Isotopic variations of germanium in iron and stony iron meteorites

Abstract

Germanium isotopic ratios for six iron meteorites of various groups, metallic phase of pallasite stony iron meteorites, three chemical reagents, and terrestrial germanite sample from Tsumeb mine, Namibia have been determined using a multiple collector-inductively coupled plasma mass spectrometer (MC-ICPMS). In this study, Ga-external correction technique, which enables us to detect the isotopic fractionation of the elements during the sample formation, was applied for the isotopic analysis of Ge. Resultant germanium isotopic data for terrestrial and meteoritic samples were found to have significant variations. Variations in germanium isotopic ratios for three chemical reagents and germanite samples were about 0.1–0.3% for 70Ge73Ge and 0.5% for 72Ge73Ge ratios, whereas germanium isotopic variations for meteoritic samples were about 4% for 70Ge73Ge and 3% for 72Ge73Ge ratios. Several points emerge from the germanium isotopic data. The first is that the germanium isotopic data for meteorites and terrestrial samples were found to define a straight line having slope of 13 on 70Ge73Ge vs. 72Ge73Ge diagram (Ge three isotopes plot). This indicates clearly that the meteoritic Ge had been isotopically fractionated during the sample formation. A second feature of the germanium isotopic data is that all the meteoritic Ge has systematically heavy isotopic compositions (low in 70Ge73Ge and 70Ge73Ge ratios) compared with those for terrestrial Ge. If the germanium isotopic ratios for germanite and chemical reagents reflect the germanium isotopic compositions for crustal materials of the Earth, this systematic deviations between meteorites and the terrestrial samples could be explained by the separation of crust from mantle. Alternatively heavy isotopic compositions for meteorites might be attributed to the larger evaporation loss of Ge from the meteoritic materials. Detailed mechanisms for the systematical difference in germanium isotopic ratios between meteorites and terrestrial materials will be given in this paper. The germanium isotopic data presented here demonstrate clearly that the isotopic fractionation data for medium- or heavy-mass range elements can give many clues to the origin and thermal evolution sequence of the metallic meteorites.