15.3 Xenon isotopes

Like terrestrial xenon, meteoritic xenon is an important isotopic tracer because it is the product of two independent decay routes. In addition, non-radiogenic xenon isotopes are of very low abundance, making radiogenic xenon from these decay routes easier to detect. Hence, xenon is a sensitive tracer of extinct radionuclide abundances.

15.3.1 I)Xe

The nuclide ¹²⁹I has a half-life of 16 Myr, and decays by $ emission to ¹²⁹Xe (Fig. 15.5). Wasserburg and Hayden (1955) searched for ¹²⁹Xe anomalies, but were unsuccessful. However, ¹²⁹Xe excesses were eventually demonstrated by Reynolds (1960), making this the first extinct nuclide to be ‘found’.

Fig. 15.5. Part of the chart of the nuclides in the region of iodine showing nucleosynthetic production and radioactive decay routes. Arrows labelled ‘r’ indicate nuclides generated entirely by the r-process.

If the excess ¹²⁹Xe signatures discovered by Reynolds were due to the decay of now-extinct ¹²⁹I in the meteorite samples, it would be expected that stable ¹²⁷I would still be present. Therefore, to test this model, it was necessary to look for correlations between the abundance of ¹²⁷I and excess ¹²⁹Xe in each sample. To do this by chemical analysis would have been labourious and inaccurate. However, Jeffrey and Reynolds (1961) conceived of an elegant means of measuring the ratio ¹²⁹Xe_excess/¹²⁷I in a single mass spectrometric analysis. By irradiating whole-rock samples of a meteorite with slow neutrons in a reactor, it was possible to generate the stable isotope ¹²⁸Xe from ¹²⁷I by the following n,( and $ decay reactions:

¹²⁷I + n 6 ¹²⁸I + (

¹²⁸I 6 ¹²⁸Xe + $ + <

Hence, the I/Xe ratio could be determined by isotopic analysis of xenon alone.

Jeffery and Reynolds made a further technical advance in their method of sample analysis. Rather than one-step outgassing of xenon from each meteorite sample by melting it (to produce a single data point), they outgassed the sample in a series of increasing temperature steps, admitting each new gas-release separately to the mass spectrometer for analysis. A very similar neutron irradiation and ‘step-heating’ mass spectrometric method was applied to K)Ar geochronology five years later, revolutionising it to the ⁴⁰Ar)³⁹Ar method (section 10.2).

It is convenient to display the xenon isotope data on a plot somewhat analogous to an Ar)Ar isochron diagram (section 10.2.3). Jeffrey and Reynolds demonstrated a correlation between ¹²⁹Xe and ¹²⁸Xe abundance in the Richardton chondrite, ratioing both of these against the non-radiogenic isotope ¹³²Xe (e.g. Fig. 15.6). If the efficiency of the activation process is calibrated, the excess ¹²⁸Xe abundance translates into the abundance of the non-radioactive iodine isotope, ¹²⁷I. Similarly, because every ¹²⁹I atom has by now been converted to ¹²⁹Xe by radioactive decay, the excess ¹²⁹Xe abundance translates into the ¹²⁹I abundance at the time when meteorite components were isolated from a common reservoir. Hence, the slope of any array of data points observed in this diagram (¹²⁹Xe_excess/¹²⁸Xe_excess) has no direct age significance. Instead, it indicates the initial ¹²⁹I/¹²⁷I ratio when the meteorite cooled to the point where its minerals became closed to diffusional loss of xenon into space. Meanwhile, the intercept of the correlation line on the y axis represents the initial ¹²⁹Xe/¹³²Xe ratio before the decay of extinct ¹²⁹I began.

Fig. 15.6. Xe)Xe plot for stepwise degassed samples of the Richardton meteorite showing the line of ‘iso-concentration’ of ¹²⁹I. Solid and open symbols indicate gas fractions released above and below 1100 ^oC respectively. After Hohenberg et al. (1967).

The slope of the array for the Richardton chondrite (Fig. 15.6) corresponds to an initial ¹²⁹I/¹²⁷I ratio of 1 H 10^!4. Subsequent work on a wide variety of meteorites has confirmed this value with only small variations. This suggests that ¹²⁹I was widely distributed through the solar nebula (e.g. Podosek, 1970; Wasserburg et al., 1977; Niemeyer, 1979). Excess ¹²⁹Xe is also found in terrestrial rocks and magmas, but in the Earth ,¹²⁹Xe abundances are not correlated with ¹²⁷I abundances. This observation is attributed to the outgassing of noble gases from a deep Earth reservoir which also once contained ‘live’ ¹²⁹I (section 11.4.1).

