15.4 Very short-lived species

15.4.1 Al)Mg

The nuclide ²⁶Al has a half-life of 0.72 Myr and decays to ²⁶Mg. The discovery of extinct ²⁶Al was a much more difficult task than ¹²⁹I, and was only made possible by the fall of the Allende carbonaceous chondrite in February 1969. Allende is an agglomerate of fine-grained debris and chondrules which also contains ‘inclusions’ with a refractory mineralogy which appear to be very early condensation products from the solar nebula. These are normally referred to as Calcium–Aluminum inclusions or ‘CAIs’.

Analysis of Mg isotope ratios in minerals separated from Allende inclusions demonstrates mass-fractionation-dependent variations in ²⁵Mg/²⁴Mg ratio. These are normally of the order of a few parts per mil (Fig. 15.12), but some inclusions (e.g. EK1-4-1 and C1) show larger effects. These were termed FUN samples (showing Fractionation and Unknown Nuclear anomalies) by Wasserburg et al. (1977). Because ²⁷Al was always a nuclide of comparatively low abundance, it was at first very difficult to demonstrate radiogenic ²⁶Mg abundances outside error of mass fractionation processes. The first such evidence for radiogenic ²⁶Mg was demonstrated by Lee et al., (1976). Subsequently, larger ²⁶Mg anomalies were found by very careful hand-picking of inclusion- and alteration-free plagioclase grains from the WA Allende inclusion. Because these grains have very high Al/Mg ratios they provide the best chance of finding ²⁶Mg anomalies. This search revealed an excess ²⁶Mg abundance of 97 parts per mil in one anorthite grain (Lee et al., 1977).

Fig. 15.12. ²⁵Mg/²⁴Mg versus ²⁶Mg/²⁴Mg isotope diagram showing deviations of Allende inclusions from the normal solar-system value, in parts per mil (*). These may be due to mass fractionation in the solar nebula (open symbols) or decay of extinct ²⁶Al (solid symbols). After Wasserburg and Papanastassiou (1982).

It is most convenient to display Al)Mg data on an isotope ratio versus element ratio plot somewhat analogous to an isochron diagram (e.g. Fig. 15.13). Hence, ²⁶Mg/²⁴Mg is plotted against an Al/Mg ratio. In a conventional isochron diagram, the ratio plotted on the abscissa would be ²⁶Al/²⁴Mg. However, since every ²⁶Al atom has been converted to ²⁶Mg by radioactive decay at the present day, the ²⁷Al/²⁴Mg ratio is plotted instead. Therefore, the slope of any array of data points observed in this diagram has no direct age significance, but indicates the initial ²⁶Al/²⁷Al ratio in the sample suite at the time when sub-systems were isolated from a common reservoir. Hence, these arrays will be called pseudochrons.

Fig. 15.13. Plot of *Mg against Al/Mg for the Allende EGG-3 inclusion showing best-fit line of constant initial ²⁶Al/²⁷Al ratio with a value of 4.9 H 10^!5. After Armstrong et al. (1984).

The data shown in Fig. 15.13 for separated minerals from the Allende EGG-3 inclusion display a good ²⁶Mg/²⁷Al correlation, yielding a well-defined initial ²⁶Al/²⁷Al ratio of 4.9 H 10^!5 (Armstrong et al., 1984). A similar array for the WA inclusion gave a ²⁶Al/²⁷Al ratio of 5.1 " 0.6 H 10^!5, (Lee et al., 1977). However, in the inclusion USNM 3529)26 (Armstrong et al., 1984), minerals from different parts of the inclusion gave different initial ratios, with a higher value in the core (3.8 H 10^!5) than the rim (2.3 H 10^!5). This spatial variation is best explained by Mg loss, particularly from the margins of the inclusion, during a later metamorphic event. The event may have resulted from the heat output of ²⁶Al decay itself.

Subsequent studies of Mg isotope ratio in Allende inclusions have made use of the ion microprobe (SIMS) for direct analysis of material with very low common Mg contents. Analysis of Allende melilites and spinels by Steele and Hutcheon (1979) gave an initial ²⁶Al/²⁷Al ratio of 2 (" 1) H 10^!5, although data on the refractory mineral hibonite (from the same inclusion) implied a ratio as high as 8 H 10^!5. Nevertheless, these results are broadly in line with the data on separated minerals.

