15.5 Short-lived species in planetary differentiation

Although the very short-lived species ²⁶Al and ⁴¹Ca are suggestive of very late addition of nucleosynthetically ‘hot’ material to the nebula, they are too short-lived to demonstrate the survival of such material into the period of early differentiation into planetessimals and planets. However, a group of slightly longer-lived species fulfils this role, and also provides additional evidence for the type of late nucleosynthetic additions.

15.5.1 Pd)Ag

¹⁰⁷Pd decays by $ emission to ¹⁰⁷Ag with a half-life of 6.5 Myr. The only objects with a Pd/Ag ratio high enough to yield measurable variations in ¹⁰⁷Ag abundance are iron meteorites. The first successful discovery of radiogenic ¹⁰⁷Ag was made by Kelly and Wasserburg (1978) on the Santa Clara meteorite. They deduced that ca. 10 Myr might have elapsed between the last nucleosynthetic event and the coalescence and differentiation of iron-cored small planets. Subsequent work has revealed radiogenic ¹⁰⁷Ag in several meteorites (Chen and Wasserburg, 1984; 1990), of which the best data are from Gibeon. These results yield an initial ¹⁰⁷Pd/¹⁰⁸Pd ratio of 2.3 H 10^!5 (Fig. 15.20). Very similar results were also seen in relatively low Pd/Ag samples from Canyon Diablo using high-precision MC–ICP–MS analysis (Carlson and Hauri, 2001). Since ¹⁰⁷Pd versus Ag correlations are observed in bodies which have clearly been melted since accretion of the solar-system, the silver isotope signatures must reflect the presence of ‘live’ ¹⁰⁷Pd in the early solar-system. They cannot have been inherited from pre-solar grains (as proposed for other systems by Clayton, 1975).

Fig. 15.20. Ag isotope versus Pd)Ag diagram showing evidence of extinct ¹⁰⁷Pd in iron meteorites. The best-fit line to seven metal samples from the Gibeon meteorite ( ! ) yields an initial ¹⁰⁷Pd/¹⁰⁸Pd ratio of 2.3 H 10^!5. Open symbols denote other group IVA iron meteorites. After Wasserburg (1985).

15.5.2 Mn)Cr

⁵³Mn decays to ⁵³Cr with a half-life of 3.7 Myr. For several reasons, this extinct nuclide reinforces the evidence from ¹⁰⁷Pd for the early history of planetary differentiation. Birck and Allegre (1985; 1988) found correlated variations in ⁵³Cr/⁵²Cr and Mn/Cr ratio in several meteorites, including Allende, Murchison, Indarch and the pallasite meteorite Eagle Station. Since the mixed silicate)iron mineralogy of the latter indicates a high temperature origin, the Mn)Cr isotope systematics must result from in situ decay of ⁵³Mn in differentiated planetary bodies. Hence ⁵³Mn provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar-system.

⁵³Mn is part of the iron group of elements (section 1.2) which are thought to be synthesised in large stars shortly before a supernova explosion. Hence it was argued (e.g. Rotaru et al., 1992) that a supernova did indeed briefly pre-date solar-system condensation, as initially proposed on the basis of ²⁶Al. From the 3.7 Myr half-life of ⁵³Mn, it would follow that such an event occurred ca. 20 Myr before planetary differentiation. However, there are other possible routes for the synthesis of ⁵³Mn (Birck and Allegre, 1985). Furthermore, curium isotope evidence (or the lack of it; see below) argues against a late r-process addition to the solar nebula. Therefore, although there is good evidence for a late nucleosynthetic addition to the solar nebula, its source remains in doubt.

Although doubts remain about the source of ⁵³Mn, its half-life of 3.7 Myr gives it great potential as a chronometer of early solar-system evolution. To test this potential, Lugmair and Shukolukov (1998) made a Mn–Cr study of a large group of achondrites of various types. These are essentially all igneous rocks produced by the very early differentiation of one or more asteroids. Firstly, Lugmair and Shukolukov analysed two ‘angrites’, Lewis Cliff (LEW) and Angra dos Reis (ADOR), which are thought to have a rapid and simple cooling history. Based on their well-defined Pb–Pb age of 4558 " 0.5 Myr, these were used to anchor the Mn–Cr relative dating method in the same way that the I–Xe method was anchored. The method was then tested by analysing the primitive achondrite Acapulco, which gave a Mn–Cr age only 3 Myr younger than the angrites, in good agreement with its Pb–Pb age of 4557 " 2 Myr. In addition, the Mn–Cr and I–Xe ages of Ste Marguerite and Richardton were also in excellent agreement (Gilmour and Saxton, 2001). Hence, the assumption of initial ⁵³Mn homogeneity, on which the Mn–Cr dating method is based, seems well justified for these materials.

