The Astrophysical Journal, 706:142–147, 2009 November 20
C 2009.
doi:10.1088/0004-637X/706/1/142
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
OXYGEN ISOTOPIC COMPOSITIONS OF SOLAR CORUNDUM GRAINS
Kentaro Makide, Kazuhide Nagashima, Alexander N. Krot1 , and Gary R. Huss
School of Ocean & Earth Science & Technology, Hawai’i Institute of Geophysics & Planetology, University of Hawai’i at Manoa, 1680 East-West Road, Honolulu,
HI 96822, USA; [email protected]
Received 2009 September 14; accepted 2009 October 2; published 2009 October 28
ABSTRACT
Oxygen is one of the major rock-forming elements in the solar system and the third most abundant element of
the Sun. Oxygen isotopic composition of the Sun, however, is not known due to a poor resolution of astronomical
spectroscopic measurements. Several Δ17 O values have been proposed for the composition of the Sun based on (1)
the oxygen isotopic measurements of the solar wind implanted into metallic particles in lunar soil (< −20‰ by
Hashizume & Chaussidon and ∼ +26‰ by Ireland et al.), (2) the solar wind returned by the Genesis spacecraft
(−27‰ ± 6‰ by McKeegan et al.), and (3) the mineralogically pristine calcium–aluminum-rich inclusions (CAIs)
(−23.3‰ ± 1.9‰ by Makide et al. and −35‰ by Gounelle et al.). CAIs are the oldest solar system solids,
and are believed to have formed by evaporation, condensation, and melting processes in hot nebular region(s)
when the Sun was infalling (Class 0) or evolved (Class 1) protostar. Corundum (Al2 O3 ) is thermodynamically
the first condensate from a cooling gas of solar composition. Corundum-bearing CAIs, however, are exceptionally
rare, suggesting either continuous reaction of the corundum condensates with a cooling nebular gas and their
replacement by hibonite (CaAl12 O19 ) or their destruction by melting together with less refractory condensates
during formation of igneous CAIs. In contrast to the corundum-bearing CAIs, isolated micrometer-sized corundum
grains are common in the acid-resistant residues from unmetamorphosed chondrites. These grains could have
avoided multistage reprocessing during CAI formation and, therefore, can potentially provide constraints on
the initial oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Here we report oxygen
isotopic compositions of ∼60 micrometer-sized corundum grains in the acid-resistant residues from unequilibrated
ordinary chondrites (Semarkona (LL3.0), Bishunpur (LL3.1), Roosevelt County 075 (H3.2)) and unmetamorphosed
carbonaceous chondrites (Orgueil (CI1), Murray (CM2), and Alan Hills A77307 (CO3.0)) measured with a Cameca
ims-1280 ion microprobe. All corundum grains, except two, are 16 O-rich (Δ17 O = −22.7‰ ± 8.5‰, 2σ ), and
compositionally similar to the mineralogically pristine CAIs from the CR carbonaceous chondrites (−23.3‰ ±
1.9‰, 2σ ), and solar wind returned by the Genesis spacecraft (−27‰ ± 6‰, 2σ ). One corundum grain is highly
17
O-enriched (δ 17 O ∼ +60‰, δ 18 O ∼ −40‰) and is probably of the presolar origin; the origin of another 17 O-rich
grain (δ 17 O ∼ −15‰, δ 18 O ∼ −35‰) is unclear. We conclude that the 16 O-rich corundum grains in the acidresistant residues from unequilibrated ordinary and unmetamorphosed carbonaceous chondrites recorded initial
oxygen isotopic composition of the solar nebula, and, hence, of the Sun. Our inferred oxygen isotopic composition
of the Sun is inconsistent with the more extreme 16 O-rich value (Δ17 O ∼ −35‰) proposed by Gounelle et al.
on the basis of two extremely 16 O-rich CAIs from the CH/CB-like chondrite Isheyevo and with the 16 O-poor
value observed as a component of the solar wind implanted into the metallic particles in lunar soil (Ireland et al.).
