U pb dating minerals
The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).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.
This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life.That would take the zircons on a straight line back to zero on the concordia diagram.The straight line takes the zircons off the concordia. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched.Prior to 1905 the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state.Although we now recognize lots of problems with that calculation, the age of 25 my was accepted by most physicists, but considered too short by most geologists. Recognition that radioactive decay of atoms occurs in the Earth was important in two respects: Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential (Energy) barrier which bonds them to the nucleus.
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Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.