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 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 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.
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
metamorphism they have experienced in parent bodies. Grades 4, 5, and 6 have experienced increasing 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 t h e
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 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 t h e
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 unbrecciated 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 t h a t
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.
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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: ba saltic a chondrite best 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 K a p o e t a ,
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,
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
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
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
0.11349
and the parent element. Consider, for ex53
ample, Cr, which is the decay product of
Allende Inclusions
53
2
Mn. The half-life of 53Mn, only 3.7 million years, is so short that any 53Mn produced by nucleosynthesis has long since de1
cayed. If 53Mn is no longer present, how do
Bulk Allende
we know that the anomalous 53Cr is due to eCr 0
0.113457 5253Cr
Cr
decay of 53Mn? We reason that the abun53
dance of Mn, 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
0
0.75
0.25
0.50
diagram (isotope ratios vs. par55Mn/52Cr
ent/daughter ratio), but use the stable iso55
53
tope, in this case Mn as a proxy for Mn. Figure 23.3. Correlation of the 53Cr/52Cr ratio with
55
An example is shown in Figure 23.3.
Mn/52Cr ratio in inclusions from the Allende CV3 meStarting from our basic equation of ra- teorite. After Birck and Allegre (1985).
dioactive decay, we can derive the following equation:
D = D0 + N0(1 – e –lt)
23.1
This is a variation on the isochron equation we derived in lecture 4. Written for the example of t h e
decay of 53Mn to 53Cr, we have:
53
53
Cr =
52
Cr
Cr
52
Cr
53
Mn
Cr
+
(1 – e–lt)
52
0
0
23.2
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 isotopic composition of Mn was homogeneous in all the reservoirs of interest; i.e., 53Mn/55Mn0 is
constant, the initial 53Mn/52Cr ratio is just:
53
Mn
Cr
=
52
0
55
Mn
Cr
52
53
Mn
Mn
0
55
23.3
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
Cr =
52
Cr
53
Cr +
52
Cr 0
55
Mn
52
Cr
53
Mn (1 – e –lt)
Mn 0
55
23.4
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:
53
Cr =
52
Cr
53
Cr +
52
Cr 0
55
Mn
52
Cr
53
Mn
Mn
55
0
23.5
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 t h e
abundance of their decay products. These include 26Al (7.2 ¥ 105 a), 41Ca (1 ¥ 105 a), 53Mn (3.7 ¥ 106 a ) ,
60
Fe (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.
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
20
129Xe
130Xe
1425
1375
1200
1350
1400
1300 1250
10
H
0
0
Average Carbonaceous Chondrite
5
1450
1550
1500
1525
1650
1475
Khairpur
(Enstatite Chondrite)
10
15
20
128Xe/130Xe
Figure 23.4. Correlation of 129Xe/130Xe with 128Xe/130Xe.
The 128Xe is produced from 127I by irradiation in a reactor,
so that the 128Xe/130Xe ratio is proportional to t h e
127
I/130Xe ratio. Numbers adjacent to data points correspond to temperature of the release step.
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 which synthesized
53
Mn occurred not more than roughly 30
million years before the meteorite
formed.
129
I–1 2 9Xe and 2 4 4Pu
Among the most useful of these short-lived radionuclides, and the first to be discovered, has been
I, 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 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 mass spectrometer. As was the case in Figure 23.3, the slope is proportion 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
244
Pu, 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 concentration 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 t h e Solar System
various systems, the 129I–129Xe decay RadioHalf-life Decay Daughter
Abundance
is perhaps most useful. Figure 23.6 nuclide
Ma
Ratio
shows relative ages based on this 26A l
26
0.7
b
Mg
decay system. These ages are calcu- 41Ca
41
41
40
0.13
b
K
Ca/ Cal < 10–6
129
127
lated from I/ I ratios, which are 53Mn
53
53
3.7
b
Cr
Mn/55Mn ~ 4 ¥ 10–5
in turn calculated from the ratio of 60Fe
60
60
1.5
b
Ni
Fe/56Fe ~ 5 ¥ 10–10
excess 129Xe to 127I. Since the initial 107Pd
107
107
9.4
b
Ag
Pd/108Pd ~ 2 ¥ 10–5
ratio of 129I/127I is not known, t h e 129I
129
129
16
b
Xe
I/127I ~ 1 ¥ 10–4
ages are relative to an arbitrary 146Sm
142
146
103
a
Nd
Sm/144Sm ~ 0.005
value, which is taken to be the age 182H f
182
182
9
b
W
Hf/180Hf ~ 2.6 ¥ 10–4
of the Bjurböle meteorite, a L4 chon- 244Pu
244
82
a, SF
Xe
Pu/238U ~ 0.005
drite.
