5.2       U)Pb (zircon) dating

 

If a mineral was available which strongly incorporated uranium at the time of formation but did not incorporate lead, equation [5.1] above could be simplified by removal of the initial 206Pb term to yield:

 

                          206Pb*    =       238U  (e8238 t ! 1)                                  [5.6]

 

where Pb* represents radiogenic lead only. Taking 238U to the other side yields the equation:

 

                          206Pb*

                          )))))    =   (e8238 t ! 1)                                           [5.7]

                          238U

 

A similar equation can be derived from [5.2] above:

 

                          207Pb*

                          )))))    =   (e8235 t ! 1)                                           [5.8]

                          235U

 

Minerals which have remained closed systems for U and Pb give concordant values of t when their isotopic compositions are inserted into the left-hand sides of equations [5.7] and [5.8]. When compositions yielding such concordant ages are plotted graphically (Fig. 5.4) they define a curve which was termed the concordia by Wetherill (1956a). The concordia curve can be drawn by substituting decay constants and successive values of t into the right-hand sides of equations [5.7] and [5.8], and plotting the results for each value of t.

 

            Uraninite and monazite were the first minerals used in U)Pb geochronology, in view of their tendency to incorporate large concentrations of uranium but very little initial (non-radiogenic) lead. However, their limited distribution restricts their usefulness. On the other hand, zircon is a U-rich mineral with a much wider distribution, which is present in most intermediate to acid rocks. This has therefore become the principal material used in U)Pb dating. A short review of alternative dating materials is given in section 5.2.7.

 

            The small fractions of initial (`common’) Pb which are incorporated by zircons are corrected by measuring the amount of (initial) 204Pb in the mineral and then using the 206Pb/204Pb and 207Pb/204Pb ratios of the whole-rock to estimate the amount of initial 206Pb and 207Pb incorporated in the zircon. These are subtracted from the present-day 206Pb and 207Pb to yield the radiogenic fractions. For zircons with very low common Pb contents, an adequate correction may be made by estimating common Pb from a general terrestrial Pb evolution model (e.g. Stacey and Kramers, 1975; section 5.4.3), rather then by direct analysis of the whole-rock sample.

 

 

5.2.1    Lead loss models

 

Early dating work on U-rich minerals soon revealed that most samples yield discordant 206Pb/238U and 207Pb/235U ages. This discordance was attributed to Pb loss by Holmes (1954). Since that time, much research in U)Pb dating has been devoted to studying the mechanism of lead loss, and to the determination of accurate dates on samples which have suffered lead loss.

 

            Ahrens (1955) found that monazite and uraninite from Zimbabwe (then called Rhodesia) yielded discordant U)Pb ages but nevertheless defined a linear array on the concordia diagram (Fig. 5.4). Such arrays were later termed discordia. Ahrens argued against lead loss from monazites by leaching, since he (erroneously ?) claimed them not to be metamict due to radiation damage. Instead, he thought that lead loss occurred by some kind of continuous diffusional process. This model was elaborated by Russell and Ahrens (1957), who postulated that intermediate members of the uranium decay series were ejected into micro-fissures in the mineral lattice (pitchblende in this case) by the ‘recoil energy from "-particle emission’. These nuclides or their decay products could subsequently be removed by diffusion or leaching.

Fig. 5.4. U)Pb concordia diagram showing the concordia line calibrated in Myr, and a discordia line generated by variable Pb loss from 2700 Myr-old U-rich minerals of Zimbabwe (Rhodesia). After Wetherill (1956a).

 

            Wetherill (1956a) advanced an alternative interpretation of the data, now called the episodic lead loss model. He agreed that the upper intersection of the discordia with the concordia corresponds to the time of formation of the minerals (t1). However, Wetherill argued that the lower intersection of the discordia and concordia (t2) also had age-significance, representing the time of a thermal event which caused lead loss from the minerals. For the ‘Rhodesian’ minerals these episodes are dated at 2700 and 500 Myr respectively. He supported his model by citing Rb)Sr and K)Ar ages of 500 Myr on lepidolite as evidence for a thermal event at that time.

 

            Wetherill (1956b) presented an algebraic proof that the episodic lead loss model could generate the graphically observed results. This can be visualized (Fig. 5.5.) by imagining that the data are plotted at the time of lead loss (500 Myr ago). When lead loss occurs, the data points move from the original composition (C) towards the origin (e.g. forming composition B). Subsequent Pb evolution simply rotates the lead loss line in proportion: A 6 D; B 6 E; C 6 F.

Fig. 5.5. Hypothetical effects of episodic lead loss from a 2500 Myr-old U-rich mineral showing formation of a discordia 500 Myr ago (A-B-C) and its rotation (D-E-F) due to subsequent closed-system evolution (see text for discussion).

 

            Tilton (1960) showed that U-rich minerals with similar formation ages from Archaean shield areas in five continents all lay near a discordia line with a lower intersection at ca. 600 Myr (Fig. 5.5). According to the episodic lead loss model this would imply a worldwide metamorphic event at 600 Myr, but geological evidence for such an event is lacking. Instead, Tilton proposed that the minerals had undergone continuous diffusional lead loss over geological time, yielding a curve on Fig. 5.6 which for much of its length closely resembles a straight line, only curving downwards to an intersection at the origin for cases of strong recent lead loss.

Fig. 5.6. Concordia diagram for Archean U-rich minerals from five continents showing common discordia lower intercept at ca. 500 Myr. Curved line shows expected effect of extreme diffusional lead loss. Dotted line shows extrapolation to an apparent episodic lead loss event. After Tilton (1960).

 

            Goldrich and Mudrey (1972) developed this diffusional lead loss model by arguing that radiation damage of a U-rich mineral was responsible for the formation of a micro-capillary network in the crystal which would become fluid filled. Pb which diffused into these fluids would be lost from the mineral when uplift of basement rock caused the mineral to dilate and expel the capillary-filling fluids. Evidence in support of this ‘dilatancy’ model was provided by the agreement of various lower intersection ages from North America with times of basement uplift derived from paleo-geographical evidence.

 

            Further understanding of lead loss mechanisms comes from consideration of the nature of the zircon crystal lattice. For example, Kober (1986) has shown that when Pb is evaporated in-situ from zircon grains in the mass spectrometer, discordant Pb can be driven off at low filament temperatures (less than 1350 oC), whereas the concordant Pb fraction is usually emitted between ca. 1400 and 1500 oC (section 5.2.4 below). Based on the high temperatures of concordant Pb emission, Kober (1987) argued that the concordant radiogenic Pb fraction is substituted into the zircon lattice itself, rather than filling defects and voids in the lattice. A stable lattice site would be difficult to envisage for the Pb2+ ion, which has an ionic radius (1.18 ) 1.29 D), much too large to allow substitution for Zr4+ (0.72 ) 0.84 D) or Hf4+ (0.71 ) 0.83 D). However, Pb4+ has an ionic radius of only 0.78 ) 0.94 D, making it a possible candidate for admission into the lattice. Kober (1987) suggested that the emission of $ particles during radioactive decay and the transformation of emitted He2+ (" particles) to neutral He can effect this oxidation.

 

            The microscopic examination of analysed zircon grains now suggests that lead loss from zircons is a fairly ‘black and white’ process. In other words, unaltered zircon lattices lose very little or no lead, whereas altered zircon (promoted by metamictisation) loses lead very readily. Any given zircon crystal may contain both kinds of material. For example, Fig. 5.7 shows alteration fronts advancing through the metamict U-rich parts of a zircon. In reality, the exact mechanism for lead loss from altered zircon may in fact be different in different circumstances. Hence it is concluded that the lower intersection of a U)Pb zircon discordia should only be attributed age- significance if this is supported by other geological evidence. However, the interpretation of the upper intersection as the age of formation of the zircons is unaffected.

Fig. 5.7. Drawing of a metamict zircon showing inward advance of alteration fronts (arrows). Unaltered material is white. From a photograph by van Breemen et al. (1986).

