Computer Science
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.264m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 264
Computer Science
Scientific paper
Closed system stepped oxidation of noble gas-rich samples at room-temperature leads to less diffusive elemental fractionation during gas-release than techniques involving high temperatures like stepped combustion or pyrolysis. Metal is generally considered to retain noble gases even better than ilmenite. Thus regolithic metal samples are likely to contain the least fractionated solar noble gases. A metal separate of the dark portion of the Fayetteville (H4-6) chondrite that contained visible amounts (~25 vol%) of attached silicates was oxidized with CuCl(sub)2xnH2O-solutions (first proposed by Vilcsek and Wanke, 1965) in a pyrex-glass extraction line. Average procedural and reagent blank values for a typical step duration of one hour are [in 10^-10 cm^3 STP]: ^4He = 500; ^20Ne = 0.7; ^40Ar = 95. Here we report data of a preliminary experiment obtained on a small 9.2 mg sample, subjected to increasingly aggressive solutions. Blank corrections are substantial and only 5 steps yielded meaningful data. At the conference, we will present results of a considerably larger Fayetteville metal sample. The Ne isotopic data are rather similar to CSSE data of ilmenite, pyroxene, and plagioclase separates from lunar soils (Wieler et al., 1986; Wieler et al., 1987; Benkert et al., 1988). (^20Ne/^22Ne)(sub)tr starts at 13.1 and decreases to 11.6. The latter value is close to the SEP-Ne point (^20Ne/^22Ne ~ 11.3 +- 0.3) indicating that we succeeded to isolate SEP-Ne in rather pure form from Fayetteville metal. The first CSSE steps release isotopically slightly heavier Ne than the first combustion steps of a Weston metal separate analyzed by Becker and Pepin (1991), whereas in the last CSSE steps Ne is consideribly closer to the SEP composition than in the last combustion/pyrolysis steps. The air corrected ^36Ar/^38Ar ratios range between 4.0 to 2.6 (assuming all ^40Ar to be atmospheric). These values are clearly lower than the ratio of SW-Ar and SEP-Ar of ~5.5 and ~4.9, respectively, and show that cosmogenic Ar interfered severely. Only a more gentle etching at the start of the run could possibly reveal the undisturbed SW-Ar signature. Due to cosmogenic ^3He we can also not derive a (^3He/^4He)(sub)tr value from our first data set. The ^20Ne/^36Ar ratio corrected for air Ar and to first order also for cosmogenic Ar is between 40 and 50 in all steps with uncertainties of 5 to 10%. This is within error identical to the solar wind value of 45 +- 10 and indicates that Fayetteville metal indeed retains rather unfractionated solar Ne and Ar. Similar ^20Ne/^36Ar values were also observed in Weston metal (Becker and Pepin, 1991). In the first two steps corrected ^4He/^36Ar ratios are 15000 and 21000, respectively, close to the solar wind value of 25000, but later this ratio decreases to 8000, which is not yet well understood. We see no evidence for (^4He/^36Ar)(sub)sw = 36000 as claimed for Weston metal (Becker and Pepin, 1991). This pilot experiment indicates that solar wind gases in metal indeed are even less disturbed by diffusion than in ilmenites and further underscores the existence of SEP noble gases as an independent component in the solar corpuscular radiation. Work supported by the Swiss National Science Foundation. References: Becker R.H. and Pepin R.O. (1991) Earth Planet. Sci. Lett. 103, 55-68. Benkert J.P. et al. (1988) Lunar Planet. Sci. (abstract) 19, 59. Geiss J. et al. (1972) NASA SP-315, 14.1- 14.10. Wieler R. et al. (1986) Geochim. Cosmochim. Acta 50, 1997- 2017. Wieler R. et al. (1987) Lunar Planet. Sci. (abstract) 18, 1080. Vilcsek E. and Wanke H. (1965) Z. Naturforsch. 20a, 1282.
Baur Holger
Murer Ch.
Signer Peter
Wieler Rainer
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