An iron isotope perspective on the origin of the nanophase metallic iron in lunar regolith

The surfaces of the Moon and other airless planetary bodies are constantly weathered by meteorite impacts and sputtering by charged particles. One of the hallmarks of this "space weathering" is the presence of nanophase metallic Fe (npFe ⁰) at the surface of airless bodies. These npFe ⁰ grains alter the surface optical spectra of planetary bodies without an atmosphere and their concentration is used to estimate the degree of maturity of lunar regolith. The origin of npFe ⁰ has been debated between in situ reduction due to the solar wind, and evaporation generated by charged particle sputtering and/or micrometeorite impact followed by re-condensation of metallic Fe. These two mechanisms will impart completely different Fe isotopic fractionation effects on the npFe ⁰. In this study we measure the Fe isotopic composition of npFe ⁰ using a step-by-step surface etching technique on lunar regolith plagioclase. Our results show that npFe ⁰ is highly enriched in the heavy isotopes of Fe (δ ⁵⁶Fe up to 0.71‰) compared to bulk plagioclase and other lunar materials such as regolith and igneous rocks. We suggest that the formation of npFe ⁰ in lunar regolith is responsible for the higher δ ⁵⁶Fe in the lunar regolith compared to lunar igneous rocks. In addition, a thermal escape model shows that the heavy Fe isotopic composition of npFe ⁰ is best explained by the preferential escape of light Fe isotopes to space in the vaporization phase of Fe. The temperature of the vapor can be inferred from our model (2750-3000K), which is compatible with those proposed by previous calculations and experiments. Therefore our results unambiguously support the vapor deposit origin of npFe ⁰, explain the origin of the heavy Fe isotopic composition of the lunar regolith and provide a temperature estimate for the impact event at the origin of the npFe ⁰.