Isotope Cosmochemistry

Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

ISOTOPE COSMOCHEMISTRY

INTRODUCTION Meteorites are our primary source of information about the early Solar System. Chemical, isotopic and petrological features of meteorites re- flect events that occurred in the first few tens of millions of years of Solar System history. Observations on meteorites, together with astro- nomical observations on the birth of stars and the laws of phys- ics, are the basis of our ideas on how the So- lar System, and the Earth, formed. Meteorites can be divided into two Figure 1. Photograph of the meteorite Allende, which fell in Mexico in broad groups: primi- 1969. Circular/spherical features are chondrules. Irregular white patches tive meteorites and are CAI’s. differentiated meteorites. The 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, by melting and brecciation, in parent bodies 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 1). The other main constituents of chondrites are the matrix, which is generally very fine grained, and refractory, or Ca-Al, inclusions (called CAI’s or RI’s), which are evaporative resides or high-temperature condensates. Chondrites are divided into carbonaceous (C), H, L, LL (collectively called ordinary, or O chondrites), and E 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 divided into classes CI, CV, CM, and CM. The CI chondrites lack chondrules and are considered the compositionally the most primitive of all objects. The classification of the remaining chondrites is based on their content of iron oxidation state of the iron. Chondrites are further assigned a petrographic grade on the basis of the extent of

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

159 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

metamorphism they have experienced in parent bodies. Grades 4, 5, and 6 have experienced increas- ing 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. 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 unclear. The oldest reliable high precision age is from CAI inclusions of Allende, a CV3 meteorite. These give a Pb isotope age of 4.568±0.003 Ga. The matrix of Allende seems somewhat younger, although this is uncertain. 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 4.504 Ga. As these phosphates are thought to be secondary and to have formed during metamorphism, these ages appar- ently 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 related 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. 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 timescale 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.

160 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

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 23.2 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 Papanastassiou and Wasserburg (1969) on basaltic achondrites (this value is known as BABI: b a- saltic a chondrite b est i nitial). 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 individual 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 dis- tinction 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 Al- lende chondrules. More recent work by Chen and Wasserburg (1983) confirms these results, i.e.: 9.3066, 10.293, and 29.475 respectively. EXTINCT RADIONUCLIDES There is evidence that certain short-lived nuclides once existed in meteorites. This evidence con- sists 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

Figure 23.2. Initial Sr isotope ratios plotted against a time scale for 87Sr/86Sr assuming a chondritic Rb/Sr ratio. After Kirsten (1978).

