13 U-series geochemistry of igneous systems

U-series dating of sedimentary rocks was discussed in the previous chapter. These isotopes can also be used as dating tools for igneous rocks; however, their application as isotopic tracers is probably more important. The short half-lives of the decay series nuclides make them ideally suited to studies of magma segregation from the mantle and magma evolution in the crust, since these processes operate over similar time periods. With a half-life of 75.4 kyr, ²³⁰Th is by far the most important of these geological tracers, and will be the main focus of this chapter. However, consideration will also be given to other shorter-lived isotopes used in conjunction with thorium. Note that all isotopic abundances of U-series nuclides referred to in this chapter are expressed as activities, unless specifically stated to be atomic.

Until recently, all U-series measurements on igneous rocks were made by " spectrometry, as for sedimentary rocks (section 12.2). Following the application of mass spectrometry to U-series dating of carbonates, it was quickly applied to igneous systems (Goldstein et al., 1989). However, analysis of ²³⁰Th in silicate rocks is made more difficult by their very large atomic ²³²Th/²³⁰Th ratios. For example, a basalt with a typical Th/U concentration ratio of 4, and in secular equilibrium, will have a ²³²Th/²³⁰Th atomic ratio of 240 000. For a single-sector mass spectrometer with an abundance sensitivity of 1 ppm at 2 a.m.u. (section 2.3.1), the ²³²Th peak will then generate a peak tail at mass 230 which is one-quarter of the size of the ²³⁰Th peak. Normally, the abundance sensitivity is improved using an energy filter (e.g. Cohen et al., 1992; section 2.3.1). However, McDermott et al. (1993) showed that relatively accurate ²³⁰Th data can be obtained without a filter if the exponentially curved baseline shape is carefully interpolated under the ²³⁰Th peak.

In addition to ²³⁰Th, there are five shorter-lived nuclides in the U and Th decay schemes which may be useful in the study of igneous systems. These are shown in Fig. 12.2, but for convenience they are summarised here. The equilibration times shown (t_Eq) represent the maximum useful range of each species, based on the assumption that its activity will be within error of secular equilibrium after five half-lives. Each arrow represents a decay transition, but only nuclides relevant to volcanic systems are shown.

²³⁸U > > > > ²³⁰Th > ²²⁶Ra > > > > > > ²¹⁰Pb > > > ²⁰⁶Pb

t_1/2, yr 75 200 1600 22

t_Eq, yr 400 000 8000 100

²³⁵U > > ²³¹Pa > > > > > > > > > ²⁰⁷Pb

t_1/2, yr 34 300

t_Eq, yr 170 000

²³²Th > ²²⁸Ra > > ²²⁸Th > > > > > > > ²⁰⁸Pb

t_1/2, yr 5.77 1.91

t_Eq, yr 30 10

Disequilibrium between short-lived nuclei in volcanic rocks was discovered very early (e.g. Joly, 1909), but has only recently been the subject of detailed study. ²¹⁰Pb was observed to be out of secular equilibrium with ²³⁰Th in ocean island lavas by Oversby and Gast (1968). This demonstrated the occurrence of Th/Pb fractionation within 100 yr of eruption. However, the chemistry of Pb is so different from the other nuclides that it is difficult to use in petrogenetic interpretations. It will not be discussed in detail here. ²²⁸Ra and ²²⁸Th can be used to measure even shorter-period changes in magma chemistry, but have only rarely been found out of isotopic equilibrium.

Of the other short-lived nuclides, ²²⁶Ra has been the most widely used. It has traditionally been measured by radioactive counting (sometimes via its shorter-lived decay products), but despite its short half-life, recent advances have allowed its measurement by mass spectrometry (e.g. Cohen and O’Nions, 1991). This permits ²²⁶Ra abundances in the femtogram range (10^!9 ppm) to be determined to better than 1% precision. ²³¹Pa abundances in igneous rocks are too low to measure by " spectrometry because of the low abundance of the parent isotope, ²³⁵U. However, with the application of mass spectrometry, ²³¹Pa measurements are also possible in the femtogram range. Hence this nuclide also shows promise as a useful geochemical and chronological tool (Goldstein et al., 1993).

13.1 Geochronology of volcanic rocks

In some ways, U-series systems in igneous rocks are simpler than carbonates, because ²³⁴U and ²³⁸U are always effectively in secular equilibrium. On the other hand, igneous systems are more complex than pure carbonates in that they invariably contain initial Th at the time of cooling. Hence, a U)Th isochron diagram must normally be used to date igneous rocks by the ²³⁰Th method.