Key words: astrochemistry – meteors, meteoroids – solar system: formation – Sun: abundances
−20‰ ± 4‰, where Δ17 O = δ 17 OSMOW − 0.52 × δ 18 OSMOW ,
and δ i O = ((i O/16 O)sample /(i O/16 O)SMOW − 1) × 1000, i =
17, 18) and 16 O-poor (Δ17 O up to ∼ +35‰). Both values are
interpreted to represent composition of the Sun (Hashizume &
Chaussidon 2005, 2009; Ireland et al. 2006). Preliminary results
of oxygen isotopic measurements of the solar wind returned by
the Genesis spacecraft (Δ17 O = −27‰ ± 6‰, 2σ , McKeegan
et al. 2009) are consistent with the 16 O-rich value, but require
further confirmation.
Calcium–aluminum-rich inclusions (CAIs) are the oldest solar system solids, and are believed to have formed by evaporation, condensation, and melting processes in hot nebular region(s) when the Sun was infalling (Class 0) or evolved (Class 1)
protostar (Amelin et al. 2002; Krot et al. 2009a). As a result, oxygen isotopic compositions of CAIs could have recorded oxygen
isotopic composition of the early solar nebula. Based on the narrow range of oxygen isotopic compositions of primary minerals
in the mineralogically pristine CAIs from the CR carbonaceous
chondrites (Δ17 O ∼ −23.3‰ ± 1.9‰, 2σ ), and mineralogical
and chemical evidence that CAIs condensed from or equilibrated with a gas of solar composition (e.g., Grossman et al.
1. INTRODUCTION
Oxygen is one of the major rock-forming elements in the
solar system and the third most abundant element of the Sun
(Lodders 2003). Oxygen isotopic composition of the Sun, however, is not known due to a poor resolution of astronomical
spectroscopic measurements (Wiens et al. 1999). The composition of the Sun has been recently inferred from the oxygen
isotopic measurements of (1) the solar wind implanted into
metallic particles in lunar soil (Hashizume & Chaussidon 2005,
2009; Ireland et al. 2006), (2) the solar wind returned by the
Genesis spacecraft (McKeegan et al. 2009), and (3) the hightemperature solar nebula condensates (e.g., Krot et al., 2008a,
2009a; Makide et al. 2009; Gounelle et al. 2009). However, no
consensus has yet been reached (e.g., Ozima et al. 2009).
The oxygen isotopic measurements of the solar wind implanted into the metallic particles in lunar soil revealed the
presence of two isotopically anomalous components relative to
the standard mean ocean water (SMOW), 16 O-rich (Δ17 O <
1
Author to whom any correspondence should be addressed.
142
No. 1, 2009
OXYGEN COMPOSITIONS OF SOLAR CORUNDUM GRAINS
2002; MacPherson et al. 2005), Krot et al. (2008a, 2009a) and
Makide et al. (2009) concluded that this value, which is consistent with the solar wind value reported by McKeegan et al.
(2009), may represent the initial composition of the solar nebula,
and, hence, of the Sun.
A more extreme value (Δ17 O ∼ −35‰) has been reported
in two grossite-rich CAIs from the CH/CB-like carbonaceous
chondrite Isheyevo (Krot et al. 2008b; Gounelle et al. 2009).
Assuming that (1) oxygen isotopic compositions of the dust and
gas in the solar system were initially similarly 16 O-rich and (2)
evolution of oxygen isotopic composition of the solar nebula
was due to CO self-shielding and was unidirectional toward
heavier compositions (Clayton 2002; Yurimoto & Kuramoto
2004; Lyons & Young 2005; Krot et al. 2005), Gounelle et al.
(2009) interpreted this extreme value to represent the initial
composition of the solar nebula and, hence, of the Sun.
Corundum (Al2 O3 ) is the first mineral thermodynamically
predicted to condense from a gas of solar composition (Tcond =
1770 K at Ptot = 10−3 bar; Ebel & Grossman 2000). At lower
temperature, corundum reacts with nebular gas to form hibonite
(CaAl12 O19 , 1728 K), grossite (CaAl4 O7 , 1698 K), perovskite
(CaTiO3 , 1680 K), melilite (Ca2 Al2 SiO7 − Ca2 MgSi2 O7 solid
solution, 1580 K), and spinel (MgAl2 O4 , 1488 K). However,
the corundum-bearing CAIs are very rare, possibly indicating
efficient reaction with the cooling solar nebula gas2 or destruction by melting together with less refractory condensates during
formation of igneous CAIs (Bar-Matthews et al. 1982; Fahey
et al. 1987; Hinton et al. 1988; MacPherson et al. 1984; Krot
et al. 2001; Simon et al. 2002; Sugiura & Krot 2007; Liu et al.
