The amount of Ar is also measured to assess how much of the total argon is atmospheric in origin. 11) the following assumptions must be true for computed dates to be accepted as representing the true age of the rock: Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample.Ar–Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem.Potassium is a common element found in many materials, such as micas, clay minerals, tephra, and evaporites.
Time since recrystallization is calculated by measuring the ratio of the amount of 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 geomagnetic polarity time scale was calibrated largely using K–Ar dating.
To obtain the content ratio of isotopes K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is melted in vacuum.
The potassium is quantified by flame photometry or atomic absorption spectroscopy.
Due to the long half-life, the technique is most applicable for dating minerals and rocks more than 100,000 years old.