Lecture 16 Spring 2007

Geol. 656 Isotope Geochemistry

ISOTOPE COSMOCHEMISTRY

INTRODUCTION Meteorites are our primary source of information about the early Solar System. Chemical, isotopic, and petrological features of meteorites reflect events that occurred in the first few tens of millions of years of Solar System history. Observations on meteorites, together with astronomical observations on the birth of stars and the laws of physics, are the basis of our ideas on how the Solar System, and the Earth, formed. Meteorites can be divided into two broad groups: primitive me- Figure 16.1. Photograph of the meteorite Allende, which fell in Mexico in teorites and differenti- 1969. Circular/spherical features are chondrules. Irregular white patches are ated meteorites. The CAI’s. chondrites constitute the primitive group: most of their chemical, isotopic, and petrological features resulted from processes that occurred in the cloud of gas and dust that we refer to as the solar nebula. All chondrites, however, have experienced at least some metamorphism on “parent bodies”, the small planets (diameters ranging from a few km to a few hundred km) from which meteorites are derived by collisions. The differentiated meteorites, which include the achondrites, stony irons, and irons, were so extensively processed in parent bodies, by melting and brecciation, that information about nebular processes has largely been lost. On the other hand, the differentiated meteorites provide insights into the early stages of planet formation. Chondrites are so called because they contain “chondrules”, small (typically a few mm diameter) round bodies that were clearly once molten (Figure 16.1). The other main constituents of chondrites are the matrix, which is generally very fine grained, amoeboid olivine aggregates (AOA’s), and refractory calcium-aluminum inclusions (generally called CAI’s). These last two groups formed by a variety of mechanisms, some of which appear to be complex, but we can generalize and say that all these are grains or aggregates of grains which are also grain that equilibrated with nebular gas at high temperature through condensation and/or evaporation. Most chondrites can be divided into carbonaceous (C), ordinary, and enstatite classes1. The carbonaceous chondrites are, as their name implies, rich in carbon (as carbonate, graphite, organic matter, and, rarely, microdiamonds) and other volatiles and are further

1 In the last decade or two, additional classes have been added that are defined by rarer meteorites.

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divided into classes CI, CV, CM, CO, CR, CH, and CB. The CI chondrites lack chondrules and are considered the compositionally the most primitive of all objects. The ordinary chondrites are divided into classes H, L, and LL based on iron content, and enstatite chondrites can be subdivided into EH and EL, also based on iron content. Chondrites are further assigned a petrographic grade on the basis of the extent of metamorphism they have experienced in parent bodies. Grades 4, 5, and 6 have experienced in- creasing degrees of high-temperature metamorphism, while grades 1 and 2 experienced low- temperature aqueous alteration. Grade 3 is the least altered. Achondrites are in most cases igneous rocks, some roughly equivalent to terrestrial basalt, others appear to be cumulates. Other achondrites are highly brecciated mixtures. Irons, as they name implies, consist mainly of Fe-Ni metal (Ni content around 5%), and can also be divided into a number of classes. Stony-irons are, as their name implies, mixtures of iron metal and silicates. In these two lectures, we focus on the question of the age of meteorites and variations in their isotopic composition. COSMOCHRONOLOGY Conventional methods Meteorite ages are generally taken to be the age of Solar System. The oft cited value for this age is 4.556 Ga. Before we discuss meteorite ages in detail, we need to consider the question of precisely what event is being dated by radiometric chronometers. Radioactive clocks record the last time the isotope ratio of the daughter element, e.g., 87Sr/86Sr, was homogenized. This is usually some thermal event. In the context of what we know of early Solar System history, the event dated might be (1) the time solid particles were removed from a homogeneous solar nebula, (2) thermal metamorphism in meteorite parent bodies, or (3) crystallization (in the case of chondrules and achondrites), or (4) impact metamorphism of meteorites or their parent bodies. In some cases, the nature of the event being dated is un- clear. The oldest reliable high precision age is from CAI inclusions of Allende, a CV3 meteorite. These give a Pb isotope age of 4.566±0.003 Ga. The matrix of Allende seems somewhat younger, although this is un- certain. Thus this age probably reflects the time of formation of the CAI’s. Precise Pb-Pb ages of 4.552 Ga have been reported by several laboratories for the St. Severin LL chondrite. The same age (4.552±0.003 Ga) has been reported for 2 L5 chondrites. U-Pb ages determined on phosphates in equilibrated (i.e., petrologic classes 4-6) ordinary chondrites range from 4.563 to 4.502 Ga. As these phosphates are thought to be secondary and to have formed during metamorphism, these ages apparently represent the age of metamorphism of these meteorites. Combined whole rock Rb-Sr ages for H, E, and LL chondrites are 4.498±0.015 Ga. However, within the uncertainty of the value of the 87Rb decay constant, this age could be 4.555 Ga (uncertainties normally reported on ages are based only on the scatter about the isochron and the uncertainty associated with the analysis, they do not include uncertainty associated with the decay constant). The age of Allende CAI’s thus seems 5 Ma older than the oldest ages obtained on ordinary chondrites. No attempt has been made at high-precision dating of CI chondrites as they are too fine-grained to separate phases. Pb isotope ages of the unusual achondrite Angra dos Reis, often classed by itself as an ‘angrite’ but re- lated to the Ca-rich achondrites, give a very precise age of 4.5578±0.0004 Ma. Ibitira, a unique un- brecciated eucrite (achondrite), has an age of 4.556±0.006 Ga. Perhaps surprisingly, these ages are the same as those of chondrites. This suggests that the parent body of these objects formed, melted, and crystallized within a very short time interval. Not all achondrites are quite so old. A few other high precision ages (those with quoted errors of less than 10 Ma) are available and they range from this value down to 4.529±0.005 Ga for Nueve Laredo. Thus the total range of the few high precision ages in achondrites is about 30 million years. K-Ar ages are often much younger. This probably reflects Ar outgassing as a result of collisions. These K-Ar ages therefore probably date impact metamorphic events rather than formation ages.

