Oxygen isotopic compositions of the high-temperatue phases in carbonaceous chondrites define a mixing line with an O-16 rich component and show little superimposed chemical isotope fractionation. Within a single inclusion in Allende, variations of delta O-18 and delta O-17 of 39% are found. The ordinary chondrites are slightly displaced from the terrestrial fractionation trend, implying that at least 0.2% of the oxygen in terrestrial rocks was derived from the O-16 rich component

Clayton, R.

Grossman, L.

Mayeda, T. K.

Onuma, N.

English

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HETEROGENEITIES IN THE SOLAR NEBULA
ROBERT CLAYTON, LAWRENCE GROSSMAN, TOSHIKO K. MAYEDA
AND NAOKI ONUMA
t- 204
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https://ntrs.nasa.gov/search.jsp?R=19750006603 2020-03-23T00:26:28+00:00Z
APR 1 i 1974
Heterogeneities in the Solar Nebula
by
Robert N. Clayton
Enrico Fermi Institute, and Departments of Chemistry and
Geophysical Sciences, University of Chicago
Lawrence Grossman
Department of Geophysical Sciences, University of Chicago
Toshiko K. Mayeda
Enrico Fermi Institute, University of Chicago
Naoki Onuma
Department of Chemistry, Tokyo Gakugei: University.
Abstract
Oxygen isotopic compositions of the high-temperature phases in carbonaceous
chondrites define a mixing line with an 160-rich component and show little
superimposed chemical isotope fractionation. Within a single inclusion in
Allende, variations of 6180 and 6170 of 39%o are found. The ordinary chondrites
are slightly displaced from the terrestrial fractionation trend, implying that
at least 0.2% of the oxygen in terrestrial rocks was derived from the 160-rich
component.
-2-
Introduction
It was recently shown that the anhydrous phases of the carbonaceous
chondrites of classes C2 and C3 display a remarkable pattern of oxygen
isotope abundances: an almost constant ratio of 10/1170 and highly
variable 160 (Clayton et al., 1973). The variations of 180/160 and
170/160 are much greater than can be accounted for by chemical fraction-
ation processes, and do not conform to a 2:1 ratio as would be expected
for any mass-dependent process. It was concluded that the variations in
the C2 and C3 meteorites are ultimately nuclear in origin, and represent
the mixing of two kinds of oxygen with different nucleosynthetic histories,
and hence different isotopic compositions. One component of the mixture
is here referred to as "ordinary oxygen" and has isotopic abundances near
the values of the mean earth, moon and ordinary chondrites; the other
component is highly depleted in the heavy isotopes, and may be nearly
pure I60. Several lines of evidence suggest that the "6 0-rich component
existed as solid grains of interstellar dust prior to condensation of
solids containing "ordinary oxygen" from the solar nebular gas.
The purpose of this paper is to present new data on meteorites,
obtained from refined analytical techinques which yield an order of
magnitude better precision in 170/160 determinations than those previously
used. It is now possible to examine the small isotopic differences among
the earth, moon and ordinary chondrites, and to distinguish between the
effects of admixture of 160-rich components into these bodies and the effects
of chemical isotope effects during condensation and accretion (Onuma et al., 1972a).
It will be shown that even a surficial sample of a differentiated olanet
(e.g. Mars) can be used to determine the content of '61-rich component in the
planet - a parameter that may be important in understanding planetary accretion.
-3-
Analytical Procedures
Sampling: In the equilibrated ordinary chondrites and in the
achondrites, isotopic equilibration has occurred in metamorphic and igneous
processes, respectively, so that the internal isotopic variations which remain
are just the chemical isotopic fractionations between minerals which reflect
the maximum temperature of heating (Onuma et. al., 1972b). Sampling of such
meteorites presents no special problems.
In the unequilibrated chondrites, and especially in the carbonaceous
chondrites, large isotopic variations are present at least down to a sub-
millimeter size range, and information is lost in analyzing larger samples.
Hence, our emphasis has been on individual chondrules, crystals, or small
clusters of crystals, particularly as exemplified by the refractory aggregates
in the C3 meteorite, Allende.
Samples, typically 3-10 mg in size, were treated by the bromine
pentafluoride technique (Clayton and Mayeda, 1963), and the oxygen thus
liberated was analyzed, as 02, using a 15 cm, 600 sector, double-collecting
mass spectrometer. Both 180/160 and 170/160 ratios were determined. The
