Geoscience Reference
In-Depth Information
DECAY SERIES AND GEOCHRONOLOGY
One might wonder, then, what good these radioactive decay series
could be in unravelling Earth history and chronology. This challenge
was taken up by two young men working either side of the
Atlantic, by a researcher at Yale University with the wonderful name
Bertram Borden Boltwood (1870-1927) (one can imagine him gracing
the croquet lawn at Blandings Castle), and by the Englishman, the
Honourable Robert John Strutt (1875-1947), better known later as the
fourth Baron Rayleigh. (Unusually and quite incidentally the Barony
was first conferred on a woman, Robert's great-grandmother the
daughter of the Duke of Leinster, and the title of Baronwas then passed
down the male line.)
In 1905 Strutt, who was working in Cambridge at the important
Cavendish Laboratory, took up Rutherford's speculation that the gas
helium was an end product of a decay series, and went further in
suggesting that if the quantity of trapped gas in rocks could be deter-
mined then it might be possible to discover the age of the rock. Over
the next five years he worked on this problem, analysing many differ-
ent rocks and minerals. He said that the deposits of iron ore haematite
at Frizington in Cumbria, which lay just above the Carboniferous
limestone, were a minimum of 141 million years old, while fossilised
sharks' teeth from Florida were 77 million years old. These determi-
nations were not bad, when compared with present estimates. The
iron ore is likely to be no more than 300 million years old while the
Miocene teeth are probably between 5 million and 23 million years
old. Strutt realised that he would have problems dating these materi-
als accurately as it was highly probable that some of the helium as it
formed was lost through seepage and leakage. Consequently by 1901
he considered that this line of research led up a cul-de-sac and so
abandoned it.
At much the same time that Strutt was labouring in Cambridge,
Boltwood thought that he had discovered a new element, ionium,
but unfortunately for him it was later discovered to be a variety of
thorium. However, his other findings received great attention. Having
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