1. Origin of 17,18O-rich materials from Acfer 094
Thank you chairman.
N. Sakamoto1, Y. Seto1, S. Itoh1, K. Kuramoto1, K. Fujino1,
K. Nagashima2, A. N. Krot2 and H. Yurimoto1.
1Hokkaido University
2University of Hawai’i

2. Formation scenario from Stars to the Solar System
Red Giant
AGB star
Supernova
This is final talk of this meeting, I will start with the rough sketch of formation scenario from stars to our solar system.
Isotopes are produced by stars in different nucleosynthetic processes and ejected to molecular cloud which form planetary systems.
Molecular Cloud
Solar System

3. Formation scenario from Stars to the Solar System
Red Giant
AGB star
Supernova
Presolar grains
The signatures of the particular nucleosynthetic processes are remained in presolar grains as large isotopic heterogeneity.
Molecular Cloud
Solar System

4. Formation scenario from Stars to the Solar System
Red Giant
AGB star
Supernova
Presolar grains
Meteorite
In contrast, it is well-known that isotopic compositions of oxygen in chondrite components have orderly variation.
This unexpected variation is considered to be an important key for the evolutioin of the Solar System.
Molecular Cloud
Solar System

5. Oxygen isotopes in meteorite
This is three oxygen isotope diagram of chondrite components.
The oxygen isotopic compositions of chondrite components varies along slope-1 line indicating mass-independent fractionation.
(Data from Clayton, 1993)

6. Oxygen isotopes in meteorite
17,18O-rich ??
This variation is generally believed to have resulted from mixing of an 16O-rich reservoir and an 17,18O-rich reservoir.
The 16O-rich reservoir has been inferred from isotopic compositions of nebular condensates.
On the other hands, the nature and composition of 17,18O-rich reservoir are still poorly constrained.
16O-rich
(Data from Clayton, 1993)

7. 17,18O-rich material from Acfer 094
Sakamoto et al., 2007
Recently, Sakamoto et al reported extremely 17,18O-enriched material from primitive meteorite Acfer 094.
The range of this diagram is from -50 permil to +200 permil.
I talk about oxygen isotope study of this material and discuss the forming condition in order to constrain the 17,18O-rich reservoir.

8. new-PCP (NEW-Poorly Characterized Phase)
BSE
FeOS
5 µm
This is a backscattered electron image of the material. Scale bar is 5 micron.
These are oxygen isotope ratio images. Color bar is -50 permil to +200 permil.
This material is cleary enriched in 17O and 18O.
This material has an unique chemical composition composed of Fe, Ni, O and S.
This is an RGB map of elements correspondng to the backscattered electron image.
Color of red is assigned to Fe, blue is O, green is S.
These elements distribute homogeneously in the material.
Seto et al revealed that this material is an assemblage of magnetite and iron-sulfide by mineralogical study.
We refered this material as new-PCP in our science paper.
18O
17O
Fe3O4 + FeS
(Seto et al., 2007)
17,18OSMOW(‰)

9. Abundance of new-PCP Abundance : 100 ppm Average size : 5x5x5 µm3
Based on the unique chemical composition, new-PCP grains have counted in an Acfer 094 matrix.
This is a backscattered electron image of matrix part of Acfer 094 thin section.
The blue circles indicate the position of new-PCP.
New-PCPs are scattered randomly throughout the Acfer 094 matrix.
The abundance of new-PCP is about 100 ppm by volume in the matrix.
The average size of new-PCP is calculated to be 5 times 5 times 5 cubic micro meter.
Abundance : 100 ppm
Average size : 5x5x5 µm3

10. Isotopography All analyzed new-PCP grains are enriched in 17,18O. 18O
We selected 10 new-PCP grains to determine oxygen isotopic composition.
The selected new-PCP are displayed by these backscattered electron images.
These color images are oxygen isotope mapping of above backscattered electron image area.
The color indicates oxygen isotopic composition using this color scale.
Left is d18O image and right is d17O image.
All analyzed new-PCP grains are enriched in 17O and 18O.
All analyzed new-PCP grains are enriched in 17,18O.
17,18OSMOW(‰)

11. Oxygen isotopic composition
new-PCP
+180‰
This is a 3 oxygen isotope diagram plotted by the new-PCP shown in the previous slide.
All analyzed new-PCPs are plotted in this region.
The representative value of new-PCP would be +180 permil.
This value is heaviest oxygen isotopic compositions of the solar System materials reported so far.

12. Thermodynamic calculation for Fe-O-S system
Fe + H2S = FeS + H2
(Fegley, 2000; Lauretta et al.,1996)
3Fe + 4H2O = Fe3O4 + 4H2
(Fegley,2000; Hong and Fegley.,1997)
3FeS + 4H2O = Fe3O4 + 3H2S + H2
In order to consider the forming conditions of new-PCP, we performed thermodynamic calculation for Fe-O-S system under the solar nebular condition.
These three chemical equations are considered.
FeS is a reaction product of Fe-metal and H2S gas.
Magnetite is a reaction product of Fe-metal and H2O gas.
Magnetite is also a reaction product of FeS and H2O gas.

