Investigation of iron sulfide impact crater residues: A combined analysis by scanning and transmission electron microscopy |
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| Authors: | Penelope J WOZNIAKIEWICZ Hope A ISHII Anton T KEARSLEY Mark J BURCHELL Philip A BLAND John P BRADLEY Zurong DAI Nick TESLICH Gareth S COLLINS Mike J COLE Sara S RUSSELL |
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| Institution: | 1. Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA;2. Impacts & Astromaterials Research Centre (IARC), Department of Mineralogy, Natural History Museum, London SW7 5BD, UK;3. Centre for Astrophysics and Planetary Sciences, School of Physical Science, University of Kent, Canterbury CT2 7NH, UK;4. IARC, Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW11 2AZ, UK;5. Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia |
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| Abstract: | Abstract– Samples returned from comet 81P/Wild 2 by the Stardust mission provided an unequaled opportunity to compare previously available extraterrestrial samples against those from a known comet. Iron sulfides are a major constituent of cometary grains commonly identified within cometary interplanetary dust particles (IDPs) and Wild 2 samples. Chemical analyses indicate Wild 2 sulfides are fundamentally different from those in IDPs. However, as Wild 2 dust was collected via impact into capture media at approximately 6.1 km s?1, it is unclear whether this is due to variation in preaccretional/parent body processes experienced by these materials or due to heating and alteration during collection. We investigated alteration in pyrrhotite and pentlandite impacted into Stardust flight spare Al foils under encounter conditions by comparing scanning and transmission electron microscope (SEM, TEM) analyses of preimpact and postimpact samples and calculating estimates of various impact parameters. SEM is the primary method of analysis during initial in situ examination of Stardust foils, and therefore, we also sought to evaluate the data obtained by SEM using insights provided by TEM. We find iron sulfides experience heating, melting, separation, and loss of S, and mixing with molten Al. These results are consistent with estimated peak pressures and temperatures experienced (approximately 85 GPa, approximately 2600 K) and relative melting temperatures. Unambiguous identification of preserved iron sulfides may be possible by TEM through the location of Al‐free regions. In most cases, the Ni:Fe ratio is preserved in both SEM and TEM analyses and may therefore also be used to predict original chemistry and estimate mineralogy. |
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