Lugmair and Shukolukov (1998) also analysed a suite of eight eucrites, attributed to a single parent body that is tentatively identified with the asteroid Vesta 4. Two of these (Chervony Kut and Juvinas gave good internal pseudochrons, with absolute ages of 4564 and 4563 Myr respectively using the calibration against Pb–Pb described above. In addition, the complete suite gave an excellent whole-rock pseudochron with a ⁵³Mn/⁵⁵Mn ratio equivalent to an age of 4565 Myr using the same calibration (Fig. 15.21). This was interpreted as the time of differentiation of the parent body, which is within error of the Pb–Pb ages of CAIs. This supports I–Xe ages in pointing to extremely early formation and differentiation of planetessimals.

Fig. 15.21. Mn)Cr correlation diagram for whole-rock samples of achondrites, yielding an initial ⁵³Mn/⁵⁵Mn ratio of 4.7 H 10^!6 at the time of differentiation of the eucrite parent body. After Lugmair and Shukolyukov (1998).

Three chondrites analysed by Lugmair and Shukolukov (1998) were found to lie perfectly on the angrite pseudochron. However, a detailed study of individual chondrules from Chainpur and Bishunpur (Nyquist et al., 2001) gave internal pseudochrons with slopes indicating an age 10 Myr older than the angrites. Nyquist et al. interpreted this as a Mn/Cr fractionation event in the chondrite precursors of the early nebula (due to variations in volatility) rather than as the actual age of chondrule formation. Therefore the calculated age of 4568 Myr is reasonable, since it is only 3 Myr older than the Eucrite parent body and is within error of the Pb–Pb ages of CAIs.

All of these data are summarised on a plot of initial Cr isotope ratios against Mn isotope ratios determined from the Mn–Cr pseudochron diagrams (Fig. 15.22). Moving backwards in time from the top left corner, the data suggest that all the achondrite samples (eucrites, angrites, and primitive achondrites) can be derived by Mn–Cr fractionation from a source which is either the Eucrite Parent Body (Vesta) or a closely related one. Some whole-rock chondrites lie on the angrite evolution line, but Indarch lies below this with a lower initial Cr isotope ratio, which however is consistent with the Bulk Earth composition. Lugmair and Shukolukov (1998) attributed this less radiogenic Cr isotope ratio to original Mn isotope heterogeneity in the nebula, but it is easier to explain it by variable Mn/Cr fractionation in the nebula due to the different volatility of these two elements (Birck et al., 1999).

Fig. 15.22. Mn–Cr isotope evolution diagram for the early solar-system. ( Î ) = eucrites; ( v ) = angrites and primitive achondrites; ( " ) = chondrite whole-rocks; ( ! ) = chondrule initial ratios. After Nyquist et al. (2001).

The latter model is supported by the new Chainpur and Bishunpur chondrule pseudochrons. However, whole-rock data for carbonaceous chondrites (Birck et al., 1999, not shown here) are more scattered. In addition, the Allende inclusion (CAI) analysed by Birck and Allegre (1985) gives an apparent Mn–Cr age up to 10 Myr older than its Pb–Pb age. However, these inclusions contain many isotopic anomalies, and are the one clear example of inhomogeneous extinct nuclide distribution, so we cannot assume that their Mn isotope systematics were homogenised with other solar-system materials. Therefore, more Mn–Cr data are needed from the very early bodies (carbonaceous chondrites and inclusions) to adequately understand this primitive stage in the evolution of the solar nebula.

