Elsevier

Geochimica et Cosmochimica Acta

Volume 116, 1 September 2013, Pages 84-95
Geochimica et Cosmochimica Acta

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

https://doi.org/10.1016/j.gca.2012.05.007Get rights and content

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.

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

References (38)

  • B.T. De Gregorio et al.

    Isotopic anomalies in organic nanoglobules from Comet 81P/Wild 2: comparison to Murchison nanoglobules and isotopic anomalies induced in terrestrial organics by electron irradiation

    Geochim. Cosmochim. Acta.

    (2010)
  • L.A.J. Garvie et al.

    Nanosized carbon-rich grains in carbonaceous chondrite meteorites

    Earth Planet. Sci. Lett.

    (2004)
  • J.P. Bradley

    Chemically anomalous, preaccretionally irradiated grains in interplanetary dust from comets

    Science

    (1994)
  • G.D. Cody et al.

    Establishing a molecular relationship between chondritic and cometary organic solids

    PNAS

    (2011)
  • B.T. De Gregorio et al.

    Isotopically anomalous organic globules from comet 81P/WILD 2

    Lunar Planet. Sci.

    (2009)
  • B.T. De Gregorio et al.

    Isotopic and chemical variation of organic nanoglobules in primitive meteorites

    Lunar Planet. Sci.

    (2010)
  • B.T. De Gregorio et al.

    Isotopic and chemical variations on the nanoscale of distinct lithologies from the Tagish Lake meteorite

    Met. Planet. Sci.

    (2010)
  • E. Dartois et al.

    Combined VLT ISAAC/ISO SWS spectroscopy of two protostellar sources. The importance of minor solid state features

    Astron. Astrophys.

    (2002)
  • L.A.J. Garvie et al.

    Carbonaceous materials in the acid residue from the Orgueil carbonaceous chondrite meteorite

    Met. Planet. Sci.

    (2006)
  • J.M. Greenberg

    Making a comet nucleus

    Astron. Astrophys.

    (1998)
  • Hashiguchi M., Kobayashi S. and Yurimoto H. (2011) Occurrences of Deuterium-rich organic matters in NWA 801 CR2...
  • Henke B. L., Gullikson E. M. and Davis J. C. (1993) X-ray interactions: photoabsorption, scattering, transmission, and...
  • Hubbell J. H. and Seltzer S. M. (1996) Tables of X-ray mass attenuation coefficients and mass energy-absorption...
  • H.A. Ishii et al.

    Nitrogen carrier identified in 15N extreme hotspots in the Isheyevo (CH/CB) meteorite

    Met. Planet. Sci.

    (2010)
  • H.M. Lee et al.

    Infrared extinction and polarization due to partially aligned spheroidal grains: models for the dust toward the BN object

    Astrophys. J.

    (1985)
  • K. Lodders

    Solar system abundances and condensation temperatures of the elements

    Astrophys. J.

    (2003)
  • G. Matrajt et al.

    Study of the C-rich phases of two cometary particles with electron microscopy and nanoSIMS

    Geochim. Cosmochim. Acta

    (2008)
  • G. Matrajt et al.

    Coordinated TEM, isotopic and heating analyses of distinctive carbonaceous phases in IDPs

    Lunar Planet. Sci.

    (2011)
  • S. Messenger et al.

    15N-rich organic globules in a cluster IDP and the bells CM2 chondrite

    Lunar Planet. Sci.

    (2008)
  • Cited by (16)

    • The origin and evolution of organic matter in carbonaceous chondrites and links to their parent bodies

      2018, Primitive Meteorites and Asteroids: Physical, Chemical, and Spectroscopic Observations Paving the Way to Exploration
    • X-ray computed tomography of planetary materials: A primer and review of recent studies

      2017, Chemie der Erde
      Citation 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.

    • Formation of an ultracarbonaceous Antarctic micrometeorite through minimal aqueous alteration in a small porous icy body

      2017, Geochimica et Cosmochimica Acta
      Citation 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.

    • Quantifying the three-dimensional shapes of spheroidal objects in rocks imaged by tomography

      2015, Journal of Structural Geology
      Citation Excerpt :

      Using CT, geologists have now measured many three-dimensional fabrics in rocks (Ketcham, 2005), studied porphyroblast growth (Carlson and Denison, 1992; Ketcham et al., 2005), textures of igneous assemblages (Philpotts et al., 1999; Carlson et al., 1999), connectivity of leucosomes in migmatites (Brown et al., 1999), vesicle formation in basalts, properties of clasts in pumice (Song et al., 2001; Polacci et al., 2006; Gualda and Rivers, 2006; Voltolini et al., 2011), and Ni-Cu-PGE sulfide mineralization (Godel et al., 2010; Barnes et al., 2011). In meteorites, the sizes, shapes, textures and modal abundances of chondrules, calcium–aluminum-rich inclusions, voids, metal and organic phases have also been imaged by CT (e.g., Gnos et al., 2002; Ebel and Rivers, 2007; Matsumoto et al., 2013). Tsuchiyama et al. (2003) separated chondrules mechanically from their host and embedded them in epoxy; classification of voxels into chondrules vs. epoxy is easy and 3-D moments of inertia could be calculated for the simple chondrules, from which ellipsoid axial ratios could then be inferred.

    • In situ observation of D-rich carbonaceous globules embedded in NWA 801 CR2 chondrite

      2013, Geochimica et Cosmochimica Acta
      Citation 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.

    View all citing articles on Scopus
    View full text