Trace element distribution between magnetic and non-magnetic portions of ordinary chondrites

Abstract

The contents of As, Au, Ni, Co, Ir, Os, Re and Pt were determined in the metal and silicates-plus-troilite portions of 10 H-, 7 L-, and 5 LL-group chondrites. All elements are strongly siderophile and concentrated in the metal phase. The different values of the As/Ni, Au/Ni and Co/Ni ratios in the magnetic and non-magnetic portions of the three chemical groups indicate that the contents of these elements in the latter fractions cannot be entirely attributed to metal contamination. Analyses of minerals separated from L-group chondrites reveal relatively high concentrations of Ni and Co in troilite and of Au in chromite, feldspar and phosphate. Iridium, Os, Re and Pt are enriched with respect to Ni in the non-magnetic fractions of ordinary chondrites, and the proportion of refractory metals, in these fractions, increases from H- to LL-group chondrites, and, within each group, in the order Ir ≳ Os ≳ Pt ≫ Re. The Ir/Re ratio in ordinary chondritic metal is about 35% below C1 ratios, but higher than C1 in the non-magnetic fractions. The Ir-Re fractionation appears to have taken place in the nebula before condensation of Fe-Ni. The Ir-rich inclusions within chondritic silicates may represent a sample of the material that was lost during the agglomeration of ordinary chondrites.