Elemental and Isotopic Abundances in Meteorites
P. Hoppe
Max-Planck-Institutefor Chemistry, Cosmochemistry Department, P.O. Box 3060, D-55020Mainz, Germany
Abstract. Abundance variations of refractory elements between different groups of undifferentiated meteorites
(chondrites) are within a factor of 2. The elemental abundance pattern of CI chondrites matches that of the solar
photosphere reasonably well except for Li, which is depleted in the Sun's convection zone, and the volatile
elements H, C, N, O, and noble gases which are incompletely condensed in chondrites. Macroscopic-scale
isotopic heterogeneities are largest for H (D/H varies by 8x) and N (15N/14N varies by 2x). Bulk C- and Oisotopic compositions do not vary by more than a few percent and isotopic heterogeneities are even much smaller
for refractory elements. The variations in bulk elemental and isotopic compositions are attributed to variations in
the nebula environment from which the chondrites formed. Some meteoritic components (calcium-aluminum-rich
inclusions and chondrules) also carry the decay products of now extinct short-lived radioactive nuclides which
were either produced locally in the early solar system or in a stellar source shortly before seeding the solar nebula.
Large isotopic anomalies with variations over more than four orders of magnitude are observed on a microscopic
scale in chondrites. These anomalies are carried by presolar grains that formed mainly in the winds of red giant
and asymptotic giant stars and in the ejecta of supernova explosions.
INTRODUCTION
Our solar system formed from the collapse of a molecular cloud about 4.57 Gy ago, possibly triggered by a
nearby supernova (SN) explosion [see 1]. The release of
gravitational energy led to the evaporation of a large fraction of dust grains present in the solar nebula and much
of the nucleosynthetic memories carried by these grains
was erased by chemical and isotopic equilibration. New
minerals condensed out of the cooling nebula and an accretion disk formed around the young Sun. Formation
of solids probably started about several 100,000 y after
onset of molecular cloud collapse and lasted for at least
several million years as inferred from the presence of the
decay products of short-lived radionuclei, such as 41Ca
(half-life - 103,000 y) and26A1 (half-life - 730,000 y)
in primitive solar system matter [e.g. 2, 3]. According
to modern models of planet formation [see 4] dust accumulates by low-velocity collisions and non-gravitational
sticking mechanisms to km-sized bodies, the planetesimals. Mutual collisions of planetesimals finally led to
the formation of the solar system planets and asteroids
and fragments of those bodies eventually may reach the
Earth as meteorites (Fig. 1). While most meteorites originate from asteroids some of them are from the Moon or
from Mars.
Although models of solar system formation do not
support large chemical gradients in the inner solar system
Presolar
cloud
, /^
* f
Collapse &
Partial
evaporation
of dust grains
Cooling &
Condensation <
of minerals
*YV* *
Formation of
accretion disk
Formation of ^K|fr
planetesimals ^P
I
Formation of
planets and asteroids
Formation
of meteorites
FIGURE 1. Origin and history of meteorites.
[5], elemental and isotopic heterogeneities may have existed among the formation locations of planetary bodies
to some extent. The magnitude of such heterogeneities
CP598, Solar and Galactic Composition, edited by R. F. Wimmer-Schweingruber
© 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00
31
in solar system matter depends on the mixing of compounds that formed at different locations or times in the
solar nebula and on the possible admixture of molecular cloud material during the early evolution of the solar
system. Isotopic heterogeneities among solar system materials are also expected from the presence of short-lived
radioactive nuclides in the early solar system whose decay will leave characteristic imprints on the isotopic patterns of the daughter elements.
Meteorites represent a sample of solar system matter from different locations in the solar nebula that can
be studied with high precision in the laboratory. Meteorites can be divided into differentiated and undifferentiated meteorites (Fig. 2). Differentiated meteorites are
further subdivided into achondrites, irons, and stonyirons. These meteorites have experienced strong postaccretionary alteration and their elemental abundances
are not representative of the bulk compositions of their
parent bodies. Undifferentiated meteorites, the chondrites, on the other hand, have preserved the bulk elemental and isotopic compositions of their parent bodies.
