12.2 Analytical methods

As noted above, the atomic abundance of a U-series nuclide in secular equilibrium is proportional to its half-life. Therefore, the very variable half-lives of the U-series radionuclides cause them to have extreme abundance ratios. Until recently, this discouraged mass spectrometric determination of U-series nuclides for dating purposes. In contrast, species in secular equilibrium have equal activities (by definition), so radioactive counting is an obvious method for their determination. Counting techniques utilising $ and ( particles are not favoured because of the low energies of $ transitions and the complexity of (-ray spectra (Yokoyama and Nguyen, 1980). Therefore, the traditional technique for measurement of U-series nuclides has been " spectrometry.

Because of the very short range of " particles in matter, samples to be counted are made into a thin film and placed under vacuum in a gridded ion chamber (a type of gas ionisation chamber with a short dead-time). If the potential between cathode and anode is within a certain range, the electrical pulses generated by " particle emission will be proportional in size to the kinetic energy of those particles. The output is then fed to a multi-channel analyser in order to register count rates as a function of energy level. This allows the " particles from different decay transitions to be distinguished as separate ‘peaks’ in the energy spectrum. To obtain 1F counting errors of 1%, total counts of 10⁴ are required on each peak (F = /n). To achieve this, counting times of at least a week are required for most natural samples. Recoil effects gradually contaminate the counter over time with U-series nuclides, raising its background. Hence the counter has a finite effective life, which is shortened if higher-than-normal count rates are measured.

Because " counting only measures the abundances of atoms that actually decay during the measurement period, it is a very inefficient measurement technique, especially for long-lived nuclides. In contrast, mass spectrometry offers the opportunity of counting every atom in the sample, and is therefore much more sensitive. In the case of U-series nuclides, mass spectrometry offers approximately an order of magnitude improvement, both in sensitivity and in precision, and has therefore largely replaced " spectrometry as a measurement technique. However, these advances also throw the emphasis of U-series dating work back onto sample collection and preparation, since open-system behaviour of samples becomes more obvious with improvements in analytical precision. These problems will be discussed below.

Given the low abundances of the U-series nuclides to be measured in natural materials (part-per-trillion to part-per-million range), chemical purification is essential, for both " counting and mass spectrometry. This normally involves dissolution of the sample in HNO₃ (carbonates) or HF (silicates) followed by anion exchange separation (section 2.1.4). Anion exchange is also used to separate between U and Th. Since chemical extractions are not expected to give a 100% yield, the sample is ‘spiked’ before chemistry with a known quantity of artificially enriched isotopes, allowing an isotope dilution determination of isotope abundances in the sample (section 2.4). In " counting analysis, short-lived radioactive species were usually chosen as spikes. For example, a widely used U)Th spike was ²³²U (t_1/2 = 72 yr), which had been allowed to naturally generate its daughter ²²⁸Th (t_1/2 = 1.9 yr). The short half-life of the latter nuclide meant that it reached secular equilibrium with its parent in ca. 20 years (Ivanovich, 1982a). For mass spectrometry, the longer lived isotopes ²²⁹Th and ²³⁶U are preferred, with half-lives of ca. 6 kyr and 70 kyr respectively.

A pre-requisite to precise and accurate dating with U-series nuclides is the availability of good half-life determinations. However, the attainment of secular equilibrium allows these half-lives to be determined relative to the very well-constrained ²³⁸U half-life. For example, the half-life of ²³⁴U can be determined very accurately relative to ²³⁸U, by measurement of the ²³⁴U/²³⁸U ratio on a sample in secular equilibrium, such as uraninite ore. Using this technique, de Bievre et al. (1971) determined a value of 244.6 " 0.7 kyr by " spectrometry, which was revised to 245.3 " 0.14 kyr by mass spectrometry (Ludwig et al., 1992). The latter result was confirmed by Cheng et al. (2000), who determined a value of 245.25 " 0.49 kyr.