13.1.1 The U)Th isochron diagram

After time t, the net ²³⁰Th activity in a silicate sample is the sum of ²³⁰Th growth from U decay and the residue of partially-decayed initial ²³⁰Th. In other words we sum equations [12.8] and [12.24]:

²³⁰Th_P = ²³⁰Th_I e^{!8230 t} + ²³⁸U (1 ! e^{!8230 t}) [13.1]

It is convenient to divide through by ²³²Th, whose activity is effectively constant between t initial and the present:

(²³⁰Th) (²³⁰Th) ²³⁸U

()))) = ()))) e^{!8230 t} + ))) (1 ! e^{!8230 t}) [13.2]

(²³²Th)_P (²³²Th)_I ²³²Th

This is the equation for a straight line, and is plotted on a diagram of ²³⁰Th/²³²Th activity ratio against ²³⁸U/²³²Th activity ratio (Fig. 13.1) which is analogous to the Rb)Sr isochron diagram. As with the Rb)Sr system, a suite of cogenetic samples of the same age define a linear array whose slope yields the age. However, because the ²³⁰Th daughter product is itself subject to decay, this leads to more complex isotope systematics.

The evolution of igneous systems on the U)Th isochron diagram depends on their composition with respect to a state of secular equilibrium. Samples in secular equilibrium must, by definition, have equal ²³⁰Th and ²³⁸U activities. Hence, they must have equal ²³⁰Th/²³²Th and ²³⁸U/²³²Th activity ratios in Fig. 13.1. Such samples lie on a slope of unity in this diagram, called the ‘equiline’ by Allegre and Condomines (1976).

Fig. 13.1. Isotopic evolution of igneous rocks on the ²³⁰Th/²³²Th versus ²³⁸U/²³²Th isochron diagram. Symbols: ( " ): time t = 0; (half-filled): samples after elapsed time t; ( ! ): samples after effectively infinite time (t >> 1/8).

Now, considering a suite of rock or mineral samples; at the time of their crystallisation they have variable U/Th ratios but a constant (initial) ²³⁰Th/²³²Th activity ratio, forming a horizontal line in Fig. 13.1. The point of intersection of this array with the equiline must by definition remain invariant, since it starts its closed-system evolution already in secular equilibrium. However, all other samples in the suite evolve with time. Those to the right of the invariant point are daughter (²³⁰Th) deficient relative to ²³⁸U, and ²³⁰Th therefore builds up with time. They move vertically upwards until they also reach the equiline (secular equilibrium). Those to the left of the equiline have daughter (²³⁰Th) excess relative to the parent. They move vertically downwards with time until they reach the equiline. The more initial thorium that is present, the higher the intersection between the initial composition and the equiline. Conversely, when no initial Th is present (as in pure carbonates), evolution begins along the x axis. Hence, in a co-genetic suite which has not yet reached secular equilibrium, the initial Th isotope ratio is given by the intersection of the isochron array with the equiline. It is not the intercept on the y axis.

The U–Th isochron method was first applied to the dating of igneous minerals by Cerrai et al. (1965), and subsequently tested by Kigoshi (1967). Kigoshi used the method to date three igneous rocks of different ages: a Cretaceous granite (effectively of infinite age), a 35.7 kyr-old pumice (dated by ¹⁴C on a wood inclusion) and a lava of historical (effectively zero) age. His results (Fig. 13.2) demonstrated the method to be effective. The old granite samples lie on the equiline, the pumice samples yield a U)Th isochron age of 38 kyr, and the historical lava yields a zero slope.

Fig. 13.2. ²³⁰Th/²³²Th versus ²³⁸U/²³²Th isochron diagram for three suites of leachates from whole-rock samples. After Kigoshi (1967).

To achieve a high-precision age from the U)Th method, a reasonable spread of ²³⁸U/²³²Th activity ratios is needed within each sample suite. Kigoshi carried over the leaching techniques of carbonate dating in order to maximise this spread of U/Th activity ratios. However, this is a potentially dangerous technique, since in the leaching process disequilibrium may be introduced between different parents and daughters in the decay series. For example, ²³⁰Th may be preferentially leached relative to ²³⁸U from radiation-damaged lattice sites, yielding spuriously old ages. This is called the ‘hot atom effect’, and is the same process that gives rise to variable ²³⁴U/²³⁸U ratios in natural waters (section 12.3.1).

