Metallic copper in ordinary chondrites

Abstract— Metallic Cu of moderately high purity (∼985 mg/g Cu, ∼15 mg/g Ni) occurs in at least 66% of ordinary chondrites (OC) as heterogeneously distributed, small (typically ≤20 μm) rounded to irregular grains. The mean modal abundance of metallic Cu in H, L and LL chondrites is low: 1.0 to 1.4 × 10−4 vol%, corresponding to only 4–5% of the total Cu in OC whole rocks. In more than 75% of the metallic‐Cu‐bearing OC, at least some metallic Cu occurs at metallic‐Fe‐Ni‐troilite grain boundaries. In some cases it also occurs within troilite, within metallic Fe‐Ni, or at the boundaries these phases form with silicates or chromite. Ordinary chondrites that contain a relatively large number of occurrences of metallic Cu/mm2 have a tendency to have experienced moderately high degrees of shock. Shock processes can cause local melting and transportation of metallic Fe‐Ni and troilite; because metallic Cu is mainly associated with these phases, it also gets redistributed during shock events. In the most common petrographic assemblage containing metallic Cu, the Cu is adjacent to small irregular troilite grains surrounded by taenite plus tetrataenite; this assemblage resembles fizzed troilite and may have formed by localized shock melting or remelting of a metal‐troilite assemblage.