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The present state of conventional meteorite chronology may be summarized by saying that it appears the meteorite parent bodies formed around 4.56±0.005 Ga, and there is some evidence that high- temperature inclusions (CAI's: calcium-aluminum inclusions) and chondrules in carbonaceous chondrites may have formed a few Ma earlier than other material. Resolving events on a finer time-scale than this has proved difficult using conventional techniques. There are, however, other techniques that help to resolve events in early solar system history, and we now turn to these. Initial Ratios Attempts have been made to use initial isotope ratios to deduce a more detailed chronology, but these have been only moderately successful. Figure 16.2 shows initial 87Sr/86Sr ratios of meteorites and lunar rocks and a time scale showing how 87Sr/86Sr should evolve in either a chondritic or solar reservoir. The reference 'initial' 87Sr/86Sr of the solar system is taken as 0.69897±3, based on the work of Pa- panastassiou and Wasserburg (1969) on basaltic achondrites (this value is known as BABI: basaltic achondrite best initial). Basaltic achondrites were chosen since they have low Rb/Sr and hence the initial ratio (but not the age) is well constrained in an isochron. Subsequent high precision analyses of in- dividual achondrites yield identical results, except for Angra Dos Reis and Kapoeta, which have slightly lower ratios: 0.69885. This suggests their parent body(ies) were isolated from the solar system somewhat earlier. CAI's and Rb-poor chondrules from Allende have an even lower initial ratio: 0.69877±3. Allende chondrules appear to be among the earliest formed objects. The parent body of the basaltic achondrites appears to have formed 10 to 20 Ma later. Note there is no distinction in the apparent age of the oldest lunar rocks and the basaltic achondrites: from this we may conclude there was little or no difference in time of formation of the moon, and presumably the Earth, and the basaltic achondrite parent body. The initial 143Nd/144Nd ratio of the solar system is taken as 0.506609±8 (normalized to 143Nd/144Nd = 0.72190) based on the work on chondrites of Jacobsen and Wasserburg (1980). Achondrites seem to have slightly higher initial ratios, suggesting they formed a bit later. The initial isotopic composition of Pb is taken from the work of Tatsumoto et al. (1973) on troilite from the Canyon Diablo iron meteorite as 206Pb/204Pb: 9.307, 207Pb/204Pb: 10.294, 208Pb/204Pb: 29.476. These values are in agreement with the best initial values determined from chondrites, including Allende chondrules. More recent work by Chen and Wasserburg (1983) confirms these results, i.e.: 9.3066,

Figure 16.2. Initial Sr isotope ratios plotted against a time scale for 87Sr/86Sr assuming a chondritic Rb/Sr ratio. After Kirsten (1978).

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10.293, and 29.475 respectively. EXTINCT RADIONUCLIDES There is evidence that certain short-lived nuclides once existed in meteorites. This evidence consists of the anomalous abundance of nuclides, for example, 129Xe, known to be produced by the decay of short-lived radionuclides, e.g., 129I, and correlations between the abundance of the radiogenic isotope and the parent element. Consider, for example, 53Cr, which is the decay product of 53Mn. The half-life of 53Mn, only 3.7 million years, is so short that any 53Mn produced by nucleosynthesis has 53 52 long since decayed. If 53Mn is no longer Figure 16.3. Correlation of the Cr/ Cr ratio with 55 52 present, how do we know that the anoma- Mn/ Cr ratio in inclusions from the Allende CV3 me- 53 53 teorite. After Birck and Allegre (1985). lous Cr is due to decay of Mn? We reason that the abundance of 53Mn, when and if it was present, should have correlated with the abundance of other isotopes of Mn. 55Mn is the only stable isotope of Mn. So we construct a plot similar to a conventional isochron diagram (isotope ratios vs. parent/daughter ratio), but use the stable isotope, in this case 55Mn as a proxy for 53Mn. An example is shown in Figure 16.3. Starting from our basic equation of radioactive decay, we can derive the following equation: !"t D = D0 + N0 (1! e ) 16.1 This is a variation on the isochron equation we derived in lecture 4. Written for the example of the decay of 53Mn to 53Cr, we have: 53Cr ! 53Cr $ ! 53 Mn$ (1 e'(t ) 16.2 52 = # 52 & + # 52 & ' Cr " Cr % 0 " Cr % 0 where the subscript naught denotes an initial ratio, as usual. The problem we face is that we do not know the initial 53Mn/52Cr ratio. We can, however, measure the 55Mn/53Cr ratio. Assuming that initial 53 55 isotopic composition of Mn was homogeneous in all the reservoirs of interest; i.e., Mn/ Mn0 is constant, the initial 53Mn/52Cr ratio is just: ! 53 Mn$ ! 55 Mn$ ! 53 Mn $ 16.3 # 52 & = # 52 & # 55 & " Cr % 0 " Cr % 0 " Mn% 0 Of course, since 55Mn and 52Cr are both non-radioactive and non-radiogenic, the initial ratio is equal to the present ratio (i.e., this ratio is constant through time). Substituting 16.3 into 16.2, we have: 53Cr ! 53Cr $ ! 55 Mn$ ! 53 Mn $ (1 e'(t ) 16.4 52 = # 52 & + # 52 & # 55 & ' Cr " Cr % 0 " Cr % 0 " Mn% 0 Finally, for a short-lived nuclide like 53Mn, the term λt is very large after 4.55 Ga, so the term e–λt is 0 (this is equivalent to saying all the 53Mn has decayed away). Thus we are left with: 53Cr ! 53Cr $ ! 55 Mn$ ! 53 Mn $ 16.5 52 = # 52 & + # 52 & # 55 & Cr " Cr % 0 " Cr % 0 " Mn% 0