13. Thermodynamic calculation for Fe-O-S system
Fe + H2S = FeS + H2
(Fegley, 2000; Lauretta et al.,1996)
3Fe + 4H2O = Fe3O4 + 4H2
(Fegley,2000; Hong and Fegley.,1997)
3FeS + 4H2O = Fe3O4 + 3H2S + H2
This system is independent on total pressure in the system because there is no change in the number of gas molecules during each chemical reaction.
These reactions can be parameterized by temperature and partial pressure of the gas spieces.
Partial pressure ratio of H2O and H2 gas is assumed to be solar composition gas ratio of 5 times 10 to the -4.
We can draw a phase diagram using parameters of temperature and H2S over H2 pressure ratio.
Canonical solar nebula
PH2S/PH2
PH2O/PH2 ~ 5 x 10-4
(Krot et al., 2000)

14. Formation temperature
This is calculated phase relationship among Fe-metal, FeS and magnetite.
Horizontal axis is temperature and vertical axis is partial pressure of H2S and H2 gas.

15. Formation temperature
Oxidation of troilite or metal to form new-PCP would occur below 360K.
Because new-PCP consist of the magnetite and iron-sulfide, the equilibrium condition is indicated by this blue line.
If the partial pressure of H2O gas increases, this blue line shift to upward.
In any case, new-PCP formation would occur below about 360K.
< 360K

16. Origin of new-PCP Solar nebula Parent body
Most abundant oxidizing agent : water (H2O)
Solar nebula
Sublimation temperature of water ice < 200K
The new-PCP could have formed around the water ice sublimation front in the solar nebula. At the sublimation front, Fe/FeS would be oxidized by the sublimated nebular water ice.
Parent body
It is believed that H2O is the most abundant oxidizing agent in the early solar system.
There are two possibilities of new-PCP formation environments in the solar system.
One is in the solar nebula and another is on the parent body.
Because the inferred new-PCP forming temperature is higher than the sublimation temperature of water ice in the solar nebula, the new-PCP could have formed around the water ice sublimation front in the solar nebula.
At the sublimation front, Fe- metal or FeS would be oxidized by the sublimated nebular water ice.

17. Origin of new-PCP Solar nebula Parent body
Most abundant oxidizing agent : water (H2O)
Solar nebula
Sublimation temperature of water ice < 200K
The new-PCP could have formed around the water ice sublimation front in the solar nebula. At the sublimation front, Fe/FeS would be oxidized by the sublimated nebular water ice.
Parent body
Conventional aqueous alteration stages (e.g. hydrous mineral formation) are excluded.
Acfer 094 shows no evidence of aqueous alteration of silicate minerals.
Another possibility is new-PCP would be formed by aqueous alteration on the parent body.
However, conventional aqueous alteration stages , for example hydrous mineral formation stage, are excluded for the new-PCP formation because Acfer 094 shows no evidence of aqueous alteration of silicate minerals.
Therefore, if the new-PCP formed on the parent body, the new-PCP have formed at the starting stage of aqueous alteration which is prior to oxygen isotopic exchange reaction between melted or sublimated water ice and matrix silicates.
(Greshake, 1997)
The new-PCP have formed at the starting stage of aqueous alteration which is prior to oxygen isotopic exchange reaction between melted/sublimated water ice and matrix silicates.

18. Origin of new-PCP The water should be extremely enriched in 17,18O.
Most abundant oxidizing agent : water (H2O)
Solar nebula
Sublimation temperature of water ice < 200K
The new-PCP could have formed around the water ice sublimation front in the solar nebula. At the sublimation front, Fe/FeS would be oxidized by the sublimated nebular water ice.
Parent body
Conventional aqueous alteration stages (e.g. hydrous mineral formation) are excluded.
Acfer 094 shows no evidence of aqueous alteration of silicate minerals.
In either case, the water should be extremely enriched in 17O and 18O.
(Greshake, 1997)
The new-PCP have formed at the starting stage of aqueous alteration which is prior to oxygen isotopic exchange reaction between melted/sublimated water ice and matrix silicates.
The water should be extremely enriched in 17,18O.

19. Summary
A chemically unique (Fe-O-S bearing) material is extremely enriched in 17O and 18O (up to +180‰). We refer to the material as new-PCP.
The O-isotopic composition of new-PCP probably corresponds to that of water component in the early solar system.
Thermodynamic calculation shows oxidation of troilite or metal could occur below 360K.
The new-PCP could have formed
in the inner solar nebula with 17,18O-enriched water vapor.
on the parent body during the initial stage of aqueous alteration before formation of hydrous silicates.
The early solar system water should be extremely enriched in 17O and 18O.
In summary,
A chemically unique (Fe-O-S bearing) material is extremely enriched in 17O and 18O (up to +180‰). We refer to the material as new-PCP.
The O-isotopic composition of new-PCP probably corresponds to that of water component in the early solar system.
Thermodynamic calculation shows oxidation of troilite or metal could occur below 360K.
The new-PCP could have formed
in the inner solar nebula with 17,18O-enriched water vapor.
on the parent body during the initial stage of aqueous alteration before formation of hydrous silicates.
The early solar system water should be extremely enriched in 17O and 18O.