The great significance of ²⁶Al for cosmochemistry is its short half-life of 0.72 Myr. Because this is only 4% of the ¹²⁹I half-life, its presence in the early solar-system constrains a nucleosynthetic event to have occurred much more imminently before the coalescence of the solar-system. Classical nucleosynthetic models (e.g. Arnett, 1969; Truran and Cameron, 1978) attribute ²⁶Al to explosive carbon burning in the envelope of a supernova, and predict a ²⁶Al/²⁷Al production ratio of ca. 10^!3. It would take only 3 Myr for this ratio to decay to the value of 5 H 10^!5 ratio found in several meteorite samples. Therefore, Lee et al. (1976) suggested that the observed anomalies were due to late addition to the solar nebula of freshly synthesised material from a nearby nova or supernova explosion. Cameron and Truran (1977) pointed out that such an explosion in the vicinity of a condensing solar nebula was a very unlikely coincidence unless the supernova itself triggered the collapse of an interstellar cloud to form the solar-system. This model is illustrated in Fig. 15.14.

Fig. 15.14. Schematic illustration of a model in which solar-system collapse is promoted by a supernova which also seeds it with short-lived nuclides. After Wasserburg (1985).

For a time, the ‘supernova trigger’ model for solar-system coalescence was widely accepted. However, spectral data from the High Energy Astronomical Observatory satellite (HEAO 3) revealed a ( line (Fig. 15.15) due to ²⁶Al decay from a diffuse galactic source (Mahoney et al., 1984). An average galactic ²⁶Al/²⁷Al ratio of ca. 10^!5 was determined, remarkably close to that deduced for the early solar-system from Allende inclusions. This high ²⁶Al abundance in the galaxy means that supernovae, which are rare, cannot be the principal sources. Indeed, recent experimental studies (Champagne et al., 1983) suggest that Red Giants can generate Al with a 26/27 production ratio of unity. Nevertheless, the isotopic data on Allende inclusions still imply late injection of ²⁶Al into the pre-solar cloud. For example, Wasserburg (1985) suggested that this injection could be supplied by rapidly evolving stars within the interstellar cloud itself, followed by rapid condensation on a ca. 1 Myr time-scale (Wasserburg, 1985).

Fig. 15.15. Galactic (-ray spectrum showing a peak at 1808 keV which is attributed to decay of excited ²⁶Mg, itself produced by the decay of ²⁶Al in interstellar space. After Mahoney et al. (1984).

A very different explanation was proposed by Clayton (1975, 1979), who argued that most isotopic anomalies observed in meteorites, including extinct radionuclide signatures, are inherited from pre-solar dust grains. If this model was true for ²⁶Al, then both Al isotope and Al/Mg ratios would have to be inherited intact from these pre-solar materials. This would in turn imply that many meteorite mineral phases (e.g. Ca)Al inclusions in Allende) are also pre-solar. Wasserburg (1985) contested this argument on mineralogical grounds, believing that most or all of the analysed meteorite phases crystallised within the solar-system. Evidence supporting Wasserburg was provided by the discovery of ²⁶Mg excesses in a clast from the chondrite Semarkona (Hutcheon and Hutchison, 1989). Hutcheon and Hutchison argued that the mineralogy and REE chemistry of the clast were the result of igneous processes, implying a planetary, rather than nebular origin.

All of the samples described above (inclusions and clasts) are now generally thought to have formed during solar-system condensation. However, evidence has recently been found for the preservation of pre-solar grains in the Murchison carbonaceous chondrite. These grains are composed of silicon carbide, and have exotic rare gas signatures which match the abundance patterns expected in carbon-burning Red Giants (Lewis et al., 1990). This means that they have escaped significant heating during solar-system coalescence. Zinner et al. (1991) found large excesses of ²⁶Mg in some of these grains, equivalent to initial ²⁶Al/²⁷Al ratios from 10^!5 up to nearly unity. These ratios probably date from the time of expulsion of the grains, in the solar wind of a Red Giant, into interstellar space. Therefore, one model suggests that the solar wind from such a star (rather than a supernova) triggered the collapse of a giant molecular cloud to form the solar-system (Nuth, 1991).

A totally different explanation of ²⁶Al signatures that has been proposed at various times (e.g. Lee, 1978) is their generation by spallation reactions caused by intense solar radiation. For example, such irradiation may have occurred if the sun went through a ‘T Tauri’ stage early in its evolution. However, Shu et al. (1997) argued that the greatest radiation intensity was reached at an earlier stage in the Sun’s evolution, as a protostar still ‘embedded’ in the nebular disc. Lee et al. (1998) used this model to explain the presence of several of the very short-lived species whose traces are observed in meteorites. This model is discussed further during an examination of the spallogenic nuclide ¹⁰Be (section 15.4.3).

