Radiometric dating definition and meaning | Collins English Dictionary Radiometric dating definition and meaning | Collins English Dictionary

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On the other hand, even if such a process is not operating for lead, the extra radium will decay rapidly to lead, and so in either case we have much too much lead in the lava and radiometric dates that are much, much too ancient!

This high melting point suggests that uranium would crystallize and fall to the bottom of magma chambers. It is not necessarily true that one will get the same number of negative as positive slopes.

Depending on their oxidation state, according to Faure, uranium minerals can be very soluble in water while thorium compounds are, generally, very insoluble.

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Progressive geochemical differentiation of the upper mantle of the Earth has resulted in the concentration of U and Th into the rocks of the continental crust compared to those of the upper mantle. For example, it's not clear to me that we need to worry about isochrons or whether U and U dates etc.

Now, after the magma is thoroughly mixed, the uranium and thorium will also be thoroughly mixed.

Radiometric dating

They assume that initially the magma was well mixed to assure an even concentration of lead isotopes, but that uranium or thorium were unevenly distributed initially.

So this implies some kind of chemical fractionation.

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Fractionation followed by mixing can create isochrons giving too old ages, without any fractionation of daughter isotopes taking place. But many minerals take in a lot of uranium. The idea of isochrons is that one has a parent element, P, a daughter element, D, and another isotope, N, of the daughter that is not generated by decay.

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So a number of applications of the thorium method are based on this chemical partition between uranium and thorium. In the last fifteen years, people have also used cosmic ray exposure ages to date rock surfaces on the Earth.