2009).
In contrast to the corundum-bearing CAIs, micrometer-sized
corundum grains in the acid-resistant residues from primitive
chondrites are more common (e.g., Huss & Lewis 1995).
These grains may represent primordial solar nebula condensates,
which could have avoided reprocessing during formation of
CAIs3 and can potentially constrain the initial oxygen isotopic
composition of the solar nebula. Most studies of the chemically
isolated corundum grains, however, have been focused on the
presolar grains, which are very rare, characterized by large
oxygen isotopic anomalies (from several hundred to several
thousand ‰), and can be easily identified even with the relatively
low precision (±50‰–100‰) isotope measurements (e.g., Huss
et al. 1994; Hutcheon et al. 1994; Nittler et al. 1997; Choi et al.
1998; Clayton & Nittler 2004; Nguyen et al. 2007). In spite
of being relatively common, the micrometer-sized corundum
grains of the solar origin in the acid-resistant residues from
chondritic meteorites have received much less attention, largely
because of their small sizes and difficulties of measuring their
oxygen isotopic compositions with high precision: in contrast to
the presolar grains, the solar corundum grains are isotopically
normal within ∼50‰ (Virag et al. 1991; Huss et al. 1993, 1995;
Hutcheon et al. 1994; Choi et al. 1998, 1999; Strebel et al. 2000).
Here we report the results of high-precision oxygen isotopic
measurements of individual micrometer-sized corundum grains
in the acid-resistant residues from three unequilibrated ordinary
chondrites (UOCs): Semarkona (LL3.0), Bishunpur (LL3.1),
and Roosevelt County 075 (H3.2), and three unmetamorphosed
2
Corundum evaporative residues are rare; e.g., corundum in a unique CAI
from Murchison resulted from extensive evaporation (MacPherson et al. 1984).
3 CAIs appear to have experienced multistage formation history, including
melting, evaporation, and re-condensation, which could have modified their
primary oxygen isotopic signatures (e.g., Yurimoto et al. 1998, 2008;
MacPherson et al. 2005; Grossman et al. 2008; Makide et al. 2009).
143
carbonaceous chondrites (CCs): Orgueil (CI1), Murray (CM2),
and Alan Hills A77307 (CO3.0) with the University of Hawai’i
(UH) Cameca ims-1280 ion microprobe.
2. SOLAR SYSTEM CORUNDUM: PREVIOUS STUDIES
Virag et al. (1991) studied 26 individual corundum grains
3–20 μm in size in the acid-resistant residue from the CM carbonaceous chondrite Murchison. On the basis of the oxygen and
magnesium isotopic compositions, and titanium and vanadium
contents, the grains were divided into three groups: Group 1 (n =
17) and Group 2 (n = 5) grains show 26 Mg excesses (26 Mg∗ , decay product of a short-lived radionuclide 26 Al (t1/2 ∼ 0.73 Ma))
corresponding to the initial 26 Al/27 Al ratios ((26 Al/27 Al)0 ) of
5 × 10−5 and 5 × 10−6 , respectively; Group 3 grains (n = 4)
show no resolvable 26 Mg∗ ((26 Al/27 Al)0 < 3 × 10−7 ). On a
three-isotope oxygen diagram (δ 17 O versus δ 18 O), most corundum grains measured plot along ∼slope-1 line, called the carbonaceous chondrite anhydrous mineral (CCAM) line. All but
one of the Group 1 grains fall in the main cluster at δ 17,18 O ∼
−50‰, whereas four out of five Group 2 grains have highly fractionated 16 O-rich compositions, resembling those of Fractionation and Unidentified Nuclear (FUN) effects inclusions (e.g.,
Lee et al. 1980; Thrane et al. 2008; Krot et al. 2008a); Group 3
grains scatter widely (see Figure 2 in Virag et al. 1991).
Huss et al. (1993) and Hutcheon et al. (1994) reported oxygen
isotopic compositions of 1–5 μm corundum grains in the acidresistant residue from the CI carbonaceous chondrite Orgueil.