161 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

and the parent element. Consider, for ex- 0.11349 ample, 53Cr, which is the decay product of Allende Inclusions 53Mn. The half-life of 53Mn, only 3.7 mil- 2 lion years, is so short that any 53Mn produced by nucleosynthesis has long since de- 1 cayed. If 53Mn is no longer present, how do 53 Bulk Allende 53Cr we know that the anomalous Cr is due to eCr 0 0.113457 decay of 53Mn? We reason that the abun- 52Cr dance of 53Mn, when and if it was present, -1 should have correlated with the abundance of other isotopes of Mn. 55Mn is the -2 only stable isotope of Mn. So we construct a plot similar to a conventional isochron 0.113423 diagram (isotope ratios vs. par- 0 0.25 0.50 0.75 ent/daughter ratio), but use the stable iso- 55Mn/52Cr 55 53 tope, in this case Mn as a proxy for Mn. Figure 23.3. Correlation of the 53Cr/52Cr ratio with An example is shown in Figure 23.3. 55Mn/ 52Cr ratio in inclusions from the Allende CV3 me- Starting from our basic equation of ra- teorite. After Birck and Allegre (1985). dioactive decay, we can derive the fol- lowing equation: –lt D = D0 + N0(1 – e ) 23.1 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: 53 53 53 Cr Cr Mn –lt 52 = 52 + 52 (1 – e ) 23.2 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 ini- 53 55 tial isotopic composition of Mn was homogeneous in all the reservoirs of interest; i.e., Mn/ Mn 0 is constant, the initial 53Mn/ 52Cr ratio is just: 53 Mn 55Mn 53Mn 52 = 52 55 23.3 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 23.3 into 23.2, we have: 53 53 55 53 Cr Cr Mn Mn –lt 52 = 52 + 52 55 (1 – e ) 23.4 Cr Cr 0 Cr Mn 0 Finally, for a short-lived nuclide like 53Mn, the term lt is very large after 4.55 Ga, so the term e–lt is 0 (this is equivalent to saying all the 53Mn has decayed away). Thus we are left with: 53Cr 53Cr 55Mn 53Mn 52 = 52 + 52 55 23.5 Cr Cr 0 Cr Mn 0 On a plot of 53Cr/52Cr vs. 55Mn/ 52Cr, the slope is proportional to the initial 53Mn/ 55Mn ratio. Thus correlations between isotope ratios such as these is evidence for the existence of extinct radionuclides. In this way, many extinct radionuclides have been identified in meteorites from variations in the abundance of their decay products. These include 26Al (7.2 ¥ 105 a), 41Ca (1 ¥ 105 a), 53Mn (3.7 ¥ 106 a), 60Fe (1.5 ¥ 106 a), 107Pd (6.5 ¥ 106 a), 129I (1.6 ¥ 107 a), 146Sm (1.03 ¥ 108 a), 182Hf (9 ¥ 106 a) and 244Pu (8.1 ¥ 107 a) (Table 23.1). Clearly, the existence of these nuclides in meteorites requires that they must have been synthesized shortly (on geological time scales) before the solar system formed.

162 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

To understand why these short-lived 1550 radionuclides require a nucleosynthetic 1500 53 20 1525 event, consider the example of Mn. Its 1650 half-life is 3.7 Ma. Hence 3.7 Ma after 1475 1425 1450 it was created only 50% of the original 1375 number of atoms would remain. After 2 129Xe 1350 1200 130Xe 1400 half-lives, or 7.4 Ma, only 25% would 10 1300 1250 remain, after 4 half-lives, or 14.8 Ma, only 6.125% of the original 53Mn would H Average Carbonaceous Chondrite Khairpur remain, etc. After 10 half lives, or 37 (Enstatite Chondrite) Ma, only 1/210 (0.1%) of the original amount would remain. The correlation 0 between the Mn/Cr ratio and the abun- 0 5 10 15 20 dance of 53Cr indicates some 53Mn was 128Xe/130Xe present when the meteorite, or its par- Figure 23.4. Correlation of 129Xe/130Xe with 128Xe/130Xe. ent body, formed. From this we can con- The 128Xe is produced from 127I by irradiation in a reactor, clude that an event which synthesized so that the 128Xe/130Xe ratio is proportional to the 53Mn occurred not more than roughly 30 127I/130Xe ratio. Numbers adjacent to data points corre- million years before the meteorite spond to temperature of the release step. formed. 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 23.4 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 ir- radiated 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 re- leased at each step analyzed in a mass spectrometer. As was the case in Figure 23.3, the slope is pro- portion to the 129I/127I ratio at the time the meteorite formed. 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 there is a statistical distribution of nuclides produced by fission. Each fissionable isotope produces a different distribution. The distribution produced by U is similar to that produced by 244Pu, but the difference is great enough to demonstrate the existence of 244Pu in meteorites, as is shown in Figure 23.5. Fission tracks in excess of the expected number of tracks for a known uranium concen- tration are also indicative of the former presence of 244Pu. These extinct radionuclides provide a means of relative dating of Table 23.1. Short-Lived Radionuclides in the Early meteorites and other bodies. Of the Solar System various systems, the 129I–129Xe decay Radio- Half-life Decay Daughter Abundance is perhaps most useful. Figure 23.6 nuclide Ma Ratio shows relative ages based on this 26 26 Al 0.7 b Mg decay system. These ages are calcu- 41 41 41 40 –6 129 127 Ca 0.13 b K Ca/ Cal < 10 lated from I/ I ratios, which are 53 53 53 55 –5 Mn 3.7 b Cr Mn/ Mn ~ 4 ¥ 10 in turn calculated from the ratio of 60 60 60 56 –10 129 127 Fe 1.5 b Ni Fe/ Fe ~ 5 ¥ 10 excess Xe to I. Since the initial 107 107 107 108 –5 129 127 Pd 9.4 b Ag Pd/ Pd ~ 2 ¥ 10 ratio of I/ I is not known, the 129 129 129 127 –4 I 16 b Xe I/ I ~ 1 ¥ 10 ages are relative to an arbitrary 146 142 146 144 Sm 103 a Nd Sm/ Sm ~ 0.005 value, which is taken to be the age 182 182 182 180 –4 Hf 9 b W Hf/ Hf ~ 2.6 ¥ 10 of the Bjurböle meteorite, a L4 chon- 244 244 238 Pu 82 a, SF Xe Pu/ U ~ 0.005 drite.

