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

Mass spectrometry
The development of chemical and mass spectrometric techniques for the measurement of U, Th, Pa and Ra isotopes has provided an exciting new approach to the understanding of many geological processes active over the last 300,000 years. Analysis of 230Th/232Th in authigenic minerals is relatively straightforward with the routine use of ion counting and has been successfully performed since Edwards et al., (1987) and Chen et al., (1990). Analysis of silicates has been routine since the development of devices to improve the abundance sensitivity of the mass spectrometers (van Calsteren & Schwieters, 1995). Techniques use samples that may contain as little as a few ng of Th, and fg of Ra (Cohen et al., 1992; McDermott et al., 1993; van Calsteren & Schwieters, 1995), and this has had a major impact in providing precise time scales. Examples include studies of climate change (Edwards et al., 1987; Baker et al., 1999; Stirling et al., 1995); McDermott et al., 1999) and of human evolution (McDermott et al., 1993; Grün & McDermott, 1994). These techniques also have considerable potential in hydrothermal and groundwater systems (Osmond & Cowart, 1992), and they have had a major impact on models for melt generation and magma differentiation processes on the timescales of 1000s to 300,000 years (McKenzie, 1985; Lundstrom et al., 1998; Cohen & O'Nions, 1993; Chabaux et al., 1999, 1994; Bourdon et al., Nature; Huang et al., 1997; Elliott et al., 1997; Hawkesworth et al., 1997; Heath et al., 1998).

The law of radioactivity states that the number of atoms disintegrating per unit time is proportional to the number of radioactive atoms N. Thus -dN/dt=λN where λis the proportionality or decay constant. On integration this becomes: N0=Netλ, where N0 is the number of radioactive parent isotopes at the time of the formation of the sample, N is the number present today and e is the base of the natural logarithm. The number of radiogenic isotopes Nr today plus the number of radioactive isotopes still present is equal to N0 and the equation above may be rewritten as:

Nr=N(et λ-1).
Equation 1

Half-life is defined as the time that is takes for N to become equal to Nr and is given by the equation: half-life=ln2/λ where ln2 is the natural logarithm of 2. Most radiogenic clocks that are of geological significance have a half-life of the order or millions or billions of years.

The main advantage of the U-series methods is that after about 5 half-lifes of 230Th, the nuclide with the longest half-life, the system is in ‘secular equilibrium’. This means that all nuclides in the chain decay at the same rate and their ratios are the same as the ratios of their decay constants. This can be expressed in terms of ‘activity’, the product of the abundance and the decay constant of a nuclide, i.e.: A230Th =λ230Th x C230Th, and in secular equilibrium all activities are equal to 1. U-series data can be conveniently displayed on an ‘activity diagram’ for instance (230Th/232Th) vs (238U/232Th) and the brackets are used to distinguish this diagram from an isotope ratio diagram. There is, of course, no difference in principle, only in scaling, and this diagram is similar to an isochron diagram where a straight line may have age significance.
The isochron equation is
Equation 2

of a line on a (230Th/232Th) vs (238U/232Th) diagram representing an age T

Both 230Th and 234U are radioactive and samples that are in secular equilibrium plot on a line with a slope of 1. Frequently, equation 1 and indeed activity diagrams, are expressed in terms of 238U rather then 234U but the underlying assumption that the two Uranium isotopes are not fractionated must be justified by the appropriate measurement. This is more than just of pedantic importance because the behaviour of the two isotopes is very different in surface leaching processes. Most sites in a crystal lattice where 234U atom reside have been damaged by the alpha-recoil process that produced 234U from in situ 238U decay. The damaged volume is approximately cylindrical and about 55 nanometer long with random orientation and it is much easier for a 230Th atom to be leached from its position than a 238U atom. In practice, the difference in leaching rates is also proportional to grain size. A (234U/238U) activity that is not unity is taken as a clear indication of an open system, and a violation of the isochron assumptions.

Ages for authigenic minerals can be calculated for single samples on the assumption that all 230Th has been produced by in situ decay of 234U. Equation 2 was derived by Kaufman and Broecker (1965)



Equation 3
assuming that all 230Th is derived from is situ radioactive decay and that neither Th nor U has been lost from the system. The equations cannot be solved for T but by inserting a time estimate the equation can be solved to any required degree of accuracy. The assumption that all 230Th is authigenic is rarely justified and it is common to assume that when significant 232Th is present in the system, a correction has to be made based on an assumed 230Th/232Th ratio. This assumed 230Th/232Th is usually taken to be the ratio in silicate and is frequently referred to as the ‘detrital correction’. Many other methods for detrital correction have been investigated, including some chemical leaching techniques, but results have been difficult to reproduce.

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© Peter van Calsteren
Last updated: 23 December, 2011 11:31