Download RECOLLECTION The discovery of the Earth`s oldest rocks Stephen

Notes Rec. R. Soc. (2009) 63, 381–392
doi:10.1098/rsnr.2009.0004
Published online 25 February 2009
RECOLLECTION
The discovery of the Earth’s oldest rocks
Stephen Moorbath*, Department of Earth Sciences, University of Oxford,
Parks Road, Oxford OX13PR, UK
INTRODUCTION
In the mid 1960s, a young New Zealand geologist named Vic McGregor (1940–2000) was
commissioned by the Copenhagen-based Geological Survey of Greenland to make a
geological map of a large region of complex ancient rocks in the southern part of West
Greenland, in and around Greenland’s capital, Godthaab (later renamed Nuuk). This scenic
wonderland of mighty fjords and mountains forms part of a strip roughly 130 km wide at this
latitude between Greenland’s coast and the edge of the huge inland ice-cap (figure 1).
For several years McGregor explored the geology of this region from a tiny, partly open
boat, with just enough room for himself, two local crew, an occasional guest, and essential
camping, hunting, fishing and geological equipment.
Before his work in Greenland, McGregor was strongly influenced by the work of the
husband-and-wife team John Sutton and Janet Watson (both later elected FRS) of Imperial
College London, who had earlier pioneered modern field-based methods for clarifying the
geological evolution and structure of the ancient rocks of the so-called Lewisian Complex of
the northwest Highlands of Scotland, as described in their landmark paper of 1950.1 By
careful and detailed geological mapping, they recognized a whole series of varied geological
events that had affected these complex rocks, most of which had been strongly altered,
deformed and recrystallized (‘metamorphosed’) at high pressures and temperatures. Sutton
and Watson showed that the Lewisian Complex comprises two main groups of geological
events, which they conjectured were widely separated in geological time and which they
termed ‘Scourian’ (the older) and ‘Laxfordian’ (the younger), after prominent localities.
However, no absolute ages for these rocks were available at that time.
In the late 1950s, my colleagues and I at Oxford began to set up the first geological dating
laboratory in the UK, using recently developed isotopic techniques based on radioactive
schemes rubidium–strontium (87Rb –87Sr), potassium–argon (40K–40Ar) and uranium–lead
(238U–206Pb and 235U–207Pb), as well as the powerful, time-dependent combination lead / lead
(207Pb / 206Pb). This led to a concerted attack on the Lewisian Complex, providing age data
for a range of Lewisian events, including the oldest and youngest. This confirmed Sutton and
Watson’s prediction of two widely separated groups of events, at ca. 2800 million years (Myr)
and 1700 Myr for the Scourian and Laxfordian, respectively.2 At first, the Scourian rocks
remained some of the oldest known, reliably dated, rocks on Earth. By this time, the work of
such great pioneers as Arthur Holmes, Fritz Houtermans and Clair Patterson on Pb isotopes in
terrestrial rocks and meteorites had already demonstrated that the Earth and Solar System
were close to 4.5 billion years (Gyr) old.3
*[email protected]
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This journal is q 2009 The Royal Society
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S. Moorbath
Figure 1. Sketch-map of southern Greenland, showing localities and rock units mentioned in the text.
My own activities in the isotopic dating of rocks arose in line with the growing
international realization in the mid twentieth century that geology (sensu lato) must in
future deal with the origin, structure and evolution of the whole Earth, involving the
quantitative evaluation of all physical, chemical and biological processes that have shaped
our planet throughout its entire history. The continuing technical and interpretational
improvement of dating methods since the 1950s, using radioisotopes with widely differing
half-lives, has produced a timescale for numerous geological, palaeobiological,
palaeoanthropological and palaeoenvironmental processes, not to mention the study of
meteorites and planetary material as well as the accretion and early differentiation history of
the Earth. As the result of huge international effort, all these fields of enquiry have been
placed on a truly realistic historical-evolutionary footing, although much work still remains
to be done.