Taddeucci et al. (1967) avoided the complexities of the ‘hot atom’ effect by using conventional physical separation and total dissolution of minerals to date five rhyolitic tuffs from the Mono Craters of California. However, U)Th dates on phenocryst)glass pairs did not display good agreement with other methods. For example, the hornblende)glass pair gave an apparent U)Th age of only 1 kyr, whereas K)Ar dating yielded an age of ca. 7 kyr, and ¹⁴C dating of undisturbed lake sediments near the volcano gave a minimum eruptive age of 2.2 kyr.

Allegre (1968) subsequently showed that separated phenocryst phases from one of the rocks studied by Taddeucci et al. defined an isochron age of 25 kyr (Fig. 13.3). The implication of these results is that the different analytical methods are dating different events. U)Th ages on phenocryst minerals probably date their crystallisation in a magma chamber, whereas the K)Ar method dates the time of eruption (if outgassing of volatiles during eruption was effective). The hornblende)glass age is meaningless, since these two systems did not close at the same time. The discordance between dating methods is therefore caused by the relatively long residence period of magma in the chamber, after phenocryst formation.

Fig. 13.3. ²³⁰Th/²³²Th versus ²³⁸U/²³²Th isochron diagram for hornblende)olivine)quartz phenocryst assemblages and glass matrix from a Mono Craters rhyolite (California), showing isotopic discordancy between phenocrysts and glass. After Allegre (1968).

In contrast to the Mono Craters case, Allegre and Condomines (1976) and Condomines and Allegre (1980) were able to achieve good linearity of phenocryst and whole-rock points in dating studies of the Irazu volcano (Costa Rica) and Stromboli volcano (Italy). This implies that in these systems phenocryst growth only briefly preceded eruption. On the other hand, Capaldi and Pece (1981) claimed to find gross Th isotope disequilibrium between different mineral phases in modern lavas from Etna, Vesuvius and Stromboli. This led Capaldi et al. (1982) to completely write off the U)Th method as a dating tool. However, in repeat analyses of the samples from the same Etna and Vesuvius lavas, Hemond and Condomines (1985) were unable to find mineralogical disequilibrium of Th isotope ratios. This suggests that Capaldi et al. (1982) over-reacted when they dismissed the method. It is true that there are quite a number of instances where Th isotope disequilibrium has been found on a mineralogical scale (Capaldi et al., 1985). However, if phenocryst phases are screened by petrographic examination to exclude entrained xenocrysts then many young lavas can yield accurate U)Th crystallisation ages (e.g. Condomines et al., 1982).

The analysis of zircon as a mineral phase for U–Th dating was proposed in very early work (e.g. Cerrai et al., 1965), but not exploited much due to the low abundances of zircons in most rocks. However, the high uranium content of zircons often generates high U/Th ratios, which can yield good isochron fits. Hence, there has been renewed interest in U–Th dating of zircons with the development of mass spectrometric methods for the analysis of small samples.

Condomines (1997) used conventional TIMS analysis to demonstrate the usefulness of U–Th zircon analysis on a young trachytic rock in the Puy de Dome area of France. In this sample, the zircon analysis lay on the same isochron as several other mineral phases, and the isochron slope was consistent with the known age of eruption from other methods. However, more commonly, U–Th dating of zircon has produced more complex age relationships. For example, Charlier and Zellmer (2000) found that zircon fractions from a rhyolite in the Taupo Volcanic Zone (New Zealand) gave variable whole-rock/zircon ages, depending on the size fraction of the zircon analysed. This suggested that the zircon grains were probably zoned, with cores about 27 kyr older than the age of eruption, overgrown by rims only 2 kyr older than the age of eruption.

Problems of internal zircon heterogeneity can be avoided by using in situ micro-analysis. This was first demonstrated by Reid et al. (1997), using the ion microprobe to analyse zircons from the Long Valley Caldera (California). Zircons were analysed from two domes, one 115 kyr old and one less than 1 kyr old, located a few km apart. Each zircon analysis was combined with the whole-rock composition to determine a two-point isochron that Reid et al. termed a ‘zircon model age’. These model ages were somewhat scattered, due to a combination of relatively large analytical errors and some geological scatter. However, it was observed that the average zircon model age for the two domes was the same (Fig. 13.4), despite their different eruption ages. This suggests that the two domes might be sampling the same relatively long-lived magma chamber, which cooled through the zircon saturation temperature about 230 kyr ago, causing the crystallisation of a crop of zircon grains that subsequently remained entrained in the viscous magma. In view of the ability of ion microprobe analysis to analyse many grains from complex heterogeneous systems, this would appear to be a technique with strong potential for studying the evolution of shallow felsic magma chambers.

