The chemical classification of iron meteorites: IV. Irons with Ge concentrations greater than 190 ppm and other meteorites associated with group I

Abstract

Concentrations of Ni, Ga, Ge, and Ir are reported for 87 iron meteorites with high Ge concentrations. Chemical group I is defined as those iron meteorites with 190–520 ppm Ge which fall within “main-sequence” fields on Ge vs. Ge and Ge vs. Ni plots. The Ni, Ga, and Ir ranges are 6.4–8.6%, 56–100 ppm, and 0.6–5.5 ppm, respectively. The Ge-Ga correlation is positive, Ge-Ni and Ga-Ni correlations negative, and Ir is not correlated with the other three elements. Members of group I have similar structures with kamacite bandwidths generally falling in the coarse-octahedrite range, and with numerous, often large inclusions which frequently contain silicates. Three additional categories of anomalous irons which show evidence of a relationship to group I are also defined. Category I-Anl includes irons which fall off the main sequence in Ge-Ga or Ge-Ni plots, but have structures which are very similar to those in group I. Category I-An2 includes main-sequence irons with Ge concentrations between 78 and 190 ppm, and other irons with Copiapo-type silicate inclusions. Category I-An3 irons are similar to the group I members in Ga and Ge contents, and insofar as they are rich in inclusions, but are anomalous in structure, and are not as likely to be genetically related to group I as are the former two categories. A consideration of available evidence indicates that group I is one of the most primitive groups of iron meteorites. The present relationship between metal and silicates appears to have formed nonigneously, perhaps as a result of metal-silicate fractionation in the solar nebula.