They thus provide information on the homogeneity of
elemental and isotopic abundances over large distances
(from Earth to Jupiter orbits) in the solar nebula at the
time of chondrite formation.
ferent types. The different carbonaceous chondrite types
are named after a meteorite of each type (e.g., Ivuna for
CI, Mighei for CM). Subdivision of ordinary and enstatite chondrites is according to Fe content (LL: low
total Fe, low metallic Fe; L: low total Fe; H: high total Fe). Chondrites are mainly composed of chondrules
and calcium-aluminum-rich inclusions (CAIs), mm- to
cm-sized objects that experiencd high temperatures during formation in the solar nebula, and the finer grained
matrix which may be considered as a glue. The relative
abundances of those compounds vary from type to type.
The matrix contains small amounts (ppb to ppm) of nmto /mi-sized refractory dust grains that are of presolar origin as indicated by large isotopic anomalies [6, 7]. The
laboratory study of these rare objects allows to obtain a
wealth of information on many astrophysical aspects.
In this paper I will discuss the elemental and isotopic
homogeneity of meteorites both on a macroscopic (section 2 and 3) and microscopic scale (section 4) and to
which extent meteorites may serve as a reference for
bulk solar system matter (sections 2 and 3). An overview
about the isotopic compositions of presolar grains will be
presented and the origin of presolar grains will be briefly
discussed (section 4).
ELEMENTAL ABUNDANCES
The most abundant elements in chondrites are O, Fe, Si,
and Mg. Variations in elemental abundances among the
different chondrites are generally small for refractory and
moderately volatile elements (Fig. 3). These variations
are typically within a factor of two. Highest refractory
element abundances are observed in the carbonaceous
chondrites and lowest in the enstatite chondrites (see
compilation of abundance data in [8]). Larger variations
are seen in the abundances of volatile elements such as
H, N, C, and noble gases.
The small but noticable heterogeneity in refractory element abundances is attributed to variations in the nebula environment from which the chondrites formed [e. g.
9]. In contrast, the relatively large variations of highly
volatile element abundances may be the result of different condensation behaviours at different locations in the
nebula or of partial loss due to thermal metamorphism on
the meteorite parent bodies [9].
The Si-normalized elemental abundance pattern of CI
chondrites is remarkably similar to that observed in the
solar photosphere [10]. The abundances of most elements agree within a factor of 1.5 and for many elements
the agreement is even within a few percent (Fig. 4). Noticable exceptions are the volatile elements, such as H, C,
N, O, and noble gases which are incompletely condensed
in chondrites, and Li which is depleted in the solar pho-
81%
1.4%
FIGURE 2. Classification of the most common meteorites.
The numbers to the left of chondrites give the abundances of
observed meteorite falls. The arrows indicate further subdivision of differentiated meteorites.
According to chemical composition, degree of oxydization, and degree of equilibration and metamorphic
recrystallization the chondrites are divided into discrete
groups. These include the carbonaceous, ordinary, and
enstatite chondrites which are further subdivided into dif-
32
10
Elemental abundances
in chondrites
•o £
W
E
:
</) CO
E5
o c75
5*"i
C /^
O *•
o
^S
—+—- CM chondrites
............ill......
C
-
t
IU
m
2 10°
(0 0)
=32 v3
~
^ 10-1-d
o
u
= a 102-.
5
CV chondrites
LL chondrites
- EH chondrites
~™~~^—
-——
-
Jf ^ * ^
1 ^*^\^* ^^
^M
^
^W"'*^ • J>m
\/^*<i
J*y
^^^^^»***ML
^^S.
lf°
10 1
•2-- ~ —w
=
** 2
——A—-
10-2-
Elemental abundances
in CI chondrites
[
H, N, C]
1
10'
c)
Decreasing Tcond
Increasing volatility
N*
1 Noble gases!
. . . 1 . . . 1 . . . 1 ...
