Immature Carbonaceous Matter in Concordia Antarctic Micrometeorites

Introduction: Antarctic MicroMeteorites (AMMs) represent the dominant source of extraterrestrial matter accreted by Earth today [1]. Some of them are ultracarbonaceous particles (UCAMMs) [2, 3, 4]. Furthermore, AMMs could have played a role in the origin of life on Earth [e.g. 5]. A better knowledge of their constituent carbonaceous matter could thus shed light on the formation of the organic matter in the early solar system as well as on the origin of life on Earth. We have analysed twenty Antarctic micrometeorites (AMMs) from the 2006 Concordia Collection [6] by Raman micro-spectroscopy, an analytical technique sensitive to the degree of structural order of the carbonaceous matter, and which can identify functional groups like cyanide [7, 8, 9]. Samples and method: AMMs were selected in order to present a gradual thermal alteration developed during hypervelocity deceleration at atmospheric entry, ranging from unmelted AMMs to partially or completely melted ones [10]. A goal of this study was to check whether the carbonaceous matter was transformed during atmospheric entry heating. We have analyzed 8 fine-grained (Fg) AMMs (including 3 ultracarbonaceous–UCAMMs), 5 intermediate (Fg-Sc), 5 scoriaceous (Sc) and 2 cosmic spherules (CS) [10] (Fig. 1). Raman spectra were obtained with two Labram micro-spectrometers, employing 514 and 244 nm excitation. Only two UCAMMs were analyzed with both excitation wavelengths, as the 244 nm excitation requires high carbon concentration within the sample. The laser power at the sample's surface was set as low as 300 μW in order to avoid thermal alteration. No effect of sample damaging by laser irradiation as a function of acquisition time was observed. The microscope optics provided a spot diameter estimated at 23 μm. The experiments were performed in an inert atmosphere (argon) to reduce and stabilize the parameter variations [7]. The most intense bands in a typical Raman spectrum of carbonaceous material are the first order carbon bands, at ~1600 cm (G-band) and ~1350 cm (D-band) [11]. Two analytical procedures were used. The first consisted in fitting the D and G bands with Lorentzian and Breit-Wigner-Fano (LBWF) profiles, respectively. The second was a Principal Component Analysis (PCA), which accounted for the whole spectral variations. Bulk meteorites and insoluble organic matters (IOM) extracted from pristine carbonaceous chondrites were analysed in the same conditions and used as standards for comparison. Results and discussion: All AMM spectra exhibit Dand Gbands, with the exception of cosmic spherules. About half of the spectra show a high fluorescence background and were therefore not taken into account. No significant degree of structural order among all textural types (Fg, UCAMMs, Fg-Sc and Sc) are observed after applying LBWF fitting and Principal Component Analysis.