Olivine Compositions and

Electron microprobe measurements of olivine and metal in about 85 percent of the known pal-lasites 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 equili-brated in the presence of other silicates. The ob-served 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 mete-orites were either formed from different parent bodies or came from isolated metal pools within the silicate mantle