The corundum grains show a wide range in oxygen isotopic
compositions (δ 17 O = −67‰ to +55‰; δ 18 O = −63‰ to
+75‰). Most grains are 16 O-rich and on a three-isotope oxygen
diagram plot along the CCAM line; four grains have normal
oxygen isotopic compositions (δ 17,18 O ∼ 0); two grains are
17,18
O-enriched (δ 17 O = +42‰; δ 18 O = +75‰) and plot along
the terrestrial fractionation (TF) line.
Huss et al. (1995), Choi et al. (1998, 1999), and Strebel
et al. (2000) reported low-precision (2σ uncertainty was typically ∼25‰–50‰) oxygen isotopic measurements of the
micrometer-sized corundum grains in the acid-resistant residues
from Semarkona (LL3.0), Krymka (LL3.1), Bishunpur (LL3.1),
and Qingzhen (EH3). The weighted means of oxygen isotopic compositions of the corundum grains from Semarkona,
Bishunpur, and Qingzhen are δ 17 O = −21‰ ± 4‰, −52‰ ±
6‰, −32‰ ± 10‰, and δ 18 O = −18‰ ± 3‰, −42‰ ± 6‰,
−43‰ ± 9‰, respectively.
In situ discovery and oxygen isotopic measurements of the
micrometer-sized isolated corundum grains and their aggregates in fine-grained matrix of the ungrouped carbonaceous
chondrite Acfer 094 have been reported by Nakamura et al.
(2007). Because of the analytical problems associated with isotopic measurements of small grains embedded in fine-grained
matrix material using spot analysis by a Cameca ims-6f ion
microprobe (overlap of ion beam with the surrounding mineralogically and isotopically diverse material), oxygen isotopic
compositions of one of the corundum aggregates were measured
only qualitatively using ion imaging technique. The aggregate
is 16 O-enriched relative to the surrounding matrix materials.
3. SAMPLES AND ANALYTICAL TECHNIQUES
The acid-resistant residues of ordinary and carbonaceous
chondrites used for our study were prepared by Huss & Lewis
(1995) and Huss et al. (2003). The residues, consisting of
1–10 μm grains, were dispersed onto gold substrates cleaned
144
MAKIDE ET AL.
by ion milling. Prior to the dispersal, the gold substrates were
scanned for a possible terrestrial contamination using the UH
JEOL JSM 5900LV scanning electron microscope equipped
with a Thermo Electron energy dispersive spectrometer (EDS).
Several relatively large (>5 μm) corundum grains embedded in
the foils by the manufacturing process were identified on each
gold substrate. These grains prior to dispersing the samples on
the mounts were mapped and were avoided during subsequent
oxygen isotopic measurements. The initial samples were diluted
by a mixture of 90% isopropanol and 10% water. After stirring
by ultrasonic cleaner, about 10 μl of the mixture was siphoned
using a micro-aspirator and dispersed on a gold substrate under a
stereomicroscope. More than 1000 micrometer-sized refractory
grains (spinel, hibonite, silicon carbide, corundum etc.) were
dispersed on each gold substrate; corundum grains constitute
<1% of the dispersed grains, which are dominantly spinel. To
locate the corundum grains, we used cathodoluminescence (CL)
imaging by the UH JEOL JXA-8500F field emission electron
microprobe equipped with the Gatan MiniCL detector. Among
the dispersed refractory grains, only corundum and silicon
carbide grains show CL; the latter are very rare and can be
easily distinguished by the EDS. The corundum grains identified
were photographed in secondary and backscattered electrons
and transferred to the UH Cameca ims-1280 ion microprobe for
oxygen isotopic measurements.
To locate micrometer-sized corundum grains on the Cameca
ims-1280 ion microprobe, scanning ion imaging was used.
