by Brian Thomas, M.S.

Age-dating a rock using its radioactive isotopes only works by assuming that the rate at which that “clock” ticks was constant in the past and essentially identical to that in the present. Not long ago, scientists discovered excess helium in crystals1 and “orphaned” polonium radiohalos,2 both of which imply that the decay rates of isotopes commonly used to date earth rocks were dramatically accelerated in the past. Even today, researchers are finding small but significant changes in isotope decay rates, and these add credibility to the idea that isotopic processes were once very different from today’s processes.

One standard isotopic clock system uses decaying uranium isotopes. Uranium spontaneously and slowly decays to lead (Pb on the Periodic Table of Elements). Two different uranium isotopes, 235U and 238U, decay into lead at different rates. Geologists assume that the ratio between these is constant and known, giving a convenient shortcut to uranium dating, which only requires that the two uranium amounts be measured.

Of course, this shortcut age-dating method assumes that 238U and 235U have decayed at today’s rates throughout the past. It also assumes that the relative amounts of the two have been constant. Physics Today editor Johanna Miller recently wrote, “Standard Pb-Pb dating protocol uses a 238U/235U ratio of 137.88 with zero uncertainty. But several recent studies have cast doubt on that number.”3

Miller cited one experiment that found that the uranium ratio (the heavier 238U to lighter weight 235U) is not constant. The study authors wrote, “Our observations have a direct impact on the U-series and U-Th-Pb chronometers,” meaning that dates “determined” by uranium decay will need revision.4….

Continue Reading on