Geoscience Reference
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35
30
Nuevo
Laredo
25
T = 4.55 × 10 9 a
20
Forest
City
15
Henbury
Modoc
10
Canyon
Diablo
5
0
0
10
20
30
40
50
60
206 Pb/ 204 Pb
Figure 12.12 Patterson's (1956) geochron is in fact an isochron ( 206 Pb
204 Pb) established from
three chondritic meteorites (solid circles) and iron sulfide of two iron meteorites (open circles).
The age established is therefore that of the Solar System.
204 Pb, 207 Pb
/
/
The oldest crustal segment on Earth (Isua, Greenland) is 3.85 Ga old. Recently, detrital
zircons dated between 3.9 and 4.4 Ga have been identified in an Australian sandstone at
Jack Hills, which require the existence by that time of some granitic crust. However, most
ages older than 4.3 Ga are mostly isolated patches in younger zircons and probably are to
be taken with a grain of salt. Very ancient ages, of more than 4.4 Ga, have also been found
for rocks and meteorites of the Moon and Mars.
Patterson's isochron has improperly been called the geochron, although almost all the
measurements of compositions of terrestrial samples lie not on the isochron but to its right.
The essential argument for dating the Earth is, in fact, an indirect one relying on what is
known as extinct radioactivities, which provide a precise but relative chronology between
planetary objects. It should be recalled therefore that the Earth has not been dated directly
but that chondrites have, and with great precision (4.568 Ga with an uncertainty
1Ma).
The question then becomes “how much younger than primitive meteorites could the Earth
be?” and the answer has been provided by extinct radioactivities.
The first evidence of an extinct radioactivity was discovered as an excess of 129 Xe
in the Richardton meteorite by John Reynolds in 1960. The parent nuclides of extinct
radioactivities, such as 26 Al, 182 Hf, 129 I, have short half-lives ( Table 4.1 ) , and therefore
large decay constants. The concept is simple: if the parent nuclide was still extant when
a planetary body went through a major differentiation event such as accretion, core seg-
regation, or magma ocean crystallization, associated parent/daughter fractionation should
show up in the isotopic variability of the daughter isotope, even though today the radioac-
tive parent is long gone. It is the physicist's way of dating by fossil remains! Depending
on the half-life of the parent, this assumption gives us access to a range of time intervals,
from a few millions of years for 26 Al to a few hundreds of millions of years for 146 Sm. Let
us try to work out the isochron equations suitable for a system in which the parent nuclide
is now extinct.
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