Oxide Minerals in Comet Wild 2: Tem and Synchrotron Characterisation

Introduction: We have used a variety of techniques – TEM, synchrotron – as part of the UK Stardust consortium in order to identify and constrain the origin of oxides and thus demonstrate some affinities between Wild 2 and other planetary materials such as chondrite groups or IDPs. We are finding that oxidised phases in Wild 2 have a variety of origins. Studying Fe-bearing oxide phases in the Stardust mission Wild 2 samples potentially allows the identification of high temperature phases (e.g. Fe-Cr oxide chromite), space weathering products (nanophase Fe), products of parent body hydrothermal alteration (magnetite), heating and oxidation effects during capture in the aerogel. Techniques and Samples: We have studied a transverse slice from the track 41 keystone (C2044,0,41,0,0), the whole track 134 (C2012,10,134,0,0) and terminal grains from track 121 (C2005,2,121,1,0 and C2005,2,121,2,0). FIB-SEM extraction of Fe oxide-bearing wafers and TEM analyses on track 121 were performed at the University of Leicester using the technique described in [1,2]. Bright field imaging, STEM EDS and Selected Area Electron Diffraction (SAED) were performed on the particle and surrounding gold foil mount. Microfocus XRS, mapping and XANES spectroscopy were performed at Beamline I18 of the Diamond Light Source, Oxfordshire on the track 41 slice, track 134 and mineral standards [1]. This beamline operates from a 3 GeV synchrotron with typical currents of 200 mA. A Si (1 1 1) and (3 1 1) double crystal monochromator was used for energy selection with resolutions of 10 and 10 respectively. A 9 element Ge based solid state detector was used which is capable of measuring the X-rays of Ca upwards. Track 41 and track 121 were analysed by microRaman at the University of Kent (C2005,2,121,2,0) [3] and Open University (C2005,2,121,1,0) and that data together with the results of Kent light gas gun experiments designed to ascertain the effects of capture heating are presented in [1]. Track 121 is a ‘carrot’ shaped, 0.9 mm length track, tracks 134 and 41 are type b ‘turnip’ of length 0.38 and 4 mm. The track 41 slice was cut out 0.8 mm from the track entrance. Results & Discussion: Fe Kα X-ray maps taken along track 134 (Fig. 1) show that there is a concentration of Fe-bearing phases towards the terminal end. There are also 2 FeNi grains in the mid track. Fe XANES of the terminal grain of this track (Fig. 2) shows a very good match to an Fe sulphide standard (pyrrhotite). However the mid track FeNi grains show absorption edges e.g. around 7140 eV which suggest that some oxidation of the metal has occurred during capture. The Track 41 slice contains a range of different minerals and oxidation states (Fig. 3a,b). XRF showed Fe hotspots, Fe-Ni compounds, Fe-Ti (oxide – e.g. ilmenite), Cr-Fe-Mn-Ti-V (oxide-chromite), an unidentified Fe-Zn compound. Fe-XANES analyses show that Fe hotspots have a marked absorption peak at 7110 – 7111 eV, before the main K edge, a peak associated with ferric oxide. Similar effects have been reported elsewhere for ferric oxide-bearing phases [4]. The Fe hotspot patterns are very close to that of the magnetite (absorption peak at 7110 eV) and hematite standards (absorption peak at 7111 eV) and is consistent with a mixture of these 2 phases, a result initially suggested by microRaman analyses [1, 3]. The Fe-Ni grain analysed shows a marked absorbance feature around 7160 eV which distinguishes it from the hematite-magnetite mixes and is consistent with Fe metal [5]. However our sample also has an absorption peak around 7111 eV, which is consistent with a mixture of FeNi metal and ferric oxide. The Fe-Zn compound also shows this absorbance feature showing that it too has been oxidised to some extent, presumably during capture.