Geol. 656 Isotope Geochemistry
Lecture 16
Spring 2007
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 meFigure 16.1. Photograph of the meteorite Allende, which fell in Mexico in
teorites and differenti1969. 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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Lecture 16
Spring 2007
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 increasing degrees of high-temperature metamorphism, while grades 1 and 2 experienced lowtemperature 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 unclear.
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 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 4.502 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 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.
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Lecture 16
Spring 2007
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 hightemperature 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 Papanastassiou 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 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 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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Geol. 656 Isotope Geochemistry
Lecture 16
Spring 2007
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
53
Mn, only 3.7 million years, is so short that
any 53Mn produced by nucleosynthesis has
16.3. Correlation of the 53Cr/52Cr ratio with
long since decayed. If 53Mn is no longer Figure
55
52
Mn/ Cr ratio in inclusions from the Allende CV3 mepresent, how do we know that the anomateorite. After Birck and Allegre (1985).
53
53
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:
D = D0 + N 0 (1 ! e! "t )
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:
Cr ! 53 Cr $ !
=
+
52
Cr #" 52 Cr &% 0 #"
53
53
52
Mn $
(1 ' e' (t )
&
Cr % 0
16.2
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:
!
#"
!
Mn $
=
52
Cr &% 0 #"
53
Mn $ !
52
Cr &% 0 #"
55
Mn $
55
Mn &% 0
53
16.3
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:
Cr ! 53 Cr $ !
=
+
52
Cr #" 52 Cr &% 0 #"
53
Mn $ !
52
Cr &% 0 #"
55
53
55
Mn $
(1 ' e' (t )
Mn &% 0
16.4
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:
Cr ! 53 Cr $ !
=
+
52
Cr #" 52 Cr &% 0 #"
53
117
Mn $ !
52
Cr &% 0 #"
55
53
55
Mn $
Mn &% 0
16.5
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Geol. 656 Isotope Geochemistry
Lecture 16
Spring 2007
On a plot of 53Cr/52Cr vs.
55
Mn/52Cr, the slope is propor- Table 16.1. Short-Lived Radionuclides in the Early
tional not to time, as in a conven- Solar System
Half-life Decay Daughter
Abundance
tional isochron diagram, but to the RadioMa
Ratio
initial 53Mn/55Mn ratio. Thus cor- nuclide
10
10
9
relations between isotope ratios 10Be
1.5
β
B
Be/ Be ~ 9 × 10–4
26
26
such as these is evidence for the ex- 26Al
0.7
β
Mg
Al/27Al ~ 5 × 10–5
41
41
41
istence of extinct radionuclides.
Ca
0.15
β
K
Ca/40Ca 1 × 10–8
53
53
In this way, many extinct radi- 53Mn
3.7
β
Cr
Mn/55Mn ~ 4 × 10–5
60
60
onuclides have been identified in 60Fe
1.5
β
Ni
Fe/56Fe ~ 6 × 10–8
107
107
meteorites from variations in the 107Pd
9.4
β
Ag
Pd/108Pd ~ 4 × 10–4
129
129
abundance of their decay products. 129I
16
β
Xe
I/127I ~ 1 × 10–4
142
146
The most important of these are 146Sm
103
α
Nd
Sm/144Sm ~ 0.008
182
182
listed in Table 16.1. On a cosmic 182Hf
9
β
W
Hf/180Hf ~ 2.6 × 10–4
244
scale, nucleosynthesis is a more or 244Pu
82
α, SF
Xe
Pu/238U ~ 0.005
less continuous process – roughly
once every second, a supernova explodes somewhere in the universe. So we might expect that interstellar 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
53
Cr 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.
129
I–129Xe and 244Pu
Among the most useful of these shortlived radionuclides, and the first to be
discovered, has been 53I, which decays to
129
Xe. 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
127
I. 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 16.3, the slope is proportion to the
129
I/127I ratio at the time the meteorite
formed.
Figure 16.4. 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 the 127I/130Xe
ratio. Numbers adjacent to data points correspond to
temperature of the release step.
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Spring 2007
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 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 16.5. Fission tracks
in excess of the expected number of
tracks for a known uranium concentration are also indicative of the former
presence of 244Pu.
Figure 16.5. Variation of 134Xe/132Xe and 136Xe/132Xe in meteThese 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).
Figure 16.6. Summary of I-Xe ages of meteorites relative to
Bjurböle. After Swindle and Podosek (1989).
119
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. Carbonaceous 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
within about 20 Ma.
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Geol. 656 Isotope Geochemistry
Lecture 16
Spring 2007
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.
107
Pd–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
107
Pd 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
× 10-5 (Chen and Wasserburg, 1990). Other
IVA irons generally fall along the same isochron (Figure 16.7). IIAB and IIAB irons,
as well as several anomalous irons show
107
Ag/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).
26
Figure 16.7. 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).
Al–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
been a significant source of heat. 26Al
decays to 26Mg; an example of the cor- Figure 16.8. Al-Mg evolution diagram for Allende CAI WA.
relation between 26Mg/24Mg and Slope of the line corresponds to an initial 26Al/27Al ratio of
27
Al/24Mg is shown in Figure 16.8.
26
Al/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
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. 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
higher 26Mg/24Mg 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
26
Mg 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.
