Formation of olivine textures in pallasites and thermal history of pallasites in their parent body

An analysis of the textures of pallasites has been made using data concerning the kinetic and rheological properties of silicates and metals. Pallasites containing rounded olivines (e.g., the Springwater and Brehnam pallasites) have been heated to above the solidus temperature of the metallic iron phases, ∼ 1270 K. The rounded olivines of grain size 0.5-1.0 cm observed in the Springwater pallasites were formed between 1270 and 1370 K. On the other hand, those of grain size 0.5-1.0 mm as found in the Brehnam pallasites may have been heated to above 1370 K; however, the duration of heating at such high temperatures must have been less than 5 × 10³ y. Pallasites containing angular olivines with microscopically rounded corners (e.g., the Eagle Station, Dora pallasites) have suffered shock events fracturing the olivine grains, which may correspond to collisions during the accretional stage of the parent body, and experienced successive annealing during cooling from a temperature between 1150 and 1270 K.