The discovery of the Earth’s oldest rocks
RESULTS OF
383
FIELDWORK AND ISOTOPIC DATING
Taking up the story in Greenland, by 1970 McGregor had single-handedly made a detailed
geological map of the Godthaabsfjord region. Using field techniques analogous to those that
Sutton and Watson had earlier applied in the Lewisian, he came up with a complex sequence
of two main groups of 10 successive magmatic (that is, igneous), supracrustal (that is, rocks
originally laid down at the surface of the Earth) and metamorphic (affecting existing rocks)
events.4 The latest major rock-forming event in this sequence had already given a potassium–
argon date of 2710G130 Myr in some unconnected reconnaissance research.5 Hence
McGregor reasoned that if the youngest event in such a complex sequence gave 2.7 Gyr, then
the very oldest event that he recognized at that time (which has since been surpassed by even
older events) could be ‘very old indeed’ and, possibly, the oldest rocks known on Earth. Indeed,
he hazarded an inspired guess of ca. 3.6 Gyr,6 which seemed very courageous at the time.
McGregor termed these potentially oldest rocks the Amı̂tsoq gneisses after a local place name.
I first heard of McGregor’s geological model for the Godthaabsfjord region in 1969, when
I contacted him. Quite early on, I sensed his frustration in trying to convince his colleagues in
the Survey of the correctness of his proposed geological succession. As a result, we decided at
Oxford to attempt to date some crucial rock units in McGregor’s complex sequence, using
methods previously applied to the Lewisian Complex of Scotland (see above).
One memorable day in 1970, we first analysed isotopically a sample of Amı̂tsoq gneiss
(see above) sent to Oxford by McGregor, representing the oldest member of his sequence. We
found that it had a more ‘unradiogenic’ lead isotope composition (that is, lower 206Pb/204Pb
and 207Pb/204Pb ratios)7 than any terrestrial ore or rock lead ever reported previously. On any
plausible terrestrial lead-isotope growth models, which were already fairly well understood
by that time,3 the analysed lead, and hence the rock, must be at least 3.7 Gyr old. By that time
the gneiss had already become highly depleted in uranium (a common observation in strongly
metamorphosed gneisses), so that radiogenic evolution of their lead from uranium had
virtually ceased.
During the early 1970s I had the good fortune of visiting the mapped Godthaabsfjord
localities with McGregor, while amassing a large collection of Amı̂tsoq gneisses and younger
rocks (figure 2). Using the dating methods 87Rb–87Sr, 207Pb/206Pb and (later) 147Sm–143Nd, it
was confirmed that the Amı̂tsoq gneisses were ca. 3.65–3.75 Gyr old, far older than any
reliably dated rocks on Earth at that time.8
From the age data it was also possible to calculate the initial Sr, Pb and Nd isotope ratios8
of the ancient gneisses at the time of their formation. These results showed that the magmatic
precursors of the Amı̂tsoq gneisses had formed by partial melting of a mantle-like source
region, and that they represented a juvenile addition of continental-crust-like material at or
near the Earth’s surface. Indeed, later work showed that much of southern West Greenland is
made up of three great, separate episodes of mantle-crust differentiation at ca. 3.7, 2.8 and
1.7 Gyr,9 all of which were tectonically quite independent, and were not produced by
remelting or reworking of earlier continental crust (figure 3). Radiogenic isotope research has
always strongly supported the view that continental crust has grown discontinuously
throughout geological time as a result of huge, widely separated (in time) episodes of mantle
differentiation.10
In 1971 McGregor and I visited the remote and, at that time, virtually unknown Isua
region, some 150 km northeast of the already mapped Godthaabsfjord region and close to the
edge of the inland ice. It proved quite an arduous trip to sail in McGregor’s tiny boat up to the
384
S. Moorbath
Figure 2. A typical rock exposure in the Godthaabsfjord region. The banded rock on the right is the Amı̂tsoq gneiss,
with an age close to 3.7 Gyr. This is cut by the black, basaltic (dolerite) dyke on the left, dated at ca. 3.4 Gyr. The
light-coloured ‘pegmatite’ vein, which cuts both the previous rock types, is associated with a nearby granite intrusion
dated at 2.6 Gyr. (Online version in colour.)
head of the iceberg-packed Godthaabsfjord (where, in the Middle Ages, the resident Vikings
practised domestic agriculture). There we were picked up by a helicopter belonging to the
Kryolit-Marcona Mining Company; the Company had started to explore a huge iron-ore
deposit at an altitude of 1240 m at the very edge of the inland ice (latitude 658 N), which had
been discovered from a major aeromagnetic anomaly (figure 4).
