Zircon dating

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If left at low surface temperatures for a geologically long time, the radioactivity within the crystal can destroy the crystal lattice structure, whereas at higher temperatures this process is self-annealing.In fact, when examined by X-ray methods, some zircons have no detectable structure, indicating that at least 25 percent of the initial atoms have been displaced by ) in biotite can cause lead to be lost in some grains.Uranium minerals themselves are too uncommon to be very useful in dating.The most common dating method involves the use of minerals like zircon and monazite that are relatively common in granitic rocks.These situations are enigmatic, given the dramatic effect of similar temperatures during contact metamorphism.

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This dating method has become very popular for dealing with Precambrian terranes where it can often be difficult to resolve relationships between rock units and the geological history. It must be assumed that when the zircon grains crystallized, no radiogenic Pb was in them, and that all the radiogenic Pb now measured was derived by radioactive decay from U and Th.For example, over time, uranium atoms lose alpha particles (each made up of two protons and two neutrons) and decay, via a chain of unstable daughters, into stable lead.Although it is impossible to predict when a particular unstable atom will decay, the decay rate is predictable for a very large number of atoms.For this reason, three or more grain types or parts of a grain are analyzed to establish that material of only one age is present.Experience with the results of the uranium–lead method for zircons has demonstrated an interesting paradox.

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