Fig. 13.4. Comparison of ‘zircon model ages’ in two domes of different eruption age from the Long Valley Caldera, California. Open symbols were omitted from the calculated averages. Errors are 1F. After Reid et al. (1997).

13.1.2 Ra–Th isochron diagrams

In view of its short equilibration time of 8000 yr, ²²⁶Ra is useful in studies of geologically rapid magmatic processes. However, a disadvantage is the lack of a longer-lived radium isotope to normalise against, in order to exclude chemical fractionation. Williams et al. (1986) proposed that this problem might be overcome by using barium as a proxy for a stable radium isotope. For this to be useful, the two elements must have similar distribution coefficients, so that they behave in the same way during partial melting and crystal fractionation.

If barium is an accurate analogue for stable radium, the Th)Ra/[Ba] method can be used in conventional isochron dating of magma fractionation events. Reagan et al. (1992) applied this method to the dating of anorthosite phenocrysts in phonolitic magmas of Mount Erebus volcano, Antarctica. The data are used here to demonstrate the use of the Th)Ra/[Ba] isochron diagram for radium (which is analogous to the U)Th isochron diagram). This is based on the equation:

(²²⁶Ra)_P (²²⁶Ra)_I ²³⁰Th

))))) = ))))) e^{!8226 t} + )))) (1 ! e^{!8226 t}) [13.3]

[ Ba ] [ Ba ] [ Ba ]

where square brackets denote concentrations. This is analogous to equation [13.2] for the U)Th system.

Because anorthoclase readily takes up divalent but not trivalent ions, it has Th/Ba ratios of effectively zero. The isochron age is then determined by the glass points, yielding crystallisation ages of ca. 2.5 kyr for samples from two recent eruptions (Fig. 13.5). However, it may be dangerous to rely on two-point phenocryst)glass ages without other supporting evidence (section 13.1.1). A more complete example of a Th)Ra/[Ba] isochron was provided by Schaefer et al. (1993) on the 1985 pumice eruption of Nevado del Ruiz volcano, Columbia. In this case the glass point was colinear with three different mineral phases (and the whole-rock), yielding a best-fit age of 6.1 " 0.5 kyr. This result was interpreted as the average age of an extended period of crystal fractionation, rather than a discrete magmatic differentiation event.

Fig. 13.5. Th)Ra/[Ba] isochron diagram for anorthosite)glass pairs from the 1984 and 1988 phonolite eruptions of Mt. Erebus, Antarctica. After Reagan et al. (1992).

Ra)Th isotope data can also be presented on an ‘alternative’ isochron diagram that was first used by Kaufman (section 12.4.3) for dating dirty calcite. We will take the opportunity here to examine this format and compare it with the conventional isochron plot, using data for MORB glasses from the East Pacific Rise (Rubin and Macdougall, 1990).

If a magma suite is extracted from a source in secular equilibrium and with homogeneous trace element chemistry, then all zero-age lavas should have a constant ratio of ²²⁶Ra activity/Ba concentration. This is equivalent in the alternative isochron diagram to a straight line through the origin (Fig. 13.6). Distribution along the line is due to fractionation of ²²⁶Ra and Ba relative to Th, which is presumed to have occurred during partial melting. The intersection of this zero-age fractionation line with the equiline defines the Ba/Th ratio of the source (on the x axis). After separation from the source, magmas evolve vertically towards the equiline. Since the half-life of ²²⁶Ra is much less than its parent (²³⁰Th), the latter can be considered as effectively stable over the time periods under consideration (< 10 kyr). Therefore, for a theoretical sample with zero initial thorium, the decay equation is as follows:

²²⁶Ra

)))) = 1 ! e^{!8226 t} [13.4]

²³⁰Th

Therefore, ages are given by the intersection of isochrons on the y axis. However, since all samples have Ba/Th ratios greater than the source, isotopic evolution is by decay of excess ²²⁶Ra. Therefore, it is convenient to show the ages as isochron lines for a given source composition (Fig. 13.6).

Fig. 13.6. Ra/Th)[Ba/Th] Alternative isochron diagram for MORB glasses from the East Pacific Rise. For discussion, see text. After Rubin and Macdougall (1990).

