Reduction during metamorphism of four ordinary chondrites

Abstract

Three type 3 chondrite finds, Willaroy, Suwahib (Buwah) and Moorabie, and one type 4 find, Cerro los Calvos, are ordinary chondrites (OC) in all respects except redox state: (a) they have OC abundances of refractory lithophiles (Cerro los Calvos data are ambiguous), Zn and other taxonomic elements, (b) they have OC matrix/chondrule abundance ratios, and their chondrule sizes are consistent with bulk-siderophile indicators of group membership—H for Willaroy and Cerro los Calvos, L for Suwahib (Buwah) and Moorabie, and (c) their oxygen isotope compositions are far closer to those of OC than to those of any other chondrite group. In each of the four chondrites the range of olivine compositions is relatively narrow; fayalite modes and means are distinctly below those of other members of their respective groups having similar or greater degrees of equilibration. The most plausible explanation of these properties is that these four OC experienced reduction during metamorphism. This is consistent with reverse zoning in olivine from one type II chondrule in Willaroy. The reducing agents may have been foreign clasts incorporated during an earlier accretional history; alternatively, these chondrites could themselves have been foreign materials incorporated into a reduced regolith. Examples of reducing clasts are the C-rich aggregates in Sharps; examples of reducing host regoliths are the host materials in Bencubbin and Cumberland Falls. Confirmation of this interpretation will require identification of the reduced foreign material or their relicts in the four reduced OC; initial searches have been unsuccessful.