 

 

5.2.2    Upper intersection ages

 

Silver and Deutsch (1963) made a pioneering case study of lead loss from different zircon fractions in a single rock sample. They found that large zircons lost less lead than small ones (due to the larger surface area/ volume ratio of the latter), and that zircons with low uranium contents lost less lead than high-U zircons. The latter effect was attributed to the greater radiation damage suffered by U-rich grains. In addition to losing lead, metamict zircons tend to incorporate impurities, including iron. Hence magnetic separation of zircons can yield fractions with variable discordance.

 

            In order to obtain the best intersection of the discordia with the Concordia, it is desirable to analyse several zircon fractions with variable discordance and perform a linear regression on the results. This regression cannot be solved algebraically to yield upper and lower intersection ages; hence these ages are usually calculated iteratively by computer. In order to calculate a regression fit to an array of data displaying some geological scatter, it is common practice to expand analytical error bars to encompass the scatter (section 2.6.3). However, if lead loss processes have operated at different times in the history of a zircon, the resulting discordia array may fan out somewhat from the upper intercept as the points become more discordant. Therefore, Davis (1982) suggested that, rather than expanding all errors equally to encompass geological scatter, the error bars should instead be magnified in proportion to their discordance (Fig. 5.8).

Fig. 5.8. Concordia diagram showing expansion of analytical errors in proportion to discordance to encompass geological scatter on a discordia line. Enlargements are shown for two parts of the discordia. After Davis (1982).

 

            Krogh (1982a, b) argued that instead of refining the mathematical treatment of lead loss models to obtain an accurate upper intersection from discordant zircons, it would be better to remove discordant Pb from the sample before analysis. In early experiments, Krogh and Davis (1975) attempted to remove altered parts of the zircon by chemical leaching prior to analysis. However, they found that Pb was also leached from other parts of the grain. Therefore, it was concluded that physical rather than chemical methods must be used to remove discordant zones of the zircon crystal.

 

            A technique of physical separation tested by Krogh (1982a) was the use of a very-high-flux magnetic separator, which removes all but the least metamict grains. This was found to be relatively successful (Fig. 5.9), but the most successful approach  was to abrade the zircons in a pneumatic mill (Krogh, 1982b). This procedure removes the outer layers of the crystals, which are usually the most U-rich, and hence metamict. Spectacular increases in concordance were obtained in this manner (Fig. 5.9), and the technique has become a standard procedure in zircon geochronology.

Fig. 5.9. The effect of selecting very non-magnetic zircons, and of abrading off the outer rims, to increase concordance. Symbols ( + , > ) indicate two different rock samples. After Krogh (1982b).

 

            An alternative method of dating zircons with complex geological histories is to break single large zircon crystals into fragments and analyse the individual fragments after a very low blank chemical separation (e.g. Scharer and Allegre, 1982). Corfu (1988) developed this method by breaking the tips off large zircons to analyse the age of new zircon crystallisation over older cores. Aleinikoff et al. (1990) achieved a similar effect by analysing the zircon dust produced by gentle air abrasion of prismatic grains with long terminations. The effect of the air abrasion process on such a grain is shown in Fig. 5.10. Cracked grains must be excluded to prevent their disintegration during abrasion, which would release core material into the dust.

Fig. 5.10. Drawings of zircon grains before and after abrasion to remove tips and interfacial edges for analysis. From photographs by Aleinikoff et al. (1990).

 

            Recently, there has been a renewed interest in stepwise dissolution experiments on zircon (e.g. Mattinson, 1994; Corfu, 2000; Davis and Krogh, 2000). However, the new work encountered the same complex fractionation effects that had been observed in early leaching studies. This is attributed to preferential release of radiogenic Pb from energetically unstable lattice sites in radiation-damaged parts of the zircon crystal (the ‘hot-atom’ effect). To solve this problem, Mattinson (2001) proposed that samples could be annealed before dissolution. This will neutralise the hot-atom effect, but still permit step-wise dissolution of different zones of the crystal, which may have suffered variable Pb loss over geological history. Hence Mattinson suggested that a U—Pb ‘spectrum’ plot could be constructed, analogous to that used in Ar—Ar dating (section 10.2.3). It remains to be seen how widely useful the method will be.

 

            With the greatly improved statistics that are now obtainable using the various methods described above, the analytical uncertainty on upper intercept ages is often lower than 0.1% (2F). This now falls within the same range as the uncertainty on the 238U and 235U decay constants (0.11% and 0.14%, 2F). Hence, Mattinson (1987) argued that these decay constant errors should not be ignored, and that the uncertainty on a typical U/Pb upper intercept age might often double if these errors are taken into account.

 

            Many workers have taken the contrary view, that provided U–Pb ages are compared only with other U–Pb ages, it is legitimate to ignore decay constant errors. Since U-Pb ages are usually more precise than those obtained with other absolute dating methods (with a few exceptions, such as Ar-Ar dating in the Tertiary period), this argument is usually reasonable. However, Ludwig (1998, 2000) argued that even different dating calculations within the U–Pb method could introduce small biases due to decay constant errors. In other words, decay constant uncertainties have slightly different effects on age errors using the 206Pb/238U, 207Pb/235U and 207Pb/206Pb systems.

 

            In general, overall age uncertainties are minimised when both U–Pb decay routes are used together in the concordia diagram. However, even in this system, error magnification can occur. An example of these effects is shown in Fig. 5.11, where the concordia curve has a finite width when decay constant uncertainties are taken into account (Ludwig, 1998). In principle there is a moderate chance (26%) that the error ellipse in this diagram is concordant, whereas the chance that the ellipse is concordant falls to only 4% if the concordia is drawn as a single line. Even if these effects are not included in reported age errors, it is important to be aware of them.

Fig. 5.11. Part of the concordia diagram, showing a nearly-concordant data point (solid ellipse) relative to a concordia band that takes decay constant uncertainties into account. After Ludwig (1998).

 

 

5.2.3    Ion-microprobe analysis

 

A completely different approach to achieving concordant U)Pb ages is the in-situ analysis of Pb isotope composition (and U/Pb ratio) of zircon grains by ion microprobe. The general configuration of such an instrument is shown in Fig. 5.12. A beam of light ions (e.g. O!) is used to bombard and sputter a polished section of the zircon grain to yield a secondary beam of Pb ions (hence the term secondary-ion mass spectrometry or ‘SIMS’). Pb ions are analysed in a double (electrostatic and magnetic)-focussing mass spectrometer. The electrostatic analyser is necessary because emitted secondary ions have a range of energies which would yield bad peak tails in the mass spectrum if not filtered.

Fig. 5.12. Schematic illustration of a secondary-ion mass spectrometer (SIMS) showing the components of the negative ion gun and double focussing analyser. After Potts (1987).

 

            A major problem in SIMS analysis is the interference of sputtered molecular ions on the masses of atomic species. In the case of Pb isotope analysis of zircon, this is caused by species such as HfO2+, which have almost exactly the same mass as the Pb isotopes, causing isobaric interference (Hinton and Long, 1979). To overcome this problem the instrument (including magnet) must have a very large physical size, in order to generate a large spatial separation between different masses (equivalent to a resolution of one mass unit in several thousand). This allows the separation of Pb from molecular ion interferences using the ‘mass defect’ phenomenon (Fig. 5.13), by which small variations in atomic mass result from the varying nuclear binding energies of different atoms. The most successful example of a SIMS instrument used for U)Pb dating is the ‘sensitive high-resolution ion microprobe’ or ‘SHRIMP’ developed at the Australian National University (Compston et al., 1984).

Fig. 5.13. Use of a high-resolution mass spectrometer to separate Pb from interfering molecular ion signals. a) Low resolution, ca. 1000; b) High resolution, ca. 3200. After Hinton and Long (1979).

 

            An important example of the use of the SHRIMP as a dating tool is provided by the reconnaissance search for very old rocks (Froude et al., 1983). Zircons were selected from a formation of Archean quartzites surrounded by 3.6 Byr-old gneisses (at Mount Narryer, western Australia) to see whether these metasediments contained any component derived from a pre-3.6 Byr-old source. Different areas of single zircon crystals were analysed using the ion microprobe, yielding quite concordant results. Many ages were in the range 3 ) 3.8 Byr, but a few grains gave ages of 4.1 ) 4.2 Byr (Fig. 5.14).

Fig. 5.14. Concordia diagram for ion micro-probe analyses of zircon from Mount Narryer quartzite. ( € ) = very old zircons (inset shows error ellipses). Isua zircons (box) were analysed as a calibration check. After Froude et al. (1983).