20
40
60
1 ..
80
Atomic number
FIGURE 4. Elemental abundances in CI chondrites, normalized to Si and solar photospheric abundances [10]. The dashed
lines represent a difference of a factor of 1.5 between CI and
solar photospheric abundances.
FIGURE 3. Si- and Cl-normalized elemental abundances of
CM, CV, LL, and EH chondrites. The elements are ordered
according to decreasing condensation temperature (increasing
volatility). For a pressure of 10~4 bar the condensation temperature on the left-hand side of this figure is 1800 K, on the
right-hand side 80 K. Data from [8]. The dashed lines indicate
a difference of a factor of 2 from CI abundances.
Hydrogen
The D/H ratio in meteoritic water varies between
9x10~5 in clay minerals and 7x10~4 in chondrules.
These variations are interpreted to be the result of progressive isotope exchange in the solar nebula between
D-rich interstellar water and protosolar H2 [11].
tosphere due to nuclear reactions at the bottom of the
Sun's convection zone. The good agreement between the
two data sets justifies use of the elemental abundances of
CI chondrites as chemical reference for bulk solar system
matter except for H, C, N, O, and the noble gases. Their
abundances in bulk solar system matter must be directly
derived from the Sun or solar wind samples or, as for Kr
and Xe, from s-process systematics and/or neighboring
element abundances.
Carbon
Carbon concentrations are between 0.1 and several
wt% in bulk chondrites. C-isotopic compositions vary by
about 3% with 813C/12C values (8-values give the permil
deviation of an isotopic ratio from a reference ratio)
between 0 and -30 permil relative to terrestrial C (Fig. 5)
[12, 13]. Each chondrite group has a restricted range in
C-isotopic composition and C concentration. Highest C
concentrations are seen in the CI chondrites. Ordinary
chondrites have comparatively low C concentrations and
exhibit the lowest 13C/12C ratios among the different
chondrite groups.
ISOTOPIC COMPOSITIONS
It is not the purpose of this paper to review the isotopic
compositions of all elements in bulk meteoritic matter or
major meteoritic compounds. In general, isotopic heterogeneities among the different chondrite groups are small
for refractory elements. Heterogeneities are larger for
volatile elements or if the decay products of short-lived
radioisotopes or the products from cosmic ray spallation
reactions add to the isotopic abundance pattern of specific elements. Here, I will briefly discuss the isotopic
compositions of H, C, N, O, and Cr and the presence of
now extinct26 Al in meteorites.
Nitrogen
Nitrogen concentrations in bulk chondrites are much
lower than C concentrations. On the other hand, Nisotopic variations are much larger than those in C. Nitrogen concentrations range from about 1 ppm to more
33
200
^ -10CD
Q
O" -20cv
O
-30-
lL/H
Carbon in chondrites
-40
10
100
1000
[N] (ppm)
FIGURE 6. Bulk N-isotopic compositions given as permil
deviation from the terrestrial air standard and N concentrations
of chondrites. The data for CH chondrites and Bencubbin are
off-scale with bulk 815N/14N values of up to 900 permil. Data
from [12, 13, 14, 15, 16].
FIGURE 5. Bulk C-isotopic compositions given as permil
deviation from the terrestrial PDB standard (the bulk Earth has
513C/12C - -6.4 permil relative to PDB) and C concentrations
of chondrites. Data taken from [12, 13].
