FRACTIONATION OF Xe, Kr, AND Ar IN THE SOLAR CORPUSCULAR RADIATION DEDUCED BY CLOSED SYSTEM ETCHING OF LUNAR SOILS

摘要

We etched lunar soil samples in several steps in a vacuum-tight device and analyzed the evolved solar noble gases on line in a mass spectrometer. This technique avoids diffusive noble gas fractionation during analysis and therefore provides reliable element abundance ratios as a function of depth in the grains. The ratios He/Ar and Ne/Ar strongly increase in the course of the etching and reach present-day solar wind ratios toward the end of the runs. This shows that the solar wind component, residing in the top few hundred angstroms, lost much of its He and Ne, whereas even these mobile light noble gases are retained nearly unfractionated in the solar energetic particle (SEP) component at larger depths. In contrast to the light gases, the Kr/Xe ratio is constant throughout all runs, and the same is essentially true also for Ar/Kr. This strongly suggests that the relative abundances of the three heavy noble gases in the incoming solar corpuscular radiation are conserved in lunar samples. The Kr/Xe ratio in samples irradiated in the past ～100 Myr is about a factor of 2.5 lower than the most probable value in the Sun. The same ratio was another factor of 2 lower 1-3 Gyr ago. Kr/Ar in the solar corpuscular radiation is also slightly fractionated. Xe is overabundant relative to Ar by about the same factor as are elements with a first ionization potential (FIP) of less than ～10 eV relative to high-FIP elements. This seems astonishing, since Xe has a FIP above 10 eV. However, Geiss, Gloeckler, & von Steiger showed recently that the Xe overabundance deduced here is expected if actually the first ionization time rather than the FIP governs the fractionation in the solar wind source region.