Returning to the question of the distribution of ²⁶Al, recent work by Russell et al. (1996) suggested that ²⁶Al addition to the solar nebula was more widespread than previously thought, and therefore that this and other radionuclides may have been homogeneously distributed. Firstly, Mg isotope anomalies were found in two inclusions from the unmetamorphosed ordinary chondrites Semarkona and Moorabie. These gave initial ²⁶Al/²⁷Al ratios of 5 H 10^!5, exactly the same as found in inclusions from carbonaceous chondrites. The observation of consistent ratios in inclusions from different chondrite groups suggests that these inclusions might have been fairly widely distributed in the nebula.

Russell et al. (1996) also detected the first evidence for extinct ²⁶Al in Al-rich chondrules (as opposed to inclusions) from Inman and Chainpur. The chondrules gave lower initial ²⁶Al/²⁷Al ratios than the inclusions, around 1 H 10^!5. If the decrease from a typical CAI value of 5 H 10^!5 to a chondrule value of 1 H 10^!5 is attributed to ²⁶Al decay in a homogeneous reservoir, the data imply a period of ca. 2 Myr between CAI formation and chondrule formation. This time span is consistent with U–Pb ages on chondrules and inclusions (section 5.3.1), thus supporting their origin from a homogeneous reservoir. Similar results were found by Kita (2000) for two ferromagnesian chondrules from Semarkona. Al–Mg pseudochrons gave initial ²⁶Al/²⁷Al ratios in a narrow range from 6 – 9 H 10^!6, again implying a period of about 2 Myr between CAI and chondrule formation.

The occurrence of ²⁶Al was finally extended to achondrites by Srinivasan et al. (1999). These workers found evidence for a small ²⁶Al signal (²⁶Al/²⁷Al = 7.5 H 10^!7) in the eucrite Piplia Kalan, showing that this nuclide survived into the period of differentiation of the Eucrite Parent Body and could have provided a heat source for the melting of such bodies. The ²⁶Al signal was actually larger than expected, and suggests that planetary differentiation could have occurred as early as 5 Myr after the formation of CAIs, assuming a homogeneous distribution of this nuclide.

15.4.2 Ca)K

⁴¹Ca decays to ⁴¹K with a half-life of only 0.1 Myr. Hence, if ⁴¹K excesses were found in solar-system material they would imply a very late addition of nucleosynthetically ‘hot’ material to the solar nebula. Such anomalies might be expected in Allende material displaying ²⁶Al signatures. Begemann and Stegmann (1976) sought ⁴¹Ca signatures in Allende samples, and believed they had found them. However, subsequent work by Hutcheon et al. (1984) attributed this signal to (⁴⁰Ca⁴²Ca)⁺⁺ dimers, creating a peak which could not be resolved in mass from ⁴¹K. Nevertheless, subsequent work by Srinivasan et al. (1994, 1996) finally moved ⁴¹Ca from a species absent to a species present in the early solar-system.

Srinivasan et al. looked for ⁴¹K, the decay product of ⁴¹Ca, in Ca–Al-rich inclusions (CAIs) from the Efremovka CV3 carbonaceous chondrite. These inclusions were regarded as an ideal place to search for ⁴¹K anomalies because of their unusually fresh petrography and the known presence of ²⁶Al anomalies. The pristine nature of the samples is very important, because ⁴¹K anomalies are only visible at extreme Ca/K ratios, which would be compromised by re-mobilisation of common potassium.

Srinivasan et al. used the ion microprobe to measure K isotope ratios in pyroxenes and other high-Ca phases from four different inclusions. Special precautions were taken to resolve the ⁴¹K signal from interfering molecular ions. ⁴⁰CaH⁺ was resolved by its excess mass at high spectral resolution. On the other hand, ⁴⁰Ca⁴²Ca⁺⁺ ions could not be resolved by mass, but were monitored and corrected via the related species ⁴⁰Ca⁴³Ca⁺⁺. The reliability of this correction is demonstrated by the constant ⁴¹K/³⁹K ratios determined in terrestrial samples with Ca/K variations spanning 9 orders of magnitude. In contrast, Efremovka inclusions showed a strong correlation between ⁴¹K/³⁹K and ⁴⁰Ca/³⁹K ratio (Fig. 15.21). If this correlation results from in situ decay of ⁴¹Ca after the inclusions were formed, its slope corresponds to an initial ⁴¹Ca/⁴⁰Ca ratio of 1.4 H 10^!8. Minerals from the same suite of inclusions also gave initial ²⁶Al/²⁷Al ratios typical of other CAIs, around 5 H 10^!5.

