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Chapter 5
At that point, the Earth appeared to be puzzlingly anomalous among differentiated planetary bodies in that silicate-metal differentiation appeared to have occurred quite late. Subsequently, Yin et al. (2002) reported W isotope measurements carried out in two laboratories, Harvard University and the Ecole Normale Supérieure de Lyon, which showed that the chondrites Allende and Murchison had W isotope ratios 1.9 to 2.6 epsilon units lower than the terrestrial standard (Figure 5.10). Yin et al. (2002) also analyzed separated metal and silicate fractions from two ordinary chondrites ( Dhurmsala and Dal- gety Downs) that allowed them to estimate the initial 182 Hf/ 180 Hf of the solar system as 1 × 10 -4. In the same issue of the journal Nature , Kleine et al., (2002) reported similarly low εW (i.e., -2) for the carbona- ceous chondrites Allende , Orgueil , Murchison , Cold Bokkeveld , Nogoya , Murray , and Karoonda measured in a third laboratory (University of Münster). Furthermore, Kleine et al. (2002) analyzed a variety of terres- trial materials and found they all had identical W isotopic composition (Figure 5.10). It thus appears that the original measurements of Lee and Halliday (1995) were wrong. The measurement error most likely relates to what was at the time an entirely new kind of instrument, namely the multi-collector ICP-MS. Yin et al. (2002) considered two scenarios for the formation of the core. In the first, which they call the two-stage model in which the Earth first accretes (stage 1) and then undergoes core formation (stage 2), induced by the giant impact that forms the moon. In this scenario, core formation occurs 29 million years after formation of the solar system. In the second scenario that they believed more likely, metal segregates continuously from a magma ocean. In this continuous model, the mean age of core forma- tion is 11 million years. In contrast, they concluded that the parent body of the eucrite class of achon- drites (suspected to be the large asteroid Vesta) underwent core forma- tion within 3 million years of formation of the solar system. Klein et al. (2002) reached similar conclusions.Yin et al. (2002) considered two scenarios for the formation of the core. In the first, which they call the two-s restrial, lunar, and chon- dritic W isotopic compo- sitions. That isochron has an age correspond- ing to 29 million years after formation of the so- lar system. In the second scenario, which they be- lieved more likely, metal segregates continuously from a magma ocean. In this continuous model, the mean age of core formation is 11 million years. In contrast, they concluded that the par- ent body of the eucrite class of achondrites (sus- pected to be the large asteroid Vesta) un- derwent core formation within 3 million years of
Figure 5.10. W isotope ratios measured in chondrites, the iron meteorite Toluca, and terrestrial materials. Data from Kleine et al. (2002), Yin et al (2002), Touboul et al (2007, 2009).
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formation of the solar system. Klein et al. (2002) reached a similar conclusion that the Moon and the Earth’s core formed early. The next twist in the story came when it was recognized that 182 W could be produced by beta decay of (^182) Ta which in turn is produced cosmogenically through the reaction 181 Ta(n,γ) 182 Ta. Because the Earth’s surface is somewhat protected from cosmic rays by the atmosphere, this reaction is entirely trivial at the surface of the Earth. However, the Moon has no atmosphere. Furthermore, the lack of lunar geologic activity means that materials are exposed at the surface for very long times (hundreds of millions of years and more). Consequently, the cosmogenic production of nuclides in lunar materials can be sig- nificant. Early studies failed to appreciate this or failed to fully correct for this effect. Touboul et al. (2007), working with W-rich, Ta-poor metal grains separated from lunar samples, showed that previous high εW values in lunar samples were due to cosmogenic effects and when these were eliminated the Moon has and isotopically homogeneous W isotopic composition that is identical to that of the silicate Earth – about 2 epsilon units higher than chondrites. The homogeneous isotopic composition of lunar materials Moon, including samples with highly variable Hf/W ratios such as anorthosites from the ancient lunar crust and mare basalts formed by later melting of the lunar mantle, implies that 182 Hf had effectively decayed completely before the Moon formed (or at least before its magma ocean solidified). Given the analytical precision with which W iso- topes can be measured, this implies an age of the Moon of greater than 60 Ma after the beginning of the solar system (the age of Allende CAI’s). The minimum age of the Moon is constrained by Sm-Nd ages of the lunar anorthositic crust. This age is 4.456±0.040 Ga, or about 100 Ma after the beginning of the solar system. Touboul et al. (2007) thus estimate an age for the Moon of 62 +90/-10 Ma after the