than 1000 ppm. Most of the chondrites have 515N/14N
values between -40 and +40 permil relative to terrestrial
N but some, such as the CH chondrites and the unique
Bencubbin meteorite, show enrichments in 15N by up to
a factor of 2 (Fig. 6) [12, 13, 14, 15, 16]. These large
variations are still not understood but might be caused by
admixture of 15N-rich interstellar (organic) material to
protosolar N in different proportions [17, 18]. Although
presolar grains of stellar origin exhibit huge variations
in 15N/14N (see below), variable admixture of different
presolar grain populations with different N-isotopic signatures is not likely to be the cause for the large heterogeneities seen in bulk chondrites as one would expect to
see also large C-isotopic heterogeneities, contrary to the
observation (see above). Because of the large spread in
N-isotopic compositions of bulk meteorites, the protosolar N-isotopic composition can be hardly derived from
the meteorite data. Distinctly lower 15N/14N ratios than
those seen in meteorites and the Earth are derived for
the Jupiter atmosphere (515N/14N - -374 +- 82 permil
[19]). Further complication is introduced from measurements of solar wind N. In-situ spacecraft studies gave
515N/14N - +360 +- 370 permil [20], measurements of
solar wind implanted N in lunar samples 515N/14N < 240 permil [17]. See also the contribution by Busemann
et al. in these proceedings.
Oxygen
Similar to C, bulk chondrites have O-isotopic variations of about 20-30 permil and each chondrite group exhibits a characteristic O-isotopic signature (Fig. 7) [21].
As O has three stable isotopes, mass-dependent isotope
fractionation processes can be distinguished from nonmass-dependent isotope fractionation processes. Massdependent fractionation in equilibrium and kineticallycontrolled processes occurs along a line with slope 0.5
in a three-isotope representation (cf. TFL in Fig. 7). Imprints of mass-dependent fractionation is seen within individual chondrite groups. The difference between the
chondrite groups points to formation from reservoirs
with different O-isotopic signatures. The CAIs show considerable enrichments in 16O of up to 5-7% relative to
bulk chondrites and the O-isotopic compositions of different minerals plot on a mixing-line with slope 1 in
a three-isotope-representation (Fig. 7). This feature is
not yet understood and represents one of the major puzzles in the field of meteoritics. Possible explanations include non-mass-dependent chemical fractionation processes [22] and presence of 16O-rich grains from supernovae (SN) (see below) in proto-CAIs. See also the contribution by Busemann et al. in these proceedings.
34
ordinary chondrites close to that of Vesta [27]. The Cr
data suggest that a radial heterogeneous distribution of
radioactive 53Mn (half-life = 3.7 My), which decays to
53
Cr, must have existed in the early inner solar system.
CNisotopic
20
10 H
in
0.10-
o 'n2 ~
0.05-
I - °-
HED (Vesta) j
Ordinary chondritesj
'SNC (Mars)
E-C h on d rites
0.004
-40-
Earth & Moon
o
*»•
-T^^T^
-30 ~28 -18
i 10
20
-0.05-
30
FIGURE 7. O-isotopic compositions of bulk chondrites and
CAIs given as permil deviation from the terrestrial SMOW
standard [21]. TFL = terrestrial fractionation line.
• Known heliocentric distance
D Inferred heliocentric distance
1
I
'
I
-0.10
0
1
'
2
3
Heliocentric distance (AU)
FIGURE 8. The 53Cr/52Cr ratio of the Earth and Moon,
SNC meteorites, and HED meteorites given as permil deviation
from terrestrial Cr as a function of heliocentric distance. The
heliocentric distance of the place of formation of enstatite and
ordinary chondrites is inferred from the Cr data, assuming that
there is a linear relationship between 53Cr/52Cr and heliocentric
distance [27].
Aluminum-26
Another characteristic feature of CAIs are enrichments
in 26Mg from the decay of now extinct 26A1. Aluminum26 was either produced locally in the solar system by
the interaction of an X-wind, emerging from the inner
edge of the young Sun's accretion disk, with proto-CAIs
[23, 24] or in a stellar source shortly before seeding
the solar nebula [25]. In the latter scenario short-lived
radioactive nuclides can be used as a chronometer to date
events in the early solar system history. In this context
CAIs are the first solids that formed in the solar system
as they exhibit the highest level of 26A1 among solar
system materials (except presolar grains) with inferred
initial 26A1/27A1 ratios of up to 5x1 (T5 [3].
