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Isotope Geology Claude J. (cambridge)

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al., 1968). (b) The two 87Rb–87Sr methods on biotite in the
generalized Rb–Sr, K–Ar concordia. (c) The 87Rb–87Sr method on feldspar. (d) Stepwise 40Ar–39Ar ages
(after Berger, 1975). The numbers indicate distances from the granite intrusion (Chapter 3).
182 Uncertainties and results of radiometric dating
probably that of the earlier granites whose zircon was eroded and redeposited in the clays
fromwhich the schists were formed. But the questioning and the answers are themselves a
measureofuncertaintyofthe result.Whatdoes theageofformationofaschistmean?
Theveryancient rocks ofGreenland
The rocks of Greenland are the oldest in the world.7 They have been studied by various
methods: U^Pb on zircon, 206Pb^207Pb on whole-rock isochrons, 86Rb^87Sr, and
147Sm^143Nd. Steve Moorbath of the University of Oxford was both the pioneer and the
principal investigator (Moorbath and Taylor, 1981; Moorbath et al., 1986). These studies,
combinedwithprecisemappingofthe terrain, led to the identi¢cation of two separate geo-
logical formations (Figure 5.18), theAmitsoq gneiss and the IsuaGroup (the mountain of
0.7
1.0 2.0
3.0 4.0
3.0 40
0.5
0.3
0.7
0.8
0.1
20
6 P
b
/2
38
U
207Pb/235U
3.5
4
2
5
8
93
7
6
3.824 +12–9
3.9
3.8
3.7
3.6
3.5
error
3.65 3.824
2.5
1.5
3
Amitsoq gneiss
t = 3640 ± 120 Ma
εNd,0(t) = 0.9 ± 1.4
t = 3776 ± 52 Ma
εNd,0(t) = 2.0 ± 0.6
Ba
ad
sga
rd 
dis
co
rdi
a l
ine
 
2
1
0.5
0.080.06 0.10 0.12 0.14
0.5102
0.5106
0.5110
0.5098
14
3 N
d
/1
44
N
d
147Sm/144Nd147Sm/144Nd
Isua boulders
0.08 0.14 0.18 0.220.06
0.514
0.512
0.510
14
3 N
d
/1
44
N
d
Figure 5.18 Results obtained by various methods for the Amitsoq and Isua gneisses. After Michard-
Vitrac et al. (1977); Moorbath et al. (1997).
7 There are older minerals (zircon) but they are detrital and separated from their parent rock in which they
crystallized.
183 Geological interpretations
references includes:Michard-Vitrac etal.,1977;Moorbath etal.,1986,1997;Nutman etal.,
1996;KamberandMoorbath,1998).
The ¢rst is dated 3.65� 0.03Ga and the second 3.74 and 3.82Ga. These are the oldest
groupsofrock in theworld.As canbe seenbyexaminingTable 5.4 andFigure5.18, summar-
izing the results obtained by the various methods, the two groups are quite distinct.
However, small di¡erences can be noticed in the absolute values obtained by the various
methods and whose precise meaning is unclear.We have no explanations other than that
the rockswere subjectedto subsequentmetamorphismandthe chronological systemswere
probablydisruptedsomewhat.
The Australian group from Canberra has used ion probe point analysis methods to
measure zircongrainbygrain anddetermine theU^Pbages on theAmitsoqgneiss.A his-
togramcanbemade (Figure 5.19). It shows the twopeaks at 3.60Gaand3.80Ga, providing
wetakethemodeofthe twoasymmetricaldistributionsas thevalue.Thetwostatistical ages
seemtobe inagreementwiththeagesmeasuredbyconventionalmethods.
Letusdiscussandinterprettheseresults.ThedistributionoftheAmitsoqandIsuaforma-
tions is clear enough.The age of the Amitsoq gneiss is 3.64� 0.05Ga, with �t/t¼ 1.3%.
The Isua formation is a little older but not as well de¢ned. The age of 3.66Ga given by
87Rb^86Sr on whole rocks from Isua must be considered to be re-homogenization at the
timetheAmitsoqgneiss formed.
There seem to be rocks dating from 3.82Ga, particularly in the form of conglomerates
whosezircon‘‘survived’’thegeologicalperturbations.
Table 5.4 Results obtained by various methods for the Amitsoq gneiss and the Isua Group
Datingmethods Amitsoqgneiss (Ga) IsuaGroup (Ga)
U^Pb concordia (zircon) 3.45� 0.05 3.824� 0.05 (conglomerate)
3.77� 0.01 (massive rock)
87Rb^87Sr (whole rock) 3.64� 0.06 3.66� 0.06
147Sm^143Nd(whole rock) 3.640� 0.12 3.776� 0.05
207Pb^206Pb (whole rock) 3.56� 0.10 3.74� 0.12
40
2.80 3.20 3.60 4.00
30
20
10
N
u
m
b
er
 o
f 
sa
m
p
le
s
Age (Ga)
Figure 5.19 Histogram of 207Pb–206Pb ages obtained by ion microprobe methods on Amitsoq zircon.