Mn–Cr data also provide evidence about the later cooling of chondrite parent bodies. This comes from the observation of ⁵³Mn excesses in carbonate fragments from two chondritic meteorites (Endress et al., 1996). These poly-crystalline fragments, mostly of dolomitic composition, are uniformly spread through the carbonaceous chondrites Orgueil and Ivuna. They are interpreted as remnants of carbonate veins that formed during very early aqueous alteration of the meteorite parent body. Five dolomite fragments, analysed by ion microprobe, define a relatively linear array on the Mn–Cr pseudochron diagram. The slope of this array corresponds to a ⁵³Mn/⁵⁵Mn ratio of 2 H 10^!6 at the time of carbonate crystallisation. The time required to achieve this ratio, starting from the value of 4.4 H 10^!5 in 4566 Myr Allende inclusions, is less than 20 Myr. This indicates that the carbonaceous chondrite parent body cooled very rapidly after aggregation to temperatures where liquid water could exist. Other ⁵³Mn evidence for very early hydrous alteration was found in the CV3 chondrite, Mokoia (Hutcheon et al., 1998).

15.5.3 Fe)Ni

⁶⁰Fe decays to ⁶⁰Ni (via ⁶⁰Co) with a half-life of 1.5 Myr. Shukolyukov and Lugmair (1993a) found ⁶⁰Ni excesses as high as 50 , units in the achondrite Chervony Kut, facilitated by the extremely high Fe/Ni ratios in this meteorite (up to 350 000). These ratios are attributed to nickel partition into the cores of differentiated planetessimals. Whole-rock samples of Chervony Kut displayed a correlation between ⁶⁰Ni/⁵⁸Ni and Fe/Ni ratio, indicating that ⁶⁰Fe was still alive at the time of differentiation, with a ⁶⁰Fe/⁵⁶Fe ratio of 3.9 H 10^!9. Whole-rock analysis of the achondrite Juvinas also generated a pseudochron relation, but with a lower ⁶⁰Fe/⁵⁶Fe ratio of ca. 4 H 10^!10 (Shukolyukov and Lugmair, 1993b). The difference between these extinct nuclide abundance ratios implies a difference in closure age of ca. 4.7 Myr between the two meteorites, if the parent body had a homogeneous ⁶⁰Fe/⁵⁶Fe ratio.

This evidence points to widespread distribution of ⁶⁰Fe in the solar nebula, and suggests that this nuclide was a major source of heat. Coupled with ²⁶Al, ⁶⁰Fe would be capable of causing planetary melting within a few million years of accretion. Furthermore, the existence of ⁶⁰Fe in the solar nebula requires a late supernova contribution, since there is no suitable target nuclide to produce this isotope by spallation. Lee et al. (1998) questioned the evidence for extinct ⁶⁰Fe in the early solar-system, and suggested instead that it could have been produced by cosmogenic irradiation of the Eucrite Parent Body before its breakup. However, more recent work has confirmed the existence of ‘live’ ⁶⁰Fe in troilites from unmetamorphosed ordinary chondrites (Tachibana and Huss, 2003). Hence, it now seems fairly certain that ⁶⁰Fe was introduced into the early Solar System by late injection from a supernova, thus reinstating the ‘supernova trigger hypothesis’.

15.5.4 Hf–W

¹⁸²Hf decays to ¹⁸²W (tungsten) by double $ decay with a half-life of 9 Myr. Norman and Schramm (1983) proposed the Hf–W system as an r-process chronometer, but its application was delayed by the technical difficulties of tungsten isotope analysis (similar to osmium, section 8.1). Harper et al. (1991) and Harper and Jacobsen (1996) successfully compared W isotope compositions in the Earth and iron meteorites by N–TIMS analysis, and these results were verified and extended by Lee and Halliday (1995; 1996) using MC–ICP–MS (section 2.5.1).

Lee and Halliday presented the combined data set from the above studies in the form of , ¹⁸²W values, representing part per 10 000 variations in ¹⁸²W/¹⁸³W (Harper and Jacobsen, 1996) or ¹⁸²W/¹⁸⁴W (Lee and Halliday, 1995; 1996), relative to a terrestrial tungsten standard, NIST 3163. The results showed , W values within error of zero in terrestrial lavas, a Lunar mare basalt, and bulk powders of the carbonaceous chondrites Allende and Murchison (Fig. 15.23). In contrast, sawn blocks from iron meteorites (as well as metal phases from ordinary chondrites) had , values clustering around ,W of –4.

