Space Plasma Ion Processing of Idp Sulfides: a Comparison to Silicates Based on In-situ Tem Ion Irradiation Experiments

Introduction: Current interest in the origin of the more primitive components of Interplanetary Dust Particles (IDPs) has focused on the solid-state processing of IDP grains by the energetic ion component of space radiation [1,2,3]. Particular attention has been paid to radiation processing of IDP silicates, since space radiation effects are clearly evident in these phases [2]. The effects range from olivine and pyrox-ene grains with amorphous rims and ion tracks that likely formed while the accreted host particle was exposed to the solar wind [2], to the more enigmatic GEMS (Glass with Embedded Metal and Sulfide), whose primitive origin (either nebular or pre-solar) as pre-accretionary grains is currently under consideration [3,5]. In addition to primitive silicate grains, both GEMS and IDPs in general contain abundant sulfides, most commonly pyrrhotite (Fe 1-x S) [4]. Although likely to be as primitive in many cases as IDP silicates, pyrrhotites either typically show little evidence of space radiation processing, or in a few possible cases appear to have become nanocrystalline as opposed to amorphous [3]. While this observation has been informally tied to assumed radiation resistance of the pyr-rhotite structure based on crystal chemical considerations , there has been little experimental work to quantitatively compare pyrrhotite's response to ion irradiation relative to silicates. Such a calibration could help in modeling scenarios for forming GEMS by radiation processing, as well as for understanding the radiation history of individual pyrrhotite grains of possible pre-solar or nebular origin. We report here the first radiation induced amorphization study of pyrrhotite performed by in-situ means using the Intermediate Voltage Electron Microscope-Tandem Irradiation facility (IVEM-Tandem) at Argonne National Laboratory. The unique capability of this facility for performing real time Transmission Electron Microscope (TEM) observations of samples concurrent with ion irradiation makes it uniquely suited for studying the dose-dependence of amorphization and other changes in irradiated samples. Experimental Approach: The irradiations were performed on a sample of terrestrial Dalnagorsk pyr-rhotite, and on a sample of San Carlos olivine (Fo 90) for comparison. TEM observations indicate the pyr-rhotitee is dominantly of the 4C monoclinc superstructure (approximate composition Fe 7 S 8). Our experiments followed standard IVEM-Tandem methodology in which sub-micron crushed grains supported on a standard holey-carbon TEM grid are irradiated by a ~2 mm-diameter ion beam introduced at 30ْ off-angle to the optic axis of a modified 300 kV Hitachi H9000