PREFERENTIAL SEGREGATION OF FE-NI METAL DURING ASTEROIDAL IMPACTS

We examined L chondrite meteorites affected by impact melting to examine how metal and sulfide become separated from silicate during impacts. Throughout one heavily brecciated meteorite, metal content and the metal/sulfide ratio (Me/S) are highly variable. Sulfide loss generally mirrors metal loss but is not as complete, with normal Me/S preserved in high-metal domains and low Me/S in depleted domains, indicating preferential extraction of metal. Differences in density and liquid viscosity may be responsible. Preferential melting of metal, and to a lesser extent FeS, occurred because impact-induced heating is maximised at interfaces between high and low density phases. Silicates were mainly melted along narrow cm-long high-strain zones, allowing linking between melt patches and formation of pressure differentials associated with local structural dilatancies. The lower viscosity of liquid metal allowed it to be rapidly and preferentially segregated into these dilational sites. FeS melt was similarly segregated but not as efficiently due to its higher viscosity and less extensive melting. Silicate melt was not effectively segregated due to its high viscosity. Away from linked high strain zones no liquids could escape, allowing typical L chondrite Me/S to be preserved. The rapidity of this process was such that metal segregation occurred under disequilibrium conditions. Although some Fe-FeS melt did develop where the two were in contact at the time of melting, generally the metal and sulfide melts dissolved into each other to a limited extent. Two types of melt conduit developed; those containing silicate melt with numerous immiscible sulfide- and metal-melt droplets, and those filled with only metal and/or sulfide melt through which more rapid transfer may have been possible. Localized meso-scale accumulations of metal (e.g., Portales Valley) are likely to be the next stage in the progression of this process. Gravity on small planetesimals is too low, and the duration of impact events is too short-lived, for Stokes' Law settling to have been the dominant control on the initial stages of metal melt segregation. We therefore suggest that a significant fraction of non-magmatic iron meteorites, which have chemistry suggesting disequilibrium metal fractionation, may have formed through this process.