Mineralogy of four Itokawa particles collected from the first touchdown site
© Noguchi et al.; licensee Springer. 2014
Received: 23 May 2014
Accepted: 4 September 2014
Published: 9 October 2014
Four Itokawa particles collected from the first touchdown site were mineralogically investigated by optical microscopy, micro-Raman (μ-Raman) spectrometry, scanning electron microscopy (SEM), electron microprobe analysis (EPMA), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). Their mineralogy has an affinity to that of LL6 chondrites based on micro-Raman spectroscopy, EPMA, and XAS analyses. However, the space weathering rims on them are less developed than those observed on the Itokawa particles collected from the second touchdown site. Solar flare tracks are rarely observed in the four particles, whose number densities were lower than those observed in the Itokawa particles from the second touchdown site.
KeywordsItokawa XANES EPMA TEM Space weathering Solar flare tracks
In 2005, the Hayabusa spacecraft sampled twice from the Muses-C Regio of the asteroid (25143) Itokawa filled with myriad of pebbles. The distance between two touchdown sites is about 100 m (Nakamura et al.2014). The Itokawa particles investigated during the initial analyses in 2011 were collected from the second touchdown site (sample number: RB-QD04-xxxx), and those from the first touchdown site (sample number: RA-QD02-xxxx) were investigated in the first announcement of opportunity (AO) consortium, which includes this study. The Itokawa particles from the second touchdown site showed mineralogical and isotopic affinities to LL5-6 chondrites (Nakamura et al.2011; Yurimoto et al.2011; Nakamura et al.2012; Tsuchiyama et al.2011,2014; Nakashima et al.2013). Nakamura et al. (2014) reported that olivine, pyroxene, and plagioclase in six particles from the first touchdown site have chemical compositions similar to those from the second touchdown site. As a part of the first AO consortium, four Itokawa particles from the first touchdown site were allocated to us.
Although the sampling sites are located just about 100 m away from each other within the MUSES-C Regio, the reflectance spectra at the MUSES-C Regio show a variation in the relative reflectance associated with wavelength (Ishiguro et al.2007). Because it is thought that the degree of spectral reddening is related to the abundance of nanophase Fe0 on the surface material of Itokawa (e.g., Binzel et al.2001; Hiroi et al.2006), the microstructures of the external surface of the Itokawa particles from the first touchdown site are worth comparing with those from the second touchdown site.
Because the number of Itokawa particles collected from the first touchdown site is limited, six particles allocated to Nakamura et al. (2014), four particles to Tsuchiyama et al. (2014), and another four particles to this study, mineralogical data of the four Itokawa particles would contribute to clarify the mineralogy of the Itokawa particles from the first touchdown site.
The four Itokawa particles collected from the first touchdown site were mineralogically investigated by optical microscopy, micro-Raman (μ-Raman) spectrometry, scanning electron microscopy (SEM), electron microprobe analysis (EPMA), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). Among these methods, XAS is the newly applied technique for the analysis of the Itokawa samples.
If the affinity of Itokawa particles to LL5-6 chondrites is common, the redox state experienced by the four Itokawa particles in their parent body should be almost identical to that in LL5-6 chondrites. Therefore, the relative abundance of Fe3+ and Fe2+ ions in ferromagnesian silicates, which reflect the redox states, in the particles from Itokawa were measured and compared with those in ferromagnesian silicates in an LL chondrite. We used Tuxtuac LL5 chondrite for comparison. The X-ray absorption near-edge structure (XANES) of Fe K-edge is an X-ray absorption spectroscopy (XAS) technique used to compare Itokawa to chondrite meteorite samples.
List of the Itokawa particles investigated in this study and their constituent minerals
Olivine, high-Ca pyroxene, troilite
Low-Ca pyroxene, plagioclase, troilite
The cross sections at the external surface of the four particles were observed by using Hitachi HF-3300 cold field-emission (FE) TEM (Hitachi High-Technologies Co., Tokyo, Japan) equipped with EDAX energy dispersive X-ray spectrometer (EDS) (Philips, Amsterdam, Netherlands) at Hitachi High-Technologies Co. An additional TEM observation was performed by using FEI Titan TEM (FEI, Hillsboro, OR, USA) at Lawrence Livermore National Laboratory, USA, and JEOL JEM-2100 TEM (JEOL Ltd., Tokyo, Japan) at Ibaraki University. Solar flare tracks in the Itokawa samples were obtained by a method similar to the weak-beam dark-field method (e.g., Williams and Carter2009).
