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Uranium-lead and thorium-lead
The transmutation of uranium to lead is the classical example of radioactive change. Uranium-238 decays to lead-206 with a half-life of 4,530,000,000 years; uranium-235 decays to lead-207, half-life 713,000,000 years; and thorium-232 decays to lead-208, half-life of 13,890,000,000 years.
238U → 206Pb+84He
235U → 207Pb+74He
232Th → 208Pb+64He
Although many rocks contain uranium, there are few really satisfactory minerals for age dating. The method is most successful in the range of 100 million years to 5 billion years. However, use of the lead methods has greatly declined as the potassium-argon method has improved.
The production of radiogenic lead from uranium or thorium is a steplike process in which a series of radioactive products are produced, each one of which has its peculiar half-life and chemical properties. Two members of the uranium radioactive decay series, thorium-230 and protactinium-231, have low solubilities in natural waters and hence are not readily available for inclusion in shells or skeletons of aquatic animals. Uranium on the other hand has been found to enter into living coral skeletons and the extent to which thorium-230 or protactinium-231 has been produced in a specimen can be used as a measure of age. The method is mostly restricted to dating of corals in the 10,000 to 400,000-year age range. Another application of the method is the dating of stalactites in caves.
Source: William Lee Stokes, 1978, ‘Introduction to Geology’, 2nd edition, pp324 |
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