Three-dimensional observation and morphological analysis of organic nanoglobules in a carbonaceous chondrite using X-ray micro-tomography
Introduction
Insoluble organic materials called organic nanoglobules are ubiquitously distributed throughout primitive extraterrestrial materials such as Tagish Lake ungrouped C2 carbonaceous chondrite (e.g., Nakamura et al., 2002, Garvie and Buseck, 2004, Nakamura-Messenger et al., 2006, Nittler et al., 2009, De Gregorio et al., 2010a, De Gregorio et al., 2010b), CI carbonaceous chondrites (Garvie and Buseck, 2006, Nittler et al., 2009, De Gregorio et al., 2010a), CM carbonaceous chondrites (Garvie and Buseck, 2004, Messenger et al., 2008, Nittler et al., 2009, De Gregorio et al., 2010a), CR chondrites (Nittler et al., 2009, De Gregorio et al., 2010a, Hashiguchi et al., 2011), Isheyevo CH/CB meteorite (Ishii et al., 2010), type 3.0 ordinary chondrites (Cody et al., 2011), stratospheric interplanetary dust particles (IDPs; Messenger et al., 2008, Matrajt et al., 2011), and particles from comet 81P/Wild 2 (Matrajt et al., 2008, De Gregorio and Nittler, 2009, De Gregorio et al., 2010). They are approximately spherical from the observation of ultra-thin sections, but their 3-D shapes have not been exactly revealed. Their sizes range from several hundred to several thousand nanometers (nm), and in many cases they have single hollow spaces in their centers. It was suggested that the nanoglobules were formed by asteroidal aqueous alteration on their parent bodies (Nakamura et al., 2002, Cody G. D. and Alexander C. M. O., 2011). However, they are enriched in 15N/14N and D/H compared with terrestrial materials and the parent body materials. These isotopic anomalies indicate that the organic nanoglobules were formed from ice gains by photochemical reaction in cold molecular clouds or low temperature regions of protoplanetary disk at 10–50 K (Nakamura-Messenger et al., 2006). In this case, the ice grains that consisted of water and simple organic materials were irradiated by ultraviolet or cosmic ray and complex refractory organic mantles were formed on the surfaces of the particles. Interstellar grains in a molecular cloud are theoretically considered to have a layer structure that is composed of a silicate core, an inner organic matter mantle and an outer ice mantle (represented by Greenberg, 1998), which are very similar in size, chemical composition and texture with nanoglobules observed in astromaterials except the silicate core. In astromaterials, organic nanoglobules with silicate cores are quite rare, there have been only three reports of such material (Nakamura-Messenger et al., 2006, Hashiguchi et al., 2011, Nakamura-Messenger. et al., 2012). One way to explain the lack of silicate core was that the silicate core could have been removed during the sample preparation such as physically plucked away during ultramicrotomy (e.g., Nakamura-Messenger et al., 2006 for details) or petrological thin section polishing (Hashiguchi et al., 2011) or chemically digested to isolate insoluble organic matters from rest of the meteoritic material (e.g., De Gregorio et al., 2010a).
Alternatively the core of the interstellar icy dust was not refractory silicate, but a more volatile material, which could evaporate away leaving less volatile mantle material as hollow nanoglobules, or the nanoglobules formed from organics-ice particles, the central hollow regions of the nanoglobules should have originally been filled with water or organic ice, which might be preserved as fluids in the hollow regions. Fluids may also have been incorporated into nanoglobules during aqueous alteration. However, fluids in the nanoglobules have not been detected so far because all of above mentioned destructive sample preparation prior to the observations. If fluids were originally preserved in the central hollow regions of the nanoglobules, they would have been lost during these destructive processes during sample separation for transmission electron microscope observation.
X-ray computed tomography (CT) is a non-destructive method to obtain three-dimensional (3-D) structures of materials. Absorption-contrast CT using X-ray transmittance of objects gives CT images, which show spatial distribution of X-ray linear absorption coefficient (LAC) of objects. Synchrotron radiation (SR)-based imaging tomography using X-ray microscope optics with a Fresnel zone plate (FZP) has been developed (e.g., Uesugi et al., 2006, Takeuchi et al., 2009). The system achieves a few hundred nanometer spatial resolution around 8 keV, and was applied to elucidate micro-textures of extraterrestrial materials, such as cometary particles collected by the Stardust mission (Zolensky et al., 2006, Nakamura et al., 2008a, Nakamura et al., 2008b, Rietmeijer et al., 2008) and particles on asteroid surface collected by the Hayabusa mission (Tsuchiyama et al., 2011). We have applied this method to organic nanoglobules in a carbonaceous chondrite. The advantages of this method for observation of nanoglobules are follows: (1) 3-D internal structure of nanoglobules can be examined non-destructively in situ, (2) nanoglobules can be observed by its high spatial resolution, and (3) materials in nanoglobules can be estimated from a quantitative LAC values in CT images using monochromatic X-rays (Tsuchiyama et al., 2005).