start of the solar system. There is a very broad consensus among scientists that the Moon formed as a result of a collision be- tween a nearly full-sized Earth and a second body (sometimes called Thera ) about the size of Mars (the Giant Impact Hypothesis). Debris thrown into orbit about the Earth as a result of the impact subse- quently coalesced to form the Moon. A collision between bodies of these sizes releases enormous amounts of energy; depending on how efficiently the gravitational-kinetic energy is converted to heat, potentially enough energy to entirely melt the Earth. At the very least, one expects significant melting would have occurred on the Earth and one of the virtues of the hypothesis is that it explains the exten- sive melting experienced by the Moon. (Because it has remained geologically active, all evidence of a magma ocean on the Earth has been erased; because it quickly became geologically inactive, evidence of a magma ocean on the Moon has been preserved.) The significance of this is that such an impact would have provided a last opportunity for the metal to segregate from the mantle and sink into the core (a process that almost certainly requires the metal, if not also the silicate, to be liquid). We expect therefore, that the Moon-forming giant impact should also be the time of final separation of the core. If, however, the core formed in a single event 60-100 Ma after the start of the solar system, we would that the silicate Earth would have a W isotopic composition very close to chondritic; instead the terres- trial value is 2 epsilon units higher than chondritic. Furthermore, if, as silver isotopes clearly reveal, cores of asteroids formed within 10 Ma of the start of the solar system, why was core formation on the Earth so delayed? We can reconcile these observations if we assume that core formation was a more or less continuous process that began very early and only ended with the giant impact. In essence, the scenario goes like this. Once planetesimals exceed a few 10’s of km in radius, heating, either from 26 Al or release of gravitational energy in collisions, caused melting that allowed and metal and silicate to separate, with the metal sinking to form cores of the planetesimals. As the bodies grew into larger ones through collision with other planetesimals, enough energy was released to allow the cores of colliding bodies to merge. This process, known as oligarchic growth , continued to build larger and larger plane- tary embryos. As the embryos merged through collision, fewer and fewer embryos remained and the collisions were less frequent. Slowing the planetary growth process, but the collisions, when they did occur, were far more energetic. The process, in the case of the Earth, culminates when the last two bod- ies in the Earth’s orbital neighborhood (or “feeding zone”) collide in the giant impact that forms the
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tle with high Sm/Nd. A study by Harper and Jacobsen (1992) re- ported a 33 ppm excess of 142 Nd in one 3.8 Ga old metavolcanic rock from Isua. This excess was based on a comparison between the rock and laboratory standards; the latter was assumed to have the same (^142) Nd/ 144 Nd ratio as chondrites. Other workers failed to find any excesses in other rocks from Isua, so these results were controversial. More recent work using advanced mass spectrometers by Caro et al. (2003) and Boyet et al. (2003), how- ever, has confirmed the original findings of Harper and Jacobsen. This means that these early parts of the crust formed from a mantle reservoir that had Sm/Nd ratios higher than the chondritic one – and importantly, that this reservoir formed very early, most likely within the first 100 Ma. Subse- quently, other 142 Nd/ 144 Nd “anomalies” were found in other Archean rocks, including ones from Australia and Labrador (Bennett et al., 2007; O’Neil et al., 2009). A yet more surprising result came when Boyet and Carlson (2005) analyzed the 142 Nd/ 144 Nd ra- tios of meteorites and found that terrestrial rocks had 142 Nd/ 144 Nd ratios that average 20 ppm or 0. epsilon units higher than chon- drites, and most eucrites as well (Figure 5.12). This implies that the accessible Earth has a significantly higher Sm/Nd ratio than chon- drites. How much higher depends on when the increase occurred. If the increase occurred 5 million years after the beginning of the so- lar system (taken as the age of CAI’s), the Sm/Nd ratio of the ac- cessible Earth would have to be 8% higher than chondrites; if the in- crease occurred at 30 million years,
Figure 5.12. Variation in ε142Nd in the Earth and meteorites. Gray region is the range measurements of laboratory stan- dards derived from terrestrial Nd. All other terrestrial mate- rials plot within this range with the exception of some sam- ples from Isua, Greenland. Chondrites have, on average, ε142Nd of - 0.2 relative to the terrestrial standards. Data from Caro et al. (2003), Boyet and Carlson (2005), Boyet and Carl- son (2006) and O’Neil et al (2008). SNC data from the compi- lation of Halliday (2001).