Fig. 15.23. Measured tungsten isotope values for different solar-system objects, relative to a terrestrial tungsten standard. Open symbol = data of Harper and Jacobsen (1996). After Lee and Halliday (1996).

In principal, these variations could be explained by either cosmogenic production, pre-solar grains, or live ¹⁸²Hf in the early solar-system. However, the negative¹⁸²W anomaly in iron meteorites is unique because it represents an isotope deficiency rather than an enrichment. This makes both cosmogenic and pre-solar origins for the anomaly quite unlikely, whereas it can be readily explained by elemental Hf–W partitioning while ¹⁸²Hf was still alive in the early solar-system. Since W is strongly siderophile, whereas Hf is lithophile, the iron cores of meteorite parent bodies developed very low Hf/W ratios. Furthermore, evidence from the Fe–Ni system (section 15.5.3) shows that these cores formed very early, within about 5 Myr of chondrite formation. Therefore, iron meteorites should preserve a nearly primordial , ¹⁸²W composition for the solar-system. In contrast, ¹⁸²W growth would continue in other solar-system bodies until ¹⁸²Hf was extinct.

Harper and Jacobsen (1996) did not have ¹⁸²W data on chondrites to use a benchmark for solar-system W isotope evolution. Instead, they estimated the initial solar-system ¹⁸²Hf/¹⁸⁰Hf value from a predicted supernova production ratio of 2 H 10^!5. This implied that the present-day chondritic tungsten composition, and hence the Bulk Earth, was essentially the same as iron meteorites. Based on this assumption, the measured 4 , unit excess in the Bulk Silicate Earth (BSE) implied that terrestrial Hf/W fractionation occurred while ¹⁸²Hf was still alive, due to very early core formation. In contrast, Lee and Halliday (1995; 1996) made direct measurements of W isotope ratios in two bulk chondrite samples, and found , values of +4 relative to iron meteorites, essentially the same as the Bulk Silicate Earth. This led to the opposite conclusion from Harper and Jacobsen (1996): that the Earth’s core segregated relatively late, after ¹⁸²Hf had become extinct.

The small amount of chondrite data analysed by Lee and Halliday (1995; 1996) implied an unexpectedly high ¹⁸²Hf/¹⁸⁰Hf ratio of 2.6 H 10^!4 in the early solar-system. However, more detailed work by Lee and Halliday (2000) appeared to support this ratio. For example, separated metal and silicate phases from the H4 chondrites Forest Vale and Ste Marguerite gave Hf–W pseudochrons equivalent to initial ¹⁸²Hf/¹⁸⁰Hf ratios of 1.9 and 1.8 H 10^!4 respectively. The previously analysed whole rock samples of Allende and Murchison also plotted near these pseudochrons.

Unfortunately, subsequent work by three different research groups did not confirm these results. First indications of this discrepancy were obtained by Schoenberg et al. (2002a), and confirmed by more detailed work by Yin et al. (2002) and Kleine et al. (2002). The latter groups obtained consistent metal-silicate pseudochrons for four different chondrites (Dalgety Downs, Dhurmsala, Forest Vale and Ste Marguerite), with a consistent initial ¹⁸²Hf/¹⁸⁰Hf ratio around 1 H 10^!4. Both groups also found that whole-rock carbonaceous chondrites plotted on the metal–silicate pseudochrons (e.g. Fig. 15.24). Finally, Yin et al. also measured a CAI composition lying on the same chondrite pseudochron, suggesting that a ¹⁸²Hf/¹⁸⁰Hf ratio around 1 H 10^!4 is also the best estimate of the starting composition of the solar nebula. The steeper pseudochron obtained by Lee and Halliday (2000) remains unexplained, but new data obtained by Halliday (2002) were consistent with the results obtained by the three other research groups. The implications of this value for the ) value since last r-process addition will be discussed further in section 15.7.

Fig. 15.24. Hf–W pseudochron for metal–silicate separates from two chondrites ( " ) and two carbonaceous chondrites ( ! ). Inset shows the lower part of the chondrite pseudochron. Dashed line shows the carbonaceous chondrite array of Lee and Halliday (2000). After Yin et al. (2002).