Aided by locality sketch-maps provided by the Company geologists, McGregor and
I made the first geological interpretation of the now well-known Isua supracrustal belt (ISB),
sometimes termed the Isua greenstone belt, which has since been closely studied by many
workers. Right from the start, we regarded the ISB as older than the adjacent gneisses, which
we provisionally equated (correctly, as it later turned out; see below) with the ca. 3.7 Gyr-old
Amı̂tsoq gneisses some 150 km to the southwest, near the coast. These gneisses are enclosed
within the broadly circular outcrop of the ISB, which is ca. 20 km in diameter and ca. 35 km
in strike (that is, parallel to the outcrop), with a variable outcrop width of 1–3 km. The ISB
rocks form an incredible contrast with the bordering gneisses of deep-seated, magmatic
origin. Despite strong, but variable, metamorphic recrystallization and deformation, the
original ISB components are mostly identifiable as a complex series of chemical (that is,
precipitated from solution) and clastic (that is, eroded as detrital components from older
rocks) sediments, as well as mafic (basaltic, silica-poor) and felsic (silica-rich) volcanics.11
Almost all the rocks were deposited by recognizably uniformitarian processes (that is, those
active throughout subsequent geological time) in a marine environment. Of particular interest
are the voluminous basaltic pillow lavas, which were erupted directly into seawater.
385
The discovery of the Earth’s oldest rocks
87Sr/ 86Sr
0.730
0.720
0.710
0.703
A
C
owth line
Upper mantle gr
0.699
4600
B
4000
3000
2000
Time (Myr)
1000
0
Figure 3. Average 87Sr/86Sr growth lines for three different groups of granitic gneisses (A, B and C) from West
Greenland, plotted in relation to an upper-mantle growth line.9 Each group of gneisses forms part of a separate major
mantle–crust differentiation episode at the times shown on the horizontal axis. Slopes of the growth lines are
proportional to measured Rb/Sr ratios multiplied by the 87Rb decay constant. The upper-mantle growth line is derived
from 87Sr/86Sr data on Rb-poor mantle-derived rocks with a wide range of ages. Its extrapolation back to 4600 Myr
ago (that is, older than any known geological age) is based on data from meteorites. For further details of Rb–Sr
theory, see note 8. This type of information can also be obtained from other decay schemes.
The banded iron-formation (BIF), which comprises the major iron-ore deposit at Isua,
with its alternating bands of chert (SiO2) and magnetite (Fe3O4), was also deposited under
marine conditions.
How old are the Isua rocks? The first age determinations, performed at Oxford, gave
rubidium–strontium and lead / lead ages of close to 3700–3750 Myr for the gneisses and for
the BIF.12 The BIF dating proved the unexpected presence of liquid water on the Earth’s
surface as early as ca. 3.7 Gyr ago. The gneiss dating showed that our provisional correlation
of the Isua gneisses with the Amı̂tsoq gneisses of the Godthaabsfjord region was valid.
In addition, McGregor had been correct right from the start of our visit in proposing that the
Isua region rocks as a whole represented a more pristine, somewhat less deformed and
metamorphosed version of the Godthaabsfjord region rocks, and that one could look even
further back in time because of the presence of the ISB rocks. (Later, McGregor reported the
discovery of rocks from the Godthaabsfjord region that were almost certainly equivalent in
type and age to the ISB rocks, but they were in not nearly such a good state of preservation.)13
Since those early years, the ISB has yielded numerous uranium–lead dates in the range
3.7–3.8 Gyr ago, particularly using the accessory mineral zircon (ZrSiO4), which contains
trace amounts of uranium and which lends itself to very precise dating.8 Published zircon
dates strongly suggest that the depositional age of at least part of the ISB could be closer to
3.8 Gyr than to 3.7 Gyr.14
Bordering directly on the southern margin of the ISB is a still little-explored terrain
of at least 100 km2 of well-exposed, low-deformation gneisses of magmatic origin.
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S. Moorbath
(a)
(b)
Figure 4. Fieldwork in the Isua region. (a) View of Isua Mountain (1240 m). The largest exposure of sedimentary
banded iron-formation 3.8 Gyr old occurs at the top of the mountain. Beyond is the inland ice, which extends all
the way to East Greenland. (b) Collecting banded iron-formation samples close to the top of Isua Mountain.
(Online version in colour.)
Zircon uranium–lead dates range up to 3.82 Gyr, and these gneisses contain abundant
inclusions of older rocks, easily identified as varied volcanic and sedimentary rocks.15 These
inclusions may be of the same age as the ISB rocks to the north, or even slightly older.