163 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

The ages ‘date’ closure of the systems 235 to Us.f. O 238 to Xe and I mobility, but it is not clear if 555 to Us.f. this occurred at condensation or during 1.00 5 metamorphism. Perhaps both are in- 5 volved. The important point is that 5 there is only slight systematic variation in age with meteorite types. Carbona- 0.75 5 ceous chondrites do seem to be older than 134Xe 5 ordinary and enstatite chondrites, 132Xe while LL chondrites seem to be the youngest. Differentiated meteorites are 0.50 Air generally younger. These are not shown, JJ except for silicate in the El Taco iron, Ave. Carb. Chondrite which is not particularly young. The 0.25 bottom line here is that all chondrites 0.25 0.50 0.75 1.00 1.25 closed to the I-Xe decay system within 136Xe/132Xe about 20 Ma. Figure 23.5. Variation of 134Xe/132Xe and 136Xe/132Xe in mete- An interesting aspect of Figure 23.6 is orites (5 ). The isotopic composition of fission products of that the achondrites, which are igneous man-made 244Pu is shown as a star (O). After Podosek and Swindle (1989). I-Xe Age (Myr after Bjurböle) younger older in nature, and the irons do not appear +40+30+20 +10 0 -10 to be substantially younger than the chondrites. Irons and achondrites are Irons both products of melting on meteorite E. Achondrites parent bodies. That they appear to 6 be little younger than chondrites in- 5 E dicates that and melting and differ- 4 Chondrites entiation of those planetismals must 3 have occurred very shortly after the 6 solar system itself formed and within 5 LL tens of millions of years of the syn- 4 Chondrites thesis of 129I. 3 107Pd–107Ag 6 L The existence of variations isotopic 5 Bjurböle composition of silver, and in particu- 4 Chondrites lar variations in the abundance of 3 107Ag that correlate with the Pd/Ag 6 ratio in iron meteorites indicates that 5 H 107Pd was present when the irons 4 Chondrites formed. The half-life of 107Pd is 9.4 3 million years, hence the irons must O Typical have formed within a few tens of mil- V C Allende lions of years of synthesis of the 107Pd. M Chondrites This in turn implies that formation of I iron cores within small planetary bodies occurred within a few tens of 0 0.5 1.0 1.4 2.0 millions of years of formation of the 129 127 -4 I/ I ¥ 10 solar system. Figure 23.6. Summary of I-Xe ages of meteorites relative to Fractions of metal from the meteor- Bjurböle. After Swindle and Podosek (1989). ite Gibeon (IVA iron) define a fossil

