Geoscience Reference
In-Depth Information
Duck Soup
Plumbing the Atom
Although Rutherford left behind his early interest in measuring mineral ages, his
subsequent research, along with that of the other pioneers, led to a series of sur-
prising discoveries that were crucial to establishing the true age ofthe Earth. These
fertile early years were an era of “little science”—research conducted on a labor-
atory bench at minor expense with apparatus that today seems not much advanced
beyond string and sealing wax. But with only string and sealing wax, Rutherford
was a virtuoso.
Rutherford's collaborator, Frederick Soddy, found that whereas the lead in
uranium minerals has an atomic weight of just over 206, the lead in thorium min-
erals is heavier, weighing close to 208. Soddy deduced that lead and other ele-
ments exist in varieties that have the same chemical properties but different atomic
weights. These he named isotopes, from the Greek for equal place.
The explanation is that lead in nature is a mixture of isotopes of different atomic
weights. Ores rich in uranium have more of the lighter lead isotopes; those rich in
thorium have more of the heavier ones. By the mid-1930s, scientists had worked
out the essential facts of the decay of uranium and thorium to lead (the half-lives
that follow are the modern measured ones): U-235 decays to Pb-207 with a half-
life of 713 million years; U-238 decays to Pb-206 with a half-life of 4,468 million
years; Th-232 decays to Pb-208 with a half-life of 14,050 million years. Lead has
one additional naturally occurring isotope, Pb-204, that is neither radioactive nor
radiogenic.
As the pioneers recognized, uranium and thorium do not decay directly to lead
but through a long chain of in-between elements, such as radium and radon. At
firstitwouldappearthatthesecomplexdecaychains,withtheirmanyintermediate
products, some of them gases that could easily escape, would invalidate the lead
methods of age determination. But in part because the half-life of each decay event
in the chain is so short, the process works as though uranium decayed directly to
lead.
Having three clocks running at once, two from uranium decay and one from
thorium decay, lead is unique among the elements used for age dating. From the
mathematics of the decay process, it turns out that if one measures the Pb-206/
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