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On a plot of 53Cr/52Cr vs. 55Mn/52Cr, the slope is propor- Table 16.1. Short-Lived Radionuclides in the Early tional not to time, as in a conven- Solar System tional isochron diagram, but to the Radio- Half-life Decay Daughter Abundance initial 53Mn/55Mn ratio. Thus cor- nuclide Ma Ratio relations between isotope ratios 10Be 1.5 β 10B 10Be/9Be ~ 9 × 10–4 such as these is evidence for the ex- 26Al 0.7 β 26Mg 26Al/27Al ~ 5 × 10–5 istence of extinct radionuclides. 41Ca 0.15 β 41K 41Ca/40Ca 1 × 10–8 In this way, many extinct radi- 53Mn 3.7 β 53Cr 53Mn/55Mn ~ 4 × 10–5 onuclides have been identified in 60Fe 1.5 β 60Ni 60Fe/56Fe ~ 6 × 10–8 meteorites from variations in the 107Pd 9.4 β 107Ag 107Pd/108Pd ~ 4 × 10–4 abundance of their decay products. 129I 16 β 129Xe 129I/127I ~ 1 × 10–4 The most important of these are 146Sm 103 α 142Nd 146Sm/144Sm ~ 0.008 listed in Table 16.1. On a cosmic 182Hf 9 β 182W 182Hf/180Hf ~ 2.6 × 10–4 scale, nucleosynthesis is a more or 244Pu 82 α, SF Xe 244Pu/238U ~ 0.005 less continuous process – roughly once every second, a supernova explodes somewhere in the universe. So we might expect that inter- stellar dust might contain some of the longer-lived of these nuclides at low concentrations. However, such events are much rarer on a local scale (fortunately for us), and the shorter-lived of these nuclides must have been synthesized nearby shortly (on geological time scales) before the solar system formed. To understand why these short-lived radionuclides require a nucleosynthetic event, consider the example of 53Mn. Its half-life is 3.7 Ma. Hence 3.7 Ma after it was created only 50% of the original number of atoms would remain. After 2 half-lives, or 7.4 Ma, only 25% would remain, after 4 half-lives, or 14.8 Ma, only 6.125% of the original 53Mn would remain, etc. After 10 half lives, or 37 Ma, only 1/210 (0.1%) of the original amount would remain. The correlation between the Mn/Cr ratio and the abundance of 53Cr indicates some 53Mn was present when the meteorite, or its parent body, formed. From this we can conclude that an event that synthesized 53Mn occurred not more than roughly 30 million years before the meteorite formed. We will return to this issue in the next lecture. 129I–129Xe and 244Pu Among the most useful of these short- lived radionuclides, and the first to be discovered, has been 53I, which decays to 129Xe. Figure 16.4 shows the example of the analysis of the meteorite Khairpur. In this case, the analysis in done in a manner very analogous to 40Ar-39Ar dating: the sample is first irradiated with neutrons so that 128Xe is produced by neutron capture and subsequent decay of 127I. The amount of 128Xe produced is proportional to the amount of 127I present (as well as the neutron flux and reaction cross section). The sample is then heated in vacuum through a series of steps and the Xe released at each step analyzed in a Figure 16.4. Correlation of 129Xe/130Xe with 128Xe/130Xe. mass spectrometer. As was the case in 128 127 Figure 16.3, the slope is proportion to the The Xe is produced from I by irradiation in a reactor, so that the 128Xe/130Xe ratio is proportional to the 127I/130Xe 129I/127I ratio at the time the meteorite ratio. Numbers adjacent to data points correspond to formed. temperature of the release step.

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In addition to 129Xe produced by decay of 129I, the heavy isotopes of Xe are produced by fission of U and Pu. 244Pu is of interest because it another extinct radionuclide. Fission does not produce a single nuclide, rather a statistical distribution of many nuclides. Each fission- able isotope produces a different distribution. The distribution produced by U is similar to that produced by 244Pu, but the difference is great enough to demon- strate the existence of 244Pu in meteorites, as is shown in Figure 16.5. Fission tracks in excess of the expected number of tracks for a known uranium concen- tration are also indicative of the former 244 presence of Pu. Figure 16.5. Variation of 134Xe/132Xe and 136Xe/132Xe in mete- These extinct radionuclides provide a orites (5). The isotopic composition of fission products of man-made 244Pu is shown as a star (✯). After Podosek and Swindle (1989).

