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International audience; 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

International audience; Introduction: Ultracarbonaceous Antarctic Micrometeorites (UCAMMs) are dominated by N-rich polyaromatic organic matter exhibiting large D enrichments [1-5]. The high abundance of organic matter exhibiting large D enrichment suggests a cometary origin for UCAMMs [1-3]. Three kinds of organic matter are identified in UCAMMs, with different nitrogen abundances and highly variable concentrations of small (typically 30-500 nanometer) mineral aggregates embedded in the organic matter [5-8]. We focus here on a mineral assemblage embedded in one UCAMM that exhibits evidence of ion irradiation, to get insight on the formation and evolution of this cometary particle. Sample and methods: A fragment of UCAMM DC06-06-43 (hereafter DC06-43) was carbon coated, observed by SEM/EDX, and a 100 nm thick FIB section of this fragment was made at IEMN Lille. The size of the initial UCAMM before fragmentation was ~ 25 x 30 µm. After STXM-XANES analysis, the mineralogy of the sample was investigated by TEM at UMET Lille using a FEI Tecnai G2 20 at 200 kV and FEI TITAN Themis at 300 kV [9]. Results : TEM examination reveals a large assemblage of crystalline minerals at the center of the DC06-43 UCAMM fragment, surrounded by organic matter (Figure 1) [9]. The assemblage consists of µm-to subµm sized Mg-rich pyroxenes, a large "triskell"-shaped Fe-sulfide, a few Mg-rich olivines, and minor Si-Al-Carich amorphous pockets and Fe-Ni metal. About 28 crystalline pyroxenes are identified whereas only 5 olivine crystals are observed in the section. GEMS are present embedded in the organic matter, close to the crystalline assemblage. We identified irradiation features (rims and tracks) in pyroxene grains. No rims or tracks were found in olivine. We observed irradiated rims around six pyroxenes at the top of the section (Figure 1, light blue labels). One pyroxene shows a continuous irradiated rim (Figure 2 top). Rim thicknesses range from 20 to 100 nm, with an average of 60 ± 20 nm (1σ) (Figure 2 bottom). Fe-rich deposits are occasionally found on top of irradiated rims. EDX mappings and profiles show that the rims are strongly depleted in Mg (Figure 3). The average track density in pyroxene grains measured over an area of 4.5 x 10-8 cm 2 is 1.3 x 10 10 cm-2 with one value at 3.8 x 10 9 cm-2 , the other values ranging from 9.5 x 10 9 to 3.2 x 10 10 cm-2 (Figure 4). Irradiation track lengths