Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.307w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 307
Astronomy and Astrophysics
Astrophysics
1
Scientific paper
We analyzed all five noble gases in an ilmenite sample from lunar soil 71501 by closed system stepped etching (CSSE), thus extending our CSSE studies of solar noble gases (Wieler et al., 1986; Benkert et al., 1988) to Kr and Xe. He, Ne, Ar isotopes: We observe the familiar presence of two solar noble gas components: step 1 shows ^4He/^3He = 2250, ^20Ne/^22Ne = 13.8, and ^36Ar/^38Ar=5.46. The first two ratios are essentially identical to modern solar wind values (SWC; Geiss, 1973), indicating an isotopically unfractionated SW noble gas reservoir in lunar ilmenite. The Ne data-points of later steps fall on a straight line and reach the SEP-Ne point (^20Ne/^22Ne = 11.3). The slope of this line indicates mass dependent fractionation between SW-Ne and SEP-Ne. SEP-He (^4He/^3He = 4650 +-100) and SEP-Ar (^36Ar/^38Ar = 4.89+-0.05) are also observed (latter two numbers slightly revised compared to Wieler et al., 1992). Kr, Xe isotopes: Kr in those two steps that release pure SW-Ne is very slightly lighter than atmospheric Kr (^86Kr/^84Kr = 0.3041; see also Wieler et al., 1992). We interpret this to be SW-Kr in soil 71501. Steps containing (isotopically heavier) SEP-Ne likewise release heavier Kr, interpreted accordingly as SEP-Kr (^86Kr/^84Kr = 0.323). Similarly, a light Xe component is released first (SW-Xe, ^136Xe/^132Xe = 0.3003), followed by heavier SEP-Xe (^136Xe/^132Xe = 0.319). The data are consistent with both Kr and Xe in SW and SEP components to be related by mass fractionation. The relation first proposed by Benkert et al. (1988) between a ratio R(m(sub)2,m(sub)1) of two isotopes with masses m(sub)2>m(sub)1 in SW and SEP now holds for all five noble gases: (R(sub)SW-R(sub)SEP)/R(sub)SW = (2+-0.13)*(m(sub)2- m(sub)1)/m(sub)2. Since m(sub)2~m(sub)1, this relation may also be written as: R(sub)SEP/R(sub)SW ~ (m(sub)1/m(sub)2)^2 (Geiss and Bochsler, 1991). Element ratios: ^4He/^36Ar and ^20Ne/^36Ar rise from values several times below SWC to essentially SWC ratios in the steps releasing nearly pure SEP gases, indicating element fractionation between SW-He/Ne, but essentially unfractionated SEP-He/Ne/Ar in lunar ilmenite. In contrast, Kr/Xe stays virtually constant (^84Kr/^132Xe = 8.4+-0.5), except in the first few steps. If the light three noble gases indeed are not fractionated in the SEP dominated steps, we would also expect no fractionation for Kr/Xe. The measured ^84Kr/^132Xe ratio is, however, about 2.4 times lower than the most recent "solar system" ratio (Anders and Grevesse, 1989), albeit only ~30% lower than the "solar" value preferred by Marti and Suess (1988). Kr/Xe ratios similar to those in ilmenites have been found earlier, e.g. in Peysanoe (Marti, 1969). They are often taken to indicate fractionated loss of solar noble gases. Based on the new CSSE results, we propose, instead, that lunar ilmenites retain a faithful record of ^84Kr/^132Xe in SW and SEP (and also ^36Ar/^84Kr(sub)SW = 1750+- 300). If so, this may indicate noble gas fractionation in the solar corpuscular radiation, perhaps depending on first ionization potential or a related parameter. Work supported by the Swiss National Science Foundation. References: Anders, E. and Grevesse, N. (1989) Geochim. Cosmochim. Acta. 53, 197-214. Benkert, J.-P. et al. (1988) Lunar Planet. Sci. (abstract) 19, 59-60. Geiss, J. (1973) Proc. 13th Int. Cosmic Ray Conf. 3375-3398. Geiss, J. and Bochsler, P. (1991) In The Sun in Time (eds. C. P. Sonett, M. S. Giampapa and M.S. Matthews), pp. 98-117. Univ. Arizona Press, Tucson, Arizona. Marti, K. (1969) Science. 166. 1263-1265. Marti, K. and Suess, H. E. (1988) Astrophys. Space Sci. 144. 507-517. Wieler, R. et al. (1986) Geochim. Cosmochim. Acta. 50. 1997-2017. Wieler, R. et al. (1992) Lunar Planet. Sci. (abstract) 23.
Baur Holger
Signer Peter
Wieler Rainer
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