Transmission Electron Microscopy of Ultracarbonaceous Antarctic Micrometeorites of Possible Cometary Origin

Introduction: UltraCarbonaceous Antarctic Micrometeorites (UCAMM) are a new type of fluffy, fragile particles, exceptionally rich in carbon. The samples were collected at Dome C near CONCORDIA station, in central Antarctica [1]. These particles are dominated by a very large amount of carbonaceous material, with concentrations up to ~ 50 wt%, even larger than that of anhydrous IDPs [2]. They are similar to those described by Nakamura et al. 2005 [3]. Large deuterium enrichments up to 30 times the solar value have been reported in UCAMMs [4], and their high carbon contents could relate them to the CHON particles detected in comet Halley [5]. In this work we performed transmission electron microscopy (TEM) analysis on two UCAMMs to study the associated silicates and sulfide mineralogy of this primitive carbonaceous material in order to shed light on the formation and evolution of these particles. Samples and analytical procedure: The samples are first classified according to their external aspect and their carbon content by analytical scanning electron microscopy (SEM and associated EDX). Chemical compositions were measured by electron microprobe at CAMPARIS (Univ. Paris VI Jussieu) at 15 kV and 10 nA. We have studied two ultracarbonaceous Antarctic Micrometeorites (UCAMMs) (DC06-09-119 and DC06-09-45) by analytical and high resolution transmission electron microscopy. Sample DC06-09-45 was mounted in epoxy (EMBed-812) and then sliced by ultramicrotomy. Thin sections were analyzed by TEM using a Tecnai G2-20 (LaB6 filament, 200 kV) equipped with EDX. Electron energy-loss spectroscopy (EELS) spectra have been acquired around the absorption edge of carbon (284 eV). A second UCAMM sample (DC06-09-119) was crushed between two glass slides. The powder was transferred onto C-lacey TEM support grids (Cu) for high resolution TEM characterization using a Jeol 2011 (200 kV). Results and discussion: From a total of 31 finegrained fluffy (FgF) AMMs, we have identified by SEM-EDX analysis five ultracarbonaceous particles that contain up to 50 wt% carbonaceous matter. The particles exhibit a fluffy texture with a high porosity. Their average size ranges from 40 to 275 μm. HRTEM investigations show that the carbonaceous matter is highly disorganized but non stricto sensu amorphous since this carbonaceous matter is made of nanometre sized misoriented graphene layers (Fig. 1). This observation is confirmed by EELS results and by Raman spectroscopy investigation [6]. Graphitic nanodomains with total sizes of ~1 000 nm, have been observed included in the amorphous-like carbonaceous material (~15 μm), but their abundance is insignificant with regard to the disorganized carbonaceous material.