Oxidation state of iron and extensive redistribution of sulfur in thermally modified Stardust particles Hugues Leroux * , Mathieu Roskosz, Damien Jacob Laboratoire de Structure et Proprie ´te ´s de l’Etat Solide, Universite ´ des Sciences et Technologies de Lille and CNRS, UMR 8008, F-59655 Villeneuve d’Ascq, France Received 4 June 2008; accepted in revised form 21 September 2008; available online 3 January 2009 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 mineral- ogy and associated redox state of iron is assessed using a Fe–Mg–S ternary diagram on which ferromagnesian silicates, sul- fides 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 sam- ples, further supported by the very low concentration of iron in the glassy matrix. Silicate glasses contain a significant con- centration 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 signif- icant part of the Fe-droplets formed in situ by reduction at high temperature of ferromagnesian silicates (olivine and pyrox- enes) 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. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION In January 2006 the Stardust mission brought to Earth samples from comet 81P/Wild 2 (Brownlee et al., 2006). The collection was performed at an encounter speed of 6.1 km/s into silica aerogel, an underdense (0.01–0.05 g/cm 3 ) capture medium. The hypervelocity impacts created deceleration tracks in the aerogel (Ho ¨rz et al., 2006). Using synchrotron-based X-ray microprobes (SXRM), Flynn et al. (2006) showed that a major fraction of the comet material is unevenly distributed along most tracks, suggest- ing that cometary materials mechanically disaggregated during the impact into aerogel (Zolensky et al., 2006; Ho ¨rz et al., 2006). This distribution suggests the impact of loosely bonded aggregates of relatively large minerals (over 1 lm) and fine grains (Zolensky et al., 2006). The large particles are crystalline and, so far, were found mainly at the ends of tracks. In contrast, the walls contain abundant fine, sub- micron scale material. A number of Stardust samples, in particular those extracted from track walls, show evidences for strong thermally induced modifications in addition to strong intermixing with melted aerogel (Zolensky et al., 2006; Leroux et al., 2008; Rietmeijer et al., 2008). This sit- uation significantly complicates the analyses and the under- standing of these samples. 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.09.037 * Corresponding author. E-mail address: Hugues.Leroux@univ-lille1.fr (H. Leroux). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 767–777