164 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

12 isochron indicating an initial 107Pd/108Pd IV-A Iron Meteorites ratio of 2.4 ¥ 10-5 (Chen and Wasserburg, 10 1990). Other IVA irons generally fall along the same isochron (Figure 23.7). 8 IIAB and IIAB irons, as well as several 107Ag anomalous irons show 107 109 108 109 109Ag 6 Ag/ Ag– Pd/ Ag correlations that indicate 107Pd/108Pd ratios between 1.5 and 2.4 ¥ 10-5. Assuming these differ- 4 107 108 ences in initial Pd/ Pd are due to time 107 2 and the decay of Pd, all of these iron Normal meteorites would have formed no more 0 than 10 million years after Gibeon (Chen 0 1 2 3 and Wasserburg, 1996). 108Pd/109Ag (¥ 10-5) 26Al–26Mg 107 109 108 109 Figure 23.7. Correlation of Ag/ Ag with Pd/ Ag Another key extinct radionuclide has in Group IVA iron meteorites, demonstrating the exis- been 26Al. Because of its short half-life 107 tence of Pd at the time these irons formed. After (0.72 Ma), it provides much stronger con- Chen and Wasserburg (1984). straints 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 been a significant source of heat. 26Al decays to 26Mg; an example of the correlation between 26Mg/ 24Mg and 27Al/24Mg is shown in Figure 23.8. Because of the relatively short half-life of 26Al and its potential importance as a heat source, con- siderable 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 spatial scales as small as 10 µ. As a result, there are some 1500 measurements on 60 meteorites reported in Allende Inclusion WA the literature, mostly on CAI’s. Anorthosite-G The reason for the focus on CAI’s is, of course, because their high Al/Mg 0.150 26 27 -5 ratios should produce higher ( Al/ Al)0 = 5.1 (±0.6) • 10 26Mg/ 24Mg ratios. Figure 23.8 summarizes these data. These measurements show a 26Mg 26 27 Anorthosite-B maximum in the Al/ Al ratio of 24Mg around 4.5 ¥ 10-5. Significant 26Mg 0.145 anomalies, which in turn provide evidence of 26Al, are mainly con- fined to CAI’s. This may in part re- Melilite flect the easy with the anomalies are detected in this material and the focus of research efforts, but it 0.140 Spinel Fassite almost certainly also reflects real differences in the 26Al/27Al ratios 0 100 27 24 200 300 between these objects and other ma- Al/ Mg terials in meteorites. This in turn Figure 23.8. Al-Mg evolution diagram for Allende CAI WA. probably reflects a difference in the Slope of the line corresponds to an initial 26Al/27Al ratio of timing of the formation of the CAI’s 26Al/27Al ratio of 5.1 ¥ 10-4. After Lee et al. (1976). and other materials, including chondrules. The evidence thus sug-

165 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

gests that CAI’s formed several million years before chondrules and other materials found in meteorites. 182Hf–182W and Core Formation The Hf-W pair is particularly interesting because Hf is litho- phile while W is moderately si- derophile. 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 advan-

tage the one can reasonably as- 26 27 sume that bodies such as the Earth Figure 23.9. Inferred initial Al/ Al for all available meteor- should have a chondritic Hf/W itic data. After MacPherson et al. (1995). ratio, but the disadvantage that both elements are difficult to analyze by conventional thermal ioni- zation. 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 rep- licated before they be accepted. Because the variations in 182W/183W ratio are quite small, they are generally presented and discussed in the same e notation used for Nd and Hf isotope ratios. There is a slight difference, however;

eW 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 uni- form W isotope composition identical to that of the standard (an assumption which has not yet been