 

            Some of the Mount Narryer zircon spots fell above the concordia line, displaying what is termed ‘reverse discordance’. This phenomenon is common if whole-rock compositions are plotted on a concordia diagram (which is sometimes done for uranium ore deposits), and in that case is usually due to uranium loss. Froude et al. considered whether a process such as U loss could have caused the data to migrate back up the concordia to yield a spuriously old age. In theory, U loss from 3.7 Byr-old zircons during a Late Archean metamorphic episode could have caused points to move to the right and above concordia. This would have to be followed by recent Pb loss (bringing them back down onto the concordia). However, Froude et al. argued that the scatter of data points was too small to be consistent with this model. In addition, Isua zircons were analysed to test the reliability of ion-microprobe analyses for complex metamorphic terranes. These gave the expected age of 3.8 Byr.

 

            Results from a SHRIMP study on zircons from Mount Sones, Enderby Land, Antarctica, help to explain the phenomenon of reverse discordance in ion-microprobe analyses (Williams et al., 1984). Uranium concentrations were found to vary quite smoothly as ion sputtering deepened the analysis spots, whereas lead concentrations varied erratically, giving rise to sudden variations in Pb/U ratio (Fig. 5.15). Hence Williams et al. suggested that reverse discordance is due to migration of radiogenic Pb between different regions within a zircon crystal, rather than U loss. Subsequent to these studies, zircons with even older ages (4.40 Byr) have been found in the Jack Hills conglomerate of Western Australia, this time with a range from concordant to normally-discordant compositions (Wilde et al., 2001).

Fig. 5.15. U and Pb concentrations measured as a function of progressive pit deepening during the ion micro-probe analysis of four different zircon spots. Pb emission is seen to be less stable than U. After Williams et al. (1984).

 

 

5.2.4    Lead 207—206 ages

 

In the dating of Phanerozoic rocks, monazites and zircons may both lie so close to the concordia, sometimes in a clump, that a good discordia line is not generated. In such circumstances it may be necessary to force a discordia line through the origin, assuming that lead loss occurred at the present. The reciprocal of the gradient of this line yields a 207Pb/206Pb age, amounting to a simple division of equation [5.8] by equation [5.7] above. 207Pb/206Pb ages are normally minimum ages, since well-defined discordia usually have slopes too shallow to go through the origin. However, if the data display reverse discordance (e.g. Fig. 5.14) then 207Pb/206Pb ages are maximum ages.

 

            Kober (1986, 1987) demonstrated a new method of zircon dating based on 207Pb/206Pb ages, in which lead is distilled directly from the zircon crystal in the mass spectrometer. Kober’s method is a two-stage process, providing an improvement on techniques previously tried by other workers (e.g. Gentry et al., 1982). A zircon is wrapped in the side filament of a multiple-filament bead, and the temperature of this filament is raised until Pb evaporates directly from the zircon. Some of this lead is redeposited on the centre filament of the bead assembly, which is mounted in front of the evaporation filament (Fig. 5.16). After a deposition period of 5 ) 10 minutes, the side filament is turned off and the centre filament is heated to re-emit the deposited lead. It is thought that other species evaporated from the zircon (mainly SiO2) may form a blanket which holds Pb on the centre filament in a manner similar to the silica gel method for direct Pb analysis (Chapman and Roddick, 1994). When the deposited Pb is exhausted, a new deposition step is performed (if possible) at a higher side filament temperature.

Fig. 5.16. Arrangement of a filament bead for Pb)Pb dating of zircon by the two-stage direct evaporation method. a) exploded view; b) plan view.

 

            Kober’s method is based on the premise that discordant lead is contained in less stable lattice sites than those occupied by concordant lead. The discordant lead is driven off (one hopes) at comparatively low temperatures, so that above 1400 oC it can be assumed that all lead is concordant. Experiments by Chapman and Roddick (1994) suggested that the release of concordant Pb occurs as a reaction front migrates into the grain, converting zircon into zirconium oxide (baddeleyite). The results of each evaporation run are best plotted against filament temperature (Fig. 5.17). If the data define a plateau of 207/206 ages while the evaporation temperature is gradually ramped up, this suggests that the Pb emission from these steps represents a single phase of lead, rather than mixtures of concordant and discordant lead. The 207Pb/206Pb ratio will then yield the true crystallisation age of the zircon. Analysis of zircon from the population previously dated by Froude et al. (1983) gave similarly old ages (Kober et al., 1989).

Fig. 5.17. Plot of measured 207Pb/206Pb ratios (corresponding to apparent age) against evaporation temperature for five Mount Narryer zircons. Low-temperature domains have suffered Pb loss but high-temperature plateaus are argued to be concordant. After Kober et al. (1989).

 

            Another development in 207Pb/206Pb dating (Feng et al., 1993) utilises a combination of laser ablation and inductively-coupled plasma ) mass spectrometry (ICP-MS). A finely focussed laser beam is used to ablate cylindrical pits from single zircon grains, in a manner analogous to the ion microprobe. However, laser ablation is performed at atmospheric pressure, yielding a molecular vapour which is carried by argon gas to the plasma torch. Temperatures of several thousand oC in the plasma cause effective atomisation of the sample, destroying potential molecular ion interferences of Pb. The sample then passes into a quadrupole mass spectrometer (section 2.2.2), where 207Pb/206Pb ratios are analysed. Feng et al. were able to obtain 207Pb/206Pb ages from twenty large zircons (> 60 :m grain size) which fell within 1% of conventional U)Pb data. A further development of this method (section 2.5.5) involves nebulising a uranium–thallium (U—Tl) spike solution at the same time as laser ablation, allowing U/Pb ratios to be determined, and hence providing a full U–Pb age from laser ablation analysis. Analysis by multi-collector ICP-MS also promises further advances in the field of U–Pb dating by laser ablation (section 2.5.5).

 

 

5.2.5    Inherited Zircon

 

If a magma is derived by partial melting of the crust, or assimilates crustal material, old zircons may be entrained into the magma. These ‘inherited’ zircons usually dissolve in per-alkaline magmas, which have a high Zr saturation level. However, they may survive in per-aluminous melts, especially if these are cool and dry, due to the low Zr saturation levels of such magmas (Watson and Harrison, 1983). Inherited zircon xenocrysts tend to lose much of their old Pb, and may be overgrown by a new zircon crystal. However, they may still retain enough old lead to yield meaningful upper intersection ages, which yield the age of inheritance. In contrast, the lower intersection yields the age of melting. Figure 5.18 shows an example from the Ben Vuirich granite of Scotland (Pankhurst and Pidgeon, 1976). The lower intersection (514 " 7 Myr) was interpreted as the age of intrusion and the upper intersection (1316 " 26 Myr) as the approximate age of assimilated old crustal material.

Fig. 5.18. Concordia diagram for Ben Vuirich granite (Scotland) showing mixing between new Caledonian Pb and inherited Mid Proterozoic Pb. After Pankhurst and Pidgeon (1976).

 

            This study was extended by Pidgeon and Aftalion (1978) to include U)Pb analysis of 24 Caledonian granites from Scotland and northern England. Of this suite, 17 plutons with inherited zircon lie north of the Highland Boundary fault in Scotland and only one (Eskdale granite) lies to the south. In contrast, all 6 granites without inherited zircon lie south of the fault. Pidgeon and Aftalion discussed various possible explanations for these observations.

 

            Because the granites north and south of the fault have similar chemistry, Pidgeon and Aftalion ruled out the dissolution of inherited zircons during magma evolution, or their removal during emplacement. They also rejected contamination of granite magmas by sedimentary zircons north of the fault, since these granites do not have the S-type (per-aluminous) chemistry characteristic of assimilation. (In contrast, inherited zircon in the S-type Eskdale granite was probably derived by assimilation of sediments containing old zircon). Hence, it was concluded that there is a fundamental difference in granite source rocks between the Scottish Highlands, and crust to the south; a model later supported by whole-rock Nd isotope analysis (Halliday, 1984).

