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Ion Microprobe Measurements of Oxygen Isotope Compositions of Artificial Chondrule and CAI: Implica- tion for Nebular Oxygen Exchange During the Formation of Chondrules and CAIs. B.-G. Choi
Ion Microprobe Measurements of Oxygen Isotope Compositions of Artificial Chondrule and CAI: Implica-
tion for Nebular Oxygen Exchange During the Formation of Chondrules and CAIs. B.-G. Choi
1
, T. Naka-
mura
2
and M. Kusakabe
3
,
1
Department of Earth Science Education, Seoul National University, Seoul, Korea 151-
748 (bchoi@snu.ac.kr),
2
Department of Earth and Planetary Science, Kyushu University, Fukuoka 81-8581, Japan
(tomoki@geo.kyushu-u.ac.jp),
3
Korea Polar Research Institute, Incheon, Korea 406-840 (mhk2314@hotmail.com).


Introduction: Oxygen isotopic heterogeneity pre-
served especially in primitive meteorites is interpreted
as results of mixing between two or more oxygen res-
ervoirs that had existed prior to or during the forma-
tion of chondritic materials [1]. Several models have
been proposed for the nature of the reservoirs, includ-
ing recent models of CO shelf shielding [2-4]. How-
ever, there are limited numbers of experimental and/or
theoretical studies for oxygen isotope exchanges
among oxygen bearing phases in the nebula [5, 6].
These studies show that, if oxygen isotopic exchange
occurred during the formation of chondrules with CO
gas as major gaseous species in the reaction, it would
be required to have substantially longer duration times
above melting temperature of chondrules and/or higher
CO pressure that suspected for the solar nebula. Thus,
it was suggested that oxygen isotopic exchange may be
controlled by the proportion of CO to H
2
O within the
nebula, since H
2
O dissociates much more rapidly than
CO at high temperature and H
2
O is able to contribute a
substantially larger proportion of O
2
or O
-2
to the
nebular environment [6]. Recently Choi and Kusakabe
[7] reported bulk oxygen isotope compositions of arti-
ficially recycled chondrules and Ca-Al-rich inclusion
(CAI) after oxygen isotopic exchange with O
2
gas dur-
ing brief (a few minutes) heating events using CO
2

laser. Here we report in situ oxygen isotope measure-
ments of minerals in artificially recycled chondrules
from Eagle Station olivine and Ca-Al-rich inclusion
synthesized by [7]. We found that, in general, the com-
positions fall along new mixing lines connecting be-
tween compositions of original solids and reacted O
2

gas.
Analytical conditions: Cameca 6f at Kyushu uni-
versity was used for in situ oxygen isotope measure-
ments. Polished thick sections having artificially recy-
cled chondrules and CAI along with San Carlos olivine
used as oxygen isotope standard were coated with gold
in order to minimize charging during the SIMS meas-
urements. Electron flood gun was turned on for the
charge compensation. Approximately 1 nA of Cs
+

beam was used that gave about 2.5 x 10
7
cps of
16
O
-
at
Faraday cup. Sputtered craters have about 15
m in
diameters with reasonably flat bottoms. Mass resolu-
tion of ~ 5500 was used in order to separate tail of
16
OH from
17
O peak. Masses 15.9 (back ground),
16
O,
17
O,
16
OH, 17.995 (tail of
18
O), and
18
O were measured
for 0.5, 1, 5, 0.2, 1 and 1.5 seconds, respectively for
one cycle (
16
O on Faraday cup while the others on
electron multiplier). Each measurement consists of 60
cycles. No effort was made in order to make matrix
effect. Typical errors in samples are slightly larger
than 1 (1
).
Results: Back scattered electron (BSE) images and
oxygen isotope compositions of the artificial chondrule
made from Eagle Station olivine and CAI from frag-
ments of Allende CAIs are shown in figures. 1, 2, 3
and 4.
Artificial chondrule made from Eagle Station oli-
vine. The Eagle Station olivine didnt melt completely,
thus has relict grains inside (Fig. 1). The relict grains
have oxygen isotope compositions of the original eagle
station olivine, i.e., - 2.8 and 6.0 in 18
O and 17
O [8], respectively. The melted area, which formed
barred olivine texture, has exchanged oxygen with gas,
thus has the isotopic composition fall between those of
original solid and reacted gas (Fig. 2); the red broken
line is the best fit for the spot analyses and nearly iden-
tical to the line connect between original solid and
reacted gas.

Fig. 1. BSE image of artificial chondrule from Eagle
Station olivine. The right side with rounded shape
was top that directly exposed to CO
2
laser. Barred
olivine texture was formed at the right side. Three
large relict grains are shown; they were relatively
far from the CO
2
laser.
Lunar and Planetary Science XXXVIII (2007)
1054.pdf
Fig. 2. Oxygen isotopic compositions of mineral
phases in artificial chondrule made from Eagle Sta-
tion olivine. Spot data fall along a mixing line that
connects original solid and reacted gas. The red
broken line is the best fit for the spot analyses.
Artificial CAI made from fragments of Allende
CAIs. The artificially recycled CAI consists of spinel
and plagioclase (+ minor olivine) in fassite mantle.
BSE image shows that this artificial CAI was melted
almost completely except some small relict spinel
grains (Fig. 3).

Fig. 3. BSE image of artificially recycled CAI from
fragments of Allende CAIs. It was almost com-
pletely melted, thus has spherical shape. Light gray
mantle is fassite, medium gray phases are either
anorthite (needle shape) or FeO-rich spinel, and
dark gray is mostly spinel with minor amount of
olivine. Circular black areas inside are holes that
probably was gas vesicles.
Relict spinel grains depleted in FeO have oxygen
isotope compositions close to many natural CAI
spinels, i.e., -40 in both 18
O and 17
O (Fig. 4).
Relatively FeO-rich (up to 10 wt.%) spinel grains, an-
orthite and fassite mantle are more depleted in
16
O.
The red broken line is the best fit for the spot analyses.
It is not identical but very similar to the line connect
between reacted gas and -40 in both 18
O and 17
O.

Fig. 4. Oxygen isotopic compositions of mineral
phases in artificially recycled CAI. The red broken
line is the best fit for the spot analyses. FeO-poor
spinel has the most
16
O-enrich composition, while
FeO-rich spinel, anorthite and fassite fall relatively
close to the reacted gas.
Discussion: Our data show that the oxygen isotope
mixing line of chondritic materials can be produced
during brief heating and melting of chondrules and
CAIs by exchanging their oxygen with surrounding
gas, if O
2
partial pressure was high enough during the
formation of them in the nebula. However O
2
gas is
not the major oxygen-bearing gas species at the nebu-
lar condition, thus H
2
O, which dissociated to form O
2,

may have played important role in the oxygen isotopic
exchange in the nebula as suspected by [6].

References: [1] Clayton R. N. (1993) Ann. Rev.
Earth Planet. Sci., 21, 115-149.. [2] Clayton R. N.
(2002) Nature 415, 860-861. [3] Yurimoto H. and
Kuramoto K. (2004) Science 305, 1763-1766. [4] Ly-
ons J. R. and Young E. D. (2005) Nature 435, 317-320.
[5] Yu Y. et al. (1995) GCA 59, 2095-2104. [6] Boe-
senberg J. S. et al. (2005) Meteoritics & Planet. Sci. 40,
A22. [7] Choi B.-G. and Kusakabe M. (2006) Meteor-
itics & Planet. Sci. 41, A38. [8] Clayton R. N. and
Mayeda T. K. (1996) GCA 60, 1999-2017.
Lunar and Planetary Science XXXVIII (2007)
1054.pdf