A trace-element study of iron meteorites

Abstract

The technique of neutron activation and γ counting on a lithium-drifted germanium semiconductor detector has been used in measuring concentrations of cobalt, copper, arsenic, gallium, iridium, and gold in 33 iron meteorites. Similar data on 25 other iron meteorites are available in the literature. These meteorites are grouped according to similarities in their trace-element content. Out of 23 samples classified as hexahedrites and coarse octahedrites, 22 can be assigned to three groups corresponding to the structural classifications as normal hexahedrites, coarse octahedrites, and a group intermediate between these two. Within these groups, as Ni increases, the elements Co, Cu, As, Ga, and Au increase while Ir decreases. This is thought to represent trace-element behavior during crystallization and crystal settling from a melt of a homogeneous chemical composition. A discontinuity involving the elements As, Au, Ga, and Ir exists between the coarse octahedrites and medium octahedrites. The medium octahedrites are also characterized by their heterogeneity. Only 8 out of 17 can be assigned to groups. Out of 10 fine octahedrites, 5 have uniform elemental compositions. This group differs from the medium octahedrites in Ni, Co, and Ga. These differences in trace-element abundance are ascribed to differences in composition of the Fe-Ni melt from which the meteorites crystallized. At least three melts, varying chiefly in Ni and Ga, are required to account for the compositions of all the iron meteorites. Six Ni-rich ataxites show no uniformity in their trace-element abundances. They probably represent the end stage of crystallization from several sources. Two structurally anomalous meteorites, Nedagolla and N'Goureyma, are unusually low in trace elements. This may have resulted from a remelting of a normal iron meteorite during break-up of the meteorite parent body.