The metallographic cooling rate method revised: Application to iron meteorites and mesosiderites

A major revision of the current Saikumar and Goldstein (1988) cooling rate computer model for kamacite growth is presented. This revision incorporates a better fit to the alpha/alpha + gamma phase boundary and to the gamma/al pha + gamma phase boundary particularly below the monotectoid temperature o f 400 degreesC. A reevaluation of the latest diffusivities for the Fe-Ni sy stem as a function of Ni and P content and temperature is made, particularl y for kamacite diffusivity below the paramagnetic to ferromagnetic transiti on. The revised simulation model is applied to several iron meteorites and seve ral mesosiderites. For the mesosiderites we obtain a cooling rate of 0.2 de greesC/Ma, about 10x higher than the most recent measured cooling rates. Th e cooling rate curves from the current model do not accurately predict the central nickel content of taenite halfwidths smaller than similar to 10 mum . This result calls into question the use of conventional kamacite growth m odels to explain the microstructure of the mesosiderites. Kamacite regions in mesosiderites may have formed by the same process as decomposed duplex p lessite in iron meteorites.