The purpose of the study is to identify nanoglobules in CT images, reveal their 3-D morphologies and 3-D distributions in a carbonaceous chondrite for the first time, and determine whether or not fluids or mineral grains are present in their interior.
Section snippets
Experiments
Tagish Lake meteorite, an ungrouped carbonaceous chondrite of C2 type (Zolensky et al., 2002), was used in this study. A particle of ∼30 × 40 × 60 μm in size from the matrix area was held on a glass fiber of 5 μm in diameter with a small amount of glue (glycol phthalate). This particle was observed using the imaging tomographic system at BL47XU in SPring-8, a synchrotron facility in Japan (Uesugi et al., 2006, Takeuchi et al., 2009). The voxel size in CT images was 40.8 nm × 40.8 nm × 40.8 nm. Effective
Simulations of CT images by considering X-ray refraction
Refraction of X-rays affected CT images of the sample (Suzuki et al., 2002) as described later (Fig. 5). In order to constrain the compositions of the nanoglobules and to determine whether or not the nanoglobules contain fluid, CT images of nanoglobules should be examined by considering the effect of X-ray refraction. Therefore, X-ray traces passing through nanoglobules were calculated by considering X-ray refraction, and CT images were reconstructed. These simulated CT images were compared
Observation of CT images
A set of CT images of the Tagish Lake meteorite sample is shown in Fig. 2. This sample is mainly composed of matrix that consists of phyllosilicates such as saponite and serpentine. These two minerals are seen as gray objects in the CT images and cannot be distinguished from each other based on their contrast in the CT images because their LAC values are similar. Iron sulfide, magnetite grains, olivine and pyroxene are embedded in the matrix as white objects. A glass fiber to hold the sample is
Three-dimensional structure and distribution of nanoglobules
Thirty-eight nanoglobules with diameters of larger than about 400 nm in the Tagish Lake meteorite sample were clearly identified from CT images. CT images of nanoglobules with diameter under 400 nm are noisy because of ring artifacts. The external shapes of these nanoglobules were extracted by binarization using the threshold of LAC = 0. A histogram of diameters equivalent to spheres with the same volume is shown in Fig. 11. The size of the nanoglobules is less than 0.9 μm except for one large grain
Acknowledgements
We thank Jim Brook, Alan Hildebrand, Peter Brown and Charley Roots for the pristine recovery of the Tagish Lake meteorite, and for permitting its study. We also thank Dr. Scott Messenger for discussion. This paper benefitted from helpful comments by Dr. Yabuta, an anonymous reviewer and associate-editor Dr. Christian Koeberl. The tomography experiment was performed under the approval of the SPring-8 Proposal Review Committee (2009A1605). A.T. was supported by a Grant-in-aid of the Japan
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2017, Chemie der ErdeCitation Excerpt :To achieve this, an X-ray magnification optics system utilizing a Fresnel zone plate (FZP) was installed on a beamline at the SPring-8 facility (Uesugi et al., 2006). This system has been successfully used to image a variety of ultra-small particles at nanometer resolution (Section 6), including Hayabusa (Ebihara et al., 2015; Nakamura et al., 2011; Tsuchiyama et al., 2011, 2013) and Stardust (Nakamura et al., 2008a, 2008b) particles as well as organic nanoglobules from the C2 chondrite Tagish Lake (Matsumoto et al., 2013). Nakamura et al. (2008a, 2008b) achieved up to 43 nm resolution using the FZP setup to image several Stardust particles at 8 keV and examine the structure of both crystalline and amorphous particles.
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2017, Geochimica et Cosmochimica ActaCitation Excerpt :The two regions are connected at the bottom-left corner of the FIB section (Fig. 5e), indicating that these regions were made of the same organic material as shown in the similar C- and N-XANES spectra (Fig. 7c and d). The globules in region 2 appear similar in size (a few hundred nm) to the organic nanoglobules ubiquitously observed in chondritic meteorites (e.g. Nakamura et al., 2002; Garvie and Buseck, 2004; Nakamura-Messenger et al., 2006; Peeters et al., 2012; De Gregorio et al., 2013; Matsumoto et al., 2013), micrometeorites (Sakamoto et al., 2010), IDPs (Busemann et al., 2009), and comet Wild 2 dust particles (De Gregorio et al., 2010, 2011). However, the organic nanoglobules in UCAMM D05IB80, forming aggregates, have more irregular shapes compared to the rounded globules in most carbonaceous chondrites.
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2013, Geochimica et Cosmochimica ActaCitation Excerpt :Some isotopically anomalous organic materials do contain other grains; however, nanoglobules in the Tagish Lake or Bells meteorites typically have hollow interiors (Garvie and Buseck, 2004, 2006; Nakamura-Messenger et al., 2006; Messenger et al., 2008). A recent three-dimensional observation of Tagish Lake nanoglobules suggests that their hollow characteristics would not result from sample preparation for transmission electron microscopy (TEM) analysis (Matsumoto et al., 2012). Therefore, the observation of silicate or oxide grains attached to D-enriched organic matter (Figs. 4a–d, and 7) is the first observation of this type and suggests a unique nature of NWA 801.