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.
40Ar/39Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of potassium (39K) into the radioactive 39Ar.
The sample is generally crushed and single crystals of a mineral or fragments of rock are hand-selected for analysis.
However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral.
Dating minerals may provide age information on a rock, but assumptions must be made.
Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of intrusion.
40Ar/39Ar geochronology assumes that a rock retains all of its 40Ar after cooling past the closing temperature and that this was properly sampled during analysis.