HE, NE, AND AR FROM THE SOLAR-WIND AND SOLAR ENERGETIC PARTICLES IN LUNAR ILMENITES AND PYROXENES

He, Ne, and Ar in ilmenite and pyroxene separates from two lunar regolith samples exposed to the solar corpuscular radiation at widely different epochs were analyzed by the closed system stepped etching technique. This method avoids element or isotope fractionation due to diffusion during the experiment and thus depth profiles of element and isotopic composition of implanted solar noble gases can be determined. Ilmenites from a relatively recently irradiated soil release in the first steps He and Ne with isotopic compositions identical to modem solar wind values. We conclude that ilmenite retains isotopically unfractionated He, Ne, and Ar from the solar wind (SW) in the outermost grain layers. The isotopic composition of SW noble gases has remained constant over the last 100 Ma but there is some evidence for a slight change over a Ga timescale. The ratio Ar-36/Ar-38 in the solar wind is 2-3% larger than that in the terrestrial atmosphere. We give further evidence for the identification of a solar energetic particle component (SEP), which was implanted with energies exceeding those in the solar wind, All ilmenites and pyroxenes contain SEP-Ne with Ne-20/Ne-22 = 11.2+/-0.2, in agreement with SEP-Ne found earlier in lunar plagioclase. (Ne-21/Ne-22)SEP is consistent with SEP-Ne being derived from SW-Ne by mass dependent fractionation. In addition, the new data reveal SEP-He and SEP-Ar, both isotopically heavier than the SW components. The isotopic fractionation factors required to relate SW and SEP gases are approximately equal to the square of the mass ratios. Element ratios in the SEP dominated steps are similar (He:Ar) or identical (Ne:Ar) to present-day solar wind values, indicating little or no element fractionation in the SEP reservoir.