Elsevier

Geochimica et Cosmochimica Acta

Volume 116, 1 September 2013, Pages 5-16
Geochimica et Cosmochimica Acta

Analytical dual-energy microtomography: A new method for obtaining three-dimensional mineral phase images and its application to Hayabusa samples

https://doi.org/10.1016/j.gca.2012.11.036Get rights and content

Abstract

We developed a novel technique called “analytical dual-energy microtomography” that uses the linear attenuation coefficients (LACs) of minerals at two different X-ray energies to nondestructively obtain three-dimensional (3D) images of mineral distribution in materials such as rock specimens. The two energies are above and below the absorption edge energy of an abundant element, which we call the “index element”. The chemical compositions of minerals forming solid solution series can also be measured. The optimal size of a sample is of the order of the inverse of the LAC values at the X-ray energies used. We used synchrotron-based microtomography with an effective spatial resolution of >200 nm to apply this method to small particles (30–180 μm) collected from the surface of asteroid 25143 Itokawa by the Hayabusa mission of the Japan Aerospace Exploration Agency (JAXA). A 3D distribution of the minerals was successively obtained by imaging the samples at X-ray energies of 7 and 8 keV, using Fe as the index element (the K-absorption edge of Fe is 7.11 keV). The optimal sample size in this case is of the order of 50 μm. The chemical compositions of the minerals, including the Fe/Mg ratios of ferromagnesian minerals and the Na/Ca ratios of plagioclase, were measured. This new method is potentially applicable to other small samples such as cosmic dust, lunar regolith, cometary dust (recovered by the Stardust mission of the National Aeronautics and Space Administration [NASA]), and samples from extraterrestrial bodies (those from future sample return missions such as the JAXA Hayabusa2 mission and the NASA OSIRIS-REx mission), although limitations exist for unequilibrated samples. Further, this technique is generally suited for studying materials in multicomponent systems with multiple phases across several research fields.

Introduction

X-ray computed tomography (CT) is a nondestructive method for determining the three-dimensional (3D) internal structure of an object. Quantitative information about the geometry, such as volume and three-axial length (Ikeda et al., 2000), can be obtained from CT images with a known voxel (i.e., pixel in 3D) size. Synchrotron radiation (SR) X-rays with high flux density and high coherence produce CT images with high signal-to-noise (S/N) ratios and high spatial resolution (e.g., Flannery et al., 1987, Bonse and Busch, 1996). If X-ray microscope optics using a Fresnel zone plate (FZP) are used in tomography to magnify X-ray beams, submicron resolution is possible (Uesugi et al., 2006, Takeuchi et al., 2009). Tomography using X-ray absorption can be used to spatially distribute X-ray linear attenuation coefficients (LACs) in a digital image (CT image). Because SR high-flux X-ray beams are easily monochromated, the LAC values in CT images can be quantified. The LAC values can then be used to identify minerals and estimate their rough chemical compositions (Tsuchiyama et al., 2005). SR-based tomography has been applied to earth and planetary materials (e.g., Uesugi et al., 2010, Matsumoto et al., 2013) as well as medical, biological and industrial samples (e.g., Mizutani et al., 2010, Toda et al., 2010). The technique is especially useful for precious samples returned from extraterrestrial bodies by spacecraft, e.g., coma dust samples of the comet Wild-2 from the NASA Stardust mission (Nakamura et al., 2008a, Nakamura et al., 2008b, Rietmeijer et al., 2008, Tsuchiyama et al., 2009, Iida et al., 2010) and regolith particle samples of the asteroid Itokawa from the JAXA Hayabusa mission (Tsuchiyama et al., 2011).

Mineral phases can be estimated using LAC values in absorption-contrast tomography; however, in this method, the LAC values of different minerals are usually superimposed. This limitation can be overcome by imaging the sample twice at two different X-ray energies. By a subtraction method, concentration images of an element can be obtained by taking the difference between CT images measured at X-ray energies slightly below and above the edge absorption of the element (e.g., Thompson et al., 1984, Hirano et al., 1989, Ikeda et al., 2004). Torikoshi et al. (2003) showed that the electron density of materials can be measured using SR-based tomography with dual monochromatic X-rays. However, these methods cannot discriminate between minerals because the concentrations or electron densities of different minerals sometimes overlap each other. An alternative method for obtaining 3D images of mineral phases is tomography using diffraction (Uesugi et al., 2013); however, this method is in its infancy.

In medicine, X-ray tomography using two different energies called “dual-energy tomography”, was originally developed to discriminate between different body tissues and is used as a diagnostic tool (e.g., Alvarez and Macovski, 1976, Flohr et al., 2006). In this method, tomographic imaging is performed at two different polychromatic X-ray energies, and a medical CT scanner is used to discriminate between different tissues with or without contrast dye. Because polychromic X-rays are used, it is difficult to obtain quantitative information about LACs, making the method unsuitable for discriminating between minerals.

In this study, we developed a new method for identifying mineral phases and obtaining images of phases, one that uses SR-based tomography with LAC values at dual monochromatic X-ray energies chosen on the basis of the absorption edge of a specific element. If we select Fe as the specific element, most of the minerals in ordinary chondrites can be discriminated. This method also allows us to determine the chemical compositions of minerals forming solid solution series. Our method, “analytical dual-energy microtomography,” has been applied to samples returned by the Hayabusa spacecraft from asteroid 25143 Itokawa (Nakamura et al., 2011, Tsuchiyama et al., 2011).

Section snippets

Basic concept of analytical dual-energy microtomography and its application to chondritic materials

The LAC value, μ, of an object is expressed as (Koch and MacGillavry, 1962)μ=ρiτi(E)wiwhere ρ is the density, τi(E) is the mass attenuation coefficient (MAC) of element i, which is a function of the X-ray energy, E, and wi is the weight fraction of element i. Thus, the LAC value of a material can be calculated as a function of E from its chemical composition and density, because we know the MAC values of the elements in a material as a function of E (e.g., Hubbel and Seltzer, 1996).

The major

Samples

Forty particles collected from asteroid Itokawa were imaged using the analytical dual-energy microtomography during the Hayabusa preliminary examination campaign (Nakamura et al., 2011, Tsuchiyama et al., 2011). The sample sizes of the allocated grains (30–180 μm) were appropriate for our method using Fe as the index element. Scientific results on these grains are reported in Tsuchiyama et al., 2011, Tsuchiyama et al., submitted for publication.

Tomography experiments

We performed absorption-contrast imaging tomography

Comparison with subtraction method

Analytical dual-energy microtomography allows the production of 3D phase images if the phases in the sample are previously known or estimated, and the LAC values of the phases are not overlapped in the 2D histogram. The spatial resolution of the mineral phase images is determined mainly by the resolutions of the 3D registration of CT image pairs and artifacts in CT images such as refraction contrast. The chemical compositions of minerals forming solid solutions can be estimated, and chemical

Acknowledgments

The tomography experiment was performed under the approval of the SPring-8 Proposal Review Committee (2010B1531 and 2011A1388). We thank the Hayabusa sample curation team and the Hayabusa project team. We also thank Dr. A. Gucsik of Max Planck Institute for Chemistry, Mainz, Germany for reading the manuscript. We are grateful to Drs. D. Hezel, J.M. Friedrich and G. Flynn for improving the manuscript in the review step. Drs. D. Hezel and J.M. Friedrich also kindly checked the English of the

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