Metallographic cooling rate
A metallographic cooling rate is estimated from the way nickel is distributed in kamacite and taenite in meteoritic metal.
A metallographic cooling rate is worked out by measuring nickel across the metal grains of an iron or stony-iron meteorite. As metal cools below about 700 to 900 °C, nickel diffuses from growing kamacite into neighbouring taenite. The slower the cooling, the further nickel can move, so taenite develops a characteristic M-shaped nickel profile, high at its edges and lower in the centre.
By comparing measured profiles with computer models of diffusion, researchers estimate how quickly the metal cooled, usually in degrees per million years. Another method uses the size of the tiny particles in the cloudy zone at the edges of taenite.
The results have been powerful. They showed that iron meteorites came from several dozen separate parent bodies, that some groups cooled in small exposed cores, and that pallasites and mesosiderites cooled through low temperatures remarkably slowly. Ordinary chondrite metal gives cooling rates that help reconstruct the layered structure of their parent asteroids.
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