A two-point pseudochron between the average chondrite composition and the Bulk Silicate Earth (taken to be , = 0) yields the ¹⁸²Hf/¹⁸⁰Hf isotope composition of the Bulk Silicate Earth at the average time of core formation. This has a value of 1.1 H 10^!5, which (when coupled with a starting composition in the solar nebula of 1 H 10^!4) leads to an average time of terrestrial core formation of ca. 30 Myr. In contrast, the data of Lee and Halliday (1995, 2000) implied a minimum age of core formation of ca. 60 Myr. However, no maximum age constraint was available, since their chondritic and BSE results were within error.

Similar calculations can be made for the time of core formation in the asteroid Vesta and for Mars, based on the tungsten pseudotope compositions of the silicate fractions of these bodies. These can be determined by analysing the compositions of basaltic achondrites or eucrites (both attributed to Vesta) and from SNC meteorites (attributed to fragments ejected from Mars during impact events). Based on these analyses, Kleine et al. (2002) argued that the time of core formation in Vesta, Mars and the Earth was consistent with their planetary radii (Fig. 15.25). The new data are also consistent with evidence from the Pd–Ag system for early core formation in the Earth and in meteorite parent bodies (Hauri et al., 2000).

Fig. 15.25. Plot of estimated times of core formation for solar-system bodies, compared to their measured or estimated radii. After Klein et al. (2002).

The estimated formation age of the Moon is also plotted in Fig. 15.25, but it can be seen that this is the same as the Earth’s core, despite the Moon’s small radius. This is because most researchers now agree that the Moon was formed in a giant impact been the proto-Earth and another planetary body around the size of Mars (e.g. Hartman, 1986). Therefore, when speaking of the age of the Moon, we are estimating the time between CAI formation and this giant impact.

Lee et al. (1997) made several analyses from the major groups of lunar rocks types, including the lunar highlands (anorthosite, troctolite and norite), Mare basalts, and so-called KREEP basalts (rich in incompatible elements). On a Hf–W pseudochron diagram, these samples display considerable scatter, which is attributed to Hf–W fractionation within each subgroup after the extinction of ¹⁸²Hf. For example, the age of the Mare basalts (around 4.2 Byr) is much younger the formation of the Moon or the extinction of ¹⁸²Hf, but we can recover the approximate composition of the Mare basalt source by averaging all of the Mare data. This average falls close to a three point pseudochron of lunar soil, which is expected to be a relic from very early lunar differentiation. Similarly, the averages of highland anorthosites, highland mafic rocks and KREEP basalts are also near to the soils pseudochron, whose slope indicates a ¹⁸²Hf/¹⁸⁰Hf ratio of 5.9 H 10^!6.

Fig. 15.26. Hf–W pseudochron diagram for Lunar samples. The regression is based on 3 Lunar soils ( ! ). Other symbols: ( <> ) = highland rocks; ( Î ) = KREEP and Mare basalts. After Lee et al. (1997).

Lee et al. (1997) interpreted the Hf–W pseudochron as the age of massive lunar differentiation. This is thought to represent crystallisation from a lunar magma ocean formed when the Moon aggregated in Earth orbit after the giant impact between the proto-Earth and the impactor. Modelling of the collision suggests that the core of the impactor was added to the Earth’s core, and much of the mantle of the impactor was added to the Earth’s mantle, but a residue of the impactor mantle was thrown into orbit to form the Moon. Using the new ¹⁸²Hf/¹⁸⁰Hf ratio of 1.0 H 10^!4 for chondrites and CAI, leads to an estimate for the Moon-forming giant impact slightly younger than the average age of the Earth’s core, which is the expected order of events.

A final application of the Hf–W method worth mentioning is in searching for evidence of the intense ‘early meteorite bombardment’ that the Earth was subjected to shortly after its accretion. This bombardment might be expected to leave traces in Early Archean terrestrial sediments. Therefore, Schoenberg et al. (2002b) searched for W isotope anomalies in the oldest (metamosphosed) sediments from Isua in western Greenland. They claimed to find negative , W values in these sediments (relative to all other terrestrial materials analysed), which can be explained by contamination of the sediments by meteoritic debris, either from iron meteorites or chondrites. In particular, they observed an inverse correlation between , W and the log Cr/Ti elemental ratio in these sediments, believed to be a good indicator of contamination by meteoritic material (Fig. 15.27).

