Pallasites—metal composition, classification and relationships with iron meteorites

Abstract

Concentrations of Au, As, Co, Ga, Ge, Ir, Ni and W were determined in the metal of 28 different pallasites plus 6 which are probably paired, to help elucidate their origin. Most divide into two clusters: No. Ni (%) Ga (μg/g) Ge (μg/g) Au (μg/g) Fa (mole %) Main group 19 7.8–11.7 16–26 29–65 1.7–3.0 11–13 Eagle Station trio 3 14–16 4.5–6 75–120 0.8–1.0 19–20 In the main group Ni is negatively correlated with Ga, Ge and positively with Au and probably with the Fa content of the olivine. The other members of the Eagle Station trio are Cold Bay and Itzawisis. Rawlinna, Springwater and Phillips County have metal compositions appropriate to high-Ni main group members but fayalite contents are too high for main group trends. Glorieta Mountain and Pavlodar appear to be unique pallasites. Ilimaes, Antofagasta, Ollague and Salta are probably pieces of Imilac, a main group member. On a Ga-Ge plot, main group pallasites overlap the field of IIIAB iron meteorites and Au, As, Ir, Ni and W contents of the metal are very close to those of the high-Ni IIIAB irons. It is likely that both groups formed in the same parent body. The Eagle Station trio might be related to certain anomalous irons with high Ge/Ga ratios and may have formed in a more oxidizing environment.