By David Coppedge
Behind the confidence of uranium-lead dates hides worry about numbers that don’t match up.
Uranium-lead (U-Pb) dating is a staple of the billions-of-years claims about igneous and metamorphic rocks and meteorites – giving rise to the consensus age of the solar system at 4.55 billion years. The technique has been refined over the last century, but “discordances” (mismatches) remain. That’s because there are two “isotopically distinct yet chemically identical” decay chains, F. Corfu described in the Geological Society of America Bulletin this month: the 235U to 207Pb pathway, and the 238U to 206Pb pathway. (The ratios of the products,207Pb/206Pb, can also be measured.) “These twin decay systems, running at different speeds, allow an immediate verification of the validity of their ages, which must be concordant to be considered valid, although under favorable circumstances, discordant data can be extrapolated to the correct age.” Since uranium is often locked in tightly-bound minerals called zircons, from which parent and daughter product are locked in, the method is thought by modern geologists to provide a reliable clock.
In “A century of U-Pb geochronology: The long quest towards concordance,” Corfu, a geologist at the University of Oslo, described geologists’ frustration at persistent discordance between the two methods. The discordances are small; around 1% or less. Still, he finds it troubling that there should be any discordance if the same physical processes are operating for the same amount of time. At the beginning of the article, he quoted T. E. Krogh: “Only one answer is the right one, and only that one is good enough.”
The progress in developments of the U-Pb method is both a history of technical discoveries and advances, as well as a history of a long struggle toward concordance, toward an understanding of the causes of discordance, and toward ways to eliminate it. Despite the enormous progress achieved in this field, the problems of U-Pb discordance have not yet been completely resolved and will be one of the main hurdles to overcome in the future….
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