A < 1 pA Cs+ primary ion beam focused to <1 μm was
rastered over the sample up to 250 × 250 μm2 area, and 16 O−
image was acquired with a multicollection electron multiplier
(EM). Each grain identified by 16 O− image was positioned at the
center of a field aperture. Oxygen isotopic compositions were
measured with a 0.3–2 nA Cs+ primary ion beam. To achieve
uniform sputtering of the grains having different morphologies
and sizes (1–5 μm), the beam was defocused to ∼30 μm. A
field aperture of 1500 × 1500 μm2 corresponding to ∼10 μm
on the sample was used to minimize contribution of oxygen
signals from the substrate and any other grains (typically
spinel) surrounding the grain of interest. The oxygen signal
from the substrate was small, and its contribution to oxygen
from the measured grain was estimated to be <1%. Secondary
16 − 17 −
O , O , and 18 O− ions were measured simultaneously
in a multicollection mode with the magnetic field controlled
by a nuclear magnetic resonance probe. 16 O− and 18 O− were
measured by a multicollector Faraday cup and EM, respectively,
with low mass resolving power (MRP ∼ 2000), and 17 O− was
measured using the axial monocollector EM with MRP ∼ 5600,
sufficient to separate the interfering 16 OH− signal. The primary
beam current was adjusted so that 16 O− signal was >106 cps.
Acquisition time was 250 s (10 s × 25 cycle). One to ten
micron-sized Burma spinel grains prepared the same way as the
samples were used as standard. The statistical uncertainty of the
measurements (2σ mean ) was ∼3‰ and ∼6‰ in δ 18 O and δ 17 O,
respectively. After the measurements, the corundum grains were
photographed, and the analyzed spots were confirmed.
4. RESULTS AND DISCUSSION
About 130 corundum grains were identified in the acidresistant residues from the UOCs (n = 103) and CCs (n = 26)
studied. The residues are dominated by spinel grains, which
are typically larger than corundum grains (5−10 μm versus
1−5 μm) and most likely represent CAI and chondrule fragments. The corundum grains are irregularly shaped (Figure 1)
Vol. 706
Figure 1. Secondary electron image of an isolated corundum RC075 02–01
from the H3.2 chondrite Roosevelt County 075.
and may represent either condensates or fragments of CAIs.
Because corundum-bearing CAIs are exceptionally rare, a condensation origin seems more likely. Based on the size range
of the isolated corundum grains in the Acfer 094 matrix and
homogeneous nucleation theory (Kozasa & Hasegawa 1987),
Nakamura et al. (2007) also concluded that the micron-sized
corundum grains formed by gas–solid condensation.
Fifty-eight corundum grains were measured for oxygen
isotopic compositions (Table 1). In Figure 2(a), oxygen isotopic
compositions of these grains are plotted on a three-isotope
diagram, δ 17 O versus δ 18 O. In Figure 2(b), the same data
are plotted as deviations from the TF line, Δ17 O. All but two
corundum grains (both from UOCs) have 16 O-rich compositions
with an average Δ17 O value of −22.7‰ ± 8.5‰ (2σ ). One
corundum grain is highly 17 O-enriched (δ 17 O ∼ +60‰, δ 18 O
∼ −40‰, Δ17 O ∼ +80‰) and is probably of presolar origin.
Another grain is less 17 O-rich (δ 17 O ∼ −15‰, δ 18 O ∼ −35‰,
Δ17 O ∼ +5‰); its origin is unclear. On a three-isotope diagram,
the data for the 16 O-rich corundum grains spread along massdependent fractionation line with a slope of ∼0.52, indicated
by the dashed line in Figure 2(a), possibly reflecting variations
in the instrumental mass fractionation caused by the different
grain morphologies. The spread around the dashed line in
Figure 2(a) is slightly larger than expected from counting
statistics, indicating some real variation in the data. This
variation may be either due to small contributions from other
grains during the measurements (i.e., experimental artifact) or
intrinsic differences among the grains.
There are no compositional differences in oxygen isotopic
compositions of the corundum grains from UOCs and CCs:
the mean Δ17 O values are −22.6‰ ± 9.0‰ and −23.2‰ ±
6.5‰ (2σ ), respectively. These compositions are in excellent
agreement with those of the Group 1 corundum grains from the
CM carbonaceous chondrite Murchison (Δ17 O = −21.0‰ ±
9.0‰; Figure 2(b)) characterized by the canonical 26 Al/27 Al
ratio of 5 × 10−5 (Virag et al. 1991). No grains with highly
fractionated oxygen isotopic compositions, such as the Group 2
grains from Murchison (Virag et al. 1991), have been observed.