means of relative dating of meteorites and other bodies. Of the various systems, the 129I–129Xe decay is perhaps most useful. Figure 16.6 shows relative ages based on this decay system. These ages are calculated from 129I/127I ratios, which are in turn calculated from the ratio of excess 129Xe to 127I. Since the initial ratio of 129I/127I is not known, the ages are relative to an arbitrary value, which is taken to be the age of the Bjurböle meteorite, a L4 chondrite. The ages ‘date’ closure of the systems to Xe and I mobility, but it is not clear if this occurred at condensation or during metamorphism. Perhaps both are involved. The important point is that there is only slight systematic variation in age with meteorite types. Carbona- ceous chondrites do seem to be older than ordinary and enstatite chondrites, while LL chondrites seem to be the youngest. Differentiated meteorites are generally younger. These are not shown, except for silicate in the El Taco iron, which is not particularly young. The bottom line here is that all chondrites closed to the I-Xe decay system Figure 16.6. Summary of I-Xe ages of meteorites relative to within about 20 Ma. Bjurböle. After Swindle and Podosek (1989).

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An interesting aspect of Figure 16.6 is that the achondrites, which are igneous in nature, and the irons are at most only slightly younger on average than the chondrites. Irons and achondrites are both products of melting on meteorite parent bodies. That they appear to be little younger than chondrites indicates that and melting and differentiation of those planetismals must have occurred very shortly after the solar system itself formed and within tens of millions of years of the synthesis of 129I. 107Pd–107Ag The existence of variations in isotopic composition of silver, and in particular variations in the abundance of 107Ag that correlate with the Pd/Ag ratio in iron meteorites indicates that 107Pd was present when the irons formed. The half-life of 107Pd is 9.4 million years; hence the irons must have formed within a few tens of millions of years of synthesis of the 107Pd. This in turn implies that formation of iron cores within small planetary bodies occurred within a few tens of millions of years of formation of the solar system. Fractions of metal from the meteorite Gibeon (IVA iron) define a fossil isochron indicating an initial 107Pd/108Pd ratio of 2.4 -5 × 10 (Chen and Wasserburg, 1990). Other Figure 16.7. Correlation of 107Ag/109Ag with 108Pd/109Ag IVA irons generally fall along the same iso- in Group IVA iron meteorites, demonstrating the exis- chron (Figure 16.7). IIAB and IIAB irons, tence of 107Pd at the time these irons formed. After Chen as well as several anomalous irons show and Wasserburg (1984). 107Ag/109Ag–108Pd/109Ag correlations that indicate 107Pd/108Pd ratios between 1.5 and 2.4 × 10-5. Assuming these differences in initial 107Pd/108Pd are due to time and the decay of 107Pd, all of these iron meteorites would have formed no more than 10 million years after Gibeon (Chen and Wasserburg, 1996). 26Al–26Mg Another key extinct radionuclide has been 26Al. Because of its short half-life (0.72 Ma), it provides much stronger constraints on the amount of time that could have passed between nucleosynthesis and processes that occurred in the early solar system. Furthermore, the abundance of 26Al was such that it’s decay could have 26 been a significant source of heat. Al decays to 26Mg; an example of the cor- 26 24 Figure 16.8. Al-Mg evolution diagram for Allende CAI WA. relation between Mg/ Mg and 26 27 27 24 Slope of the line corresponds to an initial Al/ Al ratio of Al/ Mg is shown in Figure 16.8. 26Al/27Al ratio of 5.1 × 10-4. After Lee et al. (1976).

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Because of the relatively short half-life of 26Al and its potential importance as a heat source, considerable effort has been devoted to measurement of Mg isotope ratios in meteorites. Most of this work has been carried out with ion microprobes, which allow the simultaneous measurement of 26Mg/24Mg and 27Al/24Mg on spa- tial scales as small as 10 µ. As a result, there are some 1500 measurements on 60 meteorites reported in the literature, mostly on CAI’s. The reason for the focus on CAI’s is, of course, because their

high Al/Mg ratios should produce Figure 16.9. Inferred initial 26Al/27Al for all available meteoritic 26 24 higher Mg/ Mg ratios. data. After MacPherson et al. (1995). Figure 16.9 summarizes these data. These measurements show a maximum in the 26Al/27Al ratio of around 4.5 × 10-5. Significant 26Mg anomalies, which in turn provide evidence of 26Al, are mainly confined to CAI’s. This may in part reflect the easy with the anomalies are detected in this material and the focus of research efforts, but it almost certainly also reflects real differences in the 26Al/27Al ratios between these objects and other materials in meteorites. This in turn probably reflects a difference in the timing of the formation of the CAI’s and other materials, including chondrules. The evidence thus suggests that CAI’s formed several million years before chondrules and other materials found in meteorites. Extinct Radionuclides in the Earth Several of the short-lived radionuclides listed in Table 16.1 have half-lives sufficiently long that they should have been present in the early Earth. Of greatest interest are 129I, 182Hf, and 146Sm. The decay products of these nuclides are 129Xe, 182W, and 142Nd, an atmophile, a siderophile, and a lithophile element respectively. Their distribution can tell us about the early evolution of the Earth’s atmosphere, core, and mantle. Here we’ll consider 182Hf and 142Sm. We’ll discuss 129I in the lecture on the origin and evolution of the atmosphere. 182Hf–182W and Core Formation The Hf-W pair is particularly interesting because Hf is lithophile while W is moderately siderophile. Thus the 182Hf-182W decay system should be useful in “dating” silicate-metal fractionation, including core formation in the terrestrial planets and asteroids. Both are highly refractory elements, while has the advantage the one can reasonably assume that bodies such as the Earth should have a chondritic Hf/W ratio, but the disadvantage that both elements are difficult to analyze by conventional thermal ionization. These observations have led to a series of measurements of W isotope ratios on terrestrial materials, lunar samples, and a variety of meteorites, including those from Mars. The conclusions have evolved and new measurements have become available. Among other things, the story of Hf-W illus- trates the importance of the fundamental dictum in science that results need to be independently repli- cated before they be accepted. Because the variations in 182W/183W ratio are quite small, they are generally presented and discussed