PRESOLAR GRAINS
The topic of presolar grains has been reviewed in great
detail in recent years and only a brief summary is
given here. For more detailed informations and a complete list of references the reader is referred to the papers by [28, 29, 6, 7] and to the compilation of papers in "Astrophysical Implications of the Laboratory
Study of Presolar Materials" [30]. Presolar grains identified to date in primitive meteorites include diamond,
silicon carbide (SiC) (Fig. 9), graphite, silicon nitride
(SisN^, corundum (A^Os), spinel (MgAbCU), and hibonite (CaAli2Oi9). The presolar nature of those grains
is indicated by large isotopic anomalies in the major and
many trace elements contained in the grains (Fig. 10).
The fact that the known presolar minerals are high temperature condensates implies that they formed in stellar outflows or in the ejecta of stellar explosions. These
grains thus represent a sample of stardust that can be analyzed in the laboratory.
Figure 11 illustrates the path of presolar grains from
their stellar sources to the laboratory. The isotopic compositions of presolar grains represent those in the stellar
Chromium
Heterogeneities in the solar nebula are also evident
from the Cr-isotopic composition. Although variations
in the 53Cr/52Cr ratio are less than 100 ppm in different solar system samples, this ratio apparently correlates
with the heliocentric distance of the place of formation
of planetary bodies. This is evidenced from Cr data for
the Earth, martian (SNC) meteorites, and HED meteorites (achondrites) believed to originate from the asteroid Vesta (Fig. 8) [26, 27]. If this is generally true, then
the 53Cr/52Cr ratio can be used to constrain the place of
formation of solar system bodies. Enstatite chondrites
would have formed close to the orbit of Mars and the
35
and co-workers at the University of Chicago in the 1980s.
The laboratory study of presolar grains can provide important information on stellar nucleosynthesis and evolution, mixing in supernova (SN) ejecta, the galactic chemical evolution (GCE), grain formation in stellar winds or
ejecta, and on the inventory of stars that contributed dust
to the solar nebula.
FIGURE 9. Presolar SiC grain from the Murchison meteorite. The scale bar is 100 nm. The size of w 0.5 //m represents
a typical size of SiC grains from the Murchison meteorite.
103n
i*
FIGURE 11. Path of presolar grains from their stellar
sources to the laboratory.
Although most abundant (with concentrations of >
1000 ppm in the most primitive meteorites) the diamonds
are least understood. The reason for this is their comparatively small size of only 2-3 nm that does not allow
to measure the isotopic compositions of single grains.
The other grain types are less abundant (at most a few
ppm) but they have sizes of > 100 nm (Fig. 9) that allows to measure the isotopic compositions of many elements in single grains. A lot of isotopic information is
available for SiC, graphite, and corundum. On the other
hand, only very few presolar silicon nitride, spinel, and
hibonite grains were identified so far and there are only
few isotopic data available for those grains.
1030
Si/28Si
FIGURE 10. Range of isotopic ratios observed in presolar
grains from primitive meteorites. All ratios are normalized
to solar system reference ratios (terrestrial atmosphere for N,
14N/15N = 272; terrestrial PDF for C, 12C/13C - 89; terrestrial
SMOWforO, 16O/17O = 2610,16O/18O = 499; bulk meteorites
and Earth for Si,29Si/28Si - 0.05063, 30Si/28Si = 0.03347). For
references see [6, 7].
atmosphere or in the ejecta of stellar explosions which
in turn are determined by the compositions at the time
the parent stars formed, and by the nucleosynthesis in
and evolution of the parent stars. After passage through
the interstellar medium such grains became part of the
molecular cloud from which our solar system formed.
