Geoscience Reference
In-Depth Information
Using a value for the primordial lead isotope ratios obtained from meteorites
(the lead from troilite in the iron meteorite from Canyon Diablo is frequently used
because it is the least radiogenic of all the meteorite lead) means that Eq. (6.61)
can be applied to terrestrial samples. If a sample's age
t
is known from other dating
methods,
T
can then be obtained from the present-day lead isotope ratios. Such
estimates of
T
, using the oldest lead ores known, yield values for
T
of 4520-4560
Ma, in agreement with meteorite ages.
If the meteorites and the Earth are of the same age and initially contained
lead of the same isotopic composition, the isotope ratios of average terrestrial
lead should lie on the meteorite-lead isochron. Average terrestrial lead is not a
straightforward sample to obtain, but oceanic sediments, originating as they do
from varied sources, provide an average upper-crustal estimate. In 1956 Patterson
showed that the isotope ratios of oceanic sediments lay on the meteorite-lead
isochron (Fig. 6.9). This implied that the Earth and the meteorites were the same
age and that they had the same primordial lead isotope ratios.
This concept that the Earth and meteorites are of the same age indicates that
the ages provided by the meteorite data represent not the age of the solid Earth
but rather the time when the parts of the solar system had a uniform isotopic
composition and became separate bodies, accreted. The best estimate of this
age is 4550 Ma. The differentiation of the Earth into mantle and core (and the
degassing of the atmosphere) probably then took place over the next 100 Ma.
1.3
1.2
Problems
1.1
1. Six samples of granodiorite from a pluton in British Columbia, Canada, have strontium
and rubidium isotopic compositions as follows.
1.0
0.9
0.8
87
Sr
/
86
Sr
87
Rb
/
86
Sr
0.7
0
20
40
60
80
100
86
Sr
87
Rb
0.7117
3.65
0.7095
1.80
0.7092
1.48
0.9
0.7083
0.82
0.7083
0.66
0.8
0.7082
0.74
0.7
0
10
20
30
(a) Find the age of the intrusion.
(b) Find the initial
87
Sr
/
87
Rb
86
Sr
86
Sr ratio of the magma at the time of the intrusion.
Figure 6.10.
A
Rubidium-strontium
whole-rock isochrons for
two plutons. (From
Fullagar
et al
.(1971) and
Gunner (1974).)
86
Sr ratio of 0.1 for the undif-
ferentiated Earth 4550 Ma ago, comment on the possibility that this batholith
originated in the mantle.
2. Whole-rock rubidium-strontium isochrons for two plutons are shown in Fig. 6.10.
Calculate the age of each pluton and comment on the source of the magma.
(c) Assuming an
87
Sr
/
86
Sr ratio of 0.699 and an
87
Rb
/
