Abundances of siderophile elements in H-chondrite metal grains: Implications for the origin of metal in unequilibrated ordinary chondrites

Abstract

Understanding the evolution of metal in the protoplanetary disk is necessary to constrain the first steps of metal-silicate segregation and the early stages of the evolution of the protoplanetary disk. We measured the siderophile elemental compositions (PGE, Ni, Co, Fe, Cu, Ga, Ge) of individual metal grains in H ordinary chondrites by laser ablation inductively coupled plasma mass spectrometry to investigate their formation. We analyzed unequilibrated ordinary chondrites (H3) to constrain processes affecting the metal before accretion, and inferred the effects of metamorphism by comparing their elemental compositions to those of equilibrated chondrites (H4–H6). Our results highlight large variations of refractory (Re, Os, W, Ir, Ru, Mo, Pt) and moderately volatile siderophile element (Pd, Au, Ga, Ge) concentrations among metal grains in H3 samples that permit to classify them according to their Ge/Ir ratios and HSE contents. These intergrain variations are progressively homogenized in H4–H6 samples due to their increasing degrees of metamorphism. To constrain the origin of the metal, we modeled its evolution during melting and crystallization. Our melting model of a single metallic precursor containing 1.5 wt% C and up to 12 wt% S reproduces well the observed range of siderophile element compositions in the metal. Metal grains show a range of W, Mo, and Ga compositions that we interpret to reflect various local (grain-scale) oxidation states during the melting event(s) due to the heterogeneous distribution of various oxidizing components within the precursors. The very similar HSE compositions of H and L/LL metal grains suggests that the variations of bulk metal abundance and HSE concentrations observed among the different classes of ordinary chondrites (H, L, LL) result from the heterogeneous physical distribution of a relatively chemically homogeneous metal component among OC parent bodies, and not from a chemical (sensu lato) gradient between H and LL chondrites.