Geoscience Reference
In-Depth Information
examined the uranium decay series in detail he worked out that the
end product of the series was the stable element lead. Once lead was
formed the series stopped. He then produced a momentous argument
which went as follows: if one determined the ratio of the radioactive
uranium to the stable lead in the ore, and knew the decay rate (half-
life) of uranium to lead, then it would be possible to work out how old
that ore was. This method could then be used to establish the age of
any uranium/lead-bearing rocks in the Earth's crust and could give an
age for the Earth itself.
So for clarification let us take a simple hypothetical example
with invented figures. A mineral grain is analysed and found to
contain 100 grams of radioactive element A and 300 grams of stable
element B. The half-life of the A
!
B decay is 10 years. Originally
when formed the grain must have contained 400 grams of element
A, but after 10 years' transformation left 200 grams of element A and
200 grams of element B. In the next ten years half of the remaining
element A is transformed into element B giving the ratio of 1:3 seen
in the mineral, which therefore must be 20 years old.
On the basis of his argument Boltwood examined a series of
mineral specimens and for these he obtained a suite of age determina-
tions that ranged from 400 to 2,200 million years, results he
announced in a paper in the American Journal of Science in 1907.
These figures ten years earlier would have been considered fantastic,
in the true meaning of the word, and Boltwood would have been
considered unhinged. But in 1905, while the results were startling,
the geological community sat up, took notice and began to agree.
Agreement was not universal, however, and some such as Joly
argued otherwise. He questioned whether the decay rate of uranium
has been constant throughout geological history as had been sug-
gested. He said that this assumption was without strong basis and
that the published results were based on derived radioactive products
rather than radioactive parent products. Joly was not alone in his
concerns. Becker also voiced unease with radiometric dates, as did
the Canadian Committee on the Measurement of Geological Time