Fig. 15.27. Plot of , W against Cr/Ti ratio (on a log scale), showing a possible mixing line between terrestrial sediment ( ! ) and various meteorites types ( " ). After Schoenberg et al. (2002).

15.5.5 ¹⁴⁶Sm–¹⁴²Nd

¹⁴⁶Sm is the longest lived of the extinct nuclides, with a half-life of 103 Myr. Its extant life-time of a few hundred million years would therefore have been long enough to extend into the period of early differentiation of the silicate Earth. Therefore, if strong Sm/Nd fractionation occurred during early crust formation, variations in the abundance of the daughter product, ¹⁴²Nd, might be detected in Early Archean rocks.

¹⁴²Nd isotopic variations were detected during early studies of the achondrite Angra dos Reis (Lugmair and Marti, 1977) but their occurrence in terrestrial materials has been a subject of considerable debate. Because the variations are so small, they are quoted in ppm ( : ) relative to standard terrestrial Nd. The first claim of a positive terrestrial ¹⁴²Nd anomaly was made by Harper and Jacobsen (1992), based on the analysis of one sample from Isua, western Greenland. However, other workers (Galer and Goldstein, 1992; McCulloch and Bennett, 1993) observed only normal ¹⁴²Nd in other Early Archean rocks. Therefore, a much more detailed investigation was made by Sharma et al. (1996).

Sharma et al. analysed three sample types: normal Nd, normal Nd spiked with 30 or 57 ppm ¹⁴²Nd, and 3.8 Byr old rocks from Isua, West Greenland. The latter included one sample with a claimed 32 ppm excess of ¹⁴²Nd, and one unknown sample. Samples were run in static mode on a Finnigan MAT 262 machine. Focus positions were shown to have some influence on the measured relative abundances of ¹⁴²Nd; nevertheless, results for the three sample types were generally consistent with the predicted ¹⁴²Nd excesses (Fig. 15.28). Hence, the existence of ¹⁴²Nd anomalies in the Earth was confirmed with a fairly high degree of confidence.

Fig. 15.28. Histograms of : ¹⁴²Nd data for three artificial standards, compared with two rock samples from Isua. Data from Sharma et al. (1996).

Jacobsen and Harper (1996) showed that the ¹⁴²Nd excess can be used with initial , 143 Nd values to model early differentiation of the silicate Earth. A simple two stage model assumes Bulk Earth evolution until time T, when Sm/Nd fractionation occurs in some part of the mantle. The fractionated mantle reservoir is subsequently sampled at the time of magmatism. For the 3.81 Byr old Isua rocks, this model yields an age of 4.47 Byr for the fractionation event, 100 Myr after Earth accretion (Fig. 15.29). In contrast, 4 Byr old samples of Acasta gneiss, with calculated initial ¹⁴³Nd values ( , [ t ]) of +3.5, gave no ¹⁴²Nd anomaly. However, high , ¹⁴³Nd values in these samples may be metamorphic artifacts (section 4.4.4).

Fig. 15.29. Plot of , ¹⁴²Nd against : ¹⁴³Nd for four Isua samples, only one of which has a clear ¹⁴²Nd anomaly. This composition can be explained by a differentiation event 4.47 Byr ago. After Jacobsen and Harper (1996).

15.6 Absent species

15.6.1 Cm–U

²⁴⁷Cm (curium) decays to ²³⁵U with a half-life of 16 Myr. The significance of this species is that, like ²⁴⁴Pu, it is formed by the r-process only, but unlike ²⁴⁴Pu it has a comparatively short half-life. Hence the abundance of ²⁴⁷Cm can indicate conclusively whether late additions of material to the nebula (called for above) were r-process products or not. Since the strongest evidence for late addition is represented by ²⁶Al and ⁴¹Ca anomalies in CAI, these same samples have the best chance of being enriched in the daughter product of curium, which is ²³⁵U. The search for ²⁴⁷Cm therefore consists of looking for very small variations in uranium isotope composition. Unfortunately, ²³⁵U is normally used as an enriched isotope in U abundance determination by isotope dilution, and hence most laboratories are susceptible to artificial perturbations in this ratio. Perhaps for this reason, numerous claims have been made for excess ²³⁵U, but all are suspect.

