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Geochronology is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloguing and comparing fossil floral and faunal assemblages.Biostratigraphy does not directly provide an absolute age determination of a rock, but merely places it within an interval of time at which that fossil assemblage is known to have coexisted.So far, all we seem to have done is taken the K-Ar dating method and made it much more complicated for no apparent reason.However, there are advantages to this more complex method.By measuring the amount of radioactive decay of a radioactive isotope with a known half-life, geologists can establish the absolute age of the parent material.A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods.J is not calculated on theoretical grounds, but is found experimentally; alongside the sample we're interested in, we irradiate and then heat a sample of known age (a standard).Measuring the Ar emitted from the standard, and knowing the time t that it was formed, we can put these figures into the equation above and solve it for J.

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With the exception of the radiocarbon method, most of these techniques are actually based on measuring an increase in the abundance of a radiogenic isotope, which is the decay-product of the radioactive parent isotope.Now the bad news is that there is no way we can somehow manipulate this data to give us a correct date for the sample.But the good news is that we do know that there's a problem; whereas if we'd analyzed the same rock using the K-Ar method, then it would have supplied us with a date and there'd have been no sign in the K-Ar data of anything wrong with it.Consequently, the amount of it found in rocks is negligible — unless you subject them to an artificial neutron source.A crucial point to note is that because K are isotopes of the same element, they have the same chemical properties.

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