proven), then the silicate earth has eW of 0 by definition. The basic question can posed this way: if the 182W/183W ratio in the silicate Earth is higher than in chondrites, it would mean that much of the Earth’s tungsten had been sequestered in the Earth’s core before 182Hf had entirely decayed. Since the half-life of 182Hf is 9 Ma and using our rule of thumb that a radioactive nuclide is fully decayed in 5 to 10 half-lives, this would mean the core must have formed within 45 to 90 million years of the time chondritic meteorites formed (i.e., of the formation of the solar system). If on the other hand, the 182W/183W ratio in the silicate Earth was the same as in chondrites, which never underwent silicate melt 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 was separated from Hf-bearing silicates before 182Hf had entirely decayed. Because of the difference between “terrestrial” W (the tungsten standard is presumably representative of W in the silicate Earth, but not the entire Earth), Jacobsen and Harper (1996) concluded the Earth’s core must have seg- regated 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 182 found the iron meteorites showed depletions in W (eW = -4.5 and -3.7 for Arispe and Coya Norte re-

166 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

spectively) that were simi- 15 lar to that observed in Carbonaeous Chondrite Toluca reported by Jacobsen Chondrite Achondrite (Eucrite) and Harper (1996). The chondrites, however, had 10 ew values that were only slightly positive, about +0.5, and were analytically indistinguishable from e W 5 “terrestrial” W, as was the lunar basalt. Lee and Hal- Moon Initial 182Hf/180Hf = 1.0 x 10-4 liday (1995) inferred an initial 182Hf/180Hf for the 0 -4 Silicate Earth solar nebula of 2.6 ¥ 10 , much higher than assumed 182Hf/180Hf = 1.1 x 10-5 (29.5 Ma) by Jacobsen and Harper. Based on this similarity of -5 0 5 10 15 isotopic compositions of fHf/W chondritic and terrestrial W, Lee and Halliday Figure 23.10. W isotope ratios in meteorites, the Moon and the (1995) concluded that the Earth reported by Yin et al. (2002). minimum time required for formation of the Earth’s core was 62 million years.

Subsequently, Lee and Halliday (1998) reported eW 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 undif-

ferentiated meteorites. Lee and Halliday (1998) also reported eW values in the range of +2 to +3 in 3 SNC meteorites thought to have come from Mars. These data indicated that the Martian core formed relatively early. The heterogeneity in tungsten isotopes indicates in Martian mantle was never fully homogenized. Lee et al. (1997) reported that the W isotope ratio of the Moon was about 1 epsilon unit higher than that of terrestrial W. Thus at this point, the Earth appeared to be puzzlingly anomalous among differentiated planetary bodies in that silicate-metal differentiation appeared to have occurred quite late. In the latest chapter of this story, 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 which showed that they had W isotope ratios 1.9 to 2.6 epsilon units lower than the terrestrial standard (Figure 23.10). In the same issue of the journal Nature, Kleine et

al., (2002) reported similarly low eW (i.e., -2) for the carbonaceous chondrites Allende, Orgueil, Mur- chison, Cold Bokkeveld, Nogoya, Murray, and Karoonda measured in a third laboratory (University of Munster). Furthermore, Kleine et al. (2002) analyzed a variety of terrestrial materials and found they all had identical W isotopic composition (Figure 23.11). 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 23.12). 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

