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The group IVA fractionally crystallized iron meteorites display a diverserange of metallographic cooling rates. These have been attributed to theirformation in a metallic core, approximately 150 km in radius, that cooled tocrystallization in the absence of any appreciable insulating mantle. Here webuild upon this formation model by incorporating several new constraints. Theseinclude (i) a recent U-Pb radiometric closure age of <2.5 Myr after solarsystem formation for the group IVA iron Muonionalusta, (ii) new measurementsand modeling of highly siderophile element compositions for a suite of IVAs,and (iii) consideration of the thermal effects of heating by the decay of theshort-lived radionuclide 60Fe. Our model for the thermal evolution of the IVAcore suggests that it was approximately 50 - 110 km in radius after beingcollisionally exposed. This range is due to uncertainties in the initialabundance of live 60Fe incorporated into the IVA core. Our models define arelationship between cooling rate and closure age, which is used to makeseveral predictions that can be tested with future measurements. In general,our results show that diverse cooling rates and early U-Pb closure ages canonly coexist on mantle-free bodies and that energy released by the decay of60Fe reduces the core size necessary to produce diverse metallographic coolingrates. The influence of 60Fe on cooling rates has largely been neglected inprevious core formation models; accounting for this heat source can affect sizeestimates for other iron meteorite cores that cooled to crystallization in thepresence of live 60Fe. Candidates for such a scenario of early, mantle-freeformation include the iron IIAB, IIIAB and IVB groups.

Size of the group IVA iron meteorite core: Constraints from the age and composition of Muonionalusta