Oxidation state of iron and extensive redistribution of sulfur in thermally modified Stardust particles

Abstract

Two representative thermally modified Stardust samples were investigated by analytical transmission electron microscopy in order to decipher their iron oxidation state after the strong thermal episode due to the capture in aerogel. Their dominant microstructure consists of evenly distributed rounded Fe–Ni–S nano-droplets within a silica-rich glassy matrix. The mineralogy and associated redox state of iron is assessed using a Fe–Mg–S ternary diagram on which ferromagnesian silicates, sulfides and metal can be represented and potentially compared with any other extraterrestrial material. In this diagram, all the data (bulk and local analysis of silicates, sulfide + metal) scatter along a mixing line between the Mg corner and the average composition of the iron-sulfide. There is an obvious genetic relationship between the different phases observed in such samples, further supported by the very low concentration of iron in the glassy matrix. Silicate glasses contain a significant concentration of dissolved sulfur probably present as MgS complexes. This chemical signature is typical of highly reduced environments. These secondary microstructures were established during the high temperature stage of the capture. A significant part of the Fe-droplets formed in situ by reduction at high temperature of ferromagnesian silicates (olivine and pyroxenes) during the impact. At this stage, the indigenous sulfides destabilized and sulfur readily volatilized as S 2, diffused into molten materials and condensed later onto the Fe-precipitates that formed in the silicate melt. This scenario is supported by the structure of Fe–Ni–S beads with a metal core and a sulfide rim. It will be difficult to derive reliable information on the redox state of 81P/Wild 2 particles based on bulk analyses of whole tracks because particles found along the walls of tracks suffered strong reduction reactions, contrary to terminal particles that may have preserved their pristine redox state. The capture effect must be taken into account for comparison of Wild 2 particles with other chondritic material.