The discovery of the Earth’s oldest rocks
387
Other sites of ancient rocks (more than 3.5 Gyr old) have been reported from around the
world,16 although the age evidence for some of them is still preliminary. None is as extensive,
well preserved and well exposed as the Godthaabsfjord–Isua region, with its overall age range
of ca. 3.82–3.65 Gyr (perhaps locally up to 3.85 Gyr). Here we assume (arguably) that the
ancient Greenland rocks are representative of global terrestrial processes at ca. 3.8 Gyr old.
I conclude with a brief summary of what type of scientific insights these oldest rocks
can provide.
SCIENTIFIC INSIGHTS FROM THE
OLDEST ROCKS
The most important observation is that there is no direct evidence within the Greenland
rocks for any exposed primordial crust significantly older than ca. 3.8 Gyr. All rocks in
the Isua region are of secondary origin in that they were produced from varied source
rocks by identifiable uniformitarian geological processes. Chemical sediments were
precipitated in warm ocean water fed with iron-rich chemicals derived from hydrothermal
vents discharging massive volumes of basaltic lavas into the water from the hot mantle
below. Clastic sediments were produced close to the shorelines of a low-relief volcanic
landscape. In the 3.8 Gyr sediments there is no sign of detrital fragments of any older
continental type crust of granitic character, such as are frequently seen elsewhere in
younger rock assemblages of this type that are known to postdate the existence of
continental crust. Nevertheless, there is positive evidence that the types of deep-seated
magmatic rock of broadly granitic composition that form the backbone of modern
continents were already in production only slightly later, by 3.7–3.8 Gyr, as judged from
their substantial presence in the Godthaabsfjord–Isua region. Such rocks (termed
Tonalite–Trondhjemite–Granodiorite, or TTG gneisses) were generated by well-understood
processes of global tectonics, involving plate tectonics, ocean-floor mobility, subduction
and partial melting of mafic crust, leading to growth of continental crust throughout
geological time. It seems from the Greenland rocks that these global tectonic processes
might already have commenced at ca. 3.7–3.8 Gyr ago, soon leading to the formation of
the Earth’s first true continental crust.17
Intense debate and controversy persists over whether life already existed on Earth by
3.7–3.8 Gyr ago. Such claims have been strongly voiced for the Isua sediments, and also for
possibly related rocks some 150 km to the southwest on the coast near Nuuk.18 Because of the
relatively high degree of deformation and metamorphism of these rocks, the presence of
morphological fossil evidence is most unlikely. The claims for life are therefore based on
certain carbon isotope ratios in graphite grains, which occur in some of these sediments.
These grains show the low 13C/12C ratios (13C depletion) characteristic of all biological
material. However, it is now known that low 13C/12C ratios in graphite can be produced by
thermochemical reactions involving the decomposition of non-biological carbonate rocks,
thus greatly decreasing the diagnostic value of carbon isotope ratios in ancient rocks for the
past presence of life.19 I am highly sceptical of all claims so far made for the presence of
biogenic material in the oldest Greenland rocks. Now that we know more about their
depositional environment, there is no a priori reason why life should not have got going
by Isua times, but there is simply no convincing evidence for it. The search in this
region continues for more reliable diagnostic criteria, but the best evidence so far for the
oldest life comes from almost unmetamorphosed, undeformed 3.4–3.5 Gyr sediments from
Western Australia and South Africa that contain widely accepted morphological evidence
for cellular life.20
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S. Moorbath
Despite the secondary nature of the Isua rocks themselves (see above) there is ample
evidence from their lead isotope ratios that some of the rocks, or more probably their
immediate precursors, were derived from an unexposed source region with an age of up to
4.2–4.3 Gyr, which most probably had a mafic, mantle-like composition.21 This must have
been a part of the original so-called ‘Hadean’ crust, which has not yet been found exposed
anywhere at the Earth’s surface. In addition, the Isua rocks and surrounding gneisses are so
far the only known terrestrial rocks that preserve positive anomalies in the abundance of the
isotope 142Nd, produced by a-decay of 146Sm, with a half-life of 103 Myr. Since virtually all
146
Sm produced at the formation of the Solar System would have decayed by some
4.1–4.2 Gyr ago, it means that 142Nd heterogeneities in the source of the 3.7–3.8 Gyr
Greenland rocks must have been preserved from a much earlier period in Earth history. When
the 146Sm–142Nd data are used in conjunction with Greenland age and isotope data obtained
with the 147Sm–143Nd system (the half-life of 147Sm is 106 Gyr), which is one of the principal
dating methods used in geochronology, it can be shown that major global differentiation into
the primary layers of the Earth occurred ca. 4.45 Gyr ago, only some one hundred million
years after the accretion of the Earth itself.22 The search for 142Nd isotope anomalies
elsewhere on Earth is in progress, but it is unlikely that this anomaly can be detected in rocks
younger than about 3.5 Gyr.