The ¹²⁹I/¹²⁷I ratio calculated from meteorite studies can be used as a ‘model age’ chronometer to measure the time-interval ()) between last nucleosynthesis and coalescence of the solar nebula. However, as with model ages in general, there are several major assumptions which must be made in any attempt to calculate a ) value. In particular, estimates must be made of the following quantities:

1) the ratio of ¹²⁹I/¹²⁷I originally produced by nucleosynthesis;

2) the rate of nucleosynthesis over time, prior to the ) period; and

3) in a granular model, any dilution of the last addition of radioactively ‘hot’ iodine by ‘cold’ iodine from earlier events.

These questions are best examined by comparing some extreme solutions which were summarised by Wasserburg (1985).

It has been traditionally assumed that iodine is generated by the r-process (Fig. 15.5). Production ratios (p₁₂₇/p₁₂₉) can only be determined by theoretical calculation; hence there are large uncertainties. However, they are generally assumed to be near unity. For example, values which have been used in the literature are unity (Wasserburg et al., 1960; Wasserburg, 1985), 1.3 (Cameron, 1962; quoted by Hohenberg, 1969) and 2.9 +1/!2 (Seeger et al., 1965; quoted by Schramm and Wasserburg, 1970).

The model originally conceived by Reynolds (1960) involved synthesis of iodine with a ¹²⁹I/¹²⁷I ratio of unity in a single event. This would decay to a ratio of 10^!4 over ca. 12 half-lives, yielding a maximum ) value of ca. 200 Myr (Fig. 15.7a). However, if all iodine was generated in a single supernova (i.e. zero dilution of ‘hot’ supernova iodine by ‘cold’ ¹²⁷I), then all other r-process elements would have to have formed at this time, which is incompatible with actinide evidence (as well as some short-lived extinct nuclides).

The other extreme model assumes more-or-less constant supernova activity throughout the lifetime galaxy. If their products were kept mixed then r-process production of solar-system material might be regarded as relatively constant (Dicke, 1969). Under this model, the total number of atoms of stable ¹²⁷I after time T (at the termination of nucleosynthesis, Fig. 15.1) is defined as:

n_T¹²⁷ = p¹²⁷ T [15.1]

where p is the average production rate. Similarly, the total number of ¹²⁹I atoms after time T can be approximated as:

n_T¹²⁷ = p¹²⁹ / 8 = p¹²⁹ @ mean life [15.2]

Dividing [15.2] by [15.1] we obtain:

(n¹²⁹) p₁₂₉ mean life

()))) = )) @ )))))) [15.3]

(n¹²⁷)_T p₁₂₇ T

Therefore, assuming a production ratio of unity in a model where iodine is formed by frequent and well-mixed supernovae over a period of 10 Byr, the ¹²⁹I/¹²⁷I ratio when nucleosynthesis is interrupted is 2.3 H 10^!3. This would take nearly five half-lives to decay to a value of 1 H 10^!4, yielding a ) value of ca. 80 Myr (Fig. 15.7b). However, this model has a major conceptual problem. It is very sensitive to the contributions of iodine from ‘late’ supernovae near the end of the nucleosynthetic period. If these form a significant fraction of the total iodine budget, then they are apt to de-stabilise the smooth growth model, giving rise to a ‘granular’ model (Wasserburg and Papanastassiou, 1982), as illustrated in Fig 15.7c.

In reality, consideration of the rate of supernova occurrence in the whole galaxy (about one every 100 years) relative to the size of the galaxy, suggests that in our corner of the galaxy, a granular model is almost inevitable, relative to the relatively short half-life of ¹²⁹I. In this case, the most critical quantity is the dilution factor for the last addition of hot iodine (¹²⁹I/¹²⁷I . 1) by cold or nearly-cold iodine from earlier events (¹²⁹I/¹²⁷I . ca. 0). A dilution factor of 100 has been proposed by Cameron and Truran (1977). Coupled with a production ratio of unity, this would yield a ) value of ca. 110 Myr. However, as the dilution factor approaches 10⁴, ) can approach zero. Not until the review of Wasserburg (1985) was it explicitly pointed out that such ‘extreme’ solutions are possible.

