Stardust Microcrater Residue Compositional Groups

Introduction: Impact craters found across the Stardust foils have preserved residue from the cometary grains [1, 2, 3]. It has been successfully demonstrated that stoichiometric mineral compositions have been preserved in larger craters (e.g. >15 μm) including identified mineral grains [4]. Techniques used have been SEM EDS analyses of residue in-situ within craters and TEM EDS analyses of wafers extracted with FIB-SEM techniques [4]. Here we describe a project to extend quantitative mineral analyses to the more abundant smaller craters (e.g. 1-10 μm Dc). This has the potential to provide a representative dataset of quantitative Stardust mineral compositions. Samples and Techniques: Over 260 craters within Stardust Al foils were analysed as part of the Preliminary Examination Team work [1, 2, 3]. A subset of this (105 craters with EDS analyses on foils C2008N, C2051N, C2054N, C2060N) was used in this study. Initial EDS analyses were made by SEM EDS with 15 kV, 0.5 nA beam current and acquisition times of 75 s. Analysing the residue in small craters (e.g. <10 μm Dc) is limited to qualitative results by the blocking effects of the crater walls and small residue volumes. We have used these results to define compositional groupings. Selected craters illustrating these compositional groups were then used for FIB-SEM extraction. Extracting craters and residue from out of the craters offers the chance to remove blocking effects and use EDS systems on SEM and TEM instruments. Most samples were analysed by EDS on SEM and TEM instruments in such a way to help EDS analysis rather than enhance imaging of the residue. An FEI Quanta 200 3D dual Focussed Ion Beam/SEM was used. In order to preserve the <100 nm thick residues within the craters during analyses, Pt was deposited with the electron beam over the craters. Electron beam deposition is performed at low accelerating voltage and so damage to the underlying residue is minimised. Following that a thicker layer (~1 μm) of Pt was deposited over this using ion beam deposition. Wafers (≤ 1μm thick) were cut down through the Pt caps and craters using the FEI AutoFIB software control with parameters adjusted to minimize beam damage. Beam currents of ≤3.0 nA were found to provide a sufficient sputter yield within the Al foil. Wafers were extracted using an Ascend Instruments extraction system with Mo end effectors to hold ≤1 μm thick extracted wafers in the SEM and TEM. EDS analyses on the extracted crater with residue were then performed at 15-20 kV, 0.5-0.6 nA with a variety of acquisition times on the SEM. An INCA analytical system with appropriate mineral standards were used for these analyses. The wafers attached to Mo end effectors were also analysed by TEM (Jeol 200 kV with a sample holder made of Be to minimize the contribution of X-rays from it) using a standardless EDAX routine. TEM EDS with a small working distance allows a relatively high X-ray count rate on some of the smallest volumes of residue analysed. TEM EDS calibration was checked using silicate minerals extracted from polished blocks (e.g. see Results section below). A double tilt sample holder allowed optimum count rates, allowing sample orientations in the sample chamber that avoid the blocking of X-rays by the adjacent Mo end effector. The EDS analyses were used to determine the atomic proportions. This technique has been tested with residue-bearing impact craters prepared by light gas gun shots (prepared by M. Burchell, Kent University and A. Kearsley, NHM) and the full results will be reported in a subsequent publication. Results: FIB-SEM extraction of mineral standards and light gas gun craters. Minerals from an equilibrated L-chondrite were analysed with this technique. Results accurate to 1-2 Mg# units were produced on olivine grains in sections of varying thickness, we regard this as a guide to the potential accuracy of this technique. Residue from pyroxene craters produced by a light gas gun [2] was (MgCaFe)21Si19O60 showing that stoichiometry can be preserved during impact. Further analyses are underway to determine an accurate range of compositions from practice crater residues. Table 1. Frequency of compositional groups in 115 μm Dc craters. Residue Frequency % Fe S 18 17.1 Fe Ni S 2 1.9 Silicate + sulphide 24 22.9 Cr-bearing 1 1.0 Mg Fe silicate 38 36.2 Mg Fe Ca silicate 7 6.7 K, Na silicates (+ sulphide) 3 2.9