results are reported in the 6-notation, relative to SMOW standard (Craig, 1961).
Since SMOW is a fictitious standard, defined to have an 180/160 ratio (R'8)
fixed relative to a real substance, NBS-1 water, by the equation:
SMOW = 1.008 RNBS1  (Craig, 1961),SMOW NBS-1
we have adopted a similar definition for R ,7 as follows:SMOW,
R =1 (1.008)1/2 R17
SMOW NBS
-
1
Analytical uncertainties are estimated as + 0.1%.both for 618 and 617
-4-
Results
Oxygen isotopic compositions of some terrestrial and meteoritic samples
are given in Table 1. Data from terrestrial samples and the Aliende meteorite
are shown graphically in Figure 1. It is seen that all of the terrestrial
samples lie along a line through the origin with a slope of +1/2. This is
to be expected for any samples derived from an initially homogeneous
reservoir, in which all subsequent isotopic.fractionations are mass-
dependent, and conform approximately to the relationship:
17 1/(1)
AB AB
where aAB is defined as a fractionation factor between two phases, A and B:
AB (RP7/P 7) and (2)
RA7= (017/06)A. (3)
Equation (1) does not hold rigorously for isotopic fractionation processes,
but is a good approximation. For example, if isotopic fractionation is
brought about by diffusion, with diffusion rates inversely proportional to
the square-root of the masses of the diffusing molecules, equation (1) is
in error by only about 1% if the diffusing species are oxygen atoms, and is
an increasingly better approximation for oxygen-containing molecules of
higher molecular weight. For isotopic fractionation in a chemical
equilibrium process, equation (1) implies a linear relationship between free
energy and mass difference, which is a good approximation for small relative
mass differences.
In the paper of Clayton et al., (1973), data points for the C2 and
C3 meteorites were scattered about the mixing line with deviations from the
line on the order of 1%o. It was not clear how much of the scatter was due
to analytical uncertainty in the 170/160 ineasurements, and how much was
attributable to chemical isotope fractionations superimposed on the mixing
process. It is evident from the new data plotted in Figure 1 that the
scatter was principally analytical, and that chemical shifts from the
mixing line are small, and probably <0.5%. for all of the Allende
samples plotted. Onuma et al., (1972a) have discussed the magnitudes of
the oxygen isotope effects expected for solids condensinq from the solar
nebula, in which the principal reservoir of oxygen should be carbon monoxide
at the high temperatures involved. The small variability of the chemical
isotope shifts implies a narrow range of condensation temperatures, and is
compatible with the range from 16250 to 14500K given by Grossman (1972)
for the interval between condensation of melilite and condensation of
forsterite.
Fig. 2 shows data for other C2 and C3 meteorites, with the lines from
Fig. 1 given for reference. It is seen that the points fall on the line
defined by the Allende samples. The smaller spread in the data may reflect
the sampling technique, which tended to homogenize the samDles. It should
be noted, however, that the samples from Ornans are all iron-bearin olivines,
and it may be surprising that the introduction of lower-temperature ferrous
iron did not result in a chemical displacement of the data points from the line.
It is of interest to determine whether the various minerals within a
single chondrule or inclusion have crystallized together. A large (1 cm
diameter), coarse-grained, round inclusion from Allende was crushed, and
mineral concentrates were prepared by heavy-liquid separation. Four fractions,
labeled Al-15-1 to Al-15-5 in Table 1, showed a range of 39%o in 618 and
617, falling along the mixing line rather than a fractionation line, as
would be expected if the minerals had crystallized from a melt.
-6-
The densest fraction, composed primarily of titanian clinopyroxene and
included spinel, is the most 160-rich of any sample yet analyzed: the
160-rich component makes up at least 5% of its oxygen. Even at this
high concentration, the physical nature of the 160-rich component is
not revealed by microscopic examination.,
Clayton et al., (1973) showed that samples from the moon and the
ordinary chondrites fall on or near the terrestrial fractionation line,
implying that these bodies have the same fraction of the 160-rich component,
and might, in fact, be composed entirely of "ordinary oxygen". With the
increased precision now available, it can be seen in Fig. 3 that analyses
of two ordinary chondrites, Pantar ( H5 ) and Modoc ( L6 ) define