53
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
The ages ‘date’ closure of the systems
to Xe and I mobility, but it is not clear i f
this occurred at condensation or during
metamorphism. Perhaps both are involved. The important point is t h a t
there is only slight systematic variation
in age with meteorite types. Carbonaceous chondrites do seem to be older than
ordinary and enstatite chondrites,
while LL chondrites seem to be t h e
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 within
about 20 Ma.
An interesting aspect of Figure 23.6 is
that the achondrites, which are igneous
I-Xe Age (Myr after Bjurböle)
younger
older
+40+30+20 +10
0
6
5
4
3
6
5
4
3
6
5
4
3
6
5
4
3
O
V
M
I
0
to 235Us.f.
1.00
O
5
5
5
5
5
to 238Us.f.
5
5
0.75
5
134Xe
132Xe
5
0.50 Air
JJ
Ave. Carb.Chondrite
0.25
0.25
0.50
0.75
1.00
1.25
136Xe/132 Xe
Figure 23.5. Variation of 134Xe/132Xe and 136Xe/132Xe in meteorites (5 ). The isotopic composition of fission products of
man-made 244Pu is shown as a star (O). After Podosek and
Swindle (1989).
in nature, and the irons do not appear
to be substantially younger 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 t h e
solar system itself formed and within
tens of millions of years of the synthesis of 129I .
-10
Irons
E. Achondrites
E
Chondrites
LL
Chondrites
107
Pd–1 0 7Ag
L
Chondrites
Bjurböle
H
Chondrites
Typical
C
Allende
Chondrites
0.5
1.0
129I/127I ¥ 10-4
1.4
2.0
Figure 23.6. Summary of I-Xe ages of meteorites relative to
Bjurböle. After Swindle and Podosek (1989).
164
The existence of variations isotopic
composition of silver, and in particular variations in the abundance of
107
Ag that correlate with the Pd/Ag
ratio in iron meteorites indicates t h a t
107
Pd 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 t h e
solar system.
Fractions of metal from the meteorite Gibeon (IVA iron) define a fossil
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
12
IV-A Iron Meteorites
10
107Ag
109Ag
8
6
4
2
0
Normal
0
1
2
3
108Pd/ 109Ag (¥ 10 -5 )
isochron indicating an initial 107Pd/108Pd
ratio of 2.4 ¥ 10-5 (Chen and Wasserburg,
1990). Other IVA irons generally f a l l
along the same isochron (Figure 23.7).
IIAB and IIAB irons, as well as several
anomalous
irons
show
107
Ag/109Ag–108Pd/109Ag correlations t h a t
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).
26
Al–2 6Mg
Figure 23.7. Correlation of 107Ag/109Ag with 108Pd/109Ag
in Group IVA iron meteorites, demonstrating the existence of 107Pd at the time these irons formed. After
Chen and Wasserburg (1984).
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 been a significant source of heat. 26Al decays to 26Mg; an example of t h e
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, 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
26
Mg/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
the literature, mostly on CAI’s.
Allende Inclusion WA
The reason for the focus on CAI’s is,
Anorthosite-G
of course, because their high Al/Mg
ratios should produce higher
0.150 (26Al/27Al)0 = 5.1 (±0.6) • 10-5
26
Mg/24Mg ratios.
