Abstract:
This preliminary investigation reports evidence of linear mass fractionation in Cd in lunar soil samples. The isotopic and elemental abundance of cadmium in five lunar samples has been measured with high precision using thermal ionization mass spectrometry and the stable isotope dilution technique respectively. 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 (in the sense that the heavy isotopes of Cd are enriched with respect to a laboratory standard) mass fractionation, ranging in magnitude from 0.34 to 0.63% per mass unit. Cadmium is a volatile element and its elemental abundance in these fractionated lunar soil samples 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 times greater than the mean of the other soil samples, gave zero mass fractionation. The fifth sample is an orange-colored glass spherule which gave a small negative mass fractionation. Volatilization by micrometeorite impact and ion sputtering are both likely to have contributed to the mass fractionation observed in these lunar soils. It is also possible that redeposition of isotopically fractionated Cd may have been a factor in the magnitude of the observed effect.