in the same ε notation used for Nd and Hf isotope ratios. There is a slight difference, however; εW is the

deviation in parts per 10,000 from a terrestrial tungsten standard, and ƒHf/W is the fractional deviation of the Hf/W ratio from the chondritic value. Assuming that the silicate Earth has a uniform W isotope composition identical to that of the standard (an assumption which has not yet been proven), then the

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silicate Earth has εW of 0 by definition. The basic question 15 can posed this way: if the Carbonaeous Chondrite 182 183 Chondrite W/ W ratio in the silicate Achondrite (Eucrite) Earth is higher than in chondrites, it would mean that 10 much of the Earth’s tungsten had been sequestered in the Earth’s core before 182Hf had ε entirely decayed. Since the W 5 half-life of 182Hf is 9 Ma and us- Moon ing our rule of thumb that a Initial 182Hf/180Hf = 1.0 x 10-4 radioactive nuclide is fully decayed in 5 to 10 half-lives, this 0 Silicate Earth would mean the core must have formed within 45 to 90 182Hf/180Hf = 1.1 x 10-5 (29.5 Ma) million years of the time chon- -5 dritic meteorites formed (i.e., 0 5 10 15 of the formation of the solar fHf/W system). If on the other hand, the 182W/183W ratio in the sili- Figure 16.10. W isotope ratios in meteorites, the Moon and the cate Earth was the same as in Earth reported by Yin et al. (2002). chondrites, which never underwent silicate-metal fractionation, this would mean that at least 45 to 90 million years must have elapsed (enough time for 182Hf to fully decay) between the formation of chondrites and the formation of the Earth’s core. ‘Anomalous’ W isotopic compositions were first found in the IA iron Toluca by Harper et al. (1991). They found the 182W/183W ratio in the meteorite was 2.5 epsilon units (i.e., parts in 10,000) lower than in terrestrial W. This value was revised to -3.9 epsilon units by subsequent, more precise, measurements (Jacobsen and Harper, 1996). Essentially, the low 182W/183W ratio indicates Toluca metal separated from Hf-bearing silicates before 182Hf had entirely decayed. Because of the difference between “terrestrial” W, Jacobsen and Harper (1996) concluded the Earth’s core must have segregated rapidly. At this point, however, no measurements had yet been made on chondritic meteorites, which never underwent silicate-iron fractionation, so the conclusion was tentative. Lee and Halliday (1995) reported W isotope ratios for 2 carbonaceous chondrites (Allende and Mur- chison), two additional iron meteorites (Arispe, IA, and Coya Norte, IIA) and a lunar basalt. They found 182 the iron meteorites showed depletions in W (εW = -4.5 and -3.7 for Arispe and Coya Norte respectively) that were similar to that observed in Toluca reported by Jacobsen and Harper (1996). The chondrites,

however, had εw values that were only slightly positive, about +0.5, and were analytically indistin- guishable from “terrestrial” W, as was the lunar basalt. Lee and Halliday (1995) inferred an initial 182Hf/180Hf for the solar nebula of 2.6 × 10-4, much higher than assumed by Jacobsen and Harper. Based on this similarity of isotopic compositions of chondritic and terrestrial W, Lee and Halliday (1995) concluded that the minimum time required for formation of the Earth’s core was 62 million years.

Subsequently, Lee and Halliday (1998) reported εW values of +32 and +22 in the achondrites Juvinas and ALHA78132. These large differences in W isotopic composition meant that metal-silicate fractionation, i.e., core formation, occurred quite early in the parent bodies of achondritic meteorites; in other words, asteroids or “planetismals” must have differentiated to form iron cores and silicate mantles very early, virtually simultaneous with the formation of the solar system. This is consistent with other evidence discussed above for very little age difference between differentiated and undifferentiated mete-

orites. Lee and Halliday (1998) also reported εW values in the range of +2 to +3 in 3 SNC meteorites

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thought to have come from Mars. These data indicated that the Martian core formed relatively Karooonda early. The heterogeneity in tungsten isotopes indicates in Murray Martian mantle was never fully Nogoya homogenized. Lee et al. (1997) reported that the W isotope ratio Cold Bokkeveld of the Moon was about 1 epsilon Carbonaceous chondrites unit higher than that of terres- Murchison trial W. Thus at this point, the Earth Orgueil appeared to be puzzlingly anomalous among differentiated planetary bodies in that silicate- Allende metal differentiation appeared to a have occurred quite late. In the b latest chapter of this story, Yin et al. (2002) reported W isotope IGDL-GD measurements carried out in two G1-RF Terrestrial laboratories, Harvard University BB samples and the Ecole Normale Supé- BE-N rieure de Lyon, which showed that the chondrites Allende and Murchison which showed that Toluca a they had W isotope ratios 1.9 to b 2.6 epsilon units lower than the c terrestrial standard (Figure 16.10). In the same issue of the -6 -5 -4 -3 -2 -1 0 1 2 journal Nature, Kleine et al., εW (2002) reported similarly low εW (i.e., -2) for the carbonaceous Figure 16.11. W isotope ratios measured in chondrites, the iron chondrites Allende, Orgueil, Mur- meteorite Toluca, and terrestrial materials by Kleine et al. chison, Cold Bokkeveld, Nogoya, (2002). Murray, and Karoonda measured in a third laboratory (University of Münster). Furthermore, Kleine et al. (2002) analyzed a variety of terrestrial materials and found they all had identical W isotopic composition (Figure 16.11). 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) also analyzed separated metal and silicate fractions from two ordinary chondrites (Dhurmsala and Dalgety Downs) that allowed them to estimate the initial 182Hf/180Hf of the solar system as 1 × 10-4. Yin et al. (2002) considered two scenarios for the formation of the core (Figure 16.12). 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 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 parent body of the eucrite class of achondrites (suspected to be the large asteroid Vesta) underwent core formation within 3 million years of formation of the solar system. Klein et al. (2002) reached similar conclusions.

