Two oxygen isotopic components with extra-selenial origins observed among lunar metallic grains - In search for the solar wind component

Oxygen isotopic analyses were performed in the surface layers of lunar metallic grains from lunar regolith samples 71501 and 79035, presumably exposed Lit the Moon surface at different times. We were able to reproduce the two extreme O components previously found [Hashizume K. and Chaussidon M. (2005) A non-terrestrial O-16-rich isotopic composition for the protosolar nebula. Nature 434, 619-622; Ireland T. R., Holden P., Norman M. D. and Clarke J. (2006) Isotopic enhancements of O-17 and O-18 from solar wind particles in the lunar regolith. Nature 440, 776-778], with a range observed of -12 +/- 5 < Delta O-17 < +33 +/- 3 parts per thousand (1 sigma). The relatively minor O-16-rich component corresponding to all end-member Delta O-17 Value lower than -20 parts per thousand is likely the solar component. This comes from the fact that its concentration roughly agrees with the maximum solar wind abundance expected among the grains from the two samples. At variance the O-16-poor component is 5-10 times more abundant and thus likely non-solar. The delta O-18 range found for the O-16-poor component may reflect various Processes such Lis isotope exchange reaction during oxidation of metallic iron and/or isotope fractionation by evaporation/condensation at the surface of the Moon or during implantation at depth in the lunar metallic grains. The present study suggests that planetary solid materials in bulk are systematically depleted in O-16 relative to the solar isotopic composition, suggesting the existence of non-mass-dependent isotopic fractionations associated to the formation of solids in the accretion disk. (C) 2009 Elsevier Ltd. All rights reserved.