Potassium-Argon Radiometric Dating

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I was reading on wikipedia how K-Ar dating works and it all seemed very sound except for some of the preconditions listed:
All the following preconditions must be true for computed dates to be accepted as representing the true age of the rock. Great care is needed in collecting a sample for dating to avoid samples which have been contaminated by absorption of argon from the atmosphere. Contamination is suspected when the final results are untenable. Extraneous argon may be incorporated into a rock depending on conditions during cooling. Commonly, gases are not fully removed from magma at the time of crystallization, and so not all of the measured argon will have resulted from decay of 40K since the rock crystallized– such samples should be avoided. The Ar–Ar dating method was developed to measure the presence of extraneous argon.
The sample must have remained a closed system since it cooled enough to retain argon, neither admitting nor emitting either of the isotopes of interest. A deficiency of 40Ar in a sample of a known age can indicate a full or partial melt in the thermal history of the area.
Accuracy depends on the isotopic ratios included in the sample being normal, since 40K is usually not measured directly, but is
assumed to be 0.0117% of the total potassium. Unless some other process is active at the time of cooling, this is a very good assumption for terrestrial samples.
Accuracy also requires that the nuclear decay rate be unaffected by external conditions such as temperature and pressure. Because of the energy scales involved, this is a very good assumption, though the 40K electron capture partial decay constant may be enhanced at ultrahigh pressure.
Thanks, Jesse at University of Maryland

(comments/questions/discussion below is most welcome, DrJ)

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