Radiocarbon dating is normally suitable for organic materials less than 50 000 years old because beyond that time the amount of 14C becomes too small to be accurately measured.
This scheme was developed in 1937 but became more useful when mass spectrometers were improved in the late 1950s and early 1960s.
This technique has become more widely used since the late 1950s.
Its great advantage is that most rocks contain potassium, usually locked up in feldspars, clays and amphiboles.
For an element to be useful for geochronology (measuring geological time), the isotope must be reasonably abundant and produce daughter isotopes at a good rate.
It is useful for dating very old igneous and metamorphic rocks and also meteorites and other cosmic fragments.The isotopes are then measured within the same machine by an attached mass spectrometer (an example of this is SIMS analysis).This is a common dating method mainly used by archaeologists, as it can only date geologically recent organic materials, usually charcoal, but also bone and antlers.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).Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes.