Geoscience Reference
In-Depth Information
Figure 6.9.
Lead-lead isochron for meteorites and recent oceanic
sediments. The slope of the straight line gives an age
T
of 4540 ±
70 Ma for the meteorites. That lead isotopic ratios of recent oceanic
sediments also fall on this line indicates that meteorites and the
Earth are of the same age and initially contained lead of the same
isotopic composition. (From Faure (1986), after Patterson (1956).)
Their composition is believed to be close to the original composition of the
solar nebula from which the solar system formed, and thus they provide an ini-
tial composition to use for chemical models of the Earth, such as the CHUR
(Section 6.8). An asteroid that had undergone partial melting and chemical dif-
ferentiation into crust, silicate mantle and iron core could fragment into stony and
iron meteorites. The stony meteorites, composed primarily of the silicate miner-
als olivine and pyroxene, are thus similar to the Earth's crust and mantle, whereas
the iron meteorites are made up of alloys of iron and nickel, which have been
postulated to be present in the core (see Section 8.1.5). Dating stony meteorites
by the rubidium-strontium method gives ages of about 4550 Ma (their initial
ratio is 0.699). Dates from iron meteorites are similar.
A particular iron sulphide (FeS) phase known as troilite is present in meteorites.
Because troilite contains lead but almost no uranium or thorium, its present lead
isotopic composition must be close to its original composition. Thus, lead isotope
ratios of meteorites can be used as in Eq. (6.37) (Eq. (6.61) with
t
0 because
meteorites are still closed systems) to construct a lead-lead isochron (as in
Fig. 6.4(c)):
=
[
207
Pb
/
204
Pb]
now
−
[
207
Pb
/
204
Pb]
0
e
λ
235
T
1
137
.
88
−
1
204
Pb]
0
=
(6.62)
204
Pb]
now
−
[
206
Pb
/
e
λ
238
T
−
1
[
206
Pb
/
This is the equation of a straight line passing through the point ([
206
Pb
204
Pb]
0
,
/
[
207
Pb
/
204
Pb]
0
), with a slope of
e
λ
235
T
1
137
−
1
.
88
e
λ
238
T
−
1
Therefore, plotting the lead isotope ratios of meteorites, [
207
Pb
204
Pb]
now
,against
/
[
206
Pb
204
Pb]
now
enables the time
T
to be determined from the slope of the best-
fitting straight lines. The first determination (Patterson 1956) using three stony
meteorites and two iron meteorites yielded a value for
T
of 4540 Ma (Fig. 6.9).
Many subsequent measurements have been made, all giving an age for the mete-
orites, and by inference for the Earth, of between 4530 and 4570 Ma.
/
