Distribution of Ni,Ga, Ge and Ir between metal and silicate portions of H-group chondrites

Concentrations of Ni, Ga, Ge and Ir have been determined in the metal and silicate portions of 21 chondrites, including 15 H chondrites. The H-group metal shows the following concentration ranges: 7.2–9.4 per cent Ni, 2.4–18 ppm Ga, 61–70 ppm Ge and 1.6–4.6 ppm Ir, and concentrations in H-group silicates are 94–380 ppm Ni, 3.0–9.2 ppm Ga, 0.06–0.66 ppm Ge and 0.03–0.12 ppm Ir. The Ni, Ge and Ir contents in the metal are positively correlated with each other and with the Fe content of olivine, as expected from oxidation-reduction or combined metal-silicate-fractionation/oxidation-reduction models. Metal/silicate concentration ratios for Ir are lower than for Ge and Ni, despite the fact that Ir is more easily reduced to the elemental form. This may indicate that at the formation location of the H-group chondrites in the solar nebula, substantial amounts of Ir were present in condensed form at temperatures which were so high that Fe and Ni were present mainly as vapor. The metal/silicate concentration ratios of Ga and Ge are lower in type-3 ordinary chondrites than in types 4–6. Apparently appreciable fractions of these elements condensed from the nebula in oxidized form and entered the metal during later thermal events. That Ga and Ge were redistributed during recrystallization, whereas appreciable Ir remained in the silicate fraction, probably indicates that Ir faced a substantially greater diffusional barrier than did Ga and Ge.