Optical Microscopy of Meteoritic Metal

METAL SCIENCE- Meteorite Thermal Histories Meteorite Thermal Histories | Expected Phase Transformations for Slow Cooling | Development of Chemical Zoning | Alternative Reaction Paths for Slow Cooling | Reaction Path for Fast Cooling Most of the metallographic features we observe in meteorites are the products of phase transformations that occurred during cooling in the solid state. In the majority of iron meteorites, this solid-state cooling is thought to represent a continuation of cooling from much higher temperatures where the metal was liquid (the red line in the Figure below).     Idealized thermal histories of iron meteorites (red) and chondrites (blue). In contrast, ordinary chondrites appear to have undergone a two-stage thermal history consisting of heating to sub-solidus temperatures (prograde metamorphism) followed by cooling (retrograde metamorphism) (blue line)  The equilibrated chondrites (petrographic types 4 - 6) reached peak metamorphic temperatures at which Fe-Ni interdiffusion would have rapidly homogenized individual metal particles.  In view of this likely homogenization at the peak temperature, the microstructures preserved in chondritic metal must have developed during the cooling stage (retrograde metamorphism). The simple thermal histories depicted in this figure provide a starting point for interpreting meteorite metallography.  It is important to recognize, however, that the primary microstructures which formed during slow cooling were susceptible to significant modification by secondary events such as shock-heating.