Geology Reference
In-Depth Information
By 1922 Aston had been awarded the Nobel Prize for
Chemistry, and by 1927 he had identified on his mass spectro-
graph the three known isotopes of lead previously recognised
by their atomic weights. It therefore came as quite a shock when
in 1929 some new results indicated that 'ordinary' lead could
not, after all, be the lead isotope that had been around since the
beginning of time, and that instead, it must result from the
decay of a hitherto unknown isotope of uranium. Aston dis-
cussed the problem with Rutherford who agreed with his deduc-
tions, and went on to calculate that the new uranium isotope
must have an isotopic number of 235. Indeed, it was uranium
235 which, because it represented less than one per cent of total
uranium, had so far gone unnoticed. Wild miracle? Wild cer-
tainly, as uranium 235 unleashed on an unsuspecting world the
opportunity for mass destruction. The miracle is, perhaps, that
we still endure.
By estimating the rate at which uranium 235 decayed, which
was much faster than uranium 238, and assuming that equal
amounts of uranium 235 and 238 were present when the Earth
first formed, Rutherford was able to determine the time it had
taken to increase the ratio of the two uranium isotopes from
zero to its present day value. The figure he arrived at, 3400 mil-
lion years, was the first age of the Earth to be based on isotopic
data from a mass spectrometer. Once again, Rutherford had got
there first. But for some reason no-one seems to have taken this
age very seriously and it was largely ignored. Perhaps because
there were still many uncertainties - the exact decay rate of
uranium 235 and hence the exact ratio of 235 to 238 - but more
likely was the fact that it was just 'too old', even for those work-
ing in the field, to take on board.
So, 'ordinary' lead, lead 207, was even more ordinary than
originally thought, and turned out to be just another decay
product of uranium, albeit the newly discovered uranium 235
isotope. What then, was 'ordinary' lead? Did it exist at all? Well,
yes it did. Holmes had already identified samples that contained
