaean and we suggest that
more precise definition can only be achieved if the
geochemical behaviour of the Sm-Nd system during
and after igneous and metamorphic events is better
understood
and potential pitfalls in interpretation
avoided. Nevertheless,
the majority of initial eNd
values from a variety of terrains of different ages
indicate no significant deviation from uniformitarian
models of Nd-isotope evolution.
Our specific conclusions from the present work
are as follows:
(a) Resetting of the Sm-Nd system long after
time of formation of the protolith means that precise
U-PI> dates (e.g., smuMP-zircon) cannot be used to
infer existence of either extremely heterogeneous,
transient, REE-fractionated
mantle reservoirs or of
complex crust-mantle
interactions
during earliest
Earth history (> 3.8 Ga), because calculated initial
??
Nd values will be spurious.
(b) The Acasta gneisses of northern Canada carry
a definitive U-W
and Sm-Nd
“memory”
of a
w 3.9-4.0-Ga
protolith. However, alignment of all
analysed rock types on a well-correlated
Sm-Nd
regression line (error&on)
suggests that the igneous
and/or metamorphic and/or tectonic event that produced the Acasta gneisses as we now see them is as
young as N 3.4 Ga. This is our preferred interpretation, because we regard complete (or near-complete)
Nd-isotope homogenisation
of a large body of heterogeneous rocks by a much younger regional metamorphism as highly implausible.
(c) The N 3.9-4.0 Ga protolith of the N 3.4 Ga
old Acasta gneisses may be a mafic-ultramafic
S. Moorbath et al./Chemical
proto-crustal
complex (e.g., Chase and Patchett,
19881, with a bimodal felsic component which was
the source of inherit~ed zircons in the Acasta gneisses.
Whether physical remnants of this ancient protolith
still survive remains to be established.
(d) Re-examinat:ion
of whole-rock Sm-Nd systematics of the early Archaean Akilia enclaves and
their host Amitsoq gneisses of West Greenland also
suggests that post-formational
tectonothermal disturbance of the rocks has produced open-system
behaviour leading to a gross overestimate of the range
of initial ??
Nd values at the time estimated from
associated, precise zircon U-Pb dates (Bennett et al.,
1993).
(e) An initial ??
Nt, value of + 2.0 f 0.6 for an age
of 3776 + 52 Ma obtained from a 58-point Sm-Nd
regression of published and new data for two major
in the Isua
rock types (felsites and mica-schists)
supracrustal belt of West Greenland, is regarded as a
close approximation to a depleted-mantle
eNd value
for the early Archaean.
(f) In our view, unambiguous evidence for major
heterogeneity
of initial Ed,, values within and between early Archaean,
short-term
mantle-derived
rock units still remains to be reliably established,
especially in light of the recent Hf-isotope studies of
Vervoort et al. (1996) which strongly support our
viewpoint.
(g) The cautions expressed in this paper regarding
application of the Sm-Nd system to the study of
mantle and crust evolution during the early Archaean
apply equally well to rocks of any geological age, as
well as to other radioactive decay schemes.
Acknowledgements
Field work at Acasta in 1995 was made possible
by the interest, help and generosity of Bill Padgham,
whilst Sam Bowring kindly guided us to some critical localities. Fieldwork at Isua in 1993 was made
possible by logistic and general help from Shigenori
Maruyama
and Peter Appel. We are particularly
grateful to Mike Bickle and Shigenori Maruyama for
supplying some preliminary reconnaissance
samples
from Acasta. We thank John Arden for much of the
Sm-Nd analytical chemistry, as well as Roy Goodwin and Stephen Wyatt for skilled technical assis-
Geology 135 (1997) 213-231
229
tance. Constructive comments by Gerard Gruau on
an earlier version of this manuscript are gratefully
acknowledged. Anita Tipler typed the manuscript.
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