Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.v43j..02o&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #V43J-02
Physics
[1028] Geochemistry / Composition Of Meteorites, [1041] Geochemistry / Stable Isotope Geochemistry, [6200] Planetary Sciences: Solar System Objects
Scientific paper
Since the first discovery of the mass-independently fractionated oxygen isotopes in anhydrous, high temperature Ca-Al rich inclusion minerals in carbonaceous meteorites (CAIs) by Clayton et al. (1), their common occurrence in primitive meteorites has generally been regarded to reflect some fundamental process prevalent in the early solar nebula. The CAI oxygen isotopic composition is uniquely characterized by (i) large mass independent isotopic fractionation and (ii) their isotopic data in an oxygen three isotope plot (δ17O - δ18O (δ17O ≡ {(17O/16O)/(17O/16O)SMOW - 1} × 1000) yield nearly a straight line with a slope 1.0. In establishing these characteristics, ion microprobe analyses has played a central role, especially an isotopic mapping technique (isotopography) was crucial (e.g., 2). The extraordinary oxygen isotopic ratio in CAIs is widely attributed to the self-shielding absorption of UV radiation in CO, one of the dominant chemical compounds in the early solar nebula (3). However, the self-shielding scenario necessarily leads to the unusual prediction that a mean solar oxygen isotopic composition differs from most of planetary bodies including Earth, Moon, and Mars. If the self-shielding process were indeed responsible to the CAI oxygen isotopic anomaly, this would require a fundamental revision of the current theory of the origin of the solar system, which generally assumes the initial total vaporization of nebula material to give rise to isotopic homogenization. The GENESIS mission launched in 2001(4), which collected oxygen in the solar wind was hoped to resolve the isotopic composition of the Sun. However, because of difficulties in correcting for instrumental and more importantly for intrinsic isotopic fractionation between the SW and the Sun, a final answer is yet to be seen (5). Here, we show on the basis of the oxygen isotopic fractionation systematics that the self shielding hypothesis cannot explain the key characteristics of the CAI oxygen isotopic fractionation, that is, a slope = 1.0 linear array in an oxygen three isotope plot δ17O - δ18O. Therefore, the prediction of the self-shielding hypothesis that the solar oxygen isotopic composition must be different from planetary objects is not supported by the CAI oxygen isotopic data, and we think that the general homogeneity of isotopic composition in the early solar nebula is still a valid assumption. References [1] Clayton R.N. et al, Science, 182, 485-88, 1973. [2] Yurimoto H. et al. Applied Surface Science, 793, 203-204, 2003. [3] Clayton R., Nature 415, 317, 2002. [4] Burnett D.S. et al., Space Science Review, 105, 509-543, 2003. [5] McKeegan K. et al. 2009. Abstract #2494. 40th LPSC, 2009. [5] Wieler R. Meteoritics & Planetary Science, 44, A218, 2009. [|#22#|]
Ozima Minoru
Yamada Atsushi
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