NOBLE-GASES AND NITROGEN RELEASED FROM LUNAR SOILS BY ACID ETCHING

A stepwise acid-etching technique similar to the closed system stepwis e etching (CSSE) method developed at ETH Zurich was used to examine th e solar wind reservoirs of lunar soil grains. Samples were treated wit h weak acids (H2O, H2SO3) to facilitate the release of the most shallo wly implanted gases. Noble gas abundances and isotopic compositions, i ncluding Kr and Xe in some cases, and a few nitrogen data were obtaine d for mineral or grain-size separates of three lunar soils (plagioclas e from 60051, pyroxene from 75081, and <25 mu m bulk 79035). The 60051 plagioclase grains, considered to be a possibly unique resource for d etermining the modem-day solar wind composition, show unusually low co ntents of solar wind He, Ne, and particularly Ar, but do not otherwise possess any characteristics clearly attributable to a modern-day sola r wind exposure. Initial water and acid treatments of the grains, howe ver, release an apparently pure SEP component. The 75081 pyroxene and the size separate of bulk 79035 both yield Kr and Xe compositions in i nitial etch steps that are characteristic of undiffused solar wind, si gnificantly increasing the database for measurements of solar wind Kr and Xe where possible laboratory thermal diffusion and fractionation e ffects are not a concern. Pyroxene in particular appears to be a suita ble alternative to ilmenite for the purpose of making measurements of this kind. Nitrogen release by acid etching is not at present quantita tive, and while it appears possible to obtain reasonable isotopic rati os for solar wind N, we are unable to use the technique to determine s olar nitrogen to noble gas ratios. Light noble gases in all three soil separates, other than the aforementioned behavior of 60051, appear to behave in accord with expectations based on acid-etching analyses per formed by the Zurich group.