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146Sm-142Nd As we have mentioned, geochemists generally assume that Magma ocean Two-stage model rare earth and other refractory 3.0 model elements have the same relative concentrations in the Earth as they have in chondrites. If so, the Sm/Nd ratio of the Earth should be chondritic. Thus the 11±1 Myr 29.5±1.5 Myr 147Sm/144Nd ratio of the present 2.0 Earth should be chondritic and the 146Sm/144Nd of the early Earth should have been chon- 143 144 dritic. Thus the Nd/ Nd ΔεW and 142Nd/144Nd of the bulk earth should also be chondritic. 1.0 However, recent studies of the 142 144 Lee & Halliday (1995) Nd/ Nd ratio in chondrites and terrestrial materials suggest that this may not be the case, at least that part of the Earth accessible to sampling. 0.0 This is surprising to say the least. These two elements are very similar to each other in chemical behavior, having identical configurations of elec- -1.0 trons in bonding orbitals, and 0 10 20 30 40 50 60 70 are both refractory lithophile Mean time of core formation (Myr) elements. Indeed, Nd and Sm Figure 16.12. Models for timing of core formation in the Earth. The have 50% condensation tem- figure shows how the difference between the 182W/183W between peratures of 1602 and 1590 K, the silicate Earth and chondrites, Δε , declines as a function of time respectively. It is difficult to W between formation of the chondrites and separation of the Earth’s see how processes operating in core. Yin et al. (2002) considered two scenarios: a two-stage model the solar nebula could have in which Earth first accretes completely and then the core forms, fractionated these elements and a model in which the core segregates progressively from a significantly. The total range magma ocean as the Earth accretes. In the first scenario, the mean of high precision Sm/Nd ratio age of the core is about 30 million years, in the second it is 11 mil- measurements in chondrites lion years. These results are sharply different from those of Lee varies by less than 3%, which and Halliday (1995) who found only a small difference in ε be- would seem to confirm that W tween the Earth and chondrites and consequently concluded the these elements were not frac- core formed later (at about 60 million years). tionated in the solar nebula. 142Nd is the product of α- decay of 146Sm, a nuclide with a half-life of 103 million years. As Table 16.1 shows, the initial 146Sm/144Sm ratio of the solar system about 0.008, a value deduced from 142Nd/144Nd variations in meteorites using procedures discussed above. 144Sm is the least abundant isotope of Sm, comprising only 3% of natural Sm, so even initially, 146Sm would have only constituted 0.025% of Sm. Because of this and because the range of Sm/Nd ratios in nature is small, any variations in the 142Nd/144Nd ratio should be quite small, no more than a few 10’s of ppm. Detecting such small variations is analytically challeng-

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ing, and indeed was nearly impossible before about 15 years ago. Furthermore, because the half-life of 146Sm is short, any variation in 142Nd/144Nd must be the result of fractionation occurring in at most the first few hundred million years of solar system or Earth history. However, considerable variation in the 142Nd/144Nd ratio had been detected in SNC meteorites, which suggested early mantle differentiation on Mars. It thus seemed reasonable to look for such variations on Earth. Geochemists focused their initial attention on early crustal rocks from the Isua area in Greenland. Some rocks from this area are as old as 3.8 Ga and have initial 143Nd/144Nd ratios several epsilon units above the chondritic value, suggested there were derived from an incompatible element-depleted mantle with high Sm/Nd. A study by Harper and Jacobsen (1992) reported a 33 ppm excess of 142Nd 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 142Nd/144Nd ratio as chondrites. Other workers failed to find any ex- cesses 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), however, has confirmed the original find- ings 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. A yet more surprising result came when Boyet and Carlson (2005) analyzed the 142Nd/144Nd ratios of meteorites and found that terrestrial rocks had 142Nd/144Nd ratios that average 20 ppm or 0.2 epsilon units higher than chondrites, and most eucrites as Figure 16.13. Variation in ε142Nd in the Earth and meteorites. well (Figure 16.13). This implies that Gray region is the range measurements of laboratory stan- the accessible Earth has a signifi- dards derived from terrestrial Nd. All other terrestrial mate- cantly higher Sm/Nd ratio than rials plot within this range with the exception of some sam- chondrites. How much higher de- ples from Isua, Greenland. Chondrites have, on average, pends on when the increase oc- ε142Nd of -0.2 relative to the terrestrial standards. Data from curred. If the increase occurred 5 Caro et al. (2003), Boyet and Carlson (2005), Boyet and Carl- million years after the beginning of son (2006). SNC data from the compliation of Halliday the solar system (taken as the age of (2001).