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it would have to be 10% higher. If the increase occurred later, the Sm/Nd ratio would have to be even higher. This increase in Sm/Nd might seem small; after all, we have already stated that the assumption that the Earth has chondritic abundances of refractory elements is probably only valid to 10%. Yet this small difference is very important in interpretation of Nd isotope systematics. For the two scenarios above, 5 Ma and 30 Ma, the εNd of the accessible Earth would be +6.9 and +1 1 respectively. These val- ues fall within the range of values of mid-ocean ridge basalts. Recalling that the Isua samples have a 30 ppm excess in 142 Nd relative to a terrestrial standard^ ∗, this means that the Isua samples have a 50 ppm excess in 142 Nd relative to chondrites. How might the increase in Sm/Nd come about? First, we need to recall that meteorites come from the asteroid belt and their compositions might not be representative of the composition of the inner so- lar nebula from which the Earth and the other terrestrial planets formed. Its possible the inner solar nebula had a higher Sm/Nd ratio. That said, it is very difficult to see why this should be so. The ob- servable fractionation in primitive meteorites relates to volatility and lithophile/siderophile tendency. As we have seen, Sm and Nd have quite high and very similar condensation temperatures and neither shows a significant siderophile tendency. Although the possibility cannot be excluded, there is simply no good reason to believe that the Sm/Nd ratio of the material from which the Earth should be differ- ent from chondrites. It is also possible that the early solar system was isotopically heterogeneous and that the material from which the Earth formed was richer in either 146 Sm or 142 Nd than the material that formed chondrite parent bodies. Andreasen and Sharma (2006) and Carlson et al. (2007) found that carbonaceous chondrites have a roughly 100 ppm deficit of 144 Sm. This is significant because while (^142) Nd is primarily a s-process element produced in red giants, both 146 Sm and 144 Sm are p-process-only
isotopes produced in supernova explosions. Incomplete mixing of material from different stars thus
could result in variations in 142 Nd/^144 Nd in the early solar system. However, Andreasen and Sharma
(2006) concluded that while the observed isotopic heterogeneity in 144 Sm implied enough variation in (^146) Sm to explain 142 Nd/ (^144) Nd variations observed among chondrites, it could not explain the difference between the Earth and chondrites. At present, there are two principal theories to explain the difference between terrestrial and chon- dritic 142 Nd/^144 Nd. The first postulates that the bulk Earth does have a chondritic Sm/Nd ratio (and chondritic 142 Nd/^144 Nd), but that low Sm/Nd and 142 Nd/^144 Nd material is sequestered in a hidden res- ervoir deep in the mantle and that consequently the observable Earth has high Sm/Nd and high (^142) Nd/ (^144) Nd. The second postulates that the Earth has high, non-chondritic Sm/Nd and 142 Nd/ (^144) Nd as a consequence of loss of low Sm/Nd material during its accretion as a consequence of collisional ero- sion. Let’s now explore these two possibilities. The first possibility was suggested by Boyet and Carlson (2005). They suggested that Earth under- went early differentiation forming an early enriched reservoir , such as a primordial crust that sank into the deep mantle and has not been sampled since. This differentiation might have occurred as a conse- quence of crystallization of a terrestrial magma ocean analogous to the lunar magma ocean. Alterna- tively, crystallization of the terrestrial magma ocean might have left a layer of residual melt, similar to the KREEP source on the Moon. Boyet and Carlson (2005) noted that if it were rich in Fe and Ti, as is the lunar KREEP reservoir is, once crystallized, the EER could have sunk into the deep mantle, where it remains because if its high density. As Boyet and Carlson (2005) point out, this early enriched reservoir must have formed in the upper mantle. Below the 660 km discontinuity, Mg- and Ca-perovskite would crystallize and fractionate incompatible elements in a manner much different than observed. The second possibility, “collisional erosion”, has been suggested by Caro et al. (2008), O’Neill and Palme (2008), and Caro and Bourdon (2010). As discussed earlier, planetary bodies are thought to form through the process of “oligarchic growth”. The initial stages of this process involve aggregations of dust-sized particles to form sand, which in turn aggregate to form pebble-sized particles, etc. The final
∗ (^) The two standards commonly used in Nd isotope ratios measurements are the “La Jolla” standard and the “Ames”
standard. Both are solutions created from industrially purified Nd.
