Coordinated Micro-raman and Nanosims Analysis of Micron- to Submicron- Sized Presolar Sic Grains from Murchison

Introduction: Transmission electron microscopy (TEM) studies of sub-μm presolar SiC grains showed that 3C-SiC is the dominant polytype (~80%) [1]. Although TEM is the most effective technique to determine the crystal structure and disorder, TEM is destructive and time-consuming, and may not be representative of the bulk sample. On the other hand, Raman spectrometry investigates matter at the scale of chemical bonds, and automatic Raman mapping allows a very representative view of a sample surface. Previous Raman studies, however, mainly focused on SiC grains of usually large size (>3 μm) and did not study Raman peak shifts in detail [2,3] due to limited spatial and spectral resolutions (~1 μm and ~5 cm, respectively). We previously reported use of a confocal imaging Raman microscope with much improved spatial and spectral resolutions (~400 nm and ~0.1 cm, respectively) to investigate Raman spectral characteristics of μm to sub-μm sized presolar SiC grains [4]. However, our initial results were subsequently found to be severely affected by instrument calibration problems. Here, we have quantified the precision of our Raman measurements by bracketing presolar SiC grain measurements with an external Ne lamp. In addition, we report crystal structures and Raman peak shifts of presolar SiC grains that are classified by correlated C, N, and Si isotopic data. Experimental Methods: The SiC grains in this study were extracted from the Murchison meteorite using the isolation method described in [5] and dispersed on two high purity Au mounts (mounts #1 and #3). Presolar SiC grains on mount #3 were first measured with the Carnegie NanoSIMS 50L ion microprobe for C, N, and Si isotopic compositions using a Cs beam and standard methods. Raman spectra were acquired later from the SiC grains with a WiTeC multifunction scanning probe microscope with a spatial resolution of ~400 nm, which includes near-field optical microscopy and confocal imaging Raman microscopy (532 nm frequency-doubled Nd:YAG laser). Spectral images were obtained in 13 ~60×80 μm areas on the mount #3, where each pixel contains a full Raman spectrum with ~0.1Δ cm spectral resolution. For mount #1, Raman spectra were acquired prior to the NanoSIMS analysis in 10 areas. Results: We obtained correlated Raman and isotopic data on 48 SiC grains. One SiC grain that was analyzed with the Raman microscope could not be located during the NanoSIMS analysis and is therefore unidentified. The 49 grains range from 0.5 to 2 μm, so they are larger than the grains (Murchison KJB residue with mean diameter of 0.49 μm) analyzed by TEM [1]. Isotope Data: 35 of the measured grains were isotopically anomalous, indicating a presolar origin, while the remaining 13 grains had normal isotopic compositions, e.g., terrestrial N/N ratio. Two of the anomalous grains were determined to be of type Z, three to be of type AB, two to be of type X [e.g., 6]. As shown in Fig. 1, the C, N and Si isotopic compositions of the 48 presolar SiC grains agree well with the literature data [6], e.g., the 28 mainstream grains fall on a line of δSi = (1.47±0.23)×δSi+(−16±12) in the Si three isotope plot.