Fractionation of highly siderophile elements in metal grains from unequilibrated ordinary chondrites: Implications for the origin of chondritic metals

Abstract

Abstract To investigate the formation processes of metal grains in chondrites, we measured the abundances of highly siderophile elements (HSEs: Re, Os, Ir, Ru, Pt, Rh, Pd, and Au) using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) on individual Fe-Ni metal grains from four petrologic type 3 ordinary chondrites: NWA 6910 (L3.3), NWA 4910 (LL3.1), Richfield (LL3.7), and SAH 97210 (L/LL3.2). Among HSEs, the abundances of Pd and Au in the metal grains had positive correlations with the measured Ni abundances, indicating equilibrium partitioning of Pd and Au between kamacite and taenite via thermal metamorphism. In contrast, the other HSEs (Re, Os, Ir, Ru, Pt, Rh) showed large variations in concentrations spanning nearly three orders of magnitude without evidence of redistribution between kamacite and taenite, suggesting that these elements preserved the initial compositions before kamacite-taenite segregation. The CI-normalized HSE patterns presented large depletions in Os and Ir with relatively large Os/Ir variations (0.29–3.2) and the Ru/Ir ratios also varied significantly (0.27–40). In addition, HSE abundances in fine metal grains showed wide variations compared to those of coarse metal grains. We suggest that the variation of refractory HSE compositions in Fe-Ni metal grains with characteristic Os-Ir depletions were most likely caused by solid metal-liquid metal partitioning during crystallization of a Fe-Ni metal melt containing 2 wt.% of C. The liquid metal is considered to be generated during multiple heating events related to chondrule formation. The lack of Fe-Ni metal grains exhibiting coexistence of liquid metal and solid metal composition within a single metal grain would suggest that solid metal grains were physically segregated from the liquid metal during the crystallization of Fe-Ni metals. Droplets of the segregated liquid metal collided and merged with other liquid metal droplets and solid metal grains to form coarser metal grains. The resultant larger metal grains have relatively homogeneous HSE abundances that are close to the bulk metal composition as a result of the mixing of liquid metal with solid metal. In contrast, molten metal droplets and solid metal grains that did not collide and merge formed finer metal grains formed finer metal grains with more variable HSE abundances.