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CAI’s), the Sm/Nd ratio of the accessible Earth would have to be 8% higher than chondrites; if the increase occurred at 30 million years, 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 +8.4 and +10.1 respectively. These values fall within the range of values of mid-ocean ridge basalts. Recalling that the Isua samples have a 30 ppm excess in 142Nd relative to a terrestrial standard∗, this means that the Isua samples have a 50 ppm excess in 142Nd 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 solar 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 Earth should be different from chondrites. If the Earth does have the same Sm/Nd ratio as chondrites, then the Sm/Nd ratio of the accessible Earth must be higher than that of the Earth as a whole. Since neither Sm nor Nd should be present to any significant degree in the core, this implies there is an unsampled reservoir in the mantle with a lower than chondritic Sm/Nd ratio. Furthermore, since 146Sm has a half-life of only 103 Ma, the differentiation that produced high and low Sm/Nd reservoirs must have occurred very early in Earth’s history. Fractional crystallization of a magma ocean might seem an obvious candidate for this event. In- deed, Boyet and Carlson (2005) suggested that crystallization of the terrestrial magma ocean left a layer of residual melt, similar to the KREEP source on the Moon. They termed this hypothetical reservoir the early enriched reservoir (EER) and its compliment the early depleted reservoir (EDR). The EER would be created in the upper mantle, but since it is unsampled by volcanism and tectonism, the unsampled mantle reservoir should be in the deep mantle. 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 may have sunk into the deep mantle, where it remains because if its high density. So in their scenario, the EDR forms in lower mantle but ends up becoming the part of the mantle that produced the continental crust and continues to be sam- pled by volcanism today. In other words, the EDR comprises the accessible mantle. The EER could be the product of fractional crystallization of a mantle that was initial entirely or largely molten. In that case, the principal crystallizing phases would be the deep mantle minerals, the two perovskite phases and magnesiowüstite. Judging from partition coefficients published by Corgne et al. (2005), a cumulate layer formed of Mg-perovskite should have a Sm/Nd ratio over twice that of chondrites, far too high to be appropriate for the accessible mantle. This could be mixed with unfrac- tionated mantle material. However, because Sm and Nd concentrations would be low in the perovskite cumulate, a great deal of it would be necessary: the EDR would be composed of about 75% Mg- perovskite cumulate. This reservoir would be quite depleted in most incompatible elements, with Sm and Nd concentrations only about 1/3 those of the BSE and would have ratios of some refractory elements that are very different from chondritic. Another possibility is that Mg- and Ca-perovskite crystallized together to form the cumulate. This requires about 70-75% fractional crystallization, judging from the partition coefficients of Corgne et al. (2005). Interestingly, because Sm and Nd partition into Ca-perovskite, an EDR created in this way would have concentrations of many lithophile trace elements, including the REE, U, and Th, that are close to or slightly higher than BSE. However, ratios of some elements, such as Sc/Sr and Ba/Sm, would be very different from chondritic. This problem

∗ 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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seems to preclude the possibility of creating the EDR by crystallization of a magma ocean that extended into the deep mantle. Boyet and Carlson (2005) suggested instead that crystallization of the magma ocean involved purely upper mantle phases. This would be the case if the magma ocean were relatively shallow and did not extend substantially deeper than 660 km. The problem with this idea is that region above constitutes only 25% of the mass of the mantle and it is difficult to create an enriched reservoir within it with sufficiently low Sm/Nd that the remaining mantle would have Sm/Nd 10% greater than chondritic. In the upper mantle, the phase that is likely to fractionate Sm and Nd most is majorite garnet. However, even majorite does not seem to fractionate Sm and Nd enough to do this. Using partition coefficients published by Corgne and Wood (2004), no extent of fractional crystallization of majorite from an upper mantle magma ocean produces a sufficiently enriched residual melt layer to leave the rest of the mantle with a Sm/Nd ratio that is 10% higher than chondritic. Yet another alterative is that a primordial crust was created by partial melting of an already solidified mantle. That crust would have been enriched in incompatible elements just as the modern crust is. The crust may have destabilized in some way and been recycled back into the deep mantle. Under certain circumstances, its density might be greater than that of ordinary mantle peridotite such that if forms a stable layer in the deep mantle, perhaps in D”. Boyet and Carlson (2005) calculated that if this Early En- riched Reservoir (EER) occupied the volume of D”, it would have to be as nearly enriched in incompatible elements as the present continental crust. If the EER comprises the region deeper than 1600 km, it need be only twice as enriched in incompatible elements as the bulk silicate Earth. There are significant problems with all of the scenarios; none seems entirely satisfactory. An additional problem with them all is that there is no seismic evidence for chemical layering in the deep mantle. Indeed, tomographic imaging of the mantle shows seismically fast regions extending from subduc- tion zones at the surface to the deep mantle. These regions are presumably sinking oceanic lithosphere. Other images show mantle plumes rising from near the core-mantle boundary to the surface. Both suggest whole mantle convection and consequently, whole mantle mixing. It is difficult to see how any chemical layer could survive. Thus the discovery that the 142Nd/144Nd ratio of the accessible Earth is 20 ppm higher than chondrites presents a difficult and intricate problem for geochemistry: a true conundrum. It is certainly the result of something that happened very early. If it reflects fractionation is the solar nebula, then our understanding of nebular chemistry is weaker than we realize. If it is the product of early differentiation of the Earth, then our understanding of both early planetary processes and the chemistry of the Earth may be poorer than we had thought. In particular, it potentially invalidates our estimates of the composition of the Earth, and also models of its evolution.

