A preliminary study of cadmium mass fractionation in lunar soils

This preliminary investigation reports evidence of linear mass fractionatio n in Cd in lunar soil samples. The isotopic and elemental abundance of cadm ium in five lunar samples has been measured with high precision using therm al ionization mass spectrometry and the stable isotope dilution technique r espectively. Although both positive and negative mass fractionation for Cd have been identified in meteorites, this is the first time that Cd isotope fractionation has been observed in lunar materials. Three of the four lunar soil samples analyzed gave positive tin the sense that the heavy isotopes of Cd are enriched with respect to a laboratory standard) mass fractionatio n, ranging in magnitude from 0.34 to 0.63% per mass unit. Cadmium is a vola tile element and its elemental abundance in these fractionated lunar soil s amples ranges from 57 to 112 ng/g as compared to the solar system abundance of 686 ng/g. There is a possible inverse correlation between the magnitude of the mass fractionation and the concentrations for the same samples. The fourth lunar soil sample, which has a Cd concentration approximately 11 ti mes greater than the mean of the other soil samples, gave zero mass fractio nation. The fifth sample is an orange-colored glass spherule which gave a s mall negative mass fractionation. Volatilization by micrometeorite impact a nd ion sputtering are both likely to have contributed to the mass fractiona tion observed in these lunar soils, It is also possible that redeposition o f isotopically fractionated Cd may have been a factor in the magnitude of t he observed effect. (C) 2001 Elsevier Science B.V. All rights reserved.