12.5     U-series dating of open systems

 

12.5.1  231Pa–230Th

 

Because the 231Pa and 230Th (daughter deficiency) dating methods share a common parent element (uranium), they form a single U–Th–Pa system which is analogous to the U–Pb dating system. This similarity was first discussed by Allegre (1964), who showed that the U–Th–Pa system can be studied using the same ‘concordia’ diagram used for U–Pb zircon dating (section 5.2). The significance of this observation is that a combination of U–Th and U–Pa analyses can be used to date systems that have been partially opened during their history, rather than just as a concordance test to check for closed-system behaviour.

 

            Because the U–Th–Pa system involves three different elements, rather than two, it is necessarily more complex than the U–Pb system. However, this does not have a large effect on the method in practice, because the daughter nuclides Th and Pa are largely immobile, and behave essentially as if they were the same element. On the other hand, the principal cause of open system behaviour is the mobility of uranium, which is the common element between the two systems.

 

            Despite the fact that the U–Th and U–Pa sub-systems have the same parent element, the U–Th–Pa system is more complex than the U–Pb system because the parent uranium isotopes (234U and 235U) can undergo fractionation in the environment (section 12.3.1). As a result, the U–Th–Pa system actually gives rise to a family of curves in a concordia diagram. This complication was ignored in the analysis of Allegre (1964) and in later studies by Kaufmann and Ku (1989) and Kaufmann et al. (1995), but was treated in the more detailed discussion of Cheng et al. (1998). As a result, two different concordia diagrams can be plotted, one involving the ultimate parent (238U, Fig. 12.30a) and one involving the immediate patent (234U, Fig. 12.30b). On each of these diagrams, concordia curves can then be shown for different 234U/238U activities, presented in the form of * 234U. For reference, it should be remembered that * 234U = 0 signifies secular equilibrium, while * 234U = 150 is the uranium isotope composition of seawater, corresponding to a 234U/238U activity ratio of 1.15.

Fig. 12.30. Concordia diagrams for the U–Th–Pa system; a) against 238U; and b) against 234U. Concordia curves are shown for a variety of  * 234U values. After Cheng et al. (1998).

 

            The final complicating factor in the U–Th–Pa system, relative to U–Pb, is the fact that the daughter products are themselves radioactive. Indeed, this is actually the feature which makes the combined system useful for dating partially open systems in the age range 0 – 200 kyr. It is the return of 230Th and 231Pa activities to secular equilibrium with their parents, at different rates, which allows age information to be recovered from partially open systems. As a result, when the concordia diagram is plotted (Fig. 12.30), the curvature is in the opposite sense to that of the U–Pb concordia, with the longer-lived nuclide (230Th) along the x axis and the shorter lived (231Pa) on the y axis.

 

            Uranium gain is probably the most important type of open system behaviour in U–Th–Pa dating, and causes the data points to move towards the origin in a similar way to Pb loss in the U–Pb system. The simplest scenario is a single episode of U gain. This forms a linear array through the origin at the time of open system behaviour, and the array then rotates as the system ages under closed system conditions. The result at the present time is a discordia array similar to a U–Pb discordia (Fig. 12.31). The upper intercept with the concordia line then gives the true age of the system. However, the lower intercept only gives the age of U gain for the special case where * 234U = 0 (secular equilibrium of parent uranium isotopes).

Fig. 12.31. 234U–Pa–Th plot showing the evolution of a discordia line generated by a single episode of uranium mobility. After Cheng et al. (1998).

 

            The case of continuous open system uranium gain is more complex, and gives rise to a curved ‘discordia’ array. However, as in the case of U–Pb dating, the upper end of this continuous U-gain line is relatively straight, and can give a fairly precise upper intercept age. A special case of this scenario is ‘linear uptake’ of uranium through the life of the system, which is shown on the 234U–Pa–Th diagram in Fig. 12.32. As in U–Pb dating, the best estimate for the upper intercept age is obtained by having a suite of samples with variable degrees of U gain, but with some samples near the concordia (minor late U gain). However, a minimum estimate of the upper intercept age can be calculated by dividing the U–Pa equation [12.33] by the U–Th equation [12.32], as suggested by Ivanovich (1982b). The resulting Pa–Th age is analogous to a 207/206 lead age.

Fig. 12.32. U–Pa–Th plot showing a discordia line generated by continuous uranium gain. after Cheng et al. (1998).