 

            The early study by Pankhurst and Pidgeon (1976) made use of bulk zircon separates (the total quantity of zircon separated was 8 g!). In an attempt to refine and test the old determination, Rogers et al. (1989) re-dated the pluton using modern techniques of miniature sample analysis and zircon abrasion. The results (Fig. 5.19) were startlingly different. The lower intercept was increased by 76 Myr to 590 " 2 Myr, while the upper intercept was increased by 132 Myr to 1448 " 7 Myr. The lower ages determined from the earlier study can be attributed to the effects of secondary lead loss after intrusion, from a system which already represented a two-component mixing line. This caused rotation of the apparent discordia, yielding erroneously young ages for both upper and lower intercepts.

Fig. 5.19. Concordia diagram for Ben Vuirich granite showing a discordia between needle-shaped magmatic zircon (1) and stubby inherited zircon (4). Inset shows a Pb-loss line that defines the intrusive age. 1 & 4 = strongly abraded; 2 = slightly abraded; 3 = unabraded (to control Pb-loss line). After Rogers et al. (1989).

 

            The occurrence of secondary lead loss from Ben Vuirich zircons is demonstrated by a comparison of abraded and unabraded needle-shaped grains (representing new 590 Myr-old magmatic zircons). In contrast, abraded stubby grains provided a closer approach to the inherited zircon composition than the bulk fractions of large non-magnetic grains analysed by Pankhurst and Pidgeon. The study of Rogers et al. is typical of much recent work showing the dangers of bulk zircon analysis in rocks with complex geological histories. Such samples can yield discordia of high statistical quality which nevertheless yield erroneous ages. Consequently the painstaking selection and abrasion of crack-free and inclusion-free grains is essential to ensure the reliability of U)Pb data.

 

 

5.2.6    Alternative U)Pb data presentations

 

In the classical concordia diagram the variables are strongly correlated, because of the manner in which the data are analysed. The 207Pb/235U ratio is calculated from the 206Pb/238U ratio on the basis of the constant value of 235U/238U and the measured 207Pb/206Pb ratio, which is known much more accurately than is the U/Pb ratio. The correlation of errors is taken into account when fitting discordia regression lines (section 2.6.2), but it is largely avoided in an alternative presentation of U)Pb data pioneered by Tera and Wasserburg (1973, 1974), where the 238U/206Pb ratio is plotted directly against 207Pb/206Pb. This concordia has a different curvature to the conventional presentation, and is preferred by workers dating young rocks (e.g. Scharer et al., 1984) because it displays these discordia lines more clearly than the conventional diagram (Fig. 5.20).

Fig. 5.20. ‘Tera)Wasserburg’ concordia diagram on axes of 207Pb/206Pb against 238U/206Pb showing data for Himalayan granites. After Scharer et al. (1984).

 

            Wendt (1984) further developed the Tera)Wasserburg plot into a three-dimensional U)Pb diagram by the addition of an axis in 204Pb/206Pb, representing the level of common Pb present in the samples. In this construction the discordia is a plane, and ages can be calculated without independent knowledge of the isotopic composition of the common-Pb component, subject to the assumption that only one such component is present. An example of the application of this method is the dating of Mesozoic uranium minerals from Germany by Carl and Dill (1985). The three-dimensional diagram may also allow dating of partially open whole-rock U)Pb systems because it allows more accurate correction of large common Pb fractions (e.g. Carl et al., 1989).

 

            Ludwig (1998) termed the three-dimensional U–Pb isochron the ‘Total-Pb/U isochron’ and the Tera)Wasserburg’ concordia diagram the ‘semi-total Pb/U isochron’. He argued that the Total- Pb/U isochron is useful because it allows a more explicit view of error sources. For example, a schematic three dimensional view of a Total-Pb/U isochron in Fig. 5.21 shows that it has two anchor points. The radiogenic end of the isochron plane intersects the Tera–Wasserburg concordia to define the age of the samples, while the other end of the isochron plane describes the non-radiogenic component of the samples, which should lie close to a reasonable crustal growth curve.

Fig. 5.21. Schematic illustration of a three dimensional Total-Pb/U isochron. The discordia intersects with the concordia curve at its radiogenic end, and with the common Pb growth curve at its non-radiogenic end. After Ludwig (1998).

 

            When U–Pb ages are calculated on a concordia diagram with a correction for common Pb, this is equivalent to forcing the three-dimensional isochron through a point on the common Pb growth curve. For samples with low common Pb contents, such a forced fit may actually be more reliable than a ‘free fit’. However, when common Pb contents are large, forcing the isochron through an assumed common Pb composition may introduce errors if an incorrect common Pb point is used. A good example comes from the analysis of U–Pb data from whole-rock chondrule samples from chondritic meteorites (Tera and Carlson, 1999).  In this example, raw U–Pb isotope data, without any common Pb correction, are presented on two separate diagrams (Fig. 5.22a, b), based on the x and z axes of the Total-Pb/U isochron diagram in Fig. 5.21. This is done to ease the plotting of the data, but the conclusions are the same as for the true three dimensional diagram.

Fig. 5.22. Meteorite data on Tera–Wasserburg and Pb–Pb isotope diagrams, representing the X and Z axes of the Total-Pb/U isochron diagram. The lower (radiogenic) end of the meteorite discordia shows the effect of Pb loss, while the upper (unradiogenic) end shows the effect of terrestrial contamination. ( ! ) = whole-rock chondrules; ( " ) = phosphates; ( Π ) = irons. After Tera and Carlson (1999).

 

            On the Pb–Pb isotope sub-diagram (Fig. 5.22a) the intercept of the chondrule array on the y axis gives the 207/206 age of meteorites. Hence, the most precise ages are given by radiogenic Pb data points near the axis. However, the inset shows the effects of Pb loss from meteorite phosphate grains (solid circles) at the radiogenic end of the array. This open system behaviour limits the precision on the age. On the Tera–Wasserburg sub-diagram (Fig. 5.22b) whole-rock chondrules define an array with a concordia intersection age of 4561 Myr. This age approximates the correct age of condrule formation (section 5.3.1), but many points in the array project towards a present day terrestrial composition (solid diamond) rather the expected primordial solar system composition. The conclusion from this analysis is that the chondrules must have been partially contaminated by terrestrial Pb. Therefore, to minimise these effects, chondrules should be acid-washed before analysis in order to reduce this contamination (section 5.3.1).

 

            Ludwig (2001) argued that another advantage of the total-Pb/U isochron is that it may help to eliminate mass fractionation uncertainties from the age calculation. Again, this is more significant for samples with larger common Pb contents. These calculations were included in a Microsoft Excel Geochronological Toolkit (Ludwig, 1999).

 

 

5.2.7    Alternative U–Pb dating materials

 

Zircon has been the mineral of choice for most U–Pb dating work since the earliest studies. However, other minerals may yield valuable U–Pb age data that complement U–Pb zircon ages. The most important of these other minerals are monazite, sphene (titanite) and baddeleyite. In addition, other minerals such as garnet can be used for U–Pb dating in particular circumstances.

 

            Monazite is a light rare earth element (LREE) phosphate which also incorporates significant amounts of Th and minor U. It is found in relatively Ca-poor and Al-rich granitoids and high grade metamorphic rocks. It co-exists with zircon but not with sphene in many of these rock types. Monazite can show similar Pb inheritance and Pb loss behaviour to zircon (Copeland et al., 1988), but its different chemistry means that these types of behaviour often occur under different conditions to those for co-existing zircon. Monazite has a lower blocking temperature than zircon (Dahl, 1997), which makes any inherited monazites in a crustal melt tend to lose their Pb during the melting event. Hence, monazites can be useful for dating aluminous granitoids with major zircon inheritance. Despite its lower blocking temperature, monazite seems to be more resistant to Pb loss during lower temperature events. This is probably because unlike zircon, monazite undergoes annealing at relatively low temperatures, thus healing radiation damage of the lattice (Smith and Giletti, 1997).