a fractionation trend parallel to, but displaced from, the terrestrial
line. The range of data points from Pantar results from a set of partial
fluorination reactions, such as were first carried out on lunar soils
( Epstein and Taylor, 1971 ). The variation of about 4& in 6180 is
greater than would be expected for fractionation among the major minerals
in a meteorite of petrographic grade 5 (Onuma et al., 1972b), and probably
results from prior exposure of the constituents of this gas-rich meteorite
to bombardment while in the regolith of the parent body.
If the apparent displacement of the ordinary chondrite data from the
terrestrial fractionation line is supported by subsequent analyses, the
implications for accretion of the earth are significant. Since the ordinary
chondrites are displaced in the '60-depleted direction, the amount of 160-rich
component in the earth cannot be zero. The magnitude of the displacement
sets a lower limit on the content of this component at 0.2% of the oxygen of
all terrestrial rocks.
The three-isotope plot for oxygen should be useful in future studies
of samples returned from other planets. If the two-component model of
solar system oxygen is valid, it is possible to determine the relative
proportions of the components in a planet on the basis of a single
analysis of a sample of the planet, even if the planet has undergone
extensive differentiation, so that the surface sample is not chemically
representative of the body as a whole. It would simply be necessary to
plot the data point, and construct a fractionation line with slope of 1/2
to find its intersection with the mixing line. The intersection then
determines the content of 160-rich component relative to the earth and the
various meteorite classes.
-8-
References
R.N. Clayton and T.K. Mayeda (1963): The use of bromine pentafluoride
in the extraction of oxygen from oxides and silicates for isotopic
analysis. Geochim. Cosmochim. Acta, 27, 43 - 52.
R.N. Clayton, L. Grossman, and T.K. Mayeda (1973): A component of
primitive nuclear composition in carbonaceous meteorites. Science,
182, 485 - 488.
H. Craig (1961): Standard for reporting concentrations of deuterium
and oxygen-18 in natural waters. Science, 133, 1833 - 1834.
S. Epstein and H. P. Taylor, Jr. (1971): 018/016, Si3o/Si2 8 , D/H, and
C'3 /C 2 ratios in lunar samples. Geochim. Cosmochim. Acta, Suppl.
2, vol. 2, 1421 - 1441 (M.I.T. Press).
L. Grossman (1972) Condensation in the primitive solar nebula. Geochim.
Cosmochim. Acta. 36, 597 - 619.
N. Onuma, R.N. Clayton, and T.K. Mayeda (1972 a): Oxygen isotope cosmo-
thermometer. Geochim. Cosmochim. Acta, 36, 169 - 188.
N. Onuma, R.N. Clayton, and T.K. Mayeda (1972 b): Oxygen isotope Temp-
eratures of "equilibrated" ordinary chondrites. Geochim. Cosmochim.
Acta, 36, 157 - 168.
9-
TABLE 1
Oxygen Isotopic Comoositions of Terrestrial and Meteoritic
Samples
Sample No. Description 6160 6170
Terrestrial NBS-la Yellowstone snow -24.4 -13.0
L-water Distilled water - 9.3 - 5.1
M-3 Magnetite 3.0 1.7
0-120 Quartz 9.5 4.8
0-120 Quartz 9.6 A.9
0-120 Quartz (fractionated) 13.2 6.4
S-863 Ouartz 17.6 9.1
Carbonaceous
Chondrites Al-Mx Matrix 1.3 - 3.1
Allende Al-Wr Whole-rock 1.6 - 2.6
Al- 3 Dark inclusion 3.3 - 1.3
A1-17 White inclusion -11.2 -14.1
Al-15 White inclusion -16,0 -18.2
Al-15-1 Melilite - 0.1 - 4.0
A1-15-2 Melilite A spinel - 6.7 -10.2
A1-15-4 Pyroxene 
-34.8 -35.7
Al-15-5 Pyroxene & spinel -39.0 -40.6
Murray My-2 Olivine 
- 6.9 - 9.7
Ornans 0- 4 Olivine 
- 6.5 - 9.5
0-10 Olivine - 2.1 - 5.2
0-12 Olivine - 2.7 - 6.5
0-13 Olivine - 2.8 - 6.4
0-14 Olivine - 2.1 - 5.9
0-15 Olivine - 0.9 - 5.1
Vigarano V-6 Olivine 
- 4.4 - 7.9
Ordinary Chondrites
Pantar P-Wr Whole rock 4.1 2.7
P-A Partial fluorination 7.3 4.6
P-B " " 6.5 4.3
P-II-A " " 5.4 3.8
P-II-B " " 5.6 3.9
P-II-C " " 7.8 4.9
Modoc M-Wr Whole rock 5.9 4.2
FIGURF CAPTIONS
Fig. 1 Oxygen.isotopic compositions of various terrestrial materials,
(squares) and of samples of the Allende meteorite (circles and
triangles). The samples represented by circles are density separates
from a single inclusion.
Fig. 2 Oxygen isotopic compositions of separated fractions of Murray,
Ornans and Viqarano. The lines are taken from Fig. 1.
Fig. 3 Oxygen isotopic compositions of ordinary chondrites, Pantar and
Modoc. The data from Pantar results from successive partial
fluorinations; the Modoc result is a whole-rock analysis.
0-I0
S-20
-30
0
-40-
-40 -30 -20 -10 0 10 20
18 (SMOW)
0-
-10-
S-20-
o Murray (C2)
a Ornans (C3)
0 Vigarano (C3)
-40
-40 -30 -20 -10 0 10 20
18 (SMOW)
6I I I I
o Pontor (H5)
A Modoc (L6) o
4
0 2 4 6 8 10
18 (SMOW)


Heterogeneities in the solar nebula

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