This may reflect differences in sizes and origin of the grains
in two data sets. Corundum grains studied by Virag et al.
are generally larger than those in our study (up to 20 μm
No. 1, 2009
OXYGEN COMPOSITIONS OF SOLAR CORUNDUM GRAINS
145
Table 1
Oxygen Isotopic Compositions of the Micron-sized Corundum Grains
Grain Number
Sem 01–04
Sem 01–06
Sem 01–07
Sem 01–10
Sem 01–11
Sem 01–13
Sem 01–14
Sem 02–01
Sem 02–03
Sem 02–04
Sem 02–05
Sem 02–06
Sem 02–07
Sem 03–01
Sem 03–02
Sem 03–04
Sem 03–05
Sem 05–01
Sem 05–02
Sem 05–06
Sem 05–07
Sem 05–08
Sem 06–01
Sem 06–02
Sem 06–03
Sem 06–04
Sem 06–06
Sem 21–01
Sem 21–02
Sem 21–03
Sem 21–04
Sem 21–05
Bis 21–01
Bis 21–02∗
Bis 21–09
Bis 21–11
RC075 01–01
RC075 01–05
RC075 01–03∗
RC075 01–04
RC075 01–09
RC075 02–01
RC075 02–09
RC075 02–11
RC075 02–12
ALHA 03–03
ALHA 04–01
ALHA 04–02
ALHA 04–03
ALHA 04–05
Murr 03–12
Org 01–01
Org 01–02
Org 01–03
Org 01–11
Org 01–06
Org 01–10
δ 17 OSMOW ± 2σ
δ 18 OSMOW ± 2σ
Ordinary Chondrites
−50.5 ± 5.3
−49.8 ±
−46.7 ± 5.8
−47.6 ±
−59.0 ± 7.8
−60.1 ±
−52.7 ± 6.1
−53.7 ±
−44.8 ± 5.6
−52.1 ±
−48.1 ± 3.0
−47.9 ±
−44.2 ± 3.6
−47.0 ±
−32.2 ± 7.1
−37.7 ±
−35.3 ± 6.7
−34.2 ±
−55.0 ± 5.9
−56.7 ±
−49.7 ± 5.1
−52.0 ±
−57.9 ± 6.0
−53.4 ±
−51.9 ± 5.3
−58.0 ±
−44.6 ± 6.3
−42.8 ±
−39.7 ± 10.1
−34.5 ±
−32.8 ± 5.4
−22.6 ±
−39.5 ± 6.3
−24.8 ±
−51.2 ± 4.0
−42.1 ±
−46.4 ± 4.9
−39.5 ±
−52.9 ± 5.0
−46.3 ±
−57.3 ± 4.9
−57.6 ±
−51.2 ± 4.9
−46.7 ±
−55.7 ± 6.0
−51.0 ±
−49.4 ± 6.3
−52.0 ±
−52.6 ± 3.2
−52.3 ±
−53.0 ± 5.3
−56.7 ±
−60.1 ± 6.9
−67.4 ±
−30.1 ± 3.8
−32.2 ±
−40.7 ± 4.9
−35.2 ±
−51.1 ± 8.2
−38.1 ±
−37.1 ± 4.8
−28.8 ±
−39.4 ± 4.5
−30.1 ±
−38.2 ± 6.7
−44.2 ±
−12.6 ± 4.5
−33.8 ±
−41.8 ± 3.8
−43.6 ±
−38.8 ± 5.4
−41.5 ±
−41.7 ± 6.7
−35.3 ±
−38.3 ± 5.0
−29.2 ±
+60.0 ± 6.3
−40.1 ±
−40.3 ± 4.9
−50.3 ±
−44.7 ± 3.9
−37.4 ±
−43.8 ± 3.6
−42.2 ±
−28.0 ± 4.5
−18.0 ±
−39.3 ± 6.3
−41.6 ±
−39.4 ± 4.9
−39.4 ±
Carbonaceous Chondrites
−43.4 ± 4.5
−35.9 ±
−52.1 ± 5.7
−52.6 ±
−46.5 ± 5.7
−45.4 ±
−49.6 ± 3.8
−45.1 ±
−46.3 ± 7.5
−41.0 ±
−40.6 ± 4.4
−36.9 ±
−32.4 ± 5.3
−25.4 ±
−50.7 ± 7.0
−40.1 ±
−44.2 ± 6.3
−43.8 ±
−41.9 ± 7.8
−42.1 ±
−53.5 ± 6.1
−56.6 ±
−29.4 ± 5.4
−20.6 ±
Δ17 O ± 2σ
2.3
1.9
3.2
2.8
2.2
2.2
2.3
2.9
3.0
1.5
2.3
2.2
2.6
2.1
3.4
3.2
3.9
2.1
1.4
2.8
2.1
2.2
1.7
2.2
1.9
1.6
1.7
2.2
2.8
2.4
2.2
1.6
1.2
2.5
2.2
2.9
2.9
2.4
2.3
2.4
2.7
2.2
2.4
1.7
2.3
−24.6 ± 5.4
−22.0 ± 5.8
−27.8 ± 7.9
−24.7 ± 6.3
−17.8 ± 5.8
−23.2 ± 3.2
−19.7 ± 3.8
−12.6 ± 7.3
−17.5 ± 6.9
−25.5 ± 6.0
−22.6 ± 5.2
−30.2 ± 6.1
−21.7 ± 5.4
−22.4 ± 6.3
