thorium-lead dating A radiometric dating method based on the radioactive decay of ²³²Th, to yield ²⁰⁸Pb + 6He⁴, with a half-life (see DECAY CONSTANT) of 13 900 million years. The minerals used include sphene, zircon, monazite, apatite, and other rare U/Th minerals. The method is not totally reliable and is usually employed in conjunction with other methods. In most cases the results are discordant as a result of lead loss. The ratio of ²⁰⁸Pb: ²³²Th compared with ²⁰⁷Pb: ²³⁵U ratio is particularly useful. The Th-Pb system can also be interpreted by means of isochron diagrams similar to those used in the rubidium-strontium method.

thorium–lead dating A radiometric dating method based on the radioactive decay of ²³²Th, to yield ²⁰⁸Pb + 6He⁴, with a half-life of 13 900 million years. The method is not totally reliable, in most cases because of lead loss, and it is usually employed in conjunction with other methods.