Three-dimensional observation of carbonaceous chondrites by synchrotron radiation X-ray CT – Quantitative analysis and developments for the future sample return missions
Introduction
X-ray CT is an important tool in the study of meteorites, as well as terrestrial material (e.g. Okumura et al., 2008), because it provides fully digitized three-dimensional data of meteorites non-destructively (Okazawa et al., 2002, Tsuchiyama et al., 2002, Ebel and Rivers, 2007, Ebel et al., 2007, Friedrich, 2008, Friedrich et al., 2008, Hezel et al., 2010, Alwmark et al., 2011). Because of this feature, the X-ray CT also plays an important role in the initial analysis of sample return missions such as Stardust and Hayabusa (Nakamura et al., 2008, Tsuchiyama et al., 2011). Tsuchiyama et al. (2011) demonstrated that the X-ray CT could provide the three-dimensional information of internal structure and materials of the samples required for all analyses in the initial analysis of Hayabusa. It is also possible to do a quantitative analysis of compositions of materials in meteorites by analyzing X-ray linear attenuation coefficient (LAC, μ), which appears as the contrast in CT images when using monochromatic X-rays (Tsuchiyama et al., 2005, Uesugi et al., 2010). The LAC shows attenuation rate of the X-rays intensity through a material. We can obtain the LAC by the following equation,where I0 is the initial intensity of the X-ray beam, I is the intensity at the detector and S is the sample length along the light path.
We can also obtain the LAC from chemical component, density of the material and mass attenuation coefficient. The mass attenuation coefficient (MAC) is theoretically calculated by photoelectric absorption and Compton and elastic scattering of X-rays by elements (Hubbel and Seltzer, 1996). We can then deduce the compositions of the materials by comparing the calculated and observed LAC with monochromatic X-ray. A previous study has shown that the Fe content of olivine crystal can be obtained by LAC based analysis, and is comparable in accuracy to SEM-EDS analysis if the mineral phase in the texture is already known (Uesugi et al., 2010).
However, quantitative analysis based on X-ray CT has previously rarely been used within the Earth and Planetary science field. One of the reasons for this is the difficulty to precisely determine internal minerals. This is because the range of LAC of the minerals in meteorites overlap with each other, thus we cannot uniquely determine the mineral phases only from the LAC (Table 1). Tsuchiyama et al. (2013) applied two X-ray energies, 7 and 8 keV, for the observations of Hayabusa samples. Because these energies are just above and below Fe K-edge, the relation of the LAC of materials is largely different between the energies. So they could precisely determine the chemical composition of minerals. The method, called “analytical-dual energy microtomography”, is a very powerful method for the quantitative analysis of small particles, such as Antarctic MicroMeteorites (AMMs) and interplanetary dust particles (IDPs). However, possible sample size with this method is less than 100 μm due to the very low efficiency of the penetration of X-ray through the samples with such low energies (Tsuchiyama et al., 2013). Therefore we cannot apply the method for the observation of chondrules and CAIs in primitive chondrites, as these are generally larger than a few 100 μm.
Another problem with the use of X-ray CT is that the analysis method for three-dimensional data is not commonly developed, especially in the case of quantitative analysis. Image processing tools for the three-dimensional data have been developed in several previous studies (Nakano et al., 2006, Hezel et al., 2011), but the “image processing method” has not been well developed. Because of above mentioned difficulties, X-ray CT has only been used for the structural observation of meteorites in most of the previous studies, and has not been applied to the textural observation of silicate materials. Iron inclusions or voids have been investigated in previous studies (e.g. Ebel et al., 2007, Friedrich et al., 2008), because their LAC value is largely different from the silicate materials, and are easy to analyze.
The purpose of this paper is to investigate the chemical composition and size distribution of internal materials in carbonaceous chondrites using LAC and to draw conclusions on formation processes based on the results. In Uesugi et al. (2010), the validity of the LAC based analysis was examined, and the accuracy was confirmed by SEM observation and comparison with data obtained previous studies. In this paper, we further develop the LAC based analysis. Relation between the chemical composition of the chondrules and matrix, and size of inclusions were obtained from the histogram analysis and image processing. We discuss the possible nebular environment for the formation processes of chondrites and their components based on the results.
We also investigate the possibility of a method to non-destructively classify the carbonaceous chondrites by SR-CT based on the results. Several sample return missions are planning to go to parent bodies of carbonaceous chondrites. For example, the target asteroid of Hayabusa2, a next generation mission of Hayabusa which succeeded to sample from a parent body of LL chondrite (Nakamura et al., 2011, Yurimoto et al., 2011, Ebihara et al., 2011, Noguchi et al., 2011, Tsuchiyama et al., 2011; Nagao et al., 2011), is the C-type asteroid 1999JU3. Osiris-Rex, a sample return mission of NASA, targets the asteroid 1999RQ36, which is also considered to be one of the parent bodies of carbonaceous chondrites. Marco Polo-R is also a sample return mission that has been proposed for the ESA’s Cosmic Vision, and aims to sample a primitive NEA. Our studies will provide an important tool for the analysis of returned samples of these projects.
Section snippets
Experimental settings
Optimized environments for the CT observation of mm-sized samples of various chondrite classes were obtained by Uesugi et al. (2010). In this study, we applied the environment for the observation of carbonaceous chondrites. Experiments were carried out at BL20B2, one of the bending magnet beamlines in the third generation synchrotron radiation facility SPring-8, Japan. The X-rays are monochromatized by a Si (1 1 1) double-crystal monochromator. Transmission and direct X-ray images were recorded
Slice images and histograms
Fig. 1 shows slice images and histograms of carbonaceous chondrites studied. The calculated LACs of silicate and iron materials are also shown in the figure. In the case of chondritic samples, the LAC of silicate minerals mainly depends on their Fe content. Major elements of chondrites are O, Mg, Si and Fe. Because Fe is the heaviest element, it gives largest effect on LAC. The Allende, Y-81020, Y-791717, NWA763 and Murchison meteorites show metallic iron peaks around 70 cm−1, and there are no
Fe distribution in carbonaceous chondrites
Fig. 3 shows the relation between the peaks of chondrules and matrix in the histograms. Because Ivuna does not show any chondrule peak in the histogram, we set the peak value for chondrule to zero in the plot. It seems that there is a positive correlation between the peak values of chondrule and matrix. The peak value of chondrules in Allende would contain errors due to the multicomponent Gaussian fitting. If we apply the baseline subtraction for the peak analysis, the plots of the Allende move
Summary and future works
In this paper, we performed several three-dimensional analyses of carbonaceous chondrites using SR-CT. Through the analysis of the slope and peaks of the histograms of the LAC, we could investigate Fe distribution between the constituent components in the meteorites; chondrules, matrix and iron inclusions.
However, there are still problems in our observation. The discussions of the origin of the difference of LAC of silicate material and the size distribution of the low-Fe silicate inclusions in
Acknowledgements
The authors thank Dr. Carl Alwmark and anonymous reviewer for their detailed review and comments. The synchrotron radiation experiments were performed at the BL20B2 in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2006A1797, 2006B1007, 2007A1088, 2008A1476, 2008B1298, 2009A1105, 2009B1273, 2011A1127, and 2012A1128).
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