Discovery of Presolar Sic from Comet Wild-2

Introduction The preliminary examination of Stardust samples has yielded a number of surprising findings, such as the presence of large refractory mineral grains, including materials resembling CAIs, chondrules, and AOA fragments [1-3]. Furthermore, intact organics and fine grained materials are unexpectedly rare in Stardust tracks [4,5]. The fact that intact fine grained materials are uncommon may be related to the apparent rarity of presolar grains in Stardust samples [1,6]. So far, 3 presolar grains have been found in Stardust samples – all identified by O isotopic imaging of crater residues [6]. Although the mineralogical identity of these presolar grains is uncertain, their estimated abundance (11 ppm) is strikingly lower than silicate stardust grains in primitive meteorites (100 – 300 ppm; 7,8) or interplanetary dust particles (300 – 10,000 ppm; 9-11). Furthermore, O isotopic measurements of all silicate minerals extracted from tracks have fallen within the range of meteoritic materials [1-3]. These observations suggest that either [1] comet Wild-2 materials are less pristine than meteorites and IDPs, [2] fine grained materials and presolar grains were preferentially destroyed during capture, or [3] have been segregated from the typically studied ‘terminal particles’ e.g. in bulbous track walls. Searches for presolar grains have focused on silicate stardust because this is the most abundant presolar grain phase in meteorites and IDPs. Here we report C, N, and Si isotopic measurements of a SiC grain identified within a melt-dominated particle from the wall of track #141 (hereafter Coki). Experimental: Coki is a 2 mm type B track [12] having a very large, bulbous cavity. The track was prepared by flattening a keystone extracted from aerogel block C2061, and embedding the entire flattened track in acrylic resin. Orthogonal strips were then cut from the track and microtomed into < 100 nm thick slices and placed onto Cu TEM grids with 10 nm thick carbon thin-films. The microtomed slices were examined with a 200 kV Tecnai F20 FEG TEM/STEM using standard techniques including brightand darkfield imaging, electron diffraction, high resolution imaging and EDX compositional analysis. Isotopic measurements were performed with the JSC NanoSIMS 50L ion microprobe in two separate sessions. In the first analysis, C and Si isotopic images were taken simultaneously, acquiring images of 12 C , 13 C , 28 Si , 29 Si , 30 Si , and 12 C 28 Si in multidetection with electron multipliers. The sample was subsequently analyzed for C and N isotopes by measuring 12 C , 13 C , 12 C 14 N , 12 C 15 N , 28 Si , and 12 C 28 Si The images were obtained by rastering a <1 pA, <100 nm Cs +