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

TL;DR

This study models the size and thermal history of the group IVA iron meteorite core, incorporating new age and compositional constraints, and highlights the significant impact of 60Fe decay on core cooling and size estimates.

Contribution

It introduces a refined thermal evolution model for the IVA core that includes 60Fe decay effects, constraining its size and cooling history with new radiometric and compositional data.

Findings

Core radius estimated between 50-110 km due to 60Fe decay effects.

Diverse cooling rates and early closure ages imply a mantle-free formation.

Accounting for 60Fe decay alters size estimates for iron meteorite cores.

Abstract

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