Fig. 15.7. Schematic illustration of iodine production models and consequent ) calculation. a) Single supernova event yielding maximum value of ) (ca. 200 Myr); b) constant ‘continuous’ production followed by a period ) of ca. 80 Myr; c) complex variation in production rate (‘granular model’) yielding indeterminate ) value. After Wasserburg and Papanastassiou (1982).

In addition to uncertainties about the dilution factor, there are also questions about the site of nucleosynthetic production of ¹²⁹I. Because iodine is not greatly separated from the s-process nucleosynthetic pathway, there remains a possibility that ¹²⁹I might be produced in less extreme environments than supernovae. Furthermore, it must not be forgotten that ¹²⁷I is certainly generated by the s-process, so that uncertainties about the relative s- and r-process contribution to total iodine production are also present. Therefore, ¹²⁹I cannot place tight constraints on the relative timing of nucleosynthesis and solar-system condensation (contrary to early claims). For this we must turn to other systems. However, ¹²⁹I may be useful in dating the very early evolution of solar-system objects (section 15.3.3).

15.3.2 Pu)Xe

²⁴⁴Pu has a half-life of 82 Myr. The clearest evidence of extinct ²⁴⁴Pu in meteorite materials is provided by fission products, most notably a large excess abundance of 132, 134 and 136 xenon, which has been matched to the signature of laboratory Pu fission products (Fig. 15.8). ²⁴⁴Pu is always compared to the abundance of other actinide elements in drawing conclusions about solar-system origins.

Fig. 15.8. Histogram of measured meteoritic Xe isotope abundance ratios relative to ¹³⁶Xe, compared to ratios observed in laboratory fission products (tie bars). After Wasserburg and Papanastassiou (1982).

²⁴⁴Pu is most conveniently ratioed against ²³⁸U, but this involves elemental as well as isotopic abundances, and the former are susceptible to chemical fractionation after condensation of the nebula. Hence, representative analysis has been very difficult. The first determination of the initial ²⁴⁴Pu/²³⁸U ratio, based on meteoritic phosphate (Wasserburg et al., 1969), gave a value of ca. 0.035. However, this value may not be representative of the bulk (whole-rock) meteorite due to partition effects. Whole-rock analysis of different meteorites has yielded a large range of values, but the best consensus is for a chondritic value around 0.007 (Fig. 15.9).

Fig. 15.9. Summary of Pu/U ratios for whole-rock samples of various meteorites (which are always named after their discovery site), relative to a model production ratio. ADOR = Angra dos Reis. After Hagee et al. (1990).

Plutonium data alone are not able to apply tight constraints to ), for the same reasons as were given for iodine. However, if we assume that ¹²⁹I and ²⁴⁴Pu were added at the same time, we can use the data together to constrain ). For example, a continuous model yields an equilibrium ²⁴⁴Pu/²³⁸U ratio of 0.018, which can decay to 0.007 after a ) period of ca. 100 Myr. This is in reasonable agreement with the continuous model for ¹²⁹I, which yields a ) value of 80 Myr. Similarly, if we assume a granular model in which both elements undergo 50-fold dilution of the last r-process addition with cold material, both plutonium and iodine yield ) values of ca. 130 Myr. Both of these results argue against very late addition of very dilute material (which was able to explain the iodine data alone). On the other hand, if ¹²⁹I is not an r-process nuclide, then its addition does not have to be accompanied by ²⁴⁴Pu, and these constraints disappear.

15.3.3 I)Xe chronology

Podosek (1970) used the slightly variable initial ¹²⁹I/¹²⁷I ratios of 0.7 H 10^!4 to 1.3 H 10^!4 measured in different meteorites to calculate the relative cooling times of different meteorites, by assuming that ‘hot’ and ‘cold’ iodine sources were homogenised in the solar nebula. It is not necessary to know the iodine isotope production ratio or the dilution factor of ‘hot’ by ‘cold’ iodine in the solar nebula for this calculation. However, to interpret the isotopic variations in terms of cooling times, it is necessary to assume initial ¹²⁹I/¹²⁷I homogeneity in the solar nebula. Crabb et al. (1982) argued instead that the variations in initial ratio are due to imperfect mixing of iodine from different sources (variable dilution factors). More recently, Podosek’s research group has at times felt the evidence to be equivocal (Bernatowicz et al., 1988), and at other times argued that the ‘dates’ have genuine age-significance (Swindle et al., 1991).

