1. Department of Earth and Space Sciences, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
2. CRPG-CNRS, BP 20, 54501 Vandoeuvre-lès-Nancy Cedex, France

Correspondence to: Ko Hashizume¹ Correspondence and requests for materials should be addressed to K.H. (Email: kohash@ess.sci.osaka-u.ac.jp).

The discovery in primitive components of meteorites^{1, 2} of large oxygen isotopic variations that could not be attributed to mass-dependent fractionation effects has raised a fundamental question: what is the composition of the protosolar gas from which the host grains formed? This composition is probably preserved in the outer layers of the Sun, but the resolution of astronomical spectroscopic measurements is still too poor to be useful for comparison with planetary material^{3, 4}. Here we report a precise determination of the oxygen isotopic composition of the solar wind from particles implanted in the outer hundreds of nanometres of metallic grains in the lunar regolith. These layers of the grains are enriched in ¹⁶O by >20 4 relative to the Earth, Mars and bulk meteorites, which implies the existence in the solar accretion disk of reactions—as yet unknown—that were able to change the ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios in a way that was not dependent strictly on the mass of the isotope. Photochemical self-shielding of the CO gas irradiated by ultraviolet light^{5, 6, 7} may be one of these key processes, because it depends on the abundance of the isotopes, rather than their masses.

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