Figure 23.8 summarizes these
data. These measurements show a
26Mg
maximum in the 26Al/27Al ratio of
Anorthosite-B
24Mg
around 4.5 ¥ 10-5. Significant 26Mg
anomalies, which in turn provide
0.145
evidence of 26Al, are mainly confined to CAI’s. This may in part reMelilite
flect the easy with the anomalies
are detected in this material and
the focus of research efforts, but i t
Spinel
0.140
almost certainly also reflects real
Fassite
differences in the 26Al/27Al ratios
0
100 27 24
200
300 between these objects and other maAl/ Mg
terials in meteorites. This in turn
Figure 23.8. Al-Mg evolution diagram for Allende CAI WA. probably reflects a difference in t h e
Slope of the line corresponds to an initial 26Al/27Al ratio of timing of the formation of the CAI’s
26
and other materials, including
Al/27Al ratio of 5.1 ¥ 10-4. After Lee et al. (1976).
chondrules. The evidence thus sug165
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
gests that CAI’s formed several
million years before chondrules
and other materials found in meteorites.
182
Hf–1 8 2W and Core Formation
The Hf-W pair is particularly
interesting because Hf is lithophile while W is moderately siderophile.
Thus the 182H f -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 as26
27
sume that bodies such as the Earth Figure 23.9. Inferred initial Al/ Al for all available meteorshould 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 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 illustrates the importance of the fundamental dictum in science that results need to be independently replicated 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;
e W 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 silicate earth has e W of 0 by definition. The basic question can posed this way: i f
the 182W/183W ratio in the silicate Earth is higher than in chondrites, it would mean that much of t h e
Earth’s tungsten had been sequestered in the Earth’s core before 182Hf had entirely decayed. Since t h e
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, t h e
182
W/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 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 Murchison), two additional iron meteorites (Arispe, IA, and Coya Norte, IIA) and a lunar basalt. They
found the iron meteorites showed depletions in 182W (e W = -4.5 and -3.7 for Arispe and Coya Norte re166
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
15
Carbonaeous Chondrite
Chondrite
Achondrite (Eucrite)
10
eW 5
Moon
Initial 182Hf/ 180Hf = 1.0 x 10-4
0
Silicate Earth
182Hf/180 Hf = 1.1 x 10-5 (29.5 Ma)
-5
0
5
10
15
fHf/W
Figure 23.10. W isotope ratios in meteorites, the Moon and the
Earth reported by Yin et al. (2002).
spectively) that were similar to that observed in
Toluca reported by Jacobsen
and Harper (1996). The
chondrites, however, had
e w values that were only
slightly positive, about
+0.5, and were analytically
indistinguishable from
“terrestrial” W, as was t h e
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 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 undifferentiated meteorites. Lee and Halliday (1998) also reported e W 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, Murchison, 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
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Geol. 656 Isotope Geochemistry
Lecture 23
Spring 2003
scenario, core formation occurs
29 million years after formation of the solar system. In t h e
second scenario that they believed 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 t h e
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.
Karooonda
Murray
Nogoya
Carbonaceous
chondrites
Cold Bokkeveld
Murchison
Orgueil
Allende
a
b
IGDL-GD
G1-RF
BB
BE-N
Terrestrial
samples
Origin of Short-lived Nuclides
The mere existence of radiogenic 129Xe requires the time
span between closure of the
presolar nebula to galactic nucleosynthesis and formation of
Toluca
a
the solar system be no more
b
than about 150 Ma. This time
c
constraint is further reduced by
the identification of ra-6
-5
-4
-3
-2
-1
0
1
2
diogenic 26Mg, produced by t h e
eW
decay of 26Al. Apparent
26!
Figure 23.11. W isotope ratios measured in chondrites, the iron
Al/27Al ratios in CAI's
meteorite Toluca, and terrestrial materials by Kleine et al.
around 10-5, together with t h e
(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 26A l
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 enhanced 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 astronomical 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.
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3.0
Magma ocean
model
Spring 2003
Two-stage model
REFERENCES AND
SUGGESTIONS FOR
FURTHER READING
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