Fig. 15.16. K–Ca pseudochron diagram for pyroxenes in Efremovka inclusions, indicating a ⁴¹Ca/⁴⁰Ca ratio of 1.4 H 10^!8 in the solar nebula at the time of their crystallisation. Three radiogenic points (inset) also lie within error of the best-fit line. ( <> ) = terrestrial minerals. After Srinivasan et al. (1996).

In principal, the ⁴¹K signal in Efremovka inclusions could be explained by recent cosmogenic production of ⁴¹Ca by cosmic-ray neutrons. However, Srinivasan et al. cited rare gas isotope evidence for Efremovka which pointed to a low cosmogenic neutron flux during the fragment’s 11 Myr exposure history in space. A second alternative is that ⁴¹K was inherited as a ‘fossil’ component from ⁴¹Ca decay in pre-solar grains. However, this can be ruled out because the correlation between ⁴¹K/³⁹K and Ca/K ratio is observed in grains which show clear evidence of crystallisation from a liquid. Therefore, Srinivasan et al. concluded that the ⁴¹K signal could best be explained by the presence of live ⁴¹Ca in the early solar-system.

Srinivasan et al. also considered the possibility that ⁴¹Ca was manufactured in the solar nebula by bombardment of dust grains with energetic particles from an early ‘active’ sun. However, they argued that this model cannot produce ⁴¹Ca and ²⁶Al in their correct relative abundances. Therefore, the most attractive model attributes both nuclides to a common nucleosynthetic process immediately before solar-system condensation. This argument was supported by the discovery of correlated enrichments of ²⁶Mg and ⁴¹K in several grains of hibonite in the three carbonaceous chondrites, Murchison, Allende and Efremovka (Sahijpal et al., 1998). The correlations were observed on a microscopic scale by ion microprobe analysis, and therefore imply a common source for the extinct parents, ²⁶Al and ⁴¹K.

If we assume simultaneous injection of ⁴¹Ca and ²⁶Al, and we know the production ratios relative to stable calcium and aluminium, we can solve for both ) and the dilution factor of ‘hot’, freshly injected material by ‘cold’ pre-existing material. This is the same approach that was attempted for the two longer lived r-process nuclei ¹²⁹I and ²⁴⁴Pu (section 15.3.2). Unfortunately, similar problems arise here, because the production ratios are poorly constrained. However, the half-life of ⁴¹Ca is so short (0.1 Myr), that tight constraints on ) arise from almost any model. In practice, both supernovae and Red Giants (asymptotic giant branch, or AGB stars) can be made to fit the data, implying a ) value of 1 Myr or less, and a dilution factor of 100 or more.

15.4.3 Be-10

The ‘canonical’ model involving late incorporation of ‘hot’ nucleosynthetic material into the solar nebula has recently been undermined by the evidence for the production of ¹⁰Be in the early solar-system. ¹⁰Be is one of the class of nuclides that cannot be produced by stellar nucleosynthesis because it is unstable in stars. Hence production of ¹⁰Be is attributed to spallation reactions involving cosmic rays, termed the ‘x process’ (section 1.2.2). Evidence for extinct ¹⁰Be in the early solar-system implies that other extinct nuclides might likewise have had spallogenic origins.

¹⁰Be decays to ¹⁰B with a half-life of 1.5 Myr. Evidence for the existence of extinct ¹⁰Be in the early solar-system was discovered by McKeegan et al. (2000) from the observation of ¹⁰B/¹¹B variations that were positively correlated with Be/B ratios in melilite grains from an Allende CAI (Fig. 15.17). The slope of the array indicated an initial ¹⁰Be/⁹Be of 9.5 H 10^!4 at the time of CAI crystallisation, as was much too high to be explained by cosmogenic production by galactic cosmic rays. Therefore, McKeegan suggested that ¹⁰Be resulted from very intense radiation in the solar nebula during its early history. This raises the possibility that the other very short-lived species such as ²⁶Al, ⁴¹K, and ⁵³Mn could also have been produced in this way. Such models have been proposed before (e.g. Lee, 1978), but had been largely discarded due to the success of the ‘late addition’ model. However, the observation of ¹⁰Be in the solar nebula requires that this model be re-examined.

Fig. 15.17. Be–B correlation diagram showing evidence for the existence of extinct ¹⁰Be in an Allende CAI. After McKeegan et al. (2000).

