Argon-Argon Dating

Both rest on the same decay of potassium-40 to argon-40, but they measure the parent differently. Conventional K-Ar measures potassium chemically on one sample and argon on another, so heterogeneity between aliquots limits precision and small grains cannot be dated. Argon-argon irradiates the sample to convert potassium-39 into argon-39, turning the parent into an argon isotope that can be measured alongside the daughter on a single aliquot by one mass spectrometer. This removes the two-aliquot error, allows single-crystal dating, and enables step-heating, which gives a diagnostic age spectrum that conventional K-Ar cannot.

The J-value is the irradiation parameter that calibrates how efficiently the reactor converted potassium-39 to argon-39 for a given sample position. Because neutron flux varies within the reactor, a monitor mineral of independently known age is irradiated next to the unknowns; its measured argon-40 to argon-39 ratio and its known age fix J. Applying J to an unknown's measured ratio yields its age directly. The dependence on J means an argon-argon age is only as accurate as the assumed age of the flux monitor and the nuclear constants folded into it, which is why monitor calibration is a continuing concern in the field.

Step-heating releases the sample's argon in successive temperature increments, and each increment yields its own apparent age. If the crystal has been a closed system since it cooled, those increments agree, producing a flat plateau across a large, contiguous fraction of the gas, which is taken as the crystallization age. A sloping or staircase spectrum reveals trouble: low-temperature steps that read too young indicate argon loss from later heating, while steps that read too old indicate excess or inherited argon. The plateau is thus both an age estimate and a built-in quality check, a key reason the method is trusted over single total-fusion measurements.