Olivine Compositions and Cooling Rates of Pallasitic Meteorites

Electron microprobe measurements of olivine and metal in about 85 percent of the known pallasites were made. The olivines are unzoned, and coexisting crystals within most pallasites have identical compositions. This indicates a high degree of internal equilibrium. Relative to terrestrial olivines, pallasitic olivines are depleted in Ni. Thermochemical calculations demonstrate that the low Ni contents of pallasitic olivines are consistent with metal-olivine equilibrium at 1000° C or less. Cooling rate measurements show that the pallasites cooled more slowly (0.5° to 2.0° C per million years) than the bulk of the iron meteorites and that the pallasites form a homogeneous cooling rate group. Application of a modified Prior's rule to the pallasites suggests that olivine and metal equilibrated in the presence of other silicates. The observed compositional distribution of olivine from pallasites is compatible with that predicted from a chondritic parent melt. The pallasites were probably derived from deep within their parent body, either at or near the core. Most iron meteorites were either formed from different parent bodies or came from isolated metal pools within the silicate mantle.