Geoscience Reference
In-Depth Information
are rare, they do indicate that, by 4400 Ma and without doubt by 4000 Ma,
continents were in existence and the surface temperature was cool enough
to have liquid water. The Earth itself is certainly older than these oldest rocks.
Our present knowledge of the age of the Earth comes from a study of the
isotopes of lead and from meteorites. First, consider a general model of the lead
evolution of the Earth, usually known as the
Holmes-Houtermans model
, after its
two independent creators, who built on earlier work by Holmes and by Rutherford.
They assumed that, when the Earth was formed, it was homogeneous with a
uniform internal distribution of U, Pb and Th. Very soon afterwards, the Earth
separated (
differentiated
) into a number of subsystems (e.g., mantle and core),
each of which had its own characteristic U
/
Pb ratio. After this differentiation, the
U
Pb ratio in each subsystem changed only as a result of the radioactive decay
of uranium and thorium to lead (i.e., each subsystem was closed). Finally, when
any lead mineral formed (a common one is galena), its lead separated from all
uranium and thorium; so its lead isotopic ratios now are the same as they were at
its formation. Applying Eq. (6.37)tothis model gives
/
[
207
Pb
/
204
Pb]
now
−
[
207
Pb
/
204
Pb]
0
e
λ
235
T
−
e
λ
235
t
1
137
.
88
204
Pb]
0
=
(6.61)
204
Pb]
now
−
[
206
Pb
/
[
206
Pb
/
e
λ
238
T
−
e
λ
238
t
where
T
is the age of the Earth,
t
is the time since the formation of the lead mineral,
the subscript 'now' refers to the isotope ratio of the lead mineral measured now
and the subscript 0 refers to the primordial isotope ratio of the Earth time
T
ago. This is the
Holmes-Houtermans equation
. There are three unknowns in the
equation:
T
,[
207
Pb
204
Pb]
0
.
Thus, having at least three lead minerals of known age and lead isotope ratios
from different subsystems should enable us to determine the age of the Earth and
the primordial isotope ratios from Eq. (6.61). Unfortunately, the complex history
of the crust and the fact that rocks are frequently not closed to uranium means
that in practice
T
cannot be determined satisfactorily using terrestrial samples.
However, meteorites satisfy the criteria of the Holmes-Houtermans model. They
are thought to have had a common origin with the planets and asteroids and to
have remained a separate subsystem since their separation at the time of formation
of the Earth.
Meteorites are fragments of comets and asteroids that hit the Earth. They
vary widely in size from dust upwards and can be classified into three main
types:
chondrites
,
achondrites
and
iron
. Chondrites are the most primitive and
the most common, comprising about 90% of those meteorites observed to fall
on Earth. Chondrites are characterized by chondrules (small glassy spheres of
silicate), the presence of which indicates that the material was heated, then
rapidly cooled and later coalesced into larger bodies. Achondrites are crys-
talline silicates containing no chondrules and almost no metal phases. Chondrites
and achondrites together are termed
stony meteorites
. Some of the chondritic
meteorites, termed
carbonaceous chondrites
, are the least metamorphosed of
the meteorites and still retain significant amounts of water and other volatiles.
/
204
Pb]
0
and [
206
Pb
/
