5  Lead Isotopes

 

Of the four stable isotopes of lead, only 204Pb is non-radiogenic. The other lead isotopes are the final decay products of three complex decay chains from uranium (U) and thorium (Th). However, the intermediate members of each series are relatively short-lived, so they can usually be ignored when geological time-scales of millions of years are involved. Table 5.1 shows the ultimate parent)daughter pairs, of which the highest atomic weight parent (238U) decays to the lowest atomic weight daughter (206Pb) and vice versa. It will be noted that the 238U half-life is comparable with the age of the Earth, whereas that for 235U is much shorter, so that almost all primordial 235U in the Earth has now decayed to 207Pb. The 232Th half-life is comparable with the age of the universe.

 

Table 5.1 Ultimate parent)daughter pairs of uranium and thorium.

)))))))))))))))))))))))))))))))))))))))))))))))))))))))

   Decay route                 t1/2, Byr                      Decay const. 8, yr!1

)))))))))))))))))))))))))))))))))))))))))))))))))))))))

   238U  6  206Pb                          4.47                           1.55125 H 10!10

 

   235U  6  207Pb                          0.704                         9.8485  H 10!10

 

   232Th 6  208Pb                        14.01                          0.49475 H 10!10

)))))))))))))))))))))))))))))))))))))))))))))))))))))))

Data from Jaffey et al. (1971).

 

            If we consider a system of age t, (e.g. a granite intrusion which crystallised from a magma), then we can write an equation for the nuclides involved in each decay scheme, derived from the general equation [1.10]:

 

            206PbP  =  206PbI  +   238U  (e8238 t ! 1)                          [5.1]

 

            207PbP  =  207PbI  +   235U  (e8235 t ! 1)                          [5.2]

 

            208PbP  =  208PbI  +   232Th  (e8232 t ! 1)                         [5.3]

 

where P indicates the abundance of a given nuclide at the present and I indicates the initial abundance of that nuclide. It is convenient to divide throughout by 204Pb to obtain equations containing isotope ratios rather than absolute nuclide abundances. 204Pb is chosen because it is the only non-radiogenic isotope. Hence for the two uranium decay schemes we obtain:

 

   (206Pb)           (206Pb)              238U

   ()))))   =   ()))))   +      ))))    (e8238 t ! 1)                 [5.4]

   (204Pb)P         (204Pb)I 204Pb

 

   (207Pb)           (207Pb)              235U

   ()))))   =   ()))))   +      ))))    (e8235 t ! 1)                 [5.5]

   (204Pb)P         (204Pb)I 204Pb

 

5.1       U)Pb Isochrons

 

In principle, the decay equations [5.4] and [5.5] can be used to construct isochron diagrams and hence to date rocks in a manner analogous to the Rb)Sr system (section 3.2.2). U)Pb isochrons are subject to assumptions similar to those for Rb)Sr, the most critical of which is that the samples remained closed to U and Pb during the life-time of the system being dated. Unfortunately, the U)Pb  system rarely stays closed in silicate rocks, due to the mobility of Pb, and especially U, under conditions of low-grade metamorphism and superficial weathering. For example, in a case study on the Granite Mountains batholith, Wyoming (Fig. 5.1) whole-rock samples suffered disastrous uranium losses, displacing the data points far from a  reference line defined by the intrusive age.

Fig. 5.1. U-Pb isochron diagram for the Granite Mountains batholith showing displacement of whole-rock data points far to the left of the 2.82 Byr reference line, due to disastrous uranium losses. After Rosholt and Bartel (1969).

 

            The mobility of uranium greatly limits the application of simple U)Pb isochron dating. However, the unique properties of the U–Pb system, involving two separate decay schemes with common parent and daughter nuclides, mean that age information can be obtained even from disturbed systems. Three dating techniques exploit this situation, namely the U)Pb ‘zircon’ method, the common Pb)Pb method, and the galena model age method. These methods will be discussed in the subsequent sections of the chapter. Nevertheless, there are a few systems to which conventional U–Pb isochron dating has been applied successfully. These include dating sediment deposition by means of marine carbonate and dating prograde metamorphism by means of garnet porphyroblasts. The first of these examples will be examined here. U–Pb dating of garnets will be discussed in section 5.2.7.