A problem with this study is that somewhat abnormal U)Th analyses were obtained on the same samples. However, the radium data have subsequently been confirmed by mass spectrometric ²²⁶Ra analysis of MORB glasses from the Juan de Fuca, Gorda, and East Pacific ridges (Volpe and Goldstein, 1993). Glasses from the ridge axis yield excess ²²⁶Ra activities up to 2.5 times greater than the ²³⁰Th activity. In contrast, off-axis glasses from these ridges yield ²²⁶Ra/²³⁰Th activity ratios of 1.00, as expected from their greater age. This gives us confidence that radium has not been mobilised in these glasses by sea-floor alteration, and that the disequilibrium data reflect processes of magma generation and evolution.

13.1.3 U-series model age dating

The application of ‘zircon model ages’ to igneous systems was discussed in section 13.1.1. However, the first application of U-series model ages was actually to the dating of MORB samples. It is not practical to date these using the U)Th isochron technique, since MORB glasses have a narrow range of U)Th ratios, and phenocryst phases are rare. However, the consistency of mass spectrometric Th isotope data from the crest of the Juan de Fuca and Gorda ridges prompted Goldstein et al. (1991) to apply a model age method as a dating tool for young off-axis MORB glasses. In this approach the initial activity ratio of the off-axis samples is estimated by analysing on-axis samples from the same ridge segment. Goldstein et al. demonstrated this approach for three ridge segments, of which two Juan de Fuca ridge segments are shown in Fig. 13.7.

Fig. 13.7. Plot of U)Th model ages compared with distance from the axis of the Juan de Fuca ridge, East Pacific: (a) the Endeavour segment; and (b) the Southern segment. Magnetically determined spreading rates are shown by dashed lines. Open symbols indicate abnormally young ages. After Goldstein et al. (1991).

In order to be sure that these older MORB glasses remained as closed systems for U and Th, and were not affected by sea-floor alteration, the samples were subjected to chemical screening, in addition to the normal processes of hand picking and surface leaching of glass chips (Goldstein et al., 1989; 1991). Sensitive chemical screening was provided by boron analysis, since fresh glasses have boron levels of ca. 1 ppm, whereas altered glasses have boron contents more than an order of magnitude higher (Spivak and Edmond, 1987). In addition, samples were analysed for ²³⁴U/²³⁸U activities, since alteration may cause this ratio to increase or decrease (Macdougall et al., 1979). However, this test is probably less sensitive, since seawater has a ²³⁴U/²³⁸U activity ratio only slightly above unity.

Goldstein et al. (1991) compared model U)Th ages from samples on both sides of the ridge axis with the spreading rate determined from magnetic data. Generally, the fit was found to be quite good, although some asymmetry is apparent in the U)Th ages. Because both ridges show the same sense of asymmetry, it is possible that this is a real feature of the spreading geometry. However, there are presently insufficient data for one to be confident of this interpretation. On the other hand, complexity in the age structure of the ridge is suggested by low apparent U)Th ages just below the summit of the Endeavour segment (open symbols in Fig. 13.7) . Goldstein et al. attributed these ages to young lavas that were erupted near the ridge summit and then flowed down over its flanks.

Volpe and Goldstein (1993) showed that within a given ocean ridge segment, ²³⁰Th/[Ba] ratios were constant in lavas of different ages (below 10 kyr). This implies that elemental fractionation effects were constant over this time. Hence, it is also reasonable to assume that constant ²²⁶Ra/²³⁰Th ratios pertained at the time of eruption. In this case, ²²⁶Ra/²³⁰Th ratios can be used to determine model ages for very young ocean ridge basalts, in a similar way to the use of U)Th data. Based on this approach, the variation of ²²⁶Ra/²³⁰Th ratios in a suite of lavas from the axial valley of the Juan de Fuca ridge allowed the calculation of relative age differences of up to 1200 yr between different eruptions.

The first precise ²³¹Pa analyses of igneous rocks were made by Goldstein et al. (1993) on MORB glasses from the East Pacific Rise and the Juan de Fuca and Gorda ridges. On-axis samples yielded ²³¹Pa/²³⁵U activity ratios of up to 2.9, whereas off-axis samples returned to secular equilibrium over ca. 150 kyr. Protactinium data cannot be plotted on an isochron diagram because no suitable stable analogue to ²³¹Pa has yet been identified. However, Goldstein et al. argued that within single ridge segments, U)Pa model ages could be determined in a manner analogous to U)Th and Th)Ra model ages. By assuming a zero age in the sample from each axial valley with highest ²³¹Pa/²³⁵U activity, Goldstein et al. calculated model ages for other axial and off-axis samples that were within error of their U)Th model ages.

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