added by archaeologs Group of related radiometric dating methods, based on the decay series of the uranium isotopes 238U and 235U. Principles. Each of the isotopes decays through a series of radioactive daughter isotopes until a stable isotope of lead is reached. Three daughter isotopes arc created and decay with half-lives useful for dating Ionium (230Th), Protactinium (23lPa) and Radium (226Ra). Several uranium scries dating methods exist, most of which apply to oceanbottom deposits and are not applicable to archaeology. One, however, has recently been applied in a pioneer study to bones from such important sites as Clacton. This is the ionium deficiency (230Th/234U) method. A small amount of 234U is taken up by animals as they grow and is incorporated into their skeleton (ionium is not taken up in this way). The uranium starts to decay at a slow rate, producing ionium (23(Th). The animal dies, the skeleton is preserved, and the uranium continues to decay. The ionium decays also, but there is a gradual net increase of this isotope in the material. Ionium concentration increases steadily with age until, at about 500,000 years, an equilibrium is reached between production and decay. Until this time, the 23(*Th/234U ratio is a function of age and can be used as a dating method. Range. The practical early limit for the ionium deficiency method is 300,000 years. Only dates younger than this may be obtained. This range makes the method most useful in geochronology and Palaeolithic archaeology. Accuracy. As yet, ionium deficiency dates have large probable errors (see standard deviation). The dates on bones from the British trial study that are now available are 245,000 + 35,000 or - 25,000 bp, 125,000 ± 20,000 bp; and 174,000 ± 20,000 bp. Errors this size do not matter in such an age range, where any idea at all of date is useful. Materials. Ionium deficiency has been applied to corals with success. Early trials with bones seem to produce sensible results, but attempts to date mollusc shell have proved unreliable. Problem. After death and preservation in a deposit, the system must remain ‘closed’. That is, uranium must not be lost from the material to be dated. Some adjustment may be made for an ‘open system’, allowing for migration of uranium, but this still produces difficulties.
The Macmillan dictionary of archaeology, Ruth D. Whitehouse, 1983