−21.8 ± 10.2
−21.1 ± 5.6
−26.6 ± 6.6
−29.3 ± 4.1
−25.8 ± 5.0
−28.8 ± 5.2
−27.4 ± 5.0
−26.9 ± 5.0
−29.2 ± 6.1
−22.4 ± 6.4
−25.4 ± 3.4
−23.5 ± 5.4
−25.0 ± 6.9
−13.4 ± 4.0
−22.4 ± 5.1
−31.2 ± 8.3
−22.2 ± 4.9
−23.8 ± 4.6
−15.2 ± 6.7
+5.0 ± 4.6
−19.1 ± 4.0
−17.2 ± 5.6
−23.4 ± 6.9
−23.1 ± 5.2
+80.9 ± 6.4
−14.2 ± 5.1
−25.2 ± 4.2
−21.8 ± 3.8
−18.6 ± 4.7
−17.6 ± 6.4
−18.9 ± 5.0
2.3
2.4
2.2
3.0
4.1
2.3
2.3
3.6
2.4
2.6
4.4
2.6
−24.7
−24.8
−22.9
−26.2
−25.0
−21.4
−19.2
−29.9
−21.4
−20.0
−24.1
−18.7
±
±
±
±
±
±
±
±
±
±
±
±
4.7
5.8
5.8
4.1
7.8
4.5
5.5
7.2
6.4
7.9
6.5
5.5
Notes. ALHA = ALHA77307; Bis = Bishunpur; Mur = Murray; Org =
Orgueil; RC075 = Roosevelt County 075; Sem = Semarkona; ∗ isotopically
unusual corundum grains.
versus <5 μm, respectively); some of them may represent
CAI fragments rather than gas–solid condensates. We note that
(a)
(b)
Figure 2. Oxygen isotopic compositions of the solar micrometer-sized corundum grains in the acid-resistant residues from UOCs and CCs. Error bars are
two standard errors of the mean (2σ mean ) of individual measurements and do
not include variations in the instrumental mass fractionation of the standard
spinel grains. The TF and CCAM lines are plotted for reference. (a) All but two
corundum grains measured are 16 O-rich and spread along mass-dependent fractionation line with a slope of ∼0.52, indicated by the dashed line. (b) The same
data are plotted as deviations from the TF line (Δ17 O). There are no compositional differences between the corundum grains from UOCs and CCs (Δ17 O =
−22.6‰ ± 9.0‰ and −23.2‰ ± 6.5‰, respectively; error = 2 standard deviation, 2σ ). ∗ Oxygen isotopic compositions of the isolated micrometer-sized
corundum grains from the CM2 carbonaceous chondrite Murchison (data from
Virag et al. 1991) are plotted for reference. The mean composition of the 16 Orich corundum grains (Δ17 O = −22.7‰ ± 8.5‰, 2σ ) is consistent with those
of the mineralogically pristine CAIs from the CR chondrites (−23.3‰ ± 1.9‰,
2σ ; Makide et al. 2009) and of the solar wind returned by the Genesis spacecraft (−26.5‰ ± 5.6‰, 2σ ; McKeegan et al. 2009). We infer oxygen isotopic
compositions of the 16 O-rich corundum grains represent the initial composition
of the solar nebula, and, hence, of the Sun.
the igneous CAIs with highly fractionated oxygen isotopic
compositions resulted from melt evaporation are found in
several chondrite groups—CO, CR, CV, CM, and R (e.g., Davis
et al. 2000; Ushikubo et al. 2007; Thrane et al. 2008; Krot et al.