Raman spectra of potted butts of the four samples (the epoxy-embedded sample remained after ultramicrotomy) were measured with a μ-Raman spectrometer (JASCO Inc., NRS-3100, Tokyo, Japan) at Ibaraki University to identify minerals. The wavelength of excitation laser is 532 nm. The beam diameter of the laser is 2 μm. The peak position of the strongest Raman shift peak of metallic Si was calibrated to 520 cm-1. Measuring time was 40 s × 2, and the laser power was from 1 to 10 mW. After micro-Raman measurements, the potted butts were coated by carbon, and their textures were studied with a focused ion beam combined with scanning electron microscope (FIB-SEM) JEOL JIB-4501 equipped with JED-2100 EDS (JEOL Ltd., Tokyo, Japan) at Ibaraki University. Mineral chemistry was measured with a JEOL JXA-8530F FE-EPMA (JEOL Ltd., Tokyo, Japan) at JEOL Co. Acceleration voltage and probe current were 15 kV and 9 nA, respectively. ZAF-oxide correction was applied to calculate chemical compositions of minerals.
The potted butts of the four Itokawa particles and a polished thin section of the Tuxtuac LL5 chondrite were analyzed using the Beamline I18 X-ray micro-focus spectroscopy beamline (Diamond Light Source Ltd, Oxfordshire, UK) at the Diamond Light Source, Oxfordshire, UK. Diamond is a 3-GeV synchrotron with ring currents of approximately 250 mA. Energy selection with fractional energy resolutions of 10-4 to 10-5 are achieved with an Si (111) and (311) double crystal monochromator, respectively. An Si drift vortex detector was used to measure the X-ray fluorescence and absorbance of elements with Z >40. Fe K-edge X-ray absorption spectroscopy (XAS) was performed on approximately 2.5-μm spots and also with a mapping routine over approximately 5 × 40 μm areas. Typical experimental conditions used for XAS were 1-s integration at each 5.0 eV energy step up to approximately 7,100 eV, followed by a higher resolution of 0.1 eV energy steps over the XANES features up to 7,150 eV, and continuing over the EXAFS region with steps of 2 to 4 eV up to 7,660 eV. The errors of the Fe K-edge positions were ±0.05 eV.
After the XAS, FIB sections of RB-QD04-0008 and RB-QD04-0024 were prepared from their potted butts by JEOL JIB-4501 FIB-SEM (JEOL Ltd., Tokyo, Japan) and an ultra-low energy Ar ion milling machine Fischione NanoMill (E.A. Fischione Instruments, Inc., Corporate Circle Export, PA, USA) at Ibaraki University. Because the external surfaces of the Itokawa particles existed at the bottom of these potted butt samples, the ultrathin sections and the FIB sections were prepared from the opposite sides of these particles although the thickness of these particles were unknown. The FIB sections were observed by FE-TEM JEOL JEM-2800 at JEOL Co. and JEOL JEM-2100 TEM (JEOL Ltd., Tokyo, Japan) at Ibaraki University.
Description of the four Itokawa particles
Raman shift peak positions DB1 and DB2 of olivine
DB1 position (cm-1)
DB2 position (cm-1)
Chemical compositions of olivine and pyroxenes
Representative chemical compositions of olivine and pyroxenes in the four Itokawa particles
Mol% end members
Fe K-edge XANES of olivine
Fe XANES edge positions and pre-edge centroid positions in olivine and pyroxenes in the four Itokawa particles and Tuxtuac LL5 chondrite
Edge position (eV)
Pre-edge centroid (eV)
Fe K-edge XANES of pyroxene
The edge position and the pre-edge centroid position of low-Ca pyroxene in the Itokawa grain 0024 are 7,119.7 to 7,119.8 and 7,112.6 eV, respectively (Figure 6b). These values are indistinguishable from those measured in low-Ca pyroxene in the Tuxtuac LL5 chondrite (Wo1.5 ± 0.3En75.1 ± 0.5, n = 15): 7,119.8 and 7,112.6 eV, respectively (Table 4).
Just below the nanophase-rich layer, there are Si enrichment and depletion in Mg and Fe in both of RB-QD04-0015 and RB-QD04-0024. In the case of RB-QD04-0024, the lower part of the amorphous layer (labeled as B) is more electron transparent than the upper part (labeled as A), suggestive of lower density. In addition, a slight enrichment of Al is detected in RB-QD04-0015.
Below the amorphous layer, underlying olivine and low-Ca pyroxene show partial amorphization indicated by arrows marked ‘II’ in Figure 7c,d. The partially amorphized olivine and low-Ca pyroxene contains 2-to-4-nm-size nanophases (Figure 7e,f). Their lattice fringes range from 0.19 to 0.20 nm, suggestive of (110) of α-iron metal (Noguchi et al.2014). The amorphous layer containing nanophase (Fe,Mg)S and the underlying partially amorphized minerals containing nanophase Fe are similar to the composite rims observed on the Itokawa particles investigated in the initial analysis (Noguchi et al.2014). The composite rim is composed of a redeposition zone and a partially amorphized zone, called zones I and II, respectively (Noguchi et al.2014).
