Comprehensive Analyses of Gentle Separated Presolar Sic-grains

Introduction: Presolar grains undergo a long jour­ ney through interstellar space before being incorporat­ ed into primitive meteorites from which they are recov­ ered [1]. During this journey they are exposed to parti­ cle impacts and UV-radiation which might lead to al­ teration, accretion or sputtering effects leading to changes in structure and formation of coatings on the original refractory grain. Presolar grains may therefore preserve samples of the interstellar medium which may be analyzed if the grains can be extracted from mete­ orites without alteration. Previous work has shown that the surfaces, and possibly deeper parts of the grains are altered by the acid extraction process [2]. Bernatowicz et al., [3] showed that ~60% of grains extracted using only an ul­ tra-sonic extraction method have an amorphous coat which isn’t found using acid-extraction methods. We developed an acid-free extraction method using freezethaw dissaggregation followed by size and density sep­ aration of SiC grains [4]. This is known as the gentle separation technique. Samples: For this study, 9 SiC-grains extracted from the Murchison meteorite by the gentle separation technique were analyzed (GS grains) along with 11 (AR) grains from the KJG-fraction from the Murchison meteorite [5] for comparison. Analytical Techniques: We used a time-of-flight secondary ion mass spectrometer [6] to obtain high spatial resolution and high lateral resolution analyses. The primary ion beam was finely focused and rastered over the sample surface to acquire a complete mass spectrum in every pixel of the image. From this raw data, secondary ion distribution images and mass spec­ tra for the region of interest were reconstructed. Atoms are sputtered from the sample so rastering the beam across the sample and acquiring several mea­ surements therefore leads to a depth profile enabling a quasi 3-dimensional study. Results: Morphology. The AR-grains show the typical fea­ tures as seen in previous studies and become very smooth during sputtering. By contrast, some GS-grains showed a very different behavior. Grains GS-D4-G2-1 started as an almost rectangular grain with a quite smooth surface but became very uneven during sputter­ ing and grain GS-D4-Y1-1 changed from a smooth sur­ face to a very pitted surface. Figure 1 SEM images of grain GS-D4-Y1-1 (top) before and after measurements and Grain GS-D4G2-1 (bottom) before and after measurements.