[.] Accuracy also requires that the nuclear decay rate be unaffected by external conditions such as temperature and pressure.
Dating method based on rate of decay of radioactive isotopes
Reliability in the dating of a geological feature is increased by sampling disparate areas which have been subjected to slightly different thermal histories.
[K 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.
Some additional information about the history and composition of a sample is required before K-Ar isotopic comparison may be used to produce accurate dates.
[.] Great care is needed in selecting a sample for dating, as many potential samples have been contaminated by various means.
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.
.] The quickly cooled lavas that make nearly ideal samples for K-Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature of iron.
The development of the geomagnetic polarity time scale depended largely on K-Ar dating. In practice, each of these values is scaled to the total potassium fraction as only relative, not absolute, quantities are required.
Extraneous argon is commonly incorporated into the cooling sample.
The above equation may be corrected for the presence of non-radiogenic is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem.