TY  - STD
AU  - B Guérin, C. Engrand
PY  - 2020//
TI  - STEM and STXM-XANES analysis of FIB sections of Ultracarbonaceous Antarctic Micrometeorites (UCAMMs)
N2  - Introduction: Ultracarbonaceous Antarctic Micrometeorites (UCAMMS) are extraterrestrial dust particles containing large amount of carbonaceous material with elevated D/H ratios [1] and high N/C atomic ratio (up to 0.2)[2]. UCAMMS are rare (~ 1% of the particles in the Concordia meteorite collection) but they have been identified in several collection of interplanetary dust [3, 4]. They are most probably of cometary origin. Here, we studied the association of organic matter and minerals by scanning transmission X-ray microscopy (STXM-XANES) coupled with scanning transmission electronic microscopy (TEM/STEM). Samples and Methods: The UCAMMs studied here were collected in the Antarctic snow, close to the Concordia station at Dome C [3]. FIB sections of 8 UCAMMs (DC06-18, DC06-41, DC06-43, DC06-65, DC06-308, DC06-139, DC16-30, DC16-309) were analyzed using synchrotron based STXM-XANES at the carbon, nitrogen and oxygen K-edges. The FIB sections were subsequently analyzed with transmission electron microscopy (TEM/STEM) using a FEI Tecnai G2 20 and a FEI TITAN Themis 300 [5, 6]. Peak identification of STXM-XANES spectra are based on [7]. XANES spectra are processed and quantified using Quantorxs method [8] and quantification of STEM EDS spectra has been realized using Hyperspy software [9]. Here, we mainly present results obtained on two recently identified UCAMMs (DC06-308 and DC16-309) and compare them with previous observations [1, 5, 6]. Results: The STXM-XANES analysis reveals 3 types of organic matter (OM) characterized by different carbon speciation. Figure 1 shows type I OM in blue and type II OM in green, both having spectra close to that of chondritic insoluble organic matter (IOM). The main peaks of type I and II OMs are found around 284.8 eV (aromatic and olefinic groups (C=C)), 286.4 eV (ketone and phenol C=O) and 288.4 eV (carboxyl O=C-O). Type II OM exhibits similar functional groups as type I OM but the first peak position is shifted to 285 eV, indicating a stronger contribution of the aromatic groups. The atomic N/C ratio of types I/II OMs range between 0.01 and 0.05 (1σ=0.02) similar to those of chondritic IOM. The type III, in red on Figure 1 exhibits larger differences. The main peak is at 286.4 eV (C≡N nitrile), a small peak at 284.8 eV (alkene
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ID  - BGuerin2020
ER  -