It may not be fortuitous that an age of ca. 3.82 Gyr, or just above, for the oldest
supracrustal rocks and associated gneisses in southern West Greenland is only a few tens of
millions of years (40G20 Myr?) younger than the probable age of the so-called Late Heavy
Meteorite Bombardment (LHMB), which is widely held to have affected Moon and Earth
simultaneously, although no geological evidence for it has yet been found on Earth.
Detailed measurements of isotopic age on lunar samples in combination with impact crater
counts indicate a very rapid decline in the rate of lunar basin formation during the period
3.90–3.85 Gyr ago.23 The only terrestrial evidence so far found for early meteorite
bombardment has been the discovery of tungsten isotope anomalies based on the
182
Hf–182W system (the half-life of 182Hf is 9 Myr) in some of the sedimentary rocks from
the Isua belt.24 Because tungsten isotope heterogeneities cannot have been preserved in
the Earth’s dynamic crust–mantle environment from a time when short-lived 182Hf was
still preserved, it was concluded that the sediments contain a component derived from
the meteorites.
All of this creates an enigma concerning the apparent disappearance of the primordial
crust of the Earth, including the large number of impact craters that must once have covered
the surface. One may conclude that there must have been an immense basaltic resurfacing
produced by global melting and volcanism, which may itself have been initiated by the
massive impacts.25 Much more work is required in the oldest terrains, including Isua, to track
down exactly what happened on Earth not long (in geological terms) before recognizable
geological processes started.
A simplified summary of environments and dated events on the early Earth is presented
in figure 5. Although this closely represents my own views, it is pointed out that there is
still much controversy about such important matters as the nature and composition of the
Hadean crust.
In conclusion, both the nature and environment of the oldest exposed rocks as far back as
3.8 Gyr ago have proved amenable to study by conventional geological approaches.
In addition, these oldest rocks have provided a limited amount of significant chemical,
The discovery of the Earth’s oldest rocks
389
FORMATION OF EARTH (4.56 Gyr)
CORE – MANTLE DIFFERENTIATION (c.4.45 Gyr)
DATED EVENTS (Gyr)
ENVIRONMENT
4.4
Basaltic proto-crust with minor
felsic rocks (source of zircons);
probable presence of water and
meteoritic material; continental
crust and oceans unlikely.
Oldest known detrital zircons (see below).
4.3
HADEAN
4.2
4.1
4.0
Isotopic memory of ‘crust’
of this age in some early
Archaean rocks.
Detrital zircons (4.35–3.95 Gyr)
in Archaean rocks, mostly from
W. Australia.
No undisputed exposure
of Hadean rocks.
Impact bombardment on Moon
and Earth.
3.9
3.85(?)
Global tectonics (subduction)
permits initiation of continentaltype crust (TTG gneisses).
Beginning of carbonate
sedimentation. Anoxic
atmosphere. No undisputed
evidence for life.
HADEOARCHAEAN
3.82(?)
ARCHAEAN
Resurfacing of the Earth by
global volcanism and sediment
transport. Formation of oceans.
End of Late Heavy Bombardment.
Identifiable geological processes commence.
3.8 Oldest known mafic/ultramafic lavas;
chemical and clastic sediments;
granitoid gneisses of magmatic origin
(West Greenland).
3.7 Initiation of major production of
continental crust.
Figure 5. Simplified version of processes, environments and dated events on the early Earth, some of which are
discussed in the text. Rocks very similar in character, age and extent to the Isua region rocks discussed in this paper
have recently been reported from an area east of Hudson Bay in Canada.26
390
S. Moorbath
isotopic and mineralogical information on the previous 700 Myr of Earth history, from which
no bulk rocks survive.
At any rate, we can now begin to compare and contrast the early development of the
Earth’s surface with that of our planetary neighbours the Moon, Mars and Venus, and
contemplate in surprise and amazement their increasingly divergent evolution from early on
in their respective histories. Although the planets themselves have grown apart, the sciences
of geology and planetology seem to be coming ever closer together!