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duce a very different style of convention than actually occurs. Convective layers heated mainly from below are dominated by plumes that initiate as instabilities at the base of the layer. While some plumes do form at the base of the mantle and rise through it, the dominant form of convection in the Earth’s mantle is plate tectonics, which is the kind of convection expected in systems heated from within (lava lakes, for example, convect in a way similar to plate tectonics). At present, the collisional erosion hypothesis seems to provide the best explanation for the observa- tions we now have. However, regardless of which of these explanations is correct and whether an early-formed crust was eroded by collisions or sunk into the deep mantle, the implications of the non- chondritic Sm/Nd ratio of the Earth are profound. Both hypotheses imply that at least the observable Earth is depleted in the incompatible elements that would have been concentrated in that early crust. This would include the heat producing elements, K, U, and Th, so there are geodynamic implications as well.
Origin of Short-lived Nuclides
There is some debate over exactly how the short-lived radionuclides were synthesized. As we saw earlier in this chapter, heavy element nucleosynthesis occurs mainly in red giant stars and in superno- vae. Anomalous isotopic compositions of stable elements, which we discuss below, provides clear evi- dence that meteorites contain some material synthesized in both these environments. However, they provide no constraints on when this happened. Only with the unstable nuclides can we address the question of when. On galactic scales of time and space, red giants and supernovae continually inject newly synthesized elements into the interstellar medium. Those nuclides that are unstable will steadily decay away. These two competing processes will result in steady-state abundance of these nuclides in the interstellar medium. The abundances of 107 Pd, 129 I, and 182 Hf listed in Table 16.1 roughly match the expected steady-state galactic abundances and hence do not require a specific synthesis event. How- ever, the abundances of shorter-lived 10 Be, 26 Al, 41 Ca, 53 Mn or 60 Fe in the early Solar System requires syn- thesis of these nuclides at the time of, or just before, Solar System formation. The conventional view is that these nuclides were synthesized in a red giant and/or a supernova in the region where the solar system formed just shortly before its formation. Some of these elements, such as 26 Al are most efficiently synthesized in red giants; others, such as 60 Fe are most efficiently syn- thesized in supernovae. Thus most models invoke both environments, which may or may not have been the same star at different times. From an astronomical perspective, nucleosynthesis shortly before the solar system formed is not surprising: stars usually form not in isolation, but in clusters in large clouds of gas and dust known as nebulae. The Great Nebula in Orion is a good example. Some of the stars formed in these stellar nurseries will be quite large and have short lifetimes, ending their existence in supernova explosions. Thus stellar death, including the red giant and supernova phases, goes on simultaneously with star birth in these nebulae. Indeed, one popular hypothesis is that the formation of the solar system was actually triggered by a supernova shock wave. Boss and Vanhala (2001) pro- vide a good discussion of this view. Evidence of the existence of 10 Be in some CAI’s has led to an alternative hypothesis, namely that many of the short-lived extinct radionuclides were produced by spallation within the solar system as it was forming. As we have seen, 10 Be is not synthesized in stars, hence it presence in CAI’s and other primitive chondritic components is problematic for the red giant/supernova injection hypothesis. An- other key observation is that young protostars emit X-rays. X-rays are produced by accelerating charged particles. Hence some have suggested that near the surface of the accreting protosun, mag- netic reconnection events could produce flares that accelerate ions up to very high energies – essentially creating cosmic rays. Spallation would occur when the accelerated particles encounter dust grains – the CAI’s – that happen to be close to the forming Sun (within 0.1 AU). According to this theory, some of these irradiated CAI’s would have been carried back out to the vicinity of the asteroid belt by the ener- getic “X-winds” that are associated with these protostars. This theory can readily account for the abundances of 10 Be, 26 Al, 41 Ca, and 53 Mn observed. If it is correct, it solves the problem of the age gap between CAI’s and chondrules. Based on their apparent 26 Al/^27 Al ratios, CAI’s appear to be several
