Geology Reference
In-Depth Information
At that time, however, the isotopes of potassium had only just
been identified and it was still not certain which ones were
radioactive, so Holmes was unaware that potassium 41 is in fact
the stable isotope of the potassium family and therefore cannot
decay. Once this flaw was recognised, the paper and its new key
to petrogenesis became redundant and lay ignored by the geo-
logical community, although the principles of the method were
still perfectly valid. It was not until the 1960s, when the idea was
're-invented', that 'initial ratios' became one of the foundation
stones of isotope geochemistry. True to form Holmes has never
been given the credit for it, although the idea is possibly one of
the most remarkable examples of perspicacity in geological
research. Perhaps embarrassed by his error, he himself allowed
the model to lie dormant for fifteen years, but when attempting
to extract the age of the Earth from Nier's data he resurrected
the concept again.
In principle the problem he was hoping to address was fairly
straightforward: from the moment uranium and thorium were
formed somewhere out there in the cosmos, radiogenic isotopes
of lead would be added to the 'ordinary' lead that had formed
at the same time as uranium, giving rise to a mixed composi-
tion Holmes called 'primeval lead'. The isotopic ratio of this
primeval lead would then continue to evolve as long as uranium
and thorium continued to decay, until the primeval lead became
separated from its uranium and thorium source, probably dur-
ing formation of the Earth's crust. At this point the composi-
tion of the primeval lead would become 'fossilised' or frozen for
ever more, held inside minerals within rocks forming the Earth's
crust. In theory therefore, it should be possible to find miner-
als within ancient parts of the crust that still contained this
primeval lead composition. Having identified the composition
of primeval lead, and knowing the rate at which uranium added
radiogenic lead to the system (the decay rate of uranium), it
should then be possible to calculate the time that had elapsed
between the 'fixing' of primeval lead in the crust and its