Surface modification of two different areas within a single particle
Solar flare tracks
Morphology and mineralogy of RB-QD4-0008
Because their sizes (approximately 35 to approximately 50 μm) are comparable to the smallest grains investigated during the initial analysis (whole size range: approximately 30 to approximately 180 μm) (Nakamura et al.2011,2014), the petrography of the Itokawa grains investigated in this study is simpler than that of those investigated in the initial analysis. Among them, RB-QD04-0008 has a unique morphology although the other Itokawa particles are fragments composed of an olivine grain (RB-QD04-0011 and RB-QD04-0015) or low-Ca pyroxene and plagioclase (RB-QD04-0024). The particle is composed of partially-connected equigranular olivine and high-Ca pyroxene. Itokawa particles having a morphology similar to these particles have been reported previously (Nakamura et al.2011; Matsumoto et al.2012; Noguchi et al.2014). For example, RA-QD02-0060 is composed of a large twinned low-Ca clinopyroxene and small olivine and vesiculated plagioclase. In this particle, the dislocation density of olivine grains is variable. This fact suggests that such porous aggregate particles were formed by the recrystallization of mineral particles, each of which has a different history.
The Enstatite (En) and Wo contents (mol%) in the high-Ca pyroxene in RB-QD04-0008 are 46.1 and 41.9, respectively, which are outside the range of the En and Wo contents in high-Ca pyroxene in the highly equilibrated grains investigated in the initial analysis (Nakamura et al.2011) (Figure 5). The mol% of Wo is similar to those of high-Ca pyroxene in LL6 chondrites (McSween and Patchen1989). By considering the above discussion, RB-QD04-0008 may have been formed by recrystallization and it contains mineral fragments that experienced thermal metamorphism as high as most LL6 chondrites experienced.
Mineralogy of low-Ca pyroxene in RB-QD4-0024
As described in the ‘Raman spectroscopy’ section, the low-Ca pyroxene in RB-QD04-0024 is orthopyroxene. The average Wo mol% of the low-Ca pyroxene is 2.2, which is higher than that of the equilibrated Itokawa particles (Wo1.4: Nakamura et al.2011,2014) and within the range of LL6 chondrites (Wo1.7-2.6: McSween and Patchen1989). The highly equilibrated particles (HEP) from the second touchdown site were regarded as being comparable with LL5-6 chondrites based on the Wo mol% of both low- and high-Ca pyroxene (Nakamura et al.2011,2014), and the low-Ca pyroxene in RB-QD04-0024 is plotted at the highest Wo mol% of low-Ca pyroxene in the HEP (Figure 5). These data suggest that RB-QD04-0024 is a mineral fragment that experienced thermal metamorphism comparable with the typical LL6 chondrites. Although we investigated only four Itokawa particles in this study, each particle experienced a different metamorphic history, which is consistent with the previous studies of Itokawa particles (Nakamura et al.2011,2014).
Space weathering and solar flare track density
Two of the four Itokawa particles investigated in this study show textural modification at the external surfaces based on TEM observation. They have amorphous surface layers and underlying partially amorphized olivine or low-Ca pyroxene (Figures 7 and8). In RB-QD04-0015, the amorphous surface layer contains Al, which is not contained in olivine. These features correspond to those of the composite rims, which have probably been formed by space weathering (Noguchi et al.2011,2014).
The rims of RB-QD04-0015 and RB-QD04-0024 do not have blisters that are lenticular bubbles formed by ion implantation (e.g., Igarashi et al.2002). On the other hand, blisters were observed in 3 among 12 Itokawa particles in the initial analysis (Noguchi et al.2014). Although the number of particles investigated in this study is limited, the degrees of space weathering may have been more modest than those experienced by the Itokawa particles collected at the first touchdown site that were investigated in the initial analysis.
This interpretation is consistent with the difference in the number density of solar flare tracks between the Itokawa particles investigated in this study and those in the initial analysis. Solar flare tracks were observed in RB-QD04-115, and its number density was estimated to be <1 × 109 cm-2 (Figure 10a). On the other hand, some Itokawa particles investigated in the initial analysis show much higher track density: 1 × 1010 cm-2 in RA-QD02-0033 and 2 × 109 cm-2 in RA-QD02-0009 (Figure 10b). By considering these results, the degrees of space weathering may be different between the two touchdown sites.
Nagao et al. (2011) also predicted the duration for solar wind to be shorter than 103 years based on solar 20Ne concentrations. Considering the solar flare track densities observed in RA-QD02-0033 corresponding to approximately 104 years (Bradley et al.1984), the duration for surface modification should be between 103 and 104 years. For the particles collected at the second touchdown site, the duration could be shorter and <103 years. During such a short duration, the difference in the degrees of space weathering between the two touchdown sites could have resulted.
