Briefly, all radiometric dating techniques rely on the disappearance of
a radioisotope with time or the ingrowth of a stable isotope as a
results of the decay of a radioisotope.
Carbon dating uses Carbon-14, a radioactive isotope of carbon with a
half life of 5730 years. This isotope is produced in the upper
atmosphere by the bombardment of Nitrogen-14 with neutrons at a rate of
~2.4 Carbon-14 atoms per second per square cm of the earth's surface
(Friedlander, Kennedy and Miller p. 505). This rate of production is
reasonably constant and this C-14 is mixed with the stable C-12 in the
atmosphere and the half life of C-14 is long enough to achieve thorough
mixing. The ratio of C-12/C-14 is thus constant or close enough to
being constant. Plants (e.g. trees) take up carbon dioxide and the
carbon in trees has almost the same ratio of C-12/C-14 as the
atmosphere. This ratio does not change as long as the tree is alive;
any decay of the C-14 is made up with fresh C-14. When the tree dies or
is chopped down, the C-14 continues to decay, but no new C-14 is added.
If you find an old piece of wood, and it only contains half as much C-14
as it did when the tree was alive, the tree was chopped down 5730 years
ago.. If the tree only has one quarter of the C-14 it had when it was
alive, the tree was chopped down 2 x 5730 or 11 460 years ago, etc.
Same goes for mammals because they eat plants and when the mammal dies,
the C-14 ingress stops and the C-14 in the mammal dies.
In rocks and minerals, we look at the ingrowth of a stable isotope from
a radioactive parent. Some minerals (for example, feldspar) contain
potassium. Potassium contains a small percentage (0.0117%) of K-40
(potassium 40). this radioactive isotope has a half life of 1.25
billion years. Of 100 K-40 atoms that decay, 11 decay to stable
Argon-40, which is a noble gas. So, if one finds a feldspar crystal and
heats it until it melts, any argon-40 will be released and this can be
measured. The assumption is then made that, when the feldspar crystal
was formed, it contained no argon-40 and all the argon-40 must have been
produced by the decay of K-40. You can determine the amount of
potassium in the crystal and the amount of K-40 and then the calculation
of the time it took to produce the amount of Ar-40 is relatively
straightforward. Similar studies can be done by looking at radioactive
Rubidium-87 (half life of 49 billion years) which decays to stable
Strontium-87, or by looking at the isotopes of lead. Uranium decays by
emitting alpha particles and beta particles until stable lead is formed.
A very interesting study is the Oklo phenomenon. This deals with a
naturally occurring nuclear reaction that took place 1-2 billion years
ago in Gabon, Africa (Kuroda 1982). All evidence points to an
operation of this nuclear reactor for 600 000 to 1 500 000 years.
Fission products and activation products were formed and the isotopic
signature of the elements in the deposit show evidence of this.
Hope this will do.
Dr. T.T. Vandergraaf
Geochemistry Research Branch
Pinawa, MB R0E 1L0
*(204) 753-2311 xt. 2592