Aqueous depletion of Mg from olivine surfaces enhanced by ion irradiation

Abstract

Laboratory experiments demonstrate that olivine irradiated with keV ions and exposed to water exhibits up to a 60% reduction from the original near-surface concentration of magnesium, as measured by X-ray photoelectron spectroscopy. The depth of this depletion layer is measured to be ~15 nm. Irradiations were performed with 4 keV ions at fluences from 1014–1019 ions cm–2 and water immersion times ranging from 3 s to more than 100 h in neutral (pH = 6.8), high-purity water. The depletion of Mg depends strongly on ion fluence but weakly on immersion time after 3 min, when it saturates. Remarkably, ion irradiation enhances the rate of surface depletion of Mg from olivine by a factor of 26, 000. This effect must be considered when assessing the surface composition of samples exposed to simulated space weathering in the laboratory and during the handling and analysis of irradiated extraterrestrial specimens acquired via sample return missions.