Preferential sputtering as a method of producing metallic iron, inducing major element fractionation and trace element enrichment

The mechanism responsible for the formation of finely divided metallic iron in lunar samples has been the subject of considerable debate. A review of the data currently available from laboratory simulation studies would appear to favour an origin involving preferential sputtering of oxygen atoms by the solar wind. Using a model presented here we find that it is thermodynamically predictable that preferential sputtering would lead to the formation of free iron. Likewise titanium should be enriched in particle surfaces due to the formation of Ti₂O₃ from TiO₂. Other major elements will be fractionated according to the ratio of their heat of sublimation to the heat of dissociation of their oxide. The changes induced in the surfaces of fine grains will be of considerable importance when these particles are incorporated into complex grains. Thus, we suggest that preferential sputtering may be implicated in the major element fractionations observed for glassy agglutinates. A 1% component of Fe∮ is able to account easily for the enrichment in the total iron observed. If sufficient exposure time is available preferential sputtering may be able to produce metallic iron-rich layers ca. 40 Å thick on the surfaces of lunar grains. Most of this metal would derive from helium sputtering rather than hydrogen bombardment. Provided reduction takes place predominantly in fine particles with a mean grain size <10 μm diameter which are subsequently incorporated into glassy agglutinates, preferential sputtering could account for all the free iron in the lunar soil. For elements of atomic number ca. Z = 30 and above, thermodynamic effects become insignificant compared to momentum transfer considerations. Heavy (Pb, Hg) and medium heavy elements (Rb, Sr) could all be enriched by preferential sputtering. Monovalent elements (e.g. K, Rb) in silicates will have a lower binding energy and knock-on collisions with incoming solar wind atoms could cause their increased abundance in fine fractions of lunar soil.