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• W129522539 abstract "Introduction: Presolar graphite grains were first isolated from the Murchison (CM2) meteorite by [1]. Analysis of their major and trace element isotopic compositions indicates that they are condensates from AGB-stars, Wolf-Rayet stars and Type II Supernovae [2-4]. To date all studies of presolar graphite have been carried out on grains extracted using acid dissolution techniques [5,6]. Recent evidence has suggested that the use of harsh acids may significantly alter the outer surface of presolar SiC grains extracted in this way [e.g. 7]. Also, as graphite is less chemically resistant than SiC, its separation through acid dissolution is a more complex procedure. We have previously described a technique for “gently separating” presolar SiC grains from host meteorites (i.e. isolate without the use of acids) [8] and have now adapted this procedure in order to isolate presolar graphite, providing us with pristine grains with which to study stellar environments. Samples: The Murchison meteorite, with a presolar graphite abundance of ~5ppm [9], was selected as the source of graphite grains. Approximately 116mg of CAIand chondrule-free matrix material was crushed using a stainless steel mortar and pestle. To further break down the sample it was then subjected to 138 freeze-thaw cycles, whereby it was alternately dipped between liquid nitrogen and an ultrasonic bath containing hot water (~45°C). Electron microscope analysis of a small deposit of the disaggregated sample confirmed grains were typically 20μm, B = 7-20μm, C = 1-7μm and D = <1μm. At the end of each size separation the suspended grains were siphoned from the tube, leaving behind only the sedimented grains. A series of organic heavy liquids were used to separate the size fractions according to density. The density of presolar graphite ranges between 1.6-2.2 gcm [5], so density boundaries of 1.6, 2.05, 2.15 and 2.26 gcm were applied. As presolar graphite grains are typically 1-10μm in diameter [5] only size fractions B and C were density separated. Experimental Procedure: A small aliquot of separation B.4 (7-20μm, 2.15-2.26 gcm) was distributed over a cleaned, ultra-pure gold foil. Prior to this a copper finder-grid had been imprinted onto the foil. Using the grain search feature of the electron microscope we located candidate graphite grains. Their origin as presolar needed to be confirmed by isotope analysis. High resolution images of candidate grains were taken and their locations relative to the finder-grid noted. Sample B.4. was then transferred to a Time-ofFlight Secondary Ion Mass Spectrometer (TOFSIMS) for sub-micron spatial resolution and high mass resolution analyses (m/dm ~3000) of the isotopic and elemental compositions of candidate grains. A grain was analysed by rastering a Au primary ion beam over its surface and detecting positive secondary ions. The field-of-view of the resulting secondary ion image reflected the size of the grain and each image contained 64 x 64 pixels, with each pixel containing a complete mass spectrum. The raw data was collected for offline analysis where it could be corrected for instrumental drift and spectra obtained from only the region of interest. To obtain C isotope ratios a peak deconvolution technique, as described by [10], was used to separate the C-peak from the CH interference. Results: So far only 1 presolar graphite grain (B4J0-1) has been analysed using TOFSIMS, although this is ongoing work and we expect to report results for more grains. Figure 1 shows that the grain is ~10μm x 9μm in size. It is within close proximity to a second grain, which from EDX spectra is likely MgSiO3. The spatial resolution of the primary ion beam allows the two grains to be resolved in the secondary ion images and only spectra from the graphite have been analysed." @default.
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• W129522539 date "2009-03-01" @default.
• W129522539 modified "2023-10-17" @default.
• W129522539 title "First Analysis of Gently Separated Presolar Graphite" @default.
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