Ionium-thorium dating is a technique for determining the age of marine sediments based upon the quantities present of nearly stable

thorium-232 Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its decay ...

and more radioactive

thorium-230 Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the gene ...

. (²³⁰Th was once known as ionium, before it was realised it was the same element as ²³²Th.)

Uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

(in nature, predominantly uranium-238) is soluble in water. However, when it decays into

thorium Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately soft and malleable and has a high ...

, the latter element is insoluble and so precipitates out to become part of the sediment. Thorium-232 has a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

of 14.5 billion years, but thorium-230 has a half-life of only 75,200 years, so the ratio is useful for dating sediments up to 400,000 years old. Conversely, this technique can be used to determine the rate of ocean sedimentation over time. The ionium/thorium method of dating assumes that the proportion of thorium-230 to thorium-232 is a constant during the time period that the sediment layer was formed. Likewise, both thorium-230 and thorium-232 are assumed to precipitate out in a constant ratio; no chemical process favors one form over the other. It must also be assumed that the sediment does not contain any pre-existing particles of eroded rock, known as detritus, that already contain thorium isotopes. Finally, there must not be a process that causes the thorium to shift its position within the sediment. If these assumptions are correct, this dating technique can produce accurate results.

References

{{DEFAULTSORT:Ionium-thorium dating Radiometric dating Thorium