Extended X-ray Absorption Fine Structure (EXAFS) in Stardust tracks: Constraining the origin of ferric iron-bearing minerals

Abstract

X-ray Absorption Fine Structure techniques have been used on Comet Wild2/81P tracks from the Stardust mission. Fe-XANES and EXAFS have been performed on aerogel sections from Tracks 41 and 162 as well as the mid and terminal positions of Track 134. This is the first use of EXAFS in the study of early Solar System materials. With EXAFS, we have measured Fe–O and Fe–S bond lengths and thus, together with complementary XANES measurements, identified Fe-rich phases. In particular, we show that ferric-rich phases in 2 Tracks (41, 162) have Fe–O bond 1st shell bond lengths of 1.99–2.01Å and Fe K absorption edge and pre edge centroid positions consistent with being hematite-dominated grains. These iron oxides can be clearly distinguished from a magnetite grain, present in Track 134. We also demonstrate the identification of the Mg-rich end member olivine using EXAFS with XANES in Track 162. The terminal grain of Track 134 is pyrrhotite, its first atomic shell has an Fe–S structure, with 4 nearest neighbouring S atoms at a distance of 2.29±0.05Å.Our XANES results show the presence of Fe3+-bearing grains along the Stardust tracks and suggest either flash-cooling of an Fe–S–SiO–O2 gas during capture or the presence of a Fe–Ni–S–O melt along the cometary tracks during impact capture in the aerogel, rather than the capture process being solely associated with reduction of cometary phases. Accurate determination of Comet Wild2 redox conditions requires the identification of phases, in particular terminal grains, which have not experienced this melting. For instance, the larger hematite-rich grains (>10μm) are more likely to be cometary in origin. EXAFS provides a valuable new analytical technique to study fine-grained early Solar System materials.