One variant of the spallation production model recently proposed for extinct nuclides is the ‘x-wind’ model (Shu et al., 1997; Lee et al., 1998). In this model, protosolar rock debris in a ‘Reconnection Ring’ in the stellar accretion disc is irradiated by the proto-sun, where magnetic ‘reconnection’ events provoke impulsive flares, accelerating energetic particles to very high speeds so they are capable of causing the production of extinct nuclides by spallation. After irradiation, most of this matter is funnelled into the star. However, a sudden decrease in the solar magnetic field can cause the ‘x region’ to collapse into the zone of irradiated protosolar rock debris, flinging some of this material to planetary distances (Fig. 15.18). This model can also explain the intense, but short-lived, heating episodes that are need to explain the high-temperature mineralogy of CAIs, and to a lesser extent, chondrules (Shu et al., 1996).

Fig. 15.18. Cartoon illustrating the ‘x- wind’ production of extinct nuclides by irradiation of the solar nebula. Dots = CAI precursors. R–R = re-connection ring; X = x-region. After Shu et al. (1997).

There has been extensive debate about the effectiveness of the x-wind model to explain the abundances of short-lived extinct nuclides in their correct proportions. For example, Sahijpal et al. (1998) argued that spallation cannot explain the observed abundances of ²⁶Al without over-production of ⁴¹Ca. However, Lee et al. (1998) argued that mantling of CAIs with an Fe–Mg rich covering could enhance ²⁶Al production relative to ⁴¹Ca and therefore explain the abundances of all of the studied short-lived extinct nuclides except ⁶⁰Fe. Russell et al. (2001) predicted that spallation production would produce a heterogeneous distribution of the very-short-lived extinct species in meteorites. In theory this should allow the model to be tested; however, the actual degree of isotope heterogeneity is hard to assess.

Further evidence for the existence of extinct ¹⁰Be in the early solar-system was provided by Sugiura et al. (2001) from analysis of CAIs from Allende and Efremovka. Each of four inclusions from Efremovka and two from Allende showed positive correlations between ¹⁰B/¹¹B and Be/B in analysed melilite grains. The slopes of the arrays varied slightly, but all fell within error of the average ¹⁰Be/⁹Be ratio of 6.2 H 10^!4 (Fig. 15.19a). Al–Mg measurements were made in coexisting anorthite from the same inclusions; however, these were much more scattered, and fell below the ‘canonical’ ²⁶Al/²⁷Al ratio of 5 H 10^!5 previously observed in many CAIs. This was attributed to re-equilibration during a later disturbance. Hence, it was suggested that B diffusion in melilite must be slower than Mg diffusion in anorthite. However, the strength of the Be–B pseudochron (Fig. 15.19a) raises the alternative possibility that it could be a mixing line.

To test the possibility of a mixing line, Sugiura et al. compared the data on the Be–B pseudochron diagram with a plot of ¹⁰B/¹¹B against 1/B concentration (Fig. 15.19b). The good correlation in the latter diagram shows that the data are indeed primarily a mixing line between an exotic ¹⁰B-enriched component and common boron. However, because the quality of the pseudochron correlation was better than the mixing line, Sugiura et al. suggested that the pseudochron really does represent a signature of extinct ¹⁰Be from the early solar-system, rather than cosmogenic isotope production over the subsequent 4.5 Byr.

Fig. 15.19. Plots of boron isotope ratio against a) Be/B ratio and b) 1/B ratio to evaluate pseudochron versus mixing models for aggregate data from six CAIs. Error bars are omitted for clarity. Modified after Sugiura et al. (2001).

Some doubts about the widespread importance of spallation production come from a recent study of the hibonite grains in the carbonaceous chondrite Murchison (Marhas et al., 2002). In this meteorite, two different types of hibonite grain are observed, one of which has large mass fractionation anomalies, whereas the other has extinct nuclide anomalies. Marhas investigated the former type, to see if they would nevertheless display ¹⁰Be anomalies. This was indeed the case, since ¹⁰Be/⁹Be ratios around 5 H 10^!4 were observed, but no evidence of extinct ²⁶Al or ⁴¹Ca was observed. Marhas attributed the ¹⁰Be anomalies in these and other CAI minerals to energetic particles from an early active Sun. However, they attributed the absence of ²⁶Al or ⁴¹Ca anomalies in the analysed hibonites to their crystallisation before the solar nebula was injected with ‘hot’ nucleosynthetic material by a nearby stellar source. In addition, ¹⁰Be (accompanied by ²⁶Al and ⁴¹Ca) can also be produced in the jets of heavily irradiated material that are expelled from a supernova after its explosion. Therefore, in view of these various possibilities, the present author believe that it would be premature to reject the ‘late hot addition’ model in favour of a purely spallogenic model for short-lived extinct nuclides.

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