Oxygen and light noble gas isotopic compositions of the lunar surface

Abstract

Earth's Moon is a repository of solar wind and can therefore provide information on the chemistry of the Sun. Solar wind compositions for light-noble-gases are well established from analyses on lunar silicates and ilmenites, and these are in close agreement with Genesis measurements. Unfortunately these minerals are not suitable for oxygen analysis due to their high intrinsic oxygen levels. Oxygen isotopic compositions have been measured in single lunar metal grains. These show multiple components with essentially no match to the oxygen isotopes from Genesis. This gives reason to question how much implanted solar oxygen is actually preserved in these grains. Lunar samples from all six Apollo landing sites were chosen for different exposure ages and geography. Single lunar Iron metal grains extracted from eleven Apollo samples were analysed for 16O, 17O and 18O signatures on grain surfaces. Additionally, single lunar olivine grains were analysed for noble gases applying total fusion laser gas extraction. From metal grains analysed for oxygen, a selection were chosen for helium and neon isotopic analysis, in this case using a diode laser gas extraction system. Lunar olivines have too much intrinsic oxygen for surface analysis. Oxygen isotopes were measured on the newly commissioned SHRIMP SI, designed specifically for stable isotopes. Generally, oxygen isotopes in lunar metal grains clustered around the terrestrial (lunar) mass fractionation line, but they also show a wide range in compositions from enrichment in 17O, 18O to compositions enriched in 16O. Compositions close to Genesis solar values were found in only a few of the metal grains; solar oxygen appears to be highly under-represented in lunar metal grains. Noble gas isotope measurements of olivines were performed on the VG5400 noble gas mass spectrometer. Solar helium and neon isotope ratios collected from olivines cluster around solar wind composition with some dispersion towards implantation fractionated solar wind compositions and to more cosmogenic rich compositions. The lunar iron-metal grains cluster around the solar wind composition in both helium and neon isotopes. The 3He/4He isotope ratios from metal grains are slightly elevated relative to the compositions from the silicate grains, but consistent with an additional cosmogenic He component. Lunar metals record near-solar helium and neon isotope ratios regardless of their oxygen signature. The collected data suggest that oxygen isotopes are quickly modified in lunar metal grains, whereas noble gas compositions are effectively retained, similar to silicates. The likely source of "normal" oxygen on the Moon is indigenous oxygen of silicates and oxides in the lunar soil. This oxygen could be sputtered from silicates and oxides and ejected into the metal grains. A feasibility study on oxygen diffusion in iron metal was conducted and results indicated that implanted oxygen could be expected to diffuse out on a time scale of weeks to months at lunar surface conditions. It appears that oxygen components are more mobile in lunar metal grains than SW noble gases. Isotopic compositions of surface layers of lunar grains, and modifications to them, provide valuable information on exposure history and evolution of planetary surfaces