 

            These properties of monazite were applied by Scharer (1984) to date the Himalayan Makalu granite, whose zircon systematics were complicated by a combination of inherited Pb (Fig. 5.20) and Pb loss (Fig. 5.23). Monazites in this granite were not affected by these problems. However, a complication arose because the high Th content of monazite causes uptake of a significant content of the short-lived U-series isotope 230Th. This subsequently decays to 206Pb (Fig. 12.2a), causing an excess abundance of this isotope (Ludwig, 1977). Scharer demonstrated that a correction for this excess production caused apparently discordant analyses to fall properly on the concordia (Fig. 5.23), yielding a precise age of 24 " 1 Myr for crystallisation of the Makalu pegmatitic granite.  Several other applications were described by Parrish (1990), and details of the application of monazite to dating metamorphic events were described by Foster et al. (2002).

Fig. 5.23. Tera)Wasserburg concordia diagram for the Makalu leucogranite, Himalayas, showing zircon ( ! ) which has lost Pb, and monazite ( Q , # ) before and after correction for inherited U/Th disequilibrium. After Scharer (1984).

 

            The large Th content of most monazites also allows the possibility of Th–Pb dating. This has been applied quite widely by using the electron microprobe to determine total Th–Pb ‘chemical age dates’ (e.g. Montel et al., 1996). This method is based on the principle that Th abundances in monazite are so high that radiogenic 208Pb totally dominates over uranogenic and non-radiogenic Pb. Chemical dating can only be used as a reconnaissance technique because significant errors arise from uranogenic Pb. However, more accurate Th–Pb dating of monazite can be carried out using the ion microprobe.

 

            Unlike U–Pb dating, the Th–Pb dating method does not allow for an internal correction for Pb loss events. However, in situ depth profiling of monazite grains by ion microprobe can allow cooling curves to be determined based on Pb loss from the grain surface. Grove and Harrison (1999) demonstrated this technique on Tertiary monazites from the hanging wall of the Himalayan Main Central Thrust. By matching a model for diffusional Pb loss from the grain surface with the variation of Th–Pb ages against depth, Grove and Harrison were able to model the cooling history of the hanging wall since 12 Myr ago, the average Th–U age derived from the interiors of monazite grains. Additional applications of in situ monazite analysis were described by Catlos et al. (2002).

 

            Sphene is a titanium silicate (hence often called titanite) with similar properties to zircon and monazite. It has a somewhat lower blocking temperature (ca. 625 oC) than monazite (ca. 715 oC) and zircon (ca. 900 oC; Dahl, 1997). Therefore, sphene may remain open to Pb diffusion during high temperature cooling of metamorphic terranes. However, it is much less susceptible to low temperature Pb loss than zircon because it easily recrystallises, allowing annealing of radiation damage. Sphene was first applied as a dating tool by Tilton and Grunenfelder (1968) and has since been widely applied to studies of poly-metamorphic belts (e.g. Tucker et al., 1987). A good example is the combined use of zircon and sphene ages to date both the formation age and the Caledonian metamorphic age of gneisses from western Norway (Fig. 5.24).

Fig. 5.24. Concordia diagram for migmatite of the Western Gneiss Region (Norway), showing a discordia line defined by zircons ( ! ) which have suffered partial Pb loss, with a lower intercept anchored at the time of Caledonian metamorphism by sphene ( " ). After Tucker et al. (1987).

 

            Mafic igneous rocks have very low contents of zircon, monazite and sphene, and therefore have always been difficult to date accurately. However, Krogh et al. (1987) showed that the zirconium oxide mineral baddeleyite could be used as a dating tool in these rock types. (In pronouncing ‘baddeleyite’, it should be remembered that the mineral is named after Baddeley!) This method has since been widely applied to U–Pb dating of mafic rocks (e.g. Heaman and LeCheminant, 1993). In addition, French et al. (2002) have shown that total U–Pb analysis of baddeleyite using the electron microprobe can also be used as a reconnaissance dating tool for dyke swarms, when supported by conventional U–Pb dating of selected samples.

 

            Garnet is an important mineral in the geothermometry and barometry of metamorphic rocks. Therefore, the direct dating of this material would allow constrains to be placed on heating and cooling rates during regional metamorphism. Both the Sm–Nd and U–Pb systems have potential for dating metamorphic garnets, but they have different strengths and weaknesses. Garnets grown under amphibolite facies conditions (ca. 550 oC) can give concordant ages of prograde garnet growth from the two methods (section 4.1.4). However, Mezger et al. (1992) argued that the Sm–Nd system was opened at ca. 600 oC (upper amphibolite facies) in all but very large inclusion-free grains. Therefore, they suggested that garnet Sm–Nd ages usually date cooling rather than prograde mineral growth.

 

            Mezger et al. (1991) proposed a higher closure temperature of ca. 800 oC for the U–Pb system in garnet. However, this system suffers from the tendency for uranium to be concentrated in minute inclusions, rather than in the garnet lattice. For example, in the first study of this type, on the Pikwitonei granulite terrane in northern Manitoba, Mezger et al. (1989) attempted to use the U–Pb system to date prograde garnet growth during prolonged Late Archean metamorphism. After correction for a small common Pb component, radiogenic 206Pb*/238U and 207Pb*/235U ages were calculated using equations [5.7] and [5.8]. Unfortunately, most samples gave discordant ages for the two Pb systems, indicative of open system behaviour. In a later study, DeWolf et al. (1996) showed that this was due to Pb loss from micron-sized monazite inclusions in the garnet grains.

 

 

References

 

Abouchami, W. and Goldstein, S. L. (1995). A lead isotopic study of Circum-Antarctic manganese nodules. Geochim. Cosmochim. Acta 59, 1809)20.

 

Ahrens, L. H. (1955). Implications of the Rhodesia age pattern. Geochim. Cosmochim. Acta 8, 1)15.

 

Aleinikoff, J. N., Winegarden, D. L. and Walter, M. (1990). U)Pb ages of zircon rims: a new analytical method using the air-abrasion technique. Chem. Geol. (Isot. Geosci. Section) 80, 351)63.

 

Albarede, F. and Juteau, M. (1984). Unscrambling the lead model ages. Geochim. Cosmochim. Acta 48, 207)12.

 

Allegre, C. J., Manhes, G. and Gopel, C. (1995a). The age of the Earth. Geochim. Cosmochim. Acta 59, 1445–56.

 

Allegre, C. J., Poirier, J.-P., Humler, E. and Hofmann, A. W. (1995b). The chemical composition of the Earth. Earth Planet. Sci. Lett. 134, 515–26.

Alpher, R. A. and Herman, R. C. (1951). The primeval lead isotopic abundances and the age of the Earth’s crust. Phys. Rev. 84, 1111)14.

 

Amelin, Y., Krot, A. N., Hutcheon, I. D. and Ulyanov, A. A. (2002). Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. Science 297, 1678–83.

 

Appel, P. W. U., Moorbath, S. and Taylor, P. N. (1978). Least radiogenic terrestrial lead from Isua, west Greenland. Nature 272, 524)6.

 

Armstrong, R. L. (1968). A model for the evolution of Sr and Pb isotopes in a dynamic Earth. Rev. Geophys. 6, 175)99.

 

Burton, K. W., Ling, H.-F. and O’Nions, R. K. (1997). Closure of the Central American Isthmus and its effect on deep-water formation in the North Atlantic. Nature 386, 382–5.

 

Carl, C. and Dill, H. (1985). Age of secondary uranium mineralization in the basement rocks of the north eastern Bavaria F. R. G. Chem. Geol. (Isot. Geosci. Section) 52, 295)316.

 

Carl, C., Wendt, I. and Wendt, J. I. (1989). U/Pb whole-rock and mineral dating of the Falkenburg granite in northeast Bavaria. Earth Planet. Sci. Lett. 94, 236)44.

 

Catlos, E. J., Gilley, L. D. and Harrison, T. M. (2002). Interpretation of monazite ages obtained via in situ analysis. Chem. Geol. 188, 193–215.

 

Chapman, H. J. and Roddick, J. C. (1994). Kinetics of Pb release during the zircon evaporation technique. Earth Planet. Sci. Lett. 121, 601–11.

 

Chen, J. H. and Wasserburg, G. J. (1981). The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates. Earth Planet. Sci. Lett. 52, 1–15.

 

Chow, T. J. (1970). Isotopic identification of industrial pollutant lead. In: 2nd Int. Clean Air Congress, New South Wales Univ. Press, pp. 348–52.

 

Chow, T. J. and Earl, J. L. (1972). Lead isotopes in North American coals. Science 176, 510–11.

