Relationship between cooling rate and cooling age of a mineral: Theory andapplications to meteorites

We reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system, which permits dir ect retrieval of cooling rate from cooling age when the diffusion parameter s, grain size and initial temperature are known. This formulation is used t o show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system. The cooling rates of seven H chondrites of the metamorphic types H4, H5 and H6 were retrieved f rom the available data on the Pb-Pb model ages of the phosphates and the di ffusion kinetic data of Pb in apatite. The results are in excellent agreeme nt with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites. We also addressed the problem of similar to 90 Ma younger Sm-Nd mineral isochron age, defined by orthopyrox ene, phosphate and plagioclase, of the Morristown mesosiderite compared to the Pb-Pb age of the Estherville mesosiderite. It is shown that this younge r age could have been a consequence of resetting during cooling instead of an "impulsive heating" event, as suggested earlier.