zircon rock dating site
Spontaneous breakdown or decay of atomic nuclei, termed radioactive decay, is the basis for all radiometric dating methods. Radioactivity was discovered in 1896 by French physicist Henri Becquerel. By 1907 study of the decay products of uranium (lead and intermediate radioactive elements that decay to lead) demonstrated to B. B. Boltwood that the lead/uranium ratio in uranium minerals increased with geologic age and might provide a geological dating tool.
* Note that 40 K also decays to 40 Ca with a decay constant of x 10 -10 yr -1 , but that decay is not used for dating. The half-life is for the parent isotope and so includes both decays.
The method relies on two separate decay chains , the uranium series from 238 U to 206 Pb, with a half-life of billion years and the actinium series from 235 U to 207 Pb, with a half-life of 710 million years.
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So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero. Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time. If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept. In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1). In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U = 3), and so forth.
· Keywords: helium, diffusion, radioisotopes, age of the earth, radiometric dating , zircon , lead