Three-dimensional observation and morphological analysis of organic nanoglobules in a carbonaceous chondrite using X-ray micro-tomography

Abstract

Organic nanoglobules are submicrometer spherical, often hollow organic grains ubiquitously distributed throughout primitive solar materials, such as carbonaceous chondrites. Until now, organic nanoglobules have been examined by TEM only after sectioning by ultramicrotomy so it has not been possible to determine whether fluids or mineral grains occur in the hollow cores. H2O-rich fluids might be present in hollows of the nanoglobules if they originate from dust particles composed of organic materials and ice prior to or in an early stage of the solar system formation or fluids incorporated into nanoglobules during aqueous alteration on the asteroidal parent body. In order to determine whether or not any fluids or mineral grains are present in the nanoglobules, a carbonaceous chondrite sample (Tagish Lake C2 meteorite) was observed non-destructively using synchrotron radiation-based X-ray CT (computed tomography), and then microtomed sections were observed using a transmission electron microscope (TEM). We observed three-dimensional shapes of thirty-eight organic nanoglobules in the meteorite sample. Their size and shape distributions are consistent with a hypothesis that nanoglobules originate from icy dust particles. Nanoglobule candidates observed in CT images were confirmed by the TEM images. However, the presence or absence of fluid could not be judged because CT images of nanoglobules are affected by X-ray refraction. Simulation of CT images by considering X-ray refraction shows that the presence or absence of water in nanoglobules cannot be distinguished with CT images alone. However the outer shapes of nanoglobules can be determined quantitatively and nanoglobules containing silicate cores can be easily identified. The thirty-eight nanoglobules we examined did not have silicate cores.