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Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer (such as in the argon-argon scheme).

This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks. Some do not change with time and form stable isotopes (i.e.

However, potassium is very mobile during metamorphism and alteration, and so this technique is not used much for old rocks, but is useful for rocks of the Mesozoic and Cenozoic Eras, particularly unaltered igneous rocks.

Argon-Argon dating (39Ar-40Ar) This technique developed in the late 1960s but came into vogue in the early 1980s, through step-wise release of the isotopes.

This technique is used on ferromagnesian (iron/magnesium-containing) minerals such as micas and amphiboles or on limestones which also contain abundant strontium.

However, both Rb and Sr easily follow fluids that move through rocks or escape during some types of metamorphism. The dual decay of potassium (K) to 40Ar (argon) and 40Ca (calcium) was worked out between 19.

Radiometric dating is a method of dating based on the rate of decay of radioactive isotopes present in all organic materials.

The radiometric dating technique used most widely in archaeology is radiocarbon, or C-14, dating.Any item that derives from a previously living organism—for example, a basket made of plant fibers or a scrap of animal hide—can be subjected to carbon-14 dating.The technique is useful for dating items up to about 50,000 years old. Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces.These are released as radioactive particles (there are many types).The isotopes are then measured within the same machine by an attached mass spectrometer (an example of this is SIMS analysis).