To solve these uncertainties it was necessary to test the accuracy of I–Xe ages by comparison with absolute dating methods. Until recently this was not possible because absolute dating methods (which measures ages back from the present) were not sufficiently precise to measure age differences of ca. 1 Myr in materials 4570 Myr old. However, the availability of new high-precision Pb–Pb ages (section 5.3.1) has allowed relative I–Xe ages to be anchored against absolute Pb–Pb ages and tested for their reliability.

Nichols et al. (1994) began this work by dating phosphate mineral separates (apatite) from the primitive achondrite Acapulco, whose Pb–Pb age is 4557 " 2 Myr (Gopel et al., 1994). Brazzle et al. (1999) continued this work by obtaining step heating I–Xe ages on apatite separates from three other chondrites also dated by Pb–Pb. The results showed excellent concordance between I–Xe and Pb–Pb ages (Fig. 15.10), demonstrating that the relative I–Xe ‘ages’ date real events. Since iodine is a volatile element, Brazzle et al. inferred that it could not have been incorporated into the refractory minerals that were first crystallised from the nebula. Therefore, it must have been adsorbed onto cooling surfaces and subsequently redistributed into secondary minerals (e.g. apatite) during very early post-formational metamorphism. It is this latter event that the I–Xe and Pb–Pb analysis of apatite is thought to be dating.

Fig. 15.10. Correlation diagram to test the concordancy of I–Xe and Pb–Pb ages on apatite separates from chondritic meteorites. I–Xe ages are given relative to Acapulco. After Brazzle et al. (1999).

In order to test the ability of the I–Xe method to date primary meteorite condensation, Gilmour et al. (2000) analysed individual chondrules and clasts from a variety of ‘ordinary’ (as opposed to carbonaceous) chondrites. Subsequently, Whitby et al. (2002) focussed in more detail on the enstatite chondrites Qingzhen and Kota Kota. Because the latter two chondrites are ‘unequilibrated’, they have a relatively simple history that should allow the most precise dating of their primary condensation. (The step heating method used for all I–Xe analyses can exclude the effects of late low-temperature metamorphism and shock events that most meteorites have suffered). On the other hand, the lack of equilibration of the enstatite chondrites means that there was less opportunity to homogenise any primordial iodine isotope variations between chondrules. The analysis of individual chondrules therefore provides more control on the degree of iodine isotope homogeneity in the early solar-system, compared with the earlier data from whole-rock chondrites.

Results from this study are shown in Fig. 15.11 along with results from other studies on single chondrules, summarised by Swindle et al. (1996). Ages are quoted relative to whole-rock analyses of the chondrite Bjurbole, which has traditionally been used as an irradiation standard. The results show that the meteorites Chainpur and Semarkona yield scattered chondrule ages, attributed to shock disturbance or aqueous alteration (Swindle et al., 1996). However, the analyses from Qingzhen, Kota Kota and Allende, along with most of the new Bjurbole analyses, gave more consistent results.

Fig. 15.11. I–Xe ages for individual chondrules for several meteorites relative to whole-rock samples of the standard Bjurbole. The achondrite Shallowater is shown for reference. Modified after Whitby et al. (2002).

Gilmour et al. (2000) and Whitby et al. (2002) argued that the consensus of chondrule ages similar to Bjurbole was dating the primary cooling of chondrules shortly after their formation, and was not the result of later resetting. However, they also noted that these ages overlap with the achondrite Shallowater. The latter object also gives consistent I–Xe results, leading to its recent adoption as a standard for I–Xe analysis (e.g. Brazzle et al., 1999). Because whole-rock samples of Shallowater are only 0.6 Myr younger than Bjurbole, this would imply that the differentiation of small planetessimals (as represented by the achondrites) was already occurring at the same time as primary chondrule formation. Comparison with other extinct nuclide geochronometers may be used to test this model (section 15.5.2).

More surprising recent results come form the I–Xe analysis of halite (sodium chloride) from the H (high metal) chondrite ‘Zag’. This material yields an I–Xe age 5 Myr older than Bjurbole and Shallowater, implying that aqueous fluids were present on Zag at an extremely early time. It also implies that the halite on Zag predates Calcium–Aluminum inclusions, see below, which are usually regarded as the oldest solar-system objects. However, the I–Xe step heating results for Zag indicate some disturbance, so the result may be unreliable as a true indication of age (e.g. Ott, 2000).

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