2008a; Nagashima et al. 2008; Liu et al. 2009; Makide et al.
2009; Rout et al. 2009).
Huss et al. (1993) described several corundum grains from
Orgueil with normal oxygen isotopic compositions (Δ17 O ∼ 0);
no such grains have been found in our data set.
The oxygen isotopic compositions of the micron-sized corundum grains in the acid-resistant residues from UOCs and CCs
146
MAKIDE ET AL.
are similar to those of the mineralogically pristine CAIs (Δ O =
−23.3‰ ± 1.9‰) from the CR chondrites (Makide et al. 2009).
The majority of the CR CAIs show 26 Mg∗ corresponding to the
canonical 26 Al/27 Al ratio of ∼5 × 10−5 , indicating formation
at the beginning of the solar system evolution. Based on these
observations and thermodynamic analysis of condensation from
a cooling gas of solar composition (Yoneda & Grossman 1995;
Ebel & Grossman 2000; Petaev & Wood 2005), we conclude
that the 16 O-rich micrometer-sized corundum grains in the acidresistant residues from the UOCs and unmetamorphosed CCs
most likely represent high-temperature condensates from a gas
of solar composition. As a result, their oxygen isotopic compositions must represent composition of the Sun.
Our inferred oxygen isotopic composition of the Sun
(∼−23‰) is in excellent agreement with that of the solar wind
returned by the Genesis spacecraft (Δ17 O = −26.5‰ ± 5.6‰;
McKeegan et al. 2009). It is, however, inconsistent with the
composition of the 16 O-poor component of the solar wind implanted into the metallic particles in the lunar soil (Ireland et al.
2006; Hashizume & Chaussidon 2009). It is also outside the
Δ17 O value of −35‰ proposed by Gounelle et al. (2009) to be
representative for the Sun on the basis of the extreme 16 O-rich
compositions of two grossite-rich CAIs from Isheyevo and two
assumptions: (1) oxygen isotopic compositions of the dust and
gas in the solar system were initially similarly 16 O-rich and (2)
evolution of oxygen isotopic composition of the solar nebula
was due to CO self-shielding, and it was unidirectional, toward
heavier compositions (Clayton 2002; Yurimoto & Kuramoto
2004; Lyons & Young 2005). We note, however, that the major
assumption of the CO self-shielding models that the primordial dust and gas in the solar system had similar compositions
has not been confirmed yet and was challenged by Krot et al.
(2009b). In addition, the CO self-shielding itself as a mechanism of mass-independent oxygen isotope fractionation in the
solar system remains controversial (e.g., Yurimoto et al. 2006;
Chakraborty et al. 2008; Ozima et al. 2009). Finally, this model
does not explain tight clustering of compositions of CAIs and
corundum condensates at Δ17 O value of ∼ −25‰.
The presence of the isotopically similar, solar corundum condensates in different classes of chondritic meteorites suggests
significant radial mixing of micrometer-sized solids in the inner
protoplanetary disk, most likely the accretion region of chondritic asteroids. To constrain timing of condensation of 16 O-rich
micrometer-sized corundum grains and their radial transport in
the protoplanetary disk, magnesium isotopic measurements of
these grains are required. The only reported magnesium isotopic
compositions of the 16 O-rich micron-sized corundum grains
from Murchison (Virag et al. 1991) are consistent with their
early formation. Although the exact mechanism of radial mixing
remains controversial (Shu et al. 1996, 1997; Bockelée-Morvan
et al. 2002; Gail 2001, 2004; Dullemond et al. 2006; Ciesla
2007; Boss 2008), it was probably most effective at transporting
materials outward at the very earliest stages of disk evolution,
when the rates of mass and angular momentum transport are at
their highest, allowing for the large-scale redistribution of disk
materials.
17
We thank Eric Feigelson and the anonymous reviewer for
comments and suggestions which helped us to improve the
manuscript. We also thank Eric Hellebrand for assistance with
the electron microprobe. This work was supported by NASA
grant NNX07AI81IG (ANK). This is Hawai’i Institute of
Geophysics and Planetology publication No. 1816 and School
Vol. 706
of Ocean and Earth Science and Technology publication No.
7827.
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