In an attempt to conclusively resolve this problem, Chen and Wasserburg (1981) made a very careful investigation of U isotope compositions in samples for which large ²⁶Al signatures had been demonstrated, using a double 233/236 uranium spike to correct for mass fractionation during analysis (section 2.4.2). This study placed a maximum limit of " 0.4 % on uranium isotope variation in these very favourable samples, from which Chen and Wasserburg calculated a maximum initial ²⁴⁷Cm/²³⁸U ratio of 1.5 H 10^!3.

This value can be compared with the analogous ²⁴⁴Pu/²³⁸U ratio, which sets an upper limit on the dilution factor for late r-process material. Since inferred initial ²⁴⁷Cm abundances are substantially lower than ²⁴⁴Pu, it is deduced that the late ²⁶Al addition to the solar nebula was not r-process material. Wasserburg (1985) argued that the low ²⁴⁷Cm abundance places a strong lower limit on ) (100 Myr, Wasserburg and Papanastassiou, 1982) for the last r-process addition to the solar nebula. However, it should be pointed out that this argument only excludes late heterogenous additions of ²⁴⁷Cm to the solar nebula (accompanying ²⁶Al). This is because it rests on the observed homogeneity of ²³⁵U/²³⁸U isotope ratios rather than their absolute value.

15.7 Conclusions

In order to achieve a realistic model for the formation and early evolution of the solar nebula it is necessary to use the results of several extinct nuclide tracers. In an early review of the subject, Wasserburg and Papanastassiou (1982) drew attention to the relative similarity between the ‘hot/cold’ isotope ratios of ca. 1 H 10^!4, 0.5 H 10^!4 and 0.2 H 10^!4 for iodine, aluminium and palladium in chondrites. They pointed out that if the additions of hot material occurred comparatively early (e.g. ) = ca. 200 Myr), as suggested by Schramm and Wasserburg (1970), then their different half-lives would have attenuated the short-lived nuclides to very different degrees by the time of solar-system condensation. In contrast, the comparatively similar abundance ratios actually observed may imply a late addition with similar degrees of dilution by cold material.

More recent evidence should have narrowed the options for the origin of the solar-system, but a unique model is still not available. The presence of very short-lived species can be explained by the ‘trigger hypothesis’ (Cameron and Truran, 1977; Cameron et al., 1995), by which a single event caused late nuclide injection into a molecular cloud, and also triggered the collapse of the cloud. However, it is not clear whether this event was a Red Giant or supernova. On the other hand, ¹²⁹I and ¹⁸²Hf are best explained by a supernova, while actinides must be produced in supernovae. However, more than one type of supernova may be required to fit all of the production ratios.

Wasserburg et al. (1996) argued that the effective frequency by which supernovae could contribute to galactic nuclide production could be much higher than the one per 100 Myr commonly assumed. In addition, they suggested that different types of supernovae could have contributed ‘spikes’ of different nuclides to the pre-solar nebula. In the first place, the low level of ¹²⁹I implies an early supernova source () around 100 Myr). Wasserburg et al. then grouped ¹⁸²Hf with a late actinide addition from a second supernova () around 10 Myr, but this can increase in the light of new Hf/W data). Finally, they attributed ⁴¹Ca, ²⁶Al and ⁶⁰Fe, to very late addition, either from a third supernova source or from a Red Giant. A model of multiple supernova sources was also proposed by Harper (1996), who envisaged star birth in a large molecular cloud with a complex series of injection and mixing events (Fig. 15.30).

According to the model of Shu et al. (1997) and Lee et al. (1998), some of the very short-lived species (⁴¹Ca, ²⁶Al, and also ⁵³Mn) could also have been formed by spallation reactions when the sun was an embedded protostar. However, this mechanism cannot explain the presence of ⁶⁰Fe in the early Solar System, so it seems likely that at least some of the ⁴¹Ca, ²⁶Al, and ⁵³Mn were introduced, along with ⁶⁰Fe, by a late supernova.

Fig. 15.30. Schematic illustration of a possible scenario for the formation of new star systems (including the solar-system) in a giant molecular cloud containing supernova remnants Stippled). After Harper (1996).

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