Cooling rates and parent bodies of iron meteorites from group IIICD, IAB, and IVB

Abstract

New metallographic cooling rates have been calculated for the iron meteorites Carlton (IIICD), Toluca (IAB), and Odessa (IAB) based on the central Ni concentration versus taenite width method. New kamacite bandwidth cooling rates have been calculated for the members of group IVB, the ungrouped iron meteorite Chinga, and the high-Ni members of IIICD and IAB. The cooling rates determined are up to several orders of magnitude faster than those previously reported.A difference of about a factor of 60 is found between the kamacite bandwidth cooling rate and the central Ni versus taenite width cooling rate of the same meteorite, Carlton. This discrepancy is interpreted as resulting from extensive regolith accretion on the parent body at the time of Widmanstätten pattern formation.No systematic trend is found in the cooling rates as a function of bulk meteorite Ni of the magmatic group IVB. Combined with the strong inter-element correlations found in group IVB [15], the constant cooling rate suggests a core origin of this group.No systematic cooling rate variation is found in neither of the non-magmatic groups IIICD or IAB. Combined with the weak or missing inter-element correlations in these groups, the results of the present work support the megaregolith melt pool scenario suggested by Wasson et al. [1] for the origin of the non-magmatic groups IIICD and IAB.