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  Open University Uranium-Series Laboratory  
Earth and Environmental Sciences, The Open University, Milton Keynes, UK  

Uranium series systematics
 

This is a large and complex subject and it is, alas, not the aim of this Site to be comprehensive. Some aspects of Uranium series are discussed in most isotope geochemistry textbooks (see references below) and the most comprehensive treatment can be found in a multi-authored book, Uranium-series Disequilibrium, edited by Ivanovich and Harmon (1992). Also important is a 2003 publication in REVIEWS IN MINERALOGY & GEOCHEMISTRY: 52, focussed on applications of U-series methodology edited by Bourdon B, Turner S, Henderson GM, Lundstrom CC.

The links on the left contain introductions to U-series methodologies and some successful applications, but also discuss some of the potential pitfalls and correction methods.

 

Why?

 
There are two main reasons for dating geological and archaeological materials:
A precise age for a bone or a rock underpins our understanding of its significance by establishing its historical or geological context.  
A timespan between different conditions reveals a rate of change and may indicate the process that caused the change.  

Contrary to popular believe, radiometric dating does not date geological events but can only give the time of an element fractionation process. The geological significance of the element fractionation process depends on the understanding of that process. A process that is fast relative to the time that has elapsed since then, can be ‘dated’. Information about the rate of an element fractionating process may be found if the process lasts for a time period comparable to the half-life.

 
Closed system behaviour
 
A fundamental assumption for dating a fractionation event is that the system has remained 'closed' since the event. This means that there has been no exchange of parent or daughter isotopes with the environment. Many processes, such as hydrothermal alteration, diagenesis or weathering can result in 'open-system behaviour' rendering any dates suspect. Open system behaviour can be modeled in certain circumstances, see: Closed System and also D/A in archaeological Bone.
 

Initial 230Th, the detrital correction

 

Another important assumption is that none of the daughter isotopes in the system is derived from processes other then radioactive decay of the parent, or can be accounted for. This is important for dating authigenic minerals because some 230Th may be derived from from inclusions, mostly of silicate minerals. In common silicates, there is a narrow range of U/Th ratios and therefor a restricted range of 230Th/232Th. The measured amount of 232Th in the sample, and the measured or assumed U/Th ratio can be used to correct for initial 230Th, this is usually referred to as the 'detrital' correction. Clearly, the larger the correction, the larger the uncertainty in the corrected age and for young rocks with low uranium concentration, a few percent of detrial correction can render the calculated age meaningless, see: Allogenic Th correction

Another possibility is to calculate the values used for correction by assuming that the data for a number of samples with different amounts of detrital component, define a mixing line.

 

G.M., Lundstrom C.C., (eds), 2003. Applications of U-series methodology. Reviews in Mineralogy and Geochemistry 52.

Ivanovich, M., Harmon, R. S. (Eds), 1982. Uranium Series Disequilibrium Applications to Environmental Problems. Oxford University Press.

 

© Peter van Calsteren
Last updated: date December 23, 2011 11:42