 

Chow, T. J. and Johnstone, M. S. (1965). Lead isotopes in gasoline and aerosols of Los Angeles Basin, California. Science 147, 502–3.

 

Chow, T. J. and Patterson, C. C. (1959). Lead isotopes in manganese nodules. Geochim. Cosmochim. Acta 17, 21–31.

 

Chow, T. J. and Patterson, C. C. (1962). The occurrence and significance of lead isotopes in pelagic sediments. Geochim. Cosmochim. Acta 26, 263–308.

 

Compston, W., Williams, I. S. and Meyer, C. (1984). U)Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass-resolution ion microprobe. Proc. 14th Lunar and Planet. Sci. Conf., J. Geophys. Res. 89 Supp., B525)34.

 

Copeland, P., Parrish, R. R. and Harrison, T. M. (1988). Identification of inherited radiogenic Pb in monazite and its implications for U–Pb systematics. Nature 333, 760–3.

 

Corfu, F. (1988). Differential response of U)Pb systems in coexisting accessory minerals, Winnipeg River Sub-province, Canadian Shield: Implications for Archean crustal growth and stabilization. Contrib. Mineral. Petrol. 98, 312)25.

 

Corfu, F. (2000). Extraction of Pb with artificially too-old ages during stepwise dissolution experiments on Archean zircon. Lithos 53 279)91.

 

Craig, H., Krishnaswami, S. and Somayajulu, B. L. K. (1973). 226Pb–226Ra: radioactive disequilibrium in the deep sea. Earth Planet. Sci. Lett. 17, 295–305.

 

Cumming, G. L. and Richards, J. R. (1975). Ore lead isotope ratios in a continuously changing earth. Earth Planet. Sci. Lett. 28, 155)71.

 

Dahl, P. S. (1997). A crystal-chemical basis for Pb retention and fission-track annealing systematics in U-bearing minerals, with implications for geochronology. Earth Planet. Sci. Lett. 150, 277–90.

 

Dasch, E. J., Dymond, J. R. and Heath, G. R. (1971). Isotopic analysis of metalliferous sediment from the East Pacific Rise. Earth Planet. Sci. Lett. 13, 175–80.

 

Davis, D. W. (1982). Optimum linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci. 19, 2141)9.

 

Davis, D. W. and Krogh, T. E. (2000). Preferential dissolution of 234U and radiogenic Pb from alpha-recoil-damaged lattice sites in zircon: implications for thermal histories and Pb isotopic fractionation in the near surface environment. Chem. Geol. 172, 41-58.

 

DeWolf, C. P. and Mezger, K. (1994). Lead isotope analyses of leached feldspars: constraints on the early crustal history of the Grenville Orogen. Geochim. Cosmochim. Acta 58, 5537–50.

 

DeWolf, C. P., Zeissler, C. J., Halliday, A. N., Mezger, K. and Essene, E. J. (1996). The role of inclusions in U–Pb and Sm–Nd garnet geochronology: stepwise dissolution experiments and trace uranium mapping by fission track analysis. Geochim. Cosmochim. Acta 60, 121–34.

 

Doe, B. R. and Stacey, J. S. (1974). The application of lead isotopes to the problems of ore genesis and ore prospect evaluation: a review. Econ. Geol. 69, 757)76.

 

Doe, B. R. and Zartman, R. E. (1979). Plumbotectonics: the Phanerozoic. In: Barnes, H. L. (Ed.) Geochemistry of Hydrothermal Ore Deposits. Wiley, pp. 22-70.

 

Drummond, M. S. and Defant, M. J. (1990). A model for trondhjemite-tonalite-dacite genesis and crustal growth via slab melting: Archean to modern comparisons. J. Geophys. Res. 95, 21 503–21 521.

 

Feng, R., Machado, N. and Ludden, J. (1993). Lead geochronology of zircon by Laser Probe ) Inductively Coupled Plasma Mass Spectrometry (LP)ICPMS). Geochim. Cosmochim. Acta 57, 3479)86.

 

Foley, S., Tiepolo, M. and Vannucci, R. (2002). Growth of early continental crust controlled by melting of amphibolite in subduction zones. Nature 417, 837–40.

 

Foster, G., Gibson, H. D., Parrish, R., Horstwood, M., Fraser, J. and Tindle, A. (2002). Textural, chemical and isotopic insights into the nature and behaviour of metamorphic monazite. Chem. Geol. 191, 183–207.

 

Frank, M. and O’Nions, R. K. (1998). Sources of Pb for Indian Ocean ferromanganese crusts: a record of Himalayan erosion? Earth Planet. Sci. Lett. 158, 121)30.

 

French, J. E., Heaman, L. M. and Chacko, T. (2002). Feasibility of chemical U–Th–total Pb baddeleyite dating by electron microprobe. Chem. Geol. 188, 85–104.

 

Froude, D. O., Ireland, T. R., Kinny, I. S., Williams, I. S. and Compston, W. (1983). Ion microprobe identification of 4,100)4,200 Myr-old terrestrial zircons. Nature 304, 616)8.

 

Galer, S. J. G. and Goldstein, S. L. (1996). Influence of accretion on lead in the Earth. In: Basu, A. and Hart, S. R. (Eds.) Earth Processes: Reading the Isotopic Code. Geophys. Monograph 95, American Geophysical Union, pp. 75–98.

 

Gentry, R. V., Sworski, T. J., McKown, H. S., Smith, D. H., Eby, R. E. and Christie, W. H. (1982). Differential lead retention in zircons: implications for nuclear waste containment. Science 216, 296)7.

 

Goldrich, S. S. and Mudrey, M. G. (1972). Dilatancy model for discordant U)Pb zircon ages. In: Tugarinov, A. I. (Ed.), Contributions to Recent Geochemistry and Analytical Chemistry. Moscow Nauka Publ. Office, pp. 415)8.

 

Griffin, W. L., Taylor, P. N., Hakkinea, J. W., Heier, K. S., Idea, I. K., Krogh, E. J., Malm, O., Olsen, K. I., Ormaasen, D. E. and Treten, E. (1978). Archaean and Proterozoic crustal evolution in Lofoten)Vesteraalen, Norway. J. Geol. Soc. Lond. 135, 629)47.

 

Grove, M. and Harrison, T. M. (1999). Monazite Th–Pb age depth profiling. Geology 27, 487)90.

 

Halliday, A. N. (1984). Coupled Sm)Nd and U)Pb systematics in Late Caledonian granites and the basement under northern Britain. Nature 307, 229)33.

 

Hamelin, B., Ferrand, J. L., Alleman, L., Nicolas, E. and Veron, A. (1997). Isotopic evidence of pollutant lead transport from North America to the subtropical North Atlantic gyre. Geochim. Cosmochim. Acta 61, 4423)8.

 

Harrison, R. M. and Laxen, D. P. H. (1981). Lead Pollution: Causes and Control. Chapman and Hall.

 

Heaman, L. M. and LeCheminant, A. N. (1993). Paragenesis and U–Pb systematics of baddeleyite (ZrO2). Chem. Geol. 110, 95–126.

 

Henderson, G. M. and Maier-Reimer, E. (2002). Advection and removal of 226Pb and stable Pb isotopes in the oceans: a general circulation model study. Geochim. Cosmochim. Acta 66, 257–72.

 

Hinton, R. W. and Long, J. V. P. (1979). High-resolution ion-microprobe measurement of lead isotopes: variations within single zircons from Lac Seul, Northwestern Ontario. Earth Planet. Sci. Lett. 45, 309)25.

 

Holmes, A. (1946). An estimate of the age of the Earth. Nature 157, 680)4.

 

Holmes, A. (1954). The oldest dated minerals of the Rhodesian Shield. Nature 173, 612)7.

 

Houtermans, F. G. (1946). Die isotopen-haufigkeiten im naturlichen blei und das alter des urans. Naturwissenschaften 33, 185)7.

 

Houtermans, F. G. (1947). Das alter des urans. Z. Naturforsch 29, 322)8.

 

Jacobsen, S. B. and Wasserburg, G. J. (1978). Interpretation of Nd, Sr and Pb isotope data from Archaean migmatites in Lofoten)Vesteraalen, Norway. Earth Planet. Sci. Lett. 41, 245)53.

