Geology Reference
In-Depth Information
But then in 1928 an opportunity suddenly presented itself.
Fritz Paneth who had worked with Bob Lawson at the Vienna
Institute of Radium during the First World War and who was
now Professor of Chemistry in Berlin, had developed very
precise techniques for measuring minute amounts of helium.
Helium was ubiquitous - for each uranium atom that decayed,
eight helium atoms were produced, so large concentrations of
helium were generated over geological time, even in the com-
mon rocks. The problem here was not the shortage of uranium,
but the surplus of helium which either could not be contained
in the mineral over long periods of geological time, or was lost
in the preparation process. But Paneth considered that precisely
because the concentrations of uranium in common minerals
(those not normally selected for age dating) were so low, the
much smaller volumes of helium produced would have a good
chance of being retained in the mineral. Holmes seized the
initiative and arranged to have some common rocks analysed in
Paneth's lab in Berlin.
Samples were taken from two famous rocks from northern
England which were known to be of very di¬erent geological
ages: the Whin Sill was believed to have a very late Carboniferous
age, while the Cleveland Dyke was considered to be middle to
early Tertiary. When Paneth sent the helium results back the
Whin Sill gave 182 million years while the Cleveland Dyke gave
26 million years, values Holmes considered
'to be in excellent
agreement with the geological evidence'
.
The ideal test of these results would have been to compare
the helium ages with the more reliable lead ages analysed on
the same rocks, but it was precisely because these two rocks
contained so little lead that they could not be dated by the lead
method. Controls therefore had to be found in lead ages from
other rocks
believed
to be of the same geological age. The
problem was of course, that very few reliable lead ages
were available anywhere in the world, so they had to make do
with what ever they could get.