Surface modification within a single Itokawa particle
Two TEM samples that were prepared from the opposite sides of RB-QD04-0008 do not show any detectable surface modification. On the other hand, two TEM samples that were also prepared from the opposite sides of RB-QD04-0024 have rims similar to the composite rims described in Noguchi et al. (2014), who have also reported that two TEM samples prepared from RA-QD02-0032 had a composite rim. These data suggest that the Hayabusa spacecraft was able to recover Itokawa particles without significant modification and that fine-grained particles on the surface of Itokawa were homogeneously irradiated by solar wind, which may be a result of the low surface gravity on Itokawa (approximately 10-4 ms-2; Hirata et al.2009) and levitation of fine-grained particles by electrostatic repulsion caused by a photoelectron effect (Lee1996; Hartzell and Scheeres2013).
In the initial analysis, Nagao et al. (2011) analyzed noble gas isotopes in the Itokawa particles collected at the first touchdown. They revealed that the noble gas compositions and the release profiles vary even among the particles collected at the same touchdown site. The variety was thought to reflect different trapping sites of solar wind noble gases: Noble gases released at lower temperatures could be solar winds with lower kinetic energy and hence be trapped in the outermost layer of the grains, whereas gases released at higher temperatures could reside in deeper layers due to their higher kinetic energy. In the shallower surface layer of each grain, helium could be more enriched compared to that in the deeper surface layer, as observed in the bulk metallic glass of the Genesis (Grimberg et al.2008). These noble gas signatures are consistent with the interpretation of our TEM results; the Itokawa particles could have been collected by the Hayabusa spacecraft as is on the Itokawa surface.
Redox state of the four Itokawa particles and comparison with that of the Tuxtuac LL5 chondrite
By measuring the Fe K-edge XANES of olivine and low-Ca pyroxene in the four Itokawa samples, we checked whether they were as reduced as ordinary chondrites. We found this to be the case, and this is one more piece of evidence about the similarity to LL chondrites. The Fe K-edge positions and the Fe K pre-edge centroid positions of olivine, low-Ca pyroxene, and high-Ca pyroxene in the four Itokawa grains overlap with those in the Tuxtuac LL5 chondrite (Table 4). These data indicate a negligible abundance of Fe3+ ions in ferromagnesian silicates in both the Itokawa grains and equivalent Tuxtuac minerals, which is consistent with the mineralogical, petrological, and oxygen isotopic data of the Itokawa grains investigated in the initial analyses (Nakamura et al.2011,2014; Tsuchiyama et al.2011,2014; Nakamura et al.2012; Yurimoto et al.2011; Nakashima et al.2013).
None of the Itokawa particles investigated in this study contain pentlandite inclusions, which is consistent with the low abundance of pentlandite in LL chondrites (<<1%) (Graham et al.1988; Jamsja and Ruzicka2010).
Relationships between Fo mol% estimated by Raman spectroscopy and Fo mol% obtained by EPMA
Forsterite mol% estimated by Raman spectroscopy in this study is Fo80, which is 10 percentage points higher than the Fo mol% obtained by EPMA (Figures 3 and4). On the other hand, Fo mol% estimated by Raman spectroscopy in Noguchi et al. (2012) was consistent with that obtained by EPMA in Noguchi et al. (2012). The wavelength of the excitation laser is different between these studies. A green laser (λ = 532 nm) was applied in this study, while a near infrared laser (λ = 785 nm) was used in Noguchi et al. (2012). Some Raman shift peaks are excited from epoxy by a green laser. On the other hand, no significant peaks are excited from epoxy by a near infrared laser. Because a broad peak from epoxy (approximately 830 cm-1) overlaps with the olivine doublet centered at approximately 830 cm-1, it is likely that the overlap reflects the deconvolution of the doublet.
We investigated four Itokawa particles collected from the first touchdown site. They are indistinguishable from LL6 chondrites based on Raman spectroscopy, EPMA, and XANES analyses. The space weathered rims on them are observed but less developed on the Itokawa particles collected from the second touchdown site. This result may be consistent with the difference of the solar flare track density between the particles collected during the first touchdown and those collected during the second touchdown. However, a further study is needed to confirm this estimation because of the limited numbers of the particles investigated in this study.
We thank the Hayabusa project team for the return sample. We express our thanks to N. Mori, I. Ohnishi, and K. Shimada for supporting to use FE-EPMA and FE-TEM at JEOL Corporation and Kyushu University. L. P. Keller was appreciated for the discussion on the track density at the Hayabusa symposium. We are grateful for T. Mikouchi’s and A. Tsuchiyama’s valuable referee comments. T. Noguchi was supported by JSPS KAKENHI grant number 2424408. J Bridges and L. Hicks were supported by STFC, UK.
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