Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.273p&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 273
Other
5
Scientific paper
Regolith grains dissolved by stepwise etching release a mixture of near-surface implanted Solar Wind gases (SW) and a deeper- sited, isotopically heavier component attributed to Solar Energetic Particles (SEP) (1,2,3). In all regolith materials examined so far the elemental abundance ratios in both components are distinctly different from the canonical solar values (4). The differences are generally explained to be owing to diffusive elemental fractionation although there is no strong evidence that upon their implantation the composition of the gases was indeed solar. In contrast, the solar noble gases present in the H3-H6 chondritic regolith breccia Acfer 111 appear to be nearly unfractionated and thus offer a unique chance for more accurate analyses. A magnetic fraction of Acfer 111 matrix, consisting of approx. 80% metal and 20% silicates, was etched with a 60 g/mol aqueous solution of HNO3 in a high-vacuum extraction line similar to that in (1). The gases released were drawn off in steps and analyzed; the experiment was stopped when ~97% of the metal and ~50% of the silicates were dissolved. As etching proceeds, the isotopic composition of the released gases changes in a pattern similar to that observed previously in other regolithic materials. The isotopic composition of solar neon decreases from ^20Ne/^22Ne=13.1 in the first step to ^20Ne/^22Ne=11.6, which can be interpreted as a change of the mixing ratio of SW (^20Ne/^22Ne=13.7) and SEP (^20Ne/^22Ne=11.3) neon. The isotopic compositions of solar He, Ar, and Kr are consistent with their also being mixtures of SW and SEP having compositions reported previously (2,3), although our data are compromised to some extent by the presence of planetary gases extracted from the silicates and, in the first steps, by atmospheric contamination probably present in terrestrial weathering products (mostly rust). The elemental composition of noble gases released from Acfer 111 was distinct from previous experiments: The (^4He/^20Ne)solar ratio is initially 585 (as in the Solar Wind), increases smoothly as etching proceeds, and, after reaching a maximum around 680, decreases to approximately 230 (Fig. 1, upper panel). These variations, especially in the last two steps of the experiment, where the solar gases are depleted in He, are most probably due to different depth distributions for He and Ne, with the implantation range of the lighter nuclide being shorter than that of the heavier one (5). Ignoring the implantation range effect, the variations imply that the ^4He/^20Ne ratio of the SW and the SEP do not differ by more than 20%. Although we do not recognize significant variations in the (^20Ne/^36Ar)solar ratio (Fig. 1 lower panel), possibly because of larger experimental errors, the data imply again that SW and the SEP do not differ by more than 20% in the ^20Ne/^36Ar ratio also. References (1) Wieler et al. (1986) GCA 50, 1997-2017. (2) Benkert et al. (1988) LPSC IXX. (3) Wieler et al. (1992) LPSC XXIII (4) Anders and Grevesse GCA 53, 197-214 (5) Kiko et al. (1978) Proc. Lunar Planet. Sci. Conf. 9th, 1655-1665.
Begemann Friedrich
Pedroni Anselmo
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