 

Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. C. and Essling, A. M. (1971). Precision measurement of the half-lives and specific activities of U235 and U238. Phys. Rev. C 4, 1889)907.

 

Jahn B.-M. and Cuvellier, H. (1994). Pb–Pb and U–Pb geochronology of carbonate rocks: an assessment. Chem. Geol. (Isot. Geosci. Sect.) 115, 125–51.

 

Jones, C. E., Halliday, A. N. and Lohmann, K. C. (1995). The impact of diagenesis on high-precision U–Pb dating of ancient carbonates: an example from the Late Permian of New Mexico. Earth Planet. Sci. Lett. 134, 409–23.

 

Jones, C. E., Halliday, A. N., Rea, D. K. and Owen, R. M. (2000). Eolian inputs of lead to the North Pacific. Geochim. Cosmochim. Acta 64, 1405–16.

 

Kamber, B. S. and Collerson, K. D. (1999). Origin of ocean island basalts: a new model based on lead and helium isotope systematics. J. Geophys. Res. 104, 25 479–91.

 

Kamber, B. S. and Moorbath, S. (1998). Initial Pb of the Amitsoq gneiss revisited: implication for the timing of early Archean crustal evolution in West Greenland. Chem. Geol. 150, 19)41.

 

Kober, B. (1986). Whole-grain evaporation for 207Pb/206Pb - age investigations on single zircons using a double-filament thermal ion source. Contrib. Mineral. Petrol. 93, 482)90.

 

Kober, B. (1987). Single-zircon evaporation combined with Pb+ emitter bedding for 207Pb/206Pb - age investigations using thermal ion mass spectrometry, and implications to zirconology. Contrib. Mineral. Petrol. 96, 63)71.

 

Kober, B., Pidgeon, R. T. and Lippolt, H. J. (1989). Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia. Earth Planet. Sci. Lett. 91, 286)96.

 

Kramers, J. D. and Tolstikhin, I. N. (1997). Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust. Chem. Geol. 139 75)110.

 

Krogh, T. E. (1982a). Improved accuracy of U)Pb zircon dating by selection of more concordant fractions using a high gradient magnetic separation technique. Geochim. Cosmochim. Acta 46, 631)5.

 

Krogh, T. E. (1982b). Improved accuracy of U)Pb zircon ages by the creation of more concordant systems using the air abrasion technique. Geochim. Cosmochim. Acta 46, 637)49.

 

Krogh, T. E., Corfu, F., Davis, D. W., Dunning, G. R., Heaman, L. M., Kamo, S. L. and Machado, N. (1987). Precise U)Pb isotopic ages of diabase dykes and mafic to ultramafic rocks using trace amounts of baddeleyite and zircon. In: Halls, H. C. and Fahrig, W. F. (Eds) Mafic Dyke Swarms. Geol. Assoc. Canada Spec. Pap. 34, 147)52.

 

Krogh, T. E. and Davis, G. L. (1975). Alteration in zircons and differential dissolution of altered and metamict zircon. Carnegie Inst. Washington Year Book 74, 619–23.

 

Ling., H. F., Burton, K. W., O’Nions, R. K., Kamber, B. S., von Blankenburg, F., Gibb, A. J. and Hein, J. R. (1997). Evolution of Nd and Pb isotopes in Central Pacific seawater from ferromanganese crusts. Earth Planet. Sci. Lett. 146, 1)12.

 

Ludwig, K. R. (1977). Effect of initial radioactive daughter disequilibrium on U)Pb isotope apparent ages of young minerals. J. Res. US Geol. Surv. 5, 663)7.

 

Ludwig, K. R. (1998). On the treatment of concordant uranium-lead ages. Geochim. Cosmochim. Acta 62, 665–76.

 

Ludwig, K. R. (1999). Users’ Manual for Isoplot/Ex Version 2, A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Centre Spec. Pub. 1a. 47 p.

 

Ludwig, K. R. (2000). Decay constant errors in U-Pb concordia-intercept ages. Chem. Geol. 166 315)18.

 

Ludwig, K. R. (2001). Eliminating mass-fractionation effects on U-Pb isochron ages without double spiking. Geochim. Cosmochim. Acta 65, 3139–45.

 

Mattinson, J. M. (1987). U–Pb ages of zircons: a basic examination of error propagation. Chem. Geol. 66 151)62.

 

Mattinson, J. M. (1994). A study of complex discordance in zircons using step-wise dissolution techniques. . Contrib. Mineral. Petrol. 116, 117–29.

 

Mattinson, J. M. (2001). Multi-step high resolution Pb/U and Pb/Pb zircon age spectra: combined annealing, partial dissolution and TIMS analysis. Eos Trans. AGU 82 (47), Fall Meeting Suppl. Abstract V22C-1056.

 

Manhes, G., Allegre, C. J., Dupre, B. and Hamelin, B. (1979). Lead)lead systematics, the ‘age of the Earth’ and the chemical evolution of our planet in a new representation space. Earth Planet. Sci. Lett. 44, 91)104.

 

Mezger, K., Essene, E. J. and Halliday, A. N. (1992). Closure temperatures of the Sm–Nd system in metamorphic garnets. Earth Planet. Sci. Lett. 113, 397–409.

 

Mezger, K., Hanson, G. N. and Bohlen, S. R. (1989). U–Pb systematics of garnet: dating the growth of garnet in the Late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Manitoba, Canada. Contrib. Mineral. Petrol. 101, 136–48.

 

Mezger, K., Rawnsley, C. M., Bohlen, S. R. and Hanson, G. N. (1991). U–Pb garnet, sphene, monazite, and rutile ages: implications for the duration of high-grade metamorphism and cooling histories, Adirondack Mts., New York. J. Geol. 99, 415–28.

 

Montel, J.-M., Foret, S., Veschambre, M., Nicollet, C. and Provost, A. (1996). Electron microprobe  dating of monazite. Chem. Geol. 131, 37–53.

 

Moorbath, S., Taylor, P. N. and Goodwin, R. (1981). Origin of granite magma by crustal remobilisation:  Rb)Sr and Pb/Pb geochronology and isotope geochemistry of the late Archaean Qorqut Granite complex of southern West Greenland. Geochim. Cosmochim. Acta 45, 1051)60.

 

Moorbath, S. and Taylor, P. N. (1981). Isotopic evidence for continental growth in the Precambrian. In: Kroner, A. (Ed.), Precambrian Plate Tectonics. Elsevier, pp. 491)525.

 

Nier, A. O., Thompson, R. W. and Murphy, B. F. (1941). The isotopic constitution of lead and the measurement of geological time III. Phys. Rev. 60, 112)7.

 

O’Nions, R. K., Carter, S. R., Cohen, R. S., Evensen, N. M. and Hamilton, P. J. (1978). Pb, Nd and Sr isotopes in oceanic ferromanganese deposits and ocean floor basalts. Nature 273, 435–8.

 

Oversby, V. M. (1974). A new look at the lead isotope growth curve. Nature 248, 132)3.

 

Parrish, R. R. (1990). U–Pb dating of monazite and its application to geological problems. Can. J. Earth Sci. 27, 1431–50.

 

Patterson, C. C. (1956). Age of meteorites and the Earth. Geochim. Cosmochim. Acta 10, 230)7.

 

Pankhurst, R. J. and Pidgeon, R. T. (1976). Inherited isotope systems and the source region pre-history of early Caledonian granites in the Dalradian series of Scotland. Earth Planet. Sci. Lett. 31, 55)68.

 

Pidgeon, R. T. and Aftalion, M. (1978). Cogenetic and inherited zircon U-Pb systems in granites: Palaeozoic granites of Scotland and England. In: Bowes, D. R. and Leake, B. E. (Eds), Crustal Evolution in Northwestern Britain and Adjacent Regions. Geol. Soc. Spec. Issue 10, 183)220.

 

Potts, P. J. (1987). Handbook of Silicate Rock Analysis. Blackie. 622 p.

 

Rasbury, E. T., Hanson, G. N., Meyers, W. J. and Saller, A. H. (1997). Dating of the time of sedimentation using U–Pb ages for paleosol calcite. Geochim. Cosmochim. Acta 61, 1525–9.

