Distribution of siderophile elements in CR chondrites: Evidence for evaporation and recondensation during chondrule formation

New data on the chemical composition of bulk samples, and of metallic and n onmagnetic fractions of six CR chondrites (Renazzo, Y793495, PCA91082; EET9 2042, Acfer 209 and EI Djouf 001) are reported. It is shown that volatile s iderophile element abundance patterns of metallic and nonmagnetic fractions of CR chondrites are complementary and volatility-dependent. In the metall ic fraction CI-normalized abundances of Au, As, Sb and Ga decrease with inc reasing volatility, whereas in the nonmagnetic fraction abundances increase in the same sequence as in the metallic fraction. We argue that the sidero phile element patterns of the metallic, and nonmagnetic fractions reflect t hose of the chondrule and matrix fractions of CR chondrites, respectively, based on: (1) CR metal is mostly located inside chondrules and those metal grains outside chondrules probably were also derived from chondrules; (2) e lement partitioning within chondrules has been reset during chondrule forma tion; and (3) resetting of element distribution; within chondrules.; occurr ed at sufficiently reducing conditions to allow partitioning of Au, As, Sb and Ga into CR metal. The complementary siderophile element patterns of CR chondrules and matrix are difficult to explain by gradual gas loss during condensation. The CI pr oportions of highly volatile elements in the bulk CR chondrites further arg ue against the possibility of loss of solids during condensation. Thus, the fractionation of volatile elements in CR chondrites is unlikely to be the result of gas-solid fractionation during condensation. The fractionation of volatile siderophile elements between CR chondrules an d matrix requires evaporation of volatile elements during chondrule formati on. The matrix pattern indicates recondensation of evaporated volatile elem ents. It appears, based on the composition of CR matrix, that the depletion and fractionation of moderately volatile elements in the bulk CR chondrite s is due to formation of CR chondrites before complete recondensation of vo latile elements which were evaporated during chondrule formation. This impl ies that agglomeration of CR chondrites proceeded simultaneously with chond rule formation. Copyright (C) 1999 Elsevier Science Ltd.