Fremdlinge in Leoville and Allende CAI – Clues to Post-Formation Cooling and Alteration

Abstract

Fremdlinge are perhaps the most exotic and least understood objects in CAI and their very existence places severe constraints regarding formation and cooling histories of the host CAI. Following the discovery and description of Willy, which appears to be an "Ur-Fremdling" or prototype for smaller Fremdlinge in CAI, we have begun a systematic study of CAI of different petrographic types to see if the Fremdlinge are consistent with a common mode of formation and differing degrees of reprocessing. Fifteen type B I, B2, and compact A CAI from Allende and Leoville were selected. The relative abundance of Fremdlinge varies dramatically among CAI, however some CAI of each type and from each meteorite contain abundant Fremdlinge. The range of phase assemblages and mineral chemistries of Fremdlinge from Allende and Leoville is very similar suggesting that the parent sources for Fremdlinge were quite similar. Chemically and texturally, Fremdlinge grade continuously from complex, heterogeneous Willy-like objects to altered, homogeneous metal nuggets. Some complex Fremdlinge like Willy were found in B1 CAI; none were observed in B2 CAI.