 

 

5.1.1    U–Pb dating of carbonates

 

One area where U)Pb isochron dating has been applied with moderate success is the direct dating of marine carbonates, which have proven very difficult to date by other radiometric methods. Uranium has a seawater residence time four orders of magnitude longer than Pb, leading to a very high by 238U/204Pb ratio (: value) of ca. 75 000 (Jahn and Cuvellier, 1994). Because U is also thought to have a higher carbonate/seawater partition coefficient than Pb, the : value of marine carbonates could be even higher. An upper limit of 230 000 was suggested by Jahn and Cuvellier, corresponding to a U content of 1 ) 3 ppm and Pb content of ca. 1 ppb (part per billion). The highest : value observed in an ancient biogenic calcite is 50 000 (Smith et al., 1991). However, most ancient carbonates have values far lower than this, which are therefore attributed to open system behaviour during post-depositional diagenesis and secondary alteration.

 

            An example of U–Pb dating of typical marine carbonates is the study of Smith and Farquhar (1989) on Devonian rugose (Heliophyllum) corals from Ontario. Several coral samples, together with authigenic pyrite from one specimen, formed a reasonably good linear array on a 238U–206Pb isochron diagram (Fig. 5.2). In this figure the solid points form an errorchron (MSWD = 4.7), but the age of 376 " 10 Myr (2F) compares well with a stratigraphic age of ca. 375 ) 385 Myr. However, this result was only achieved by omitting one Heliophyllum coral and three out of four Cystiphylloides corals, which lie well off the regression line. This scatter was probably caused by open-system behaviour during recrystallisation, since the Cystiphylloides corals have a more porous structure which would facilitate the movement of fluids. In subsequent work the diagenetic calcite overgrowths were studied, but reliable ages were difficult to determine, due to the effects of multiple growth stages (Smith et al., 1991). Hence it is concluded that only primary depositional carbonates yield reliable age information.

Fig. 5.2. U)Pb isochron diagram for Devonian corals from SW Ontario, Canada. ( ! , " ) = Heliophyllum; ( > , Î ) = Cystiphylloides. Open symbols were omitted from the regression. After Smith and Farquhar (1989).

 

            Jones et al. (1995) showed that in the Capitan Limestone of New Mexico, some samples had : values over 3000 which appear to be a primary depositional feature. The sample suite came from a Permian reef complex and consisted of massive abiotic botryoids (ca. 1 cm in size), made of acicular aragonite recrystallised to low-Mg calcite. However, micro-sampling for oxygen and carbon isotope analysis suggested that relatively pristine and diagenetically disturbed domains were intimately interfingered. Samples preserving possible primary : values had high U (3 ppm), high Sr (3000 ppm) and low Mn contents (< 10 ppm). The high : values allowed the calculation of radiogenic 206Pb*/238U ages, as in equation [5.7], after correction for a small common Pb component. Using this method, six samples gave an average age of 250 " 3 Myr which is consistent with the age of deposition (Fig. 5.3). However, many other samples gave scattered ages, including some with : values above 2000, within the same range as the ‘undisturbed’ samples. Since there is no a priori way of recognising the disturbed samples, the method cannot be considered as reliable.

Fig. 5.3. Plot of radiogenic 206Pb*/238U ages against U/Pb ratio for the Capitan Limestone, New Mexico. Texturally well-preserved carbonates with high U/Pb ratios ( ! ) yield ages consistent with the known age of deposition. However, other samples are badly scattered. After Jones et al. (1995).

 

            Another kind of carbonate material that has been proposed for dating stratigraphic sections is paleosol calcite (Rasbury et al. 1997). This type of calcite grows on the sediment surface when marine sequences are subjected to sub-aerial exposure, forming a soil horizon. Under these conditions sparry calcite grows as a cement, and has a brown colour which is caused by organic material. This brown calcite can develop high : values (ca. 1000) which are suitable for U/Pb isochron dating. Rasbury et al. analysed seven calcite samples from a horizon of this type near the Carboniferous–Permian boundary in Texas. These samples define a U–Pb isochron with an age of 298 " 1.5 Myr (2F, MSWD = 1.5) which is in good agreement with the estimated stratigraphic age of 295 " 5 Myr. Hence, this method has potential to be a useful stratigraphic calibration tool.

 

 

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