Metamorphic Control of Noble Gas Abundances in Pristine Chondrites

Introduction: One of the most enduring enigmas of meteoritics is the nature of the Q (or P1) phase carrier with its associated trapped "planetary Q-gases" (He, Ne, Ar, Kr and Xe), defined by [1]. Many evidences suggest the Q-phase carrier is carbonaceous [e.g. 2], but the location of noble gas atoms in the carbon network has not been yet identified. Among the suggested structural forms of Q-phase carrier are carbynes , various macromolecular carbonaceous substances, turbostratic carbon, graphene sheets, adsorption sites in a labyrinth of pores of amo rphous carbon, closed carbon structures such as fullerenes, carbon nanotubes, or carbon onions (ref. in [3], [4]). Many studies suggested that the abundance of the Q phase in pristine chondrites is controlled by thermal metamorphism [5,6], and that this parameter should be used as a petrologic indicator. But, this general trend was established using petrologic types derived by Induced Thermo -Luminescence, which may be erronated [7], and in any case which do not provide a direct information on the structure of OM where the Qphase is sited. In this study, the structure and the texture of chondritic OM was studied by High Resolution Transmission Electron Microscopy (HRTEM) in Kaba, Leoville, Mokoia, Allende and Tieschitz. HRTEM allows direct imaging of the polyaromatic layers and reveals their multiscale organization. It provides a deaveraged structural information at the nanometric scale, and clues on the microtexture. Structural improvement in polyaromatic matter is mostly controlled by metamorphic conditions (temperature, time), whereas microtexture may witness the chemical nature of the precursor. These parameters have been considered along with previous results obtained by Raman spectrosocopy, which does provide a structural information averaged at the micrometric scale [7,8]. Using noble gas abundances available in literature, we have revisited the question of the metamorphic control of the Q (P1), P3 and P6 components, the carrier of the Q phase and the chemical homogeneity/heterogeneity of the organic precursors accreted by chondrites. Experimental: A Jeol 2010 microscope operating at 200 kV (resolution in fringe mode : 0.14 nm) was used. Image analysis techniques developed by [9] allows to obtain semi-quantitative structural data: the fringe length (i.e. the polyaromatic layer extent) L; the interlayer spacing d, the height and the diameter of the coherent domains, Lc and La, respectively. Results: Fig. 1a, b, c, d are representative images from IOM of Kaba (a), Leoville (b), Mokoia (c) and of the best organized parts of Allende (d). Unfortunatley, the structural parameters vary noticeably from a carbon nanoparticle to an other. Differences between the studied CV3 are evidenced. The least metamorphosed Kaba exhibit a microporous texture. Leoville still exhibit a poorly organized texture, but the length of the aromatic layers is larger. Mokoia (PT~3.6) is definitely more organized, the layer exhibiting different shapes, and is no longer microporous. Allende, the more metamorphosed object (PT > 3.6), exhibit circular shapes, and very organized nanometric area. These results demonstrate that the texture is controlled by thermal metamo rphism. The least metamorphosed objects have a microporous texture, in which the Q(P1) phase is originally located.