REFERENCES AND SUGGESTIONS FOR FURTHER READING Birck, J. L. and C. J. Allègre. 1985. Evidence for the presence of 53Mn in the early solar system. Earth Planet. Sci. Lett. Geophys. Res. Lett.: 745-748. Boyet, M., J. Blichert-Toft, M. Rosing, M. Storey, P. Telouk and F. Albarede, 142Nd evidence for early Earth differentiation, Earth Planet. Sci. Lett., 214:427-442, 2003. Boyet, M. and R. L. Carlson, 142Nd evidence for early (>4.3 Ga) global differentiation of the silicate Earth, Science, 309: 576-581, 2005. Caro, G., B. Bourdon, J.-L. Birck and S. Moorbath, 146Sm-142Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth's mantle, Nature, 423: 428-432, 2003. Corgne, A., C. Liebske, B. J. Wood, D. C. Rubie and D. J. Frost, Silicate perovskite-melt partitioning of trace elements and geochemical signature of a deep perovskitic reservoir, Geochim Cosmochim Acta, 69:485-496, 2005. Corgne A. and Wood B. J. Trace element partitioning betweenmajoritic garnet and silicate melt at 25 GPa. Phys. Earth Planet. Inter., 143–144, 407–419, 2004.

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Chen, J. H. and G. J. Wasserburg, 1983. The least radiogenic Pb in iron meteorites. Fourteenth Lunar and Planetary Science Conference, Abstracts, Part I, Lunar & Planet Sci. Inst., Houston, pp. 103-104. Chen, J. H. and G. J. Wasserburg. 1990. The presence of 107Pd in the early solar system. Lunar Planet. Sci. Conf. Absts. 21: 184-185. Chen, J. H. and G. J. Wasserburg. 1996. Live 107Pd in the early solar system and implications for planetary evolution. In Earth Processes: Reading the Isotope Code, Vol. 95, S. R. Hart and A. Basu. ed., pp. 1-20. Washington: AGU. Halliday, A. N., H. Wanke, J.-L. Birck and R. N. Clayton, The accretion, composition, and early differentiation of Mars, Space Sci. Rev., 96: 197-230, 2001. Harper, C. L., J. Volkening, K. G. Heumann, C.-Y. Shih and H. Wiesmann. 1991. 182Hf-182W: New cos- mochronometric constraints on terrestrial accretion, core formation, the astrophysical site of the r- process, and the origina of the solar system. Lunar Planet Sci. Conf Absts. 22: 515-516. Harper, C. L. and S. B. Jacobsen, Evidence from coupled 147Sm-143Nd and 146Sm-142Nd systematics for very early (4.5-Gyr) differentiation of the Earth's mantle, Nature, 360: 728-732, 1992. Jacobsen, S. B. and C. L. Harper. 1996. Accretion and early differentiation history of the Earth based on extinct radionuclides. In Earth Processes: Reading the Isotope Code, Vol. 95, S. R. Hart and A. Basu. ed., pp. 47-74. Washington: AGU. Jacobsen, S. and G. J. Wasserburg, 1980, Sm-Nd isotopic evolution of chondrites, Earth Planet. Sci. Lett., 50, 139-155. Kleine, T., C. Münker, K. Mezger and H. Palme, 2002, Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry, Nature, 418:952-954. Lee, D. C. and A. N. Halliday. 1995. Hafnium-tungsten chronometry and the timing of terrestrial core formation. Nature. 378: 771-774. Lee, D. C. and A. N. Halliday. 1998. Hf-W evidence for early differentiation of Mars and the Eucrite parent body. Lunar Planet. Sci. Conf. Absts. 28: 79. Lee, T., D. A. Papanastassiou and G. J. Wasserburg, 1976. Demonstration of 26Mg excess in Allende and evidence for 26Al, Geophys. Res. Lett., 3: 41-44. MacPherson, G. J., A. Davis and E. Zinner. 1995. The distribution of aluminum-26 in the early Solar System-A reappraisal. Meteoritics. 30: 365-385. Papanastassiou, D. A., and G. J. Wasserburg, 1969. Initial strontium isotopic abundances and the reso- lution of small time differences in the formation of planetary objects. Earth Planet. Sci. Lett., 5: 361-376. Podosek, F. A., 1970. Dating of meteorites by high temperature release of iodine correlated 129Xe, Geo- chim. Cosmochim. Acta, 34: 341-365. Podosek, F. and T. D. Swindle.1989. Extinct Radionuclides. in Meteorites and the Early Solar System, ed. 1093-1113. Tuscon: Univ. of Arizona Press. Shuloyukov, A., and G. W. Lugmair, 1993. 60Fe in eucrites, Earth Planet. Sci. Lett., 119: 159-166. Swindle, T. D. and F. Podosek.1989. Iodine-Xenon Dating. in Meteorites and the Early Solar System, ed. 1093-1113. Tuscon: Univ. of Arizona Press. Tatsumoto, M., R. J. Knight, and C. J. Allègre, 1973. Time differences in the formation of meteorites ad determined from the ratio of lead-207 to lead-206, Science, 180: 1279-1283. Yin, Q., S. B. Jacobsen, Y. K., J. Blichert-Toft, P. Télouk and F. Albarède, 2002. A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites, Nature, 418:949-951.

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