They survived the events that led to the formation of
the solar system inside small planetary bodies (asteroids)
and comets. They are carried to the Earth by meteorites
from which they can be separated by chemical and physical treatments which were invented by Edward Anders
Silicon carbide, graphite, and silicon nitride
Based on the isotopic compositions of C, N, Si, and
the abundance of radiogenic 26Mg the SiC grains were
divided into six distinct populations. Of particular importance are the so-called mainstream [31] and the X
grains [32, 33, 34]. The mainstream grains make up the
majority (> 90%) of the SiC grains. They are characterized by lower than solar system 12C/13C ratios (typically 40-80; bulk meteorites: 89-92) and higher than
solar system 14N/15N ratios (up to 20,000; bulk mete-
36
orites and planets: 140-430) (Fig. 12) [e.g. 35, 36, 37];
heavier elements (e.g., Kr, Sr, Zr, Mo, Xe, Ba, Nd, Sm)
show the signature of s-process nucleosynthesis [e.g.
38, 39, 40, 41, 42, 43]. From a comparison between
the grain data and astronomical observations and stellar models, low-mass (1-3 M0) asymptotic giant branch
(AGB) stars are considered the most likely stellar sources
of the mainstream grains.
*
SiC Mainstream Grains
:
/slope 1.3
line
SIC
-200
-200
FIGURE 13.
300
Si-isotopic ratios of SiC mainstream grains
given as permil deviation from the solar system reference (terrestrial) ratios. Data are from [35]. The slope 1.3 line most
likely reflects the GCE of the Si isotopes. The average Siisotopic composition of presolar SiC (including all sub-types)
is not solar, indicating that there may exist yet unidentified
presolar mineral types with isotopically light Si on average.
12C/13C
FIGURE 12.
N- and C-isotopic compositions of different
populations of presolar SiC grains. The dashed lines represent
solar system reference ratios (N: terrestrial atmosphere; C: terrestrial PDB standard). The mainstream grains are believed to
originate from AGB stars, the X grains from SN. For references
see [6, 7].
Most mainstream grains have enrichments in the
heavy Si isotopes of up to 20% relative to their solar system abundances (Fig. 13) [cf. 31]. In a Si-three-isotoperepresentation the data fall along a line with slope 1.3.
In low-mass AGB stars Si is affected by the s-process
in the He shell. He shell matter is mixed outward in the
so-called third dredge-up, leading to enrichments of 29Si
and 30Si in the star's envelope. However, the expected
shifts in 29Si/28Si and 30Si/28Si are at most a few percent
[e. g. 44] and evolution of the Si-isotopic composition in
a Si-three-isotope representation is expected along a line
with slope « 0.5 [e.g. 45], at variance with the grain
data. It is the preferred interpretation today that the slope
1.3 Si correlation line does not result from the dredgeup of He shell matter in AGB stars but reflects the GCE,
both in time and space, of the Si isotopes and represents
a range of Si starting compositions of a large number of
AGB stars [46, 47, 44].
Most SiC X grains are characterized by isotopically
light C (with 12C/13C of up to 7000), heavy N (with
14
N/15N down to 13), and light Si (with enrichments in
28
Si of up to a factor of 5). Other isotopic features of X
37
grains are high inferred initial 26A1/27A1 ratios and the
presence of radiogenic 44Ca from the decay of now extinct 44Ti (half-life 60 y) in some grains [32, 33, 34].
On the basis of the enrichments in 28Si and presence of
44
Ti at the time of grain formation, type II SN have been
proposed as the most likely stellar sources of X grains.
The same holds for the majority of the graphite grains
and all silicon nitride grains. Most of these grains exhibit
isotopic signatures that resemble those of X grains indicative for a close relationship between these types of
presolar grains [e. g. 48, 49, 33, 50]. A small fraction of
the SiC and graphite grains apparently is from novae [51]
and for some of the graphite grains also a Wolf-Rayet star
origin cannot be excluded [48].
The presolar SN grain data do not only allow to test
models of nucleosynthesis in SN but also provide information on the mixing in SN ejecta. In the context of a
type II SN origin, the isotopic compositions of presolar grains require mixing of matter from the innermost
Ni- and Si-rich zones and the C-rich outer layers, indicative of deep mixing in the ejecta. This confirms similar
conclusions derived from astronomical observations of
SN light curves [e. g. 52, 53]. Although carbonaceous
grains might form in the ejecta of type II SN explosions
even while C/O < 1 [54], some kind of selective mixing, which limits contributions of matter from the intermediate, extremely O-rich zones to the condensation
site in the ejecta, is indicated. This supports hydrodynamical models of SN explosions that predict fingers and
mushroom-like structures rising from the interior into the
outer portions of the ejecta as a result from RayleighTaylor instabilities [e. g. 55, 56, 57].
