Space Plasma Ion Processing of Ilmenite in the Lunar Soil: Insights from In-situ Tem Ion Irradiation Experiments

Introduction: Space weathering on the moon and asteroids results largely from the alteration of the outer surfaces of regolith grains by the combined effects of solar ion irradiation and other processes that include deposition of impact or sputter-derived vapors. Although no longer considered the sole driver of space weathering, solar ion irradiation remains a key part of the space weathering puzzle, and quantitative data on its effects on regolith minerals are still in short supply. For the lunar regolith, previous transmission electron microscope (TEM) studies performed by ourselves [1] and others [2] have uncovered altered rims on ilmenite (FeTiO 3) grains that point to this phase as a unique " witness plate " for unraveling nanoscale space weathering processes. Most notably, the radiation processed portions of these ilmenite rims consistently have a crystalline structure, in contrast to radiation damaged rims on regolith silicates that are characteristically amorphous. While this has tended to support informal designation of ilmenite as a " radiation resistant " regolith mineral, there are to date no experimental data that directly and quantitatively compare ilmenite's response to ion radiation relative to lunar silicates. Such data are needed because the radiation processed rims on ilmen-ite grains, although crystalline, are microstructurally and chemically complex, and exhibit changes linked to the formation of nanophase Fe metal, a key space weathering process. We report here the first ion radiation processing study of ilmenite performed by in-situ means using the Intermediate Voltage Electron Microscope Tandem Irradiation facility (IVEM-Tandem) at Argonne National Laboratory. The capability of this facility for performing real time TEM observations of samples concurrent with ion irradiation makes it uniquely suited for studying the dose-dependence of amorphization and other changes in irradiated samples. Experimental Approach: The irradiations were performed on an ion milled thin section of lunar basalt 70215. This is a high-Ti mare basalt that contains 13 modal percent ilmenite grains ranging from 50µm up to 3 mm in size [3]. The other major phases are clinopy-roxene (58%), plagioclase (18%), and olivine (6%) [3]. The ilmenite has 10-16 atom % substitution of Mg for Fe but is otherwise a fully-reduced end-member ilmen-ite (i.e., no Fe 3+ substitution). Our experiments followed standard IVEM-Tandem methodology in which ion-milled samples are irradiated by a ~2 mm-diameter