 

 

12.5.2  ESR–230Th

 

            Some of the most interesting applications of U-series dating involve human bones and cultural deposits, but these materials are notorious for their open-system behaviour of U-series isotopes, as well as their small sample size. In the past, the ages of this type of material were based on speleothem deposits which pre-dated and post-dated a ‘cultural’ layer (e.g. Schwarcz, 1989). However, with the advent of mass-spectrometric U-series analysis, 230Th ages can be used in combination with electron spin resonance (ESR) to obtain reliable dates for teeth (Grun et al., 1988; Grun and McDermott, 1994). Human teeth are not dated directly, but human bones are often found in deposits with large numbers of bovoid teeth (from the cow family). The relatively large size of these teeth provides sufficient material for ESR and U-series dating.

 

            Both 230Th (daughter deficiency) and ESR dating are based on U-series isotopes, but in different ways. ESR measures the accumulation of trapped electrons at defect sites, caused by the time integrated radiation dose experienced by the sample, mainly derived from 238U. (For reviews of ESR dating see Jonas (1997) and Rink (1997).) On the other hand, 230Th dating measures the return of this nuclide to secular equilibrium with 238U, via its daughter product 234U. In both cases, a dating signal is derived largely from a common parent (238U), and in both cases the dating signal itself is largely immobile (trapped electrons versus 230Th nuclides). On the other hand, the dating signal has a different half-life in the two methods: infinite for trapped electrons but 72.5 kyr for 230Th. Therefore, the combined ESR–230Th method can be used to date systems partially open to uranium mobility in a similar way to the U–Th–Pa dating system.

 

            Actually, ESR–230Th dates are based on a system that is almost completely open to uranium, because the entire uranium inventory of a tooth is acquired after deposition (live teeth have essentially no uranium). However, experience has shown that the U-uptake history of a tooth can be approximated by three types of model, termed ‘early uptake’ (EU), ‘linear uptake’ (LU) and ‘recent uptake’ (RU), as shown in Fig. 12.33. Of these three models, early uptake is the best scenario for dating because it approximates a case of closed system evolution, whereby the age of the tooth is essentially the same as that of the uptake event that occurred soon after burial. In this case, the ESR and U-series (230Th) age will be concordant. On the other hand, a Recent Uptake scenario is the worst for dating because the U-series method cannot see back any further than the recent uranium enrichment event. ESR dating can only give a useful date in these circumstances if the majority of the radiation dose experienced by the tooth comes from the sedimentary environment outside the tooth. Linear uptake is the approximation which represents all scenarios between these two extremes. It generates relatively large uncertainties, but may be susceptible to analysis if a large enough data set is available. In detail, the uptake histories of three uranium reservoirs must be considered: tooth enamel, tooth dentine, and surrounding sediment. The ESR measurement is made on the enamel and dentine, and its signal is a time integrated function of the radiation dose from the three reservoirs.

Fig. 12.33. Schematic illustration of different uptake models experience by buried teeth.

 

            A example of age concordance between U-series analyses and ESR ages is provided by a study of bovoid dental fragments associated with the skeletons of early modern humans (Fig. 12.34). In this case, agreement between most of the U-series and ESR dates indicated that uranium uptake occurred soon after deposition. Hence the U-series ages confirmed ESR dates for the appearance of early modern humans at least 100 kyr ago in Israel (McDermott et al., 1993). Two points in Fig. 12.34 (samples a and b) display markedly lower U-series ages relative to their ESR ages, outside the limits of error. These are indicative of models approximated by recent and linear uptake respectively (Grun and McDermott, 1994).

Fig. 12.34. Plot of apparent ESR and U-series ages for enamel ( ! ) and dentine ( " ) samples of bovoid teeth from Israel, assuming an Early Uptake model. Error bars are 1F. After Grun and McDermott (1994).

 

            Grun et al. (1988) proposed a complex parametric calculation to model the uptake history of a tooth, based on the relative ESR and U-series ages of open system samples. They demonstrated the effectiveness of this technique on samples from Hoxne, England, which have very young U-series ages relative to their ESR ages, and are therefore indicative of relatively recent U uptake. However, Grun and McDermott (1994, p. 123) admitted that ‘The mathematical formulation of these steps is very complex and we have not yet been able to establish a rigorous error calculation procedure’. Therefore, for practical purposes, a simpler method used by Rink et al. (2001) may be adequate. These authors categorised the uptake model (Fig. 12.35) based on the ratio between the U-series age and the ESR age calculated from an early uptake model. They then interpolated between the different uptake models to obtain a best estimate of the age of the tooth.

Fig. 12.35. Categories of uranium uptake model based on the ratio between apparent U-series and ESR ages (calculated assuming early uptake for both systems). After Rink et al. (2001).

 

An alternative approach might be to plot a concordia diagram of the 230Th/234U ratio against the ESR age. The upper intercept of this concordia diagram should give the approximate age of the tooth, in a similar way to the U–Th–Pa and U–Pb concordia diagrams. Unfortunately, the quality of experimental data has not yet been adequate to demonstrate this approach.

 

 

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