Geology Reference
In-Depth Information
put his ideas into print. But no other discovery has subsequently
proved so critical to radiometric dating, and the significance of
it was immediately recognised by Arthur Holmes.
So far he had assumed that any 'ordinary' lead in a rock was
present in such small quantities as to be insignificant when
compared with the amount produced by the decay of uranium,
but in fact it was impossible to tell. To muddy the waters even
further, Soddy had shown that an isotope of lead was the final
decay product of thorium as well, so now there were three
possible sources of lead - uranium, thorium and 'ordinary'. It
really was a problem that needed to be resolved, and clearly
isotopes were the path to enlightenment. Unfortunately, at that
time, the only way to distinguish one isotope of lead from
another was by measuring their atomic weights, painstaking
work that required even more fastidious care than separating
uranium and lead.
But the rewards for success in this field were high, and by 1914
the team working in the Vienna Radium Institute had become
world experts on atomic weight determinations. So despite the
di~culty of communications with Austria during the war - there
frequently weren't any - at the instigation of Holmes, Lawson
and the Viennese team accurately determined the atomic weight
of the three lead isotopes in a number of samples and showed
that the results were both consistent within themselves, and with
other workers in the field. So now when a new mineral was dated
it became possible to calculate the proportions of each type of
lead from the atomic weight of the sample and adjust the age
accordingly, ensuring that it was not over-estimated. It was a
significant breakthrough and Holmes was gratified to report:
'that the uranium-lead ratios may be used as before and with
greater certainty for the determination of geological time and
the gradual construction of a complete geological time-scale'.
It must have been quite a relief to find that after all their previ-
ous hard work, the system still worked with only minor adjust-
ments required. Or so they thought.