182
Hf–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 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 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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Geol. 656 Isotope Geochemistry
Lecture 16
Spring 2007
silicate Earth has εW of 0 by
15
definition. The basic question
Carbonaeous Chondrite
can posed this way: if the
Chondrite
182
183
W/ W ratio in the silicate
Achondrite (Eucrite)
Earth is higher than in chon10
drites, it would mean that
much of the Earth’s tungsten
had been sequestered in the
Earth’s core before 182Hf had
εW 5
entirely decayed. Since the
182
half-life of Hf is 9 Ma and usMoon
ing our rule of thumb that a
Initial 182Hf/ 180Hf = 1.0 x 10-4
radioactive nuclide is fully de0
cayed in 5 to 10 half-lives, this
Silicate Earth
would mean the core must
182Hf/180 Hf = 1.1 x 10-5 (29.5 Ma)
have formed within 45 to 90
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,
Figure 16.10. W isotope ratios in meteorites, the Moon and the
the 182W/183W ratio in the siliEarth reported by Yin et al. (2002).
cate Earth was the same as in
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 Murchison), two additional iron meteorites (Arispe, IA, and Coya Norte, IIA) and a lunar basalt. They found
the iron meteorites showed depletions in 182W (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 indistinguishable from “terrestrial” W, as was the lunar basalt. Lee and Halliday (1995) inferred an initial
182
Hf/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 meteorites. Lee and Halliday (1998) also reported εW values in the range of +2 to +3 in 3 SNC meteorites
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Lecture 16
Spring 2007
thought to have come from Mars.
These data indicated that the
Karooonda
Martian core formed relatively
early.
The heterogeneity in
Murray
tungsten isotopes indicates in
Martian mantle was never fully
Nogoya
homogenized. Lee et al. (1997)
reported that the W isotope ratio
Cold Bokkeveld
Carbonaceous
of the Moon was about 1 epsilon
chondrites
unit higher than that of terresMurchison
trial W.
Thus at this point, the Earth
Orgueil
appeared to be puzzlingly
anomalous among differentiated
planetary bodies in that silicateAllende
a
metal differentiation appeared to
b
have occurred quite late. In the
latest chapter of this story, Yin et
al. (2002) reported W isotope
IGDL-GD
measurements carried out in two
Terrestrial
G1-RF
laboratories, Harvard University
BB
samples
and the Ecole Normale SupéBE-N
rieure de Lyon, which showed
that the chondrites Allende and
Toluca
Murchison which showed that
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
Figure 16.11. W isotope ratios measured in chondrites, the iron
(i.e., -2) for the carbonaceous
meteorite Toluca, and terrestrial materials by Kleine et al.
chondrites Allende, Orgueil, Mur(2002).
chison, Cold Bokkeveld, Nogoya,
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 multicollector 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 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 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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Lecture 16
146
Spring 2007
Sm-142Nd
As we have mentioned, geochemists generally assume that
Magma ocean
Two-stage model
3.0
rare earth and other refractory
model
elements have the same relative
concentrations in the Earth as
they have in chondrites. If so,
the Sm/Nd ratio of the Earth
11±1 Myr
29.5±1.5 Myr
should be chondritic. Thus the
2.0
147
144
Sm/ Nd ratio of the present
Earth should be chondritic and
the 146Sm/144Nd of the early
Earth should have been chondritic. Thus the 143Nd/144Nd
ΔεW
and 142Nd/144Nd of the bulk
1.0
earth should also be chondritic.
However, recent studies of the
Lee & Halliday (1995)
142
Nd/144Nd ratio in chondrites
and terrestrial materials suggest that this may not be the
case, at least that part of the
0.0
Earth accessible to sampling.
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 temfigure shows how the difference between the 182W/183W between
peratures of 1602 and 1590 K,
the silicate Earth and chondrites, ΔεW, declines as a function of time
respectively. It is difficult to
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 milmeasurements 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 εW bewould seem to confirm that
tween the Earth and chondrites and consequently concluded the
these elements were not fraccore formed later (at about 60 million years).
tionated in the solar nebula.
142
Nd is the product of αdecay of 146Sm, a nuclide with a half-life of 103 million years. As Table 16.1 shows, the initial
146
Sm/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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Spring 2007
ing, and indeed was nearly impossible before about 15 years ago. Furthermore, because the half-life of
146
Sm 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
142
Nd/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
143
Nd/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
142
Nd/144Nd ratio as chondrites.
Other workers failed to find any excesses in other rocks from Isua, so
these results were controversial.
More recent work using advanced
mass spectrometers by Caro et al.
(2003) and Boyet et al. (2003), however, has confirmed the original findings of Harper and Jacobsen. This
means that these early parts of the
crust formed from a mantle reservoir
that had Sm/Nd ratios higher than
the chondritic one – and importantly,
that this reservoir formed very early,
most likely within the first 100 Ma.
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
Figure 16.13. Variation in ε142Nd in the Earth and meteorites.
than chondrites, and most eucrites as
Gray region is the range measurements of laboratory stanwell (Figure 16.13). This implies that
dards derived from terrestrial Nd. All other terrestrial matethe accessible Earth has a signifirials plot within this range with the exception of some samcantly higher Sm/Nd ratio than
ples from Isua, Greenland. Chondrites have, on average,
chondrites. How much higher deε142Nd of -0.2 relative to the terrestrial standards. Data from
pends on when the increase ocCaro et al. (2003), Boyet and Carlson (2005), Boyet and Carlcurred. If the increase occurred 5
son (2006). SNC data from the compliation of Halliday
million years after the beginning of
(2001).
the solar system (taken as the age of
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Spring 2007
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 observable 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. Indeed, 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 sampled 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 unfractionated 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% Mgperovskite 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 Enriched 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 subduction 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.
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