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The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace.Heating causes the crystal structure of the mineral (or minerals) to degrade, and, as the sample melts, trapped gases are released.The mineral sanidine, the high-temperature form of potassium feldspar, is the most desirable.But micas, plagioclase, hornblende, clays, and other minerals can yield good data, as can whole-rock analyses.
Dating minerals may provide age information on a rock, but assumptions must be made.
The rock sample to be dated must be chosen very carefully.
Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C.
Thus, a granite containing all three minerals will record three different "ages" of emplacement as it cools down through these closure temperatures.
Because Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clear indicator of the potassium content.