Space weathering processes on airless bodies: Fe isotope fractionation in the lunar regolith

Nanophase Fe metal grains (np-Fedegrees) are a product of space weathering, formed by processes related to meteorite impacts, and solar-wind sputtering on airless planetary bodies, such as the Moon. Iron isotopes of lunar soils are fractionated during these processes, and the np-Fedegrees in the finest ( < 10 mum), mature, size fractions of the soil become enriched in heavier isotopes by similar to0.3 parts per thousand in Fe-56/Fe-54 in comparison to the bulk rocks (0.03+/-0.05 parts per thousand), from which the soil was formed. A positive correlation of delta(56)Fe values with the soil maturity index, I-S/FeO, suggests that the high delta(56)Fe values reflect production of nanophase Fe metal that is produced by space weathering that occurs on airless planetary bodies. Furthermore, the enrichment of delta(56)Fe in the smallest size fraction of lunar soils supports a model for creation of np-Fedegrees through vapor deposition induced by micrometeorites, as well as that by solar-wind sputtering. (C) 2003 Elsevier B.V. All rights reserved.