167 9/9/03

a Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

scenario, core formation occurs 29 million years after formation of the solar system. In the Karooonda second scenario that they be- Murray lieved more likely, metal segregates continuously from a Nogoya magma ocean. In this continu- ous model, the mean age of core Carbonaceous Cold Bokkeveld formation is 11 million years. chondrites In contrast, they concluded Murchison that the parent body of the eucrite class of achondrites Orgueil (suspected to be the large as- teroid Vesta) underwent core formation within 3 million years of formation of the solar Allende a b system. Klein et al. (2002) reached similar conclusions. Origin of Short-lived Nu- IGDL-GD clides G1-RF Terrestrial The mere existence of ra- BB samples diogenic 129Xe requires the time BE-N span between closure of the presolar nebula to galactic nu- Toluca a cleosynthesis and formation of b the solar system be no more c than about 150 Ma. This time constraint is further reduced by the identification of ra- -6 -5 -4 -3 -2 -1 0 1 2 26 e diogenic Mg, produced by the W decay of 26Al. Apparent Figure 23.11. W isotope ratios measured in chondrites, the iron 26!Al/27Al ratios in CAI's meteorite Toluca, and terrestrial materials by Kleine et al. around 10-5, together with the (2002). half-life of 26Al of 0.72 Ma and theoretical production ratios for 26Al/27Al of around 10-3 to 10-4, suggests nucleosynthesis occurred less than several million years before formation of these CAI's. What this nucleosynthetic “event” was remains a matter of debate. The most likely site of 26Al synthesis is in asymptotic giant branch stars (sometimes called AGB stars; they are a subclass of red giants). Red giants inject an enormous amount of material into surrounding space through greatly en- hanced solar winds. Thus the 26Al may have been injected into the cloud that ultimately collapsed to form the solar system by a red giant. 107Pd is produced principally in the s process, and so may also have originated in a red giant. However, other extinct nuclides, such as 60Fe, 129I, 182Hf, and 244Pu are “r” nuclides and therefore likely to have been produced in supernova explosions. From an astronomi- cal perspective, such nucleosynthesis shortly before the solar system formed is not surprising: stars usually form not in isolation, but in large numbers in large clouds of gas and dust known as nebulae. The Great Nebula in Orion is a good example. Many of the stars formed in these stellar nurseries will be quite large and have short lifetimes and end their existence in supernova explosions. Thus stellar death, including the red giant and supernova phases, goes on simultaneously with star birth in these nebulae.

168 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

REFERENCES AND SUGGESTIONS FOR Magma ocean Two-stage model 3.0 model FURTHER READING Birck, J. L. and C. J. Allègre. 1985. Evidence for the presence of 53Mn in the early solar system. Earth Planet. Sci. 11±1 Myr 29.5±1.5 Myr Lett. Geophys. Res. Lett.: 745- 2.0 748. Chen, J. H. and G. J. Wasser- burg, 1983. The least radiogenic Pb in iron meteorites. DeW Fourteenth Lunar and Plane- tary Science Conference, Ab- 1.0 stracts, Part I, Lunar & Lee & Halliday (1995) Planet Sci. Inst., Houston, pp. 103-104. Chen, J. H. and G. J. Wasser- burg. 1990. The presence of 107 0.0 Pd in the early solar system. Lunar Planet. Sci. Conf. Absts. 21: 184-185. Chen, J. H. and G. J. Wasser- burg. 1996. Live 107Pd in the early solar system and im- -1.0 plications for planetary evo- 0 10 20 30 40 50 60 70 lution. In Earth Processes: Mean time of core formation (Myr) Reading the Isotope Code, Figure 23.12. Models for timing of core formation in the Earth. The Vol. 95, S. R. Hart and A. figure shows how the difference between the 182W/183W between Basu. ed., pp. 1-20. Wash- ington: AGU. the silicate Earth and chondrites, DeW, declines as a function of time between formation of the chondrites and separation of the Harper, C. L., J. Volkening, K. Earth’s core. Yin et al. (2002) considered two scenarios: a two- G. Heumann, C.-Y. Shih and H. Wiesmann. 1991. 182Hf- stage model in which Earth first accretes completely and then the 182 core forms, and a model in which the core segregates progressively W: New cos- from a magma ocean as the Earth accretes. In the first scenario, mochronometric constraints the mean age of the core is about 30 million years, in the second it on terrestrial accretion, core is 11 million years. These results are sharply different from those formation, the astrophysi- of Lee and Halliday (1995) who found only a small difference in cal site of the r-process, and the origina of the solar sys- eW between the Earth and chondrites and consequently concluded the core formed later (at about 60 million years). tem. Lunar Planet Sci. Conf Absts. 22: 515-516. 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.

169 9/9/03 Geol. 656 Isotope Geochemistry Lecture 23 Spring 2003

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, Geochim. 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.

170 9/9/03