Geochronology - Four Revolutions in the Earth Sciences - page 40

In 1908 the Imperial College of London appointed Strutt its professor of physics.
He soon spotted in his classroom a student with a special aptitude for the new
subject of radioactivity. Strutt invited the young man, Arthur Holmes, to join his
research. 11 He must have felt his confidence justified when only a year later, at
age twenty-one, Holmes published his first scientific article, “The Association of
Lead with Uranium in Rock-Minerals, and Its Application to the Measurement of
Geological Time.” 12 Holmes would go on to become an instructor at Imperial Col-
lege and professor of geology first at Durham and then at Edinburgh, and he would
write many more articles and a topic that became a classic. No one would do more
tovalidatetheagesdeterminedusingradioactivity thanHolmes.Hewouldbecome
the pivotal figure not only in establishing the age of the Earth, but as we will see
in part 3 , in keeping the theory of continental drift on life support after nearly the
entire community of geologists had pronounced it dead and good riddance.
Holmes began his first scientific article by rephrasing the three assumptions ne-
cessary for radioactivity to give accurate geologic ages, the same three that Joly
noted. Let us state them as:
1. No original daughter atoms
2. No gain or loss of parent or daughter atoms: a “closed system”
3. Constant decay rate
Had a mineral contained original lead, the first problem, its measured age would
appear to be older than its true age. The way to avoid or minimize this problem,
Holmes said, was to select specimens that incorporate much more uranium than
lead when they crystallize, so that the amount of original lead becomes negligible
in comparison with the lead produced by radioactive decay (radiogenic lead). One
such mineral is zircon.
To avoid the second problem, the scientist should select “fresh, stable, primary
rock-minerals.” Since it is inconceivable that different minerals would lose parent
anddaughteratomsinthesameproportion,“iftheanalyses[ofthosedifferentmin-
erals] give consistent results one can only assume that any alteration has been in-
appreciable.” Conversely, if a suite of minerals does not give consistent ages, then,
whatever the reason, that suite cannot be used for age dating.
Madame Curie and Rutherford had been unable to change the rate of radioactive
decay, but no one could say it was impossible. The most that could be said, as
Holmes put it, was that “experimental evidence consistently agrees in suggest-
ing that [radioactive] processes are quite independent of the temperatures and
pressures which igneous rocks can have sustained without becoming metamorph-
osed.” 13