The ²³⁰Th half-life can also be determined by analysis of uraninite in secular equilibrium, using a mixed ²²⁹Th)²³⁶U spike. However, this determination has a larger uncertainty because it incorporates the errors of spike calibration, based on gravimetric U and Th standards. (Many labs calibrate their spikes against uraninite, which would lead to a circular argument in this case). Meadows et al. (1980) determined a half-life of 75.4 " 0.6 kyr from " counting, which was revised to 75.69 " 0.23 kyr by mass spectrometry (Cheng et al., 2000) using four different types of sample considered to be in secular equilibrium. Fortunately, both half-life determinations increased by a similar proportion (on moving from the " counting to the mass spectrometric values), so the overall effect on calculated ages is small.

12.2.1 Mass spectrometry

Uranium-series dating by mass spectrometry was one of the missed opportunities of 1970s isotope geology, since the analytical equipment available at that time was equal to this task, but was not applied until the late 1980s. This omission can be explained by a communications gap between workers in the two fields, and by exaggerated estimates of the problems which might be posed by large nuclide-abundance ratios. The gap was closed in two stages, by Chen et al. (1986) who performed the first precise mass spectrometric analysis on ²³⁴U, and by Edwards et al. (1987), who made the first ²³⁰Th measurements. These workers showed that mass spectrometric U-series dating offered great improvements in precision over the best " counting determinations.

Edwards et al. avoided the difficulty of measuring large ²³⁸U/²³⁴U ratios by measuring ²³⁵U/²³⁴U instead. Since ²³⁸U/²³⁵U has a constant ratio of 137.88 in normal rocks, the conversion is simple. Furthermore, by analysing pure corals with a low detrital ²³²Th content (see below) it was possible to obtain ²³²Th/²³⁰Th atomic abundance ratios as low as 1.1 (compared with typical ratios of over 250 000 in silicate rocks). These techniques allowed Edwards et al. to determine the age of a typical Pleistocene coral to a precision of 123 " 1.5 kyr (2F), compared with an " counting determination of 129 " 9 kyr.

Thorium has a relatively high ionisation potential. Therefore thermal ionisation mass spectrometry (TIMS) analysis of this element is relatively inefficient. Li et al. (1989) used the conventional double-filament technique employed for Nd isotope analysis (section 2.2.1), with a very hot centre filament to promote the formation of Th metal ions. This method is not very demanding of chemical purity but is relatively inefficient. Edwards et al. loaded both U and Th (separately) on graphite-coated single rhenium filaments, and analysed them as the metal species. This method is more efficient for very small samples, but the ionisation efficiency drops rapidly as the size of loaded sample increases (Fig. 12.3), from 0.1 % with very small samples to 0.001% with large samples. This is due to a failure to make proper contact with the heated metal filament as the size of the sample load increases. Asmerom and Edwards (1995) described a new method for loading Th as the fluoride. When used with a normal double filament technique, this method improved the ionisation efficiency of large thorium samples (such as igneous rocks). Using the new method, a 200 ng Th sample had an ionisation efficiency of 2 H 10^!4, an order of magnitude better than that achieved with the previous technique (Fig. 12.3).

Fig. 12.3. Plot of ionisation efficiency for Th isotope analysis against the total size of Th sample loaded in a single Re filament. The typical sample size of clean corals is shown. After Edwards et al. (1987).

Ionisation problems are avoided using an ICP source, which achieves nearly complete ionisation of all elements (section 2.2.2). As a result, MC-ICP-MS seems likely to supersede TIMS analysis for Th analysis (section 2.5.3). It also has the capability of performing in situ U-series analysis of uranium-rich samples using the laser microprobe (Stirling et al., 2000). However, sampling of the plasma by the mass spectrometer is only about 1% efficient, so MC-ICP-MS offers only a moderate advantage over TIMS. Therefore, TIMS analysis is expected to find continued use for some time into the future.

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