 

Reynolds, P. H. and Dasch, E. J. (1971). Lead isotopes in marine manganese nodules and the ore-lead growth curve. J. Geophys. Res. 76, 5124–9.

 

Rogers, G., Dempster, T. J., Bluck, B. J. and Tanner, P. W. G. (1989). A high precision U)Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup. J. Geol. Soc. Lond. 146, 789)98.

 

Rosholt, J. N. and Bartel, A. J. (1969). Uranium, thorium and lead systematics in Granite Mountains, Wyoming. Earth Planet. Sci. Lett. 7, 141)7.

 

Rosman, K. J. R., Chisholm, W., Boutron, C. F., Candelone, J. P. and Gorlach, U. (1993). Isotopic evidence for the source of lead in Greenland snows since the late 1960s. Nature 362, 333–5.

 

Russell, R. D. (1956). Lead isotopes as a key to the radioactivity of the Earth’ s mantle. Ann. N. Y. Acad. Sci. 62, 435)48.

 

Russell, R. D. (1972). Evolutionary model for lead isotopes in conformable ores and in ocean volcanics. Rev. Geophys. Space Phys. 10, 529)49.

 

Russell, R. D. and Ahrens, L. H. (1957). Additional regularities among discordant lead-uranium ages. Geochim. Cosmochim. Acta 11, 213)18.

 

Russell, R. D. and Farquhar, R. M. (1960). Lead Isotopes in Geology. Interscience Pub., 243 p.

 

Scharer, U. (1984). The effect of initial 230Th disequilibrium on young U)Pb ages: the Makalu case, Himalaya. Earth Planet. Sci. Lett. 67, 191)204.

 

Scharer, U. and Allegre, C. J. (1982). Uranium-lead system in fragments of a single zircon grain. Nature 295, 585)7.

 

Scharer, U., Xu, R. H. and Allegre, C. J. (1984). U)Pb geochronology of Gangdese (Transhimalaya) plutonism in the Lhasa)Xigaze region, Tibet. Earth Planet. Sci. Lett. 69, 311)20.

 

Silver, L. T. and Deutsch, S. (1963). Uranium)lead isotopic variations in zircons: a case study. J. Geol. 71, 721)58.

 

Smith, H. A. and Giletti, B. J. (1997). Lead diffusion in monazite. Geochim. Cosmochim. Acta 61,  1047–55.

 

Smith, P. E. and Farquhar, R. M. (1989). Direct dating of Phanerozoic sediments by the 238U)206Pb method. Nature 341, 518)21.

 

Smith, P. E., Farquhar, R. M. and Hancock, R. G. (1991). Direct radiometric age determination of carbonate diagenesis using U)Pb in secondary calcite. Earth Planet. Sci. Lett. 105, 474)91.

 

Stacey, J. S. and Kramers, J. D. (1975). Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207)21.

 

Stanton, R. L. and Russell, R. D. (1959). Anomalous leads and the emplacement of lead sulfide ores. Econ. Geol. 54, 588)607.

 

Sturges, W. T. and Barrie, L. A. (1987). Lead 206/207 isotope ratios in the atmosphere of North America as tracers of US and Canadian emissions. Nature 329, 144–6.

 

Tatsumoto, M., Knight, R. J. and Allegre, C. J. (1973). Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206. Science 180, 1279)83.

 

Tatsumoto, M. and Patterson, C. C. (1963). The concentration of common lead in sea water. In: Geiss, J. and Goldberg, E. D. (Eds.), Earth Science and Meteoritics. North-Holland Pub. Co., pp. 74)89.

 

Taylor, P. N. (1975). An early Precambrian age for migmatitic gneisses from Vikan i Bo, Vesteraalen, North Norway. Earth Planet. Sci. Lett. 27, 35)42.

 

Taylor, P. N., Moorbath, S., Goodwin, R. and Petrykowski, A. C. (1980). Crustal contamination as an indicator of the extent of early Archaean continental crust: Pb isotopic evidence from the late Archaean gneisses of West Greenland. Geochim. Cosmochim. Acta 44, 1437)53.

 

Tera, F. and Carlson, R. W. (1999). Assessment of the Pb–Pb and U–Pb chronometry of the early solar system. Geochim. Cosmochim. Acta 63, 1877)89.

 

Tera, F. and Wasserburg, G. J. (1973). A response to a comment on U)Pb systematics in lunar basalts. Earth Planet. Sci. Lett. 19, 213)17.

 

Tera, F. and Wasserburg, G. J. (1974). U)Th)Pb systematics on lunar rocks and inferences about lunar evolution and the age of the Moon. Proc. 5th Lunar Sci. Conf. (Supp. 5, Geochim. Cosmochim. Acta) 2, 1571)99.

 

Tilton, G. R. (1960). Volume diffusion as a mechanism for discordant lead ages. J. Geophys. Res. 65, 2933)45.

 

Tilton, G. R. and Grunenfelder, M. H. (1968). Sphene: uranium–lead ages. Science 159, 1458–61.

 

Tucker, R. D., Raheim, A., Krogh, T. E. and Corfu, F. (1986/87). Uranium)lead zircon and titanite ages from the northern portion of the Western Gneiss Region, south-central Norway. Earth Planet. Sci. Lett. 81, 203–11.

 

van Breemen, O., Davidson, A., Loveridge, W. D. and Sullivan, R. W., (1986). U)Pb zircon geochronology of Grenville tectonites, granulites and igneous precursors, Parry Sound, Ontario. In: Moore, J. M., Davidson, A. and Baer, A. J. (Eds), The Grenville Province. Geol. Assoc. Canada Spec. Pap. 31, 191)207.

 

Vlastelic, I., Abouchami, W., Galer, S. J. G. and Hofmann, A. W. (2001). Geographical control on Pb isotope distribution and sources in Indian Ocean Fe)Mn deposits. Geochim. Cosmochim. Acta 65, 4303–19.

 

von Blanckenburg, F. and O’Nions, R. K. (1999). Response of beryllium and radiogenic isotope ratios in northern Atlantic deep water to the onset of Northern Hemisphere glaciation. Earth Planet. Sci. Lett. 167, 175–82.

 

von Blanckenburg, F., O’Nions, R. K. and Hein, J. R. (1996). Distribution and sources of pre-anthropogenic lead isotopes in deep ocean water from Fe)Mn crusts. Geochim. Cosmochim. Acta 60, 4957)63.

 

Watson, E. B. and Harrison, T. M. (1983). Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth Planet. Sci. Lett. 64, 295)304.

 

Wendt, I. (1984). A three-dimensional U)Pb discordia plane to evaluate samples with common lead of unknown isotopic composition. Isot. Geosci. 2, 1)12.

 

Wetherill, G. W. (1956a). An interpretation of the Rhodesia and Witwatersrand age patterns. Geochim. Cosmochim. Acta 9, 290)2.

 

Wetherill, G. W. (1956b). Discordant uranium)lead ages. Trans. Amer. Geophys. Union 37, 320)7.

 

Whitehouse, M. (1990). Isotopic evolution of the southern Outer Hebridean Lewisian gneiss complex: constraints on Late Archean source regions and the generation of transposed Pb)Pb palaeoisochrons.  Chem. Geol. (Isot. Geosci. Section) 86, 1)20.

 

Wilde, S. A., Valley, J. W., Peck, W. H. and Graham, C. M. (2001). Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409, 175–8.

 

Williams, I. S., Compston, W., Black, L. P., Ireland, T. R. and Foster, J. J. (1984). Unsupported radiogenic Pb in zircon: a cause of anomalously high Pb)Pb, U)Pb and Th)Pb ages. Contrib. Mineral. Petrol. 88, 322)7.

 

Wu, J. and Boyle, E. A. (1997). Lead in the western North Atlantic Ocean: completed response to leaded gasoline phase-out.  Geochim. Cosmochim. Acta 61, 3279–83.

 

Zartman, R. E. and Doe, B. R. (1981). Plumbotectonics ) the model. Tectonophys. 75, 135)62.

 

Zartman, R. E. and Haines, S. M. (1988). The plumbotectonic model for Pb isotopic systematics among major terrestrial reservoirs ) a case for bi-directional transport. Geochim. Cosmochim. Acta 52, 1327)39.