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

Introduction

 
The term Uranium-series is short-hand for the decay chains of 238U, 235U and 232Th.

There are important differences between Uranium-series and other decay schemes used in Earth and Environmental Sciences such as Rb-Sr, Sm-Nd, K-Ar Lu-Hf, Re-Os, including:

 
the half-lifes of the U-series nuclides are relatively short.    
the daughter isotopes are also radioactive and decay further till, ultimately, stable isotopes of Lead.    
the U-series decay schemes can be exploited to further our understanding of processes that operate on short time-scales, much shorter than 1 my.    

Nuclides in the Uranium-series belong to a number of elements with sometimes very different geochemical characteristics and these differences can results in element fractionations during geochemical processes. One important aspect of a decay series is that after about 5-6 half-lifes of the nuclide with the longest half-life have elapsed, all nuclide in the series decay at the same rate, and the abundance of each isotope is proportional the decay constant of its nuclide. After five half-lifes the ‘steady-state’ of radioactive decay throughout the chain is described as ‘secular equilibrium’.

 
Geochemical processes may fractionate some of the U-series elements and the result is radioactive disequilibrium, although the geochemical process itself may well have been a thermodynamic equilibrium process. Table 1 is a list of U-series elements with half-life and decay energy for each isotope. Decay energy is related to the number of protons and neutrons in the decaying nucleus and in secular equilibrium the number of decay events integrated over a sufficiently long period of time is the same for each nucleus.
 
   
   
Table 1
     
 

The observation that the abundances of isotope with shorter half-lifes are lower than abundances of isotopes with longer half-lifes has the practical consequence that isotopes with short half-lifes are best analysed with radio-activity detectors such as a or g spectrometers, while the higher abundance isotopes can be analysed by mass spectrometry.

The half-life of the 234U daughter 230Th is about 75,000 years and dates and processes between a few dozen and 400,000 years can be determined. U-series dating is particularly suited for carbonates, phosphates, some silicate phases such as clinopyroxe and volcanic glass, and water. The half-life of 226Ra is about 1600 years and it is useful to 8000y.

 
   
   
   
   
Table 2 Simplified U-series decay chains. 238U progeny in red, 235U in black and 232Th in blue.
   
   
   
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© Peter van Calsteren
Last updated: date December 23, 2011 11:42