The composition of the stone meteorites and the origin of the meteorites

Abstract

A study has been made of some 350 chemical analyses of stone meteorites available in the literature, and criteria have been applied in an attempt to select those which are more reliable. It is found that the chondrites fall into two distinct groups with regard to both the total iron content and the oxidation state of the iron. These groups cannot be related by any simple mixing process and thus indicate that the meteorites have been derived from two rather different parent objects rather than from a single body. It appears that preferential volatilization of silicates and retention of nickel-iron operating to a different extent during a high temperature stage in the evolution of the asteroids may explain the different compositions of the two groups of chondrites. The implications of these data with respect to the cosmic abundances of the elements are discussed, and it is concluded that Goldschmidt's abundances for iron and nickel are too high. It is postulated that the parent asteroids went through a low temperature accumulation process, a high temperature stage of partial melting and evaporation, a stage of collisions with smaller objects which produced brecciated and tuffitic agglomerations of material and chondrules; and finally a collision of two such objects compacted the material into the meteorites, preserving however the essential differences in composition between the two groups.