Chondrites contain small quantities of presolar grains.
These grains exhibit large isotopic anomalies in the major and many trace elements indicative of a circumstellar origin. Presolar minerals identified to date include diamond, silicon carbide, graphite, silicon nitride, corundum, spinel, and hibonite. Most of the grains apparently
formed in the winds of red giant and AGB stars and in the
ejecta of SN explosions. A small fraction of the grains
appears to come from novae and possibly also from
Wolf-Rayet stars. The isotopic compositions of presolar
grains reveal the signature of different nuclear burning
processes and of the GCE. The non-solar average isotopic compositions of Si and O in presolar grains indicate that there may be yet unidentified presolar mineral
types in primitive meteorites.
An important merit of meteorites is that they preserve
a record of presolar components and of processes in the
early solar system. On the other hand, they represent
only a small fraction of the matter that went into the
making of the solar system and they cannot a priori
be considered to be a good reference for the average
elemental and isotopic compositions of the protosolar
nebula. However, knowledge of average elemental and
isotopic compositions is important in order to understand
isotopic variations in meteorites, e.g., those observed
in N and O. There are many opportunities where the
meteorite community could get input from the wider
SOHO/ACE community and these are outlined in the
contribution of the results from the working group on
"Applications in Cosmochemistry" (H. Busemann, these
proceedings).
Oxides
Presolar corundum (and spinel and hibonite) grains
have 16O/17O ratios between 70 and 30,000 (meteorites:
2580-2740) and 16O/18O ratios between 200 and 50,000
(bulk meteorites and CAIs: 490-520) [e. g., 58, 59, 60,
61,62,63]. Most grains are characterized by enrichments
in 17O and depletions in 18O as compared to solar system
abundances. This is consistent with astronomical observations of red giant and AGB stars and with theoretical
predictions of those types of stars. Oxide grains from SN
are apparently rare among meteoritic stardust. Up to now
only one corundum grain was found that shows the expected enrichment in 16O for oxide grains from SN [61].
SUMMARY
Undifferentiated meteorites (chondrites) have preserved
the bulk elemental and isotopic compositions of their
parent bodies. Variations in elemental abundances between different chondrite groups are within a factor of
2 for the refractory elements and are attributed to variations in the nebula environment from which the chondrites formed. The CI chondrites, which represent the
most primitive meteoritic matter, have elemental abundances that are compatible to those in the solar photosphere except for Li, which is destroyed in the Sun's convection zone, and the highly volatile elements H, C, N,
O, and noble gases which are incompletely condensed in
chondrites. This justifies use of CI chondrites as chemical reference for bulk solar system matter for most of the
elements. Largest bulk isotopic heterogeneities are seen
for H (variations in D/H of a factor of 8), followed by N
(15N/14N varies by 2x), and C and O (C- and O-isotopic
ratios vary by a few percent). Much smaller isotopic variations are observed for more refractory elements. The
53
Cr/52Cr varies by less than 100 ppm and this ratio appears to be correlated with the place of formation of
planetary bodies. Short-lived radioactive nuclides such
as 26A1 and 41Ca existed live in the early solar system
as evidenced from the presence of the radiogenic daughter nuclides in CAIs and chondrules. Provided that the
radionuclides were injected into the solar nebula at the
time of solar system formation, they can be used as a
chronometer to date events in the early solar system.
ACKNOWLEDGMENTS
I thank the organizers of the SOHO/ACE workshop on
"Solar and Galactic Composition" for the invitation to
present this paper. Critical and helpful reviews by P.
Eberhardt and V. Heber are acknowledged.
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2.
3.
4.
38
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