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million years older than chondrules. Yet the time required for these particles to drift through the solar nebula is only about 10,000 years. The spallation hypothesis means that initial isotope ratios are not simple functions of time, but might also vary in space, particularly radial distance from the protosun, because of variations in radiation flux. Russell et al. (2001) provide a concise summary of this hypothe- sis. Proponents of the red giant/supernova injection hypothesis point out that the spallation hypothesis cannot explain the presence of 60 Fe, which cannot be produced by spallation. They concede that spalla- tion is the only way to produce 10 Be, but argue that it can be produced by collisions with particles accel- erated in the enhanced solar winds of red giants and/or in the expanding envelopes of supernovae. Thus debate continues on this subject. We point out only that the two sets of ideas are not mutually ex- clusive, and it is very possible that both spallation and stellar/explosive nucleosynthesis were in- volved.
STARDUST AND ISOTOPIC ANOMALIES IN METEORITES In addition to the isotopic anomalies that resulted from decay of short-lived radionuclides, there are other isotopic anomalies in meteorites that are not due to such in situ decay. Many of these anomalies, like those created by decay of extinct radionuclides, may reflect the injection of newly synthesized ma- terial into the cloud of dust and gas from which the solar system ultimately formed. Others, however, may reflect isotopic inhomogeneity within this cloud, and the variable abundance of exotic gas and grains of material synthesized at various times and places in the galaxy. Still other isotopic anomalies may reflect chemical fractionations within this cloud. It is these anomalies we focus on in this section.
Neon Alphabet Soup and ‘Pre-Solar’ Noble Gases in Meteorites
Noble gases were the first group of elements in which isotopic variations were identified, and they occur in virtually all of the carbonaceous chon- drites that have not experienced extensive metamorphism. In contrast to the isotopic anomalies of metals mentioned above, most of the isotopically distinct noble gas is contained in the matrix that accreted at low temperature (be- low 100-200° C), specifically in highly unreac- tive carbon species, including organic carbon, graphite, diamond, and silicon carbide. Noble gases are present in meteorites at concentrations that are often as low as 1 part in 10^10. Although they can be isolated and analyzed at these con- centrations, their isotopic compositions are nonetheless partly sensitive to change due to processes such as radioactive decay (for He, Ar, and Xe), spallation and other cosmic-ray in- duced nuclear processes, and solar wind im- plantation. In addition, mass fractionation can significantly affect the isotopic compositions of the lighter noble gases (He and Ne). Up to the late 1960’s, it was thought that all isotopic varia- tions in meteoritic noble gases were related to these processes. For example, Ne isotopic varia- tions could be described as mixtures of three components, “Neon A” or “planetary” (similar in composition to the Earth’s atmosphere),
Figure 5.13. Neon isotopic compositions in a step- heating experiment on Orgueil CI chondrite, which produced the first evidence of ‘pre-solar’ or exotic Ne. The points connected by the line show the changing Ne isotope ratios with increasing temperature. Shaded area was the original esti- mate of the composition of the pure Ne-E compo- nent. Also shown are the compositions of Ne-A (‘solar’), Ne-B (‘planetary’), and Ne-E (‘spallo- genic’). After Black and Pepin (1969).