ACKNOWLEDGEMENTS
The early stages of the age and isotope work at Oxford on the Lewisian Complex of northwest
Scotland were carried out in particular collaboration with B. J. Giletti, R. St J. Lambert and
H. J. Welke, and on the ancient Greenland rocks with R. K. O’Nions, R. J. Pankhurst and
P. N. Taylor. I am deeply indebted to them and, in particular, to our Chief Technician Roy
Goodwin (1942–2002), who was an indispensable member of the Geological Age and
Isotope Research Group for 43 years.
NOTES
1
2
3
4
5
6
7
8
9
10
11
J. Sutton and J. Watson, ‘The pre-Torridonian metamorphic history of the Loch Torridon and
Scourie areas in the North-West Highlands, and its bearing on the chronological classification of
the Lewisian’, Q. J. Geol. Soc. Lond. 106, 241–307 (1950).
B. J. Giletti, S. Moorbath and R. St J. Lambert, ‘A geochronological study of the metamorphic
complexes of the Scottish Highlands’, Q. J. Geol. Soc. Lond. 117, 233–264 (1961).
G. Brent Dalrymple, The age of the Earth (Stanford University Press, 1991).
V. R. McGregor, ‘The early Precambrian gneisses of the Godthaab district, West Greenland’,
Phil. Trans. R. Soc. A 273, 343–358 (1973).
R. L. Armstrong, ‘K–Ar dates from West Greenland’, Bull. Geol. Soc. Am. 74, 1189–1192 (1963).
V. R. McGregor, ‘Field evidence of very old Precambrian rocks in the Godthaab area, West
Greenland’, Rep. Geol. Surv. Greenland 15, 31–35 (1968).
In the mass-spectrometric analysis of lead isotope ratios, the radiogenic isotopes 206Pb and 207Pb
(produced respectively by the radioactive decay of 238U and 235U) are compared with the nonradiogenic isotope 204Pb, which has always been constant in amount.
Radiogenic isotope ratios are used to date and to identify individual episodes of mantle
differentiation and continental crust formation throughout geological time. The isotope ratios
referred to here are 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 143Nd/144Nd; that is, the radiogenic
isotope divided by the non-radiogenic isotope (see note 7). Full descriptions of dating techniques
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applications, 3rd edn (John Wiley & Sons, New York, 2005).
S. Moorbath, ‘Ages, isotopes and evolution of Precambrian continental crust’, Chem. Geol. 20,
151–187 (1977).
This huge subject is briefly reviewed by C. J. Hawkesworth and A. I. S. Kemp, ‘Evolution of the
continental crust’, Nature 443, 811–817 (2006).
A. P. Nutman, J. H. Allaart, D. Bridgwater, E. Dimroth and M. T. Rosing, ‘Stratigraphic and
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belt southern West Greenland’, Precambr. Res. 25, 365–396 (1984); P. W. U. Appel, C. M. Fedo,
S. Moorbath and J. S. Myers, ‘Recognisable primary volcanic and sedimentary features in a lowstrain domain of the highly deformed, oldest known (w3.7–3.8 Gyr) greenstone belt, Isua, West
The discovery of the Earth’s oldest rocks
12
13
14
15
16
17
18
19
20
21
22
391
Greenland’, Terra Nova 10, 57–62 (1998); C. M. Fedo, J. S. Myers and P. W. U. Appel,
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the O3.7 Ga Isua greenstone belt, West Greenland’, Sediment. Geol. 141/142, 61–77 (2001).
S. Moorbath, R. K. O’Nions, R. J. Pankhurst, N. H. Gale and V. R. McGregor, ‘Further
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Precambrian crustal rocks at Isua, West Greenland—geochemical and isotopic evidence’, Earth
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V. R. McGregor and B. Mason, ‘Petrogenesis and geochemistry of metabasaltic and
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(1977).
A. P. Nutman, V. C. Bennett, C. R. L. Friend and M. T. Rosing, ‘w3710 and R3790 Ma volcanic
sequences in the Isua (Greenland) supracrustal belt; structural and Nd isotope implications’,
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A. P. Nutman, V. C. Bennett, C. R. L. Friend and M. D. Norman, ‘Meta-igneous (non-gneissic)
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B. S. Kamber, S. Moorbath and M. J. Whitehouse, ‘The oldest rocks on Earth: time constraints
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