After Nutman et al. (1996).
184 Uncertainties and results of radiometric dating
But we may surmise that the Isua surface formation is rather aged 3.77� 0.08Ga (�t/
t¼ 2%).Theageof3.82Ga is the resultof inheritedprocesses.Along the same lines,we can
infer that some old zircons have been incorporated into the Amitsoq formation, either
through the erosion^sedimentation cycle or by processes of magmatic assimilation or
metamorphism yielding the old values in the statistical distribution. As can be seen not
everything is clear.But itdidall happen nearly4billionyearsago!
Archean komatiites
Komatiites are associations of basic and ultrabasic lavas found in Archean rocks alone (see,
e.g.,Hamilton etal.,1979;Zindler,1982;Bre¤ vartetal.,1986;Dupre¤ andArndt,1986).Theyare
theonlyevidenceofwhatthemantlewas likeatthattime.Theseassociationsofrockhavebeen
datedmainlyby Sm^Nd and Pb^Pb systems since the other geochronometers, particularly
Rb^Sr,Ar^Ar,andU^Pb,aregenerally verydisturbedsystems. Inaddition, theU^Pb,con-
cordiamethod is di⁄culttouseasuranium-richmineralsarevery rare inthese rocks.Theold-
estwell-identi¢edkomatiitebelt is theBarbertonGreenstoneBelt in SouthAfrica. It is dated
3.4� 0.12Ga (almost as old as Amitsoq!). Several datings havebeen obtained on these rocks
byvariousmethods,allofthemmoreorless concordant.Herearetheresults.
The whole-rock isochron method gives: 87Rb^87Sr¼ 3.35� 0.2Ga; 147Sm^143Nd¼
3.54� 0.07Ga; 40Ar^39Ar¼ 3.49� 0.01Ga; 206Pb^207Pb¼ 3.46� 0.07Ga.Thisposes the
question ofthe exact ageofemplacementofthekomatiites. Is it 3.35Gaor 3.53Ga?There is
agapofsome200Mabetween the twodates,which is as longas the time separatingus from
the Jurassic.Given the datawe currentlyhave, wehave no criterion fordecidingonewayor
another, and so choose the value of 3.45� 0.10Ga as the most likely age.The resolution of
such problems will answer the question of the duration of the emplacement episode of
komatiites.
An entirely di¡erent situation is found at Kambalda in Western Australia. Both the
147Sm^143Nd and 206Pb^207Pb methods give very handsome alignments on the isochron
diagrams.Unfortunately, these alignments donotyield the sameage.The147Sm^143Ndage
is 3.26Gawhile the 206Pb^207Pb is 2.72Ga. Both methods are reputed tobe robust.Which
shouldwe choosewhenthey fail toagree?
Dupre¤ and Arndt (1987), then working together at the Max-Planck Institute in Mainz,
showed thatthe147Sm^143Ndstraight lineswere in fact straight lines ofmixing, as shown in
the ("Nd,1/Nd) plot (Figure 5.20).Themost likelyage is therefore 2.72Ga, which is consis-
tent with the local geological context and datings of other associated terrains. Dupre¤ and
Arndt (1987)generalizedthediscussionofcomparativeSm^NdandPb^Pbagesonkoma-
tiites andmadeasystematic compilation (seeTable 5.5).
Therearethreecaseswheretheageis¢xedtowithin� 20Ma:Barberton inSouthAfrica,
theAbitibi komatiitebeltofCanada, andZimbabwe.This assertion isbasedonthe concor-
dance of ages determined by both methods and on the geological context and dating of
neighboringgranitic rocks.Notice that�t/t¼ 0.7%.
CapeSmith is aspecial casebecausethe147Sm^143Ndand207Pb^206Pbagesarenotverydif-
ferentandhaveoverlappingmarginsofuncertainty.Forwantofanyother information,wemust
putdownanageof1.73� 0.1Gawith�t/t¼ 5%,which isnotbadcomparedwiththeothers.
The case ofWest Pilbara in Australia is rather similar.The two 207Pb^206Pb measure-
ments seemweakerand incorrectbecause thegeological contextargues rather foranageof
185 Geological interpretations
3.56Ga. It is the context alone that allows anyconclusion, butthe 207Pb^206Pb results indi-
cate therewasasecondary,disruptivephenomenon.
As said, Kambalda is the opposite case. The geological context