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evidence: Argon -- argon -LRB- or 40Ar / 39Ar -RRB- dating is a radiometric dating method invented to supersede potassium-argon -LRB- K / Ar -RRB- dating in accuracy .. radiometric dating. radiometric dating. Potassium. Argon. 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 -LRB- 39K -RRB- into the radioactive 39Ar .. As long as a standard of known age is co-irradiated with unknown samples , it is possible to use a single measurement of argon isotopes to calculate the 40K / 40Ar * ratio , and thus to calculate the age of the unknown sample .. 40Ar * refers to the radiogenic 40Ar , i.e. the 40Ar produced from radioactive decay of 40K .. 40Ar * does not include atmospheric argon adsorbed to the surface or inherited through diffusion and its calculated value is derived from measuring the 36Ar -LRB- which is assumed to be of atmospheric origin -RRB- and assuming that 40Ar is found in a constant ratio to 36Ar in atmospheric gases .