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Ken-ichi Bajo, Noriyuki Kawasaki, [Isao Sakaguchi](https://orcid.org/0000-0003-4382-2509), [Taku T. Suzuki](https://orcid.org/0000-0001-6041-4297), Satoru Itose, Miyuki Matsuya, Morio Ishihara, Kiichiro Uchino, Hisayoshi Yurimoto

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[In Situ Helium Isotope Microimaging of Meteorites](https://mdr.nims.go.jp/datasets/f3eb422f-cb98-4911-bd06-d47c98a50d76)

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Template for Electronic Submission to ACS JournalsIn situ helium isotope micro-imaging of meteoritesKen-ichi Bajo1*, Noriyuki Kawasaki1, Isao Sakaguchi2, Taku T. Suzuki2, Satoru Itose3, Miyuki Matsuya3, Morio Ishihara4, Kiichiro Uchino5, and Hisayoshi Yurimoto11Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan. 2National Institute for Materials Science, Tsukuba 305-0044, Japan. 3JEOL Ltd., Akishima 196-8558, Japan. 4Department of Physics, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan. 5Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan. ABSTRACT: Isotope imaging is commonly used to investigate the localization of trace elements and their isotopes. In situ noble gas analysis of meteorites revealed the distribution of primordial noble gases that were trapped in the building blocks of asteroids and planets during the early stage of the Solar System evolution. Solar wind noble gases are among the primordial gases present in meteorites and were trapped through exposure to solar wind. Micrometer resolution in situ noble gas analysis has not been achieved due to the lack of sensitivity and spatial resolution. Microscale imaging technique is crucial for identifying the carrier phase of the solar wind noble gases. We have developed 4He isotope imaging utilizing secondary neutral mass spectrometry with strong field post-ionization. This technique achieved a lateral resolution of 2 µm and a 4He detection limit of 2  1017 cm–3. This development allows for the study of a solar wind gas-rich meteorite, Northwest Africa 801 carbonaceous chondrite, with micrometer resolution. The solar wind 4He carriers are fine-grained particles and are sparsely scattered in the matrix region. PAGE  22INTRODUCTIONNoble gas isotopes have been used as tracers for solar wind to facilitate understanding of the chemical abundance of the sun and the physical properties of solar wind, owing to their chemically inert nature and extremely low solubility in solid materials. In situ noble gas analysis of minerals has been performed using conventional noble gas mass spectrometers with laser ablation1–3 to study noble gas distributions in rock samples. This approach can identify the carrier phases of the trapped noble gases. The ablation pits, i.e., the spatial resolution, are limited to ~50 μm in width and 50–500 μm in depth, primarily because of the scarcity of noble gases in minerals. In situ helium measurements with a lateral resolution of ~25 μm have been performed using secondary ion mass spectrometry (SIMS),4 while solar wind He is concentrated on the surface (<100 nm) of the particles because the typical energy of solar wind 4He is ~4 keV.5 The carriers of the abundant 4He are preserved in regolithic rocks, which were exposed to the solar wind on the meteorite parent body.6 The small regolith particles are the main reservoirs of solar wind because their large surface-to-volume ratio increases the efficiency of solar wind implantation. Thus, the spatial resolution of >20 μm by conventional SIMS is too large to study the distribution of the solar wind in the particles. Furthermore, the detection limit of conventional SIMS is ~1019 cm–3,4 while the most abundant bulk 4He concentration in meteorites is ~10–2 cm3 STP g–1,7, 8 corresponding to a 4He concentration of ~1018 cm–3. Therefore, conventional SIMS is insufficient for identifying solar wind He in meteorites. Helium-4 imaging with higher spatial resolution and sensitivity has the potential to reveal the location of the solar wind noble gases. We developed a secondary neutral mass spectrometry technique using laser post-ionization and a focused ion beam, to examine the micrometer- and nanometer-scale noble gas distributions.5,9–17 The lateral resolution of the 4He measurements was ~3 µm and the detection limit was 3  1018 cm–3,5 which is close to the highest 4He concentration observed in meteorites. If the 4He distribution in meteorites is homogeneous, then the concentration is below the detection limit, and it is difficult to observe 4He there. Therefore, the detection limit of 4He needs to be improved for in situ noble gas analysis of the meteorites. In this study, we reduced the 4He detection limit by improving the vacuum and laser systems. Using the improved systems, we conducted two-dimensional (2D) mapping of 4He on the polished surface of a solar wind gas-rich meteorite, Northwest Africa 801 (NWA 801) Renazzo-type carbonaceous chondrite (CR chondrite).7 INSTRUMENTAL MODIFICATIONSA time-of-flight secondary neutral mass spectrometer (TOF-SNMS) with a femtosecond (fs) laser, (Laser Ionization MAss nanoScope, LIMAS, JEOL) at Hokkaido University was used in this study.5,11–17 The instrumental and analytical conditions of LIMAS were modified to improve the He analysis as follows. The ionization efficiency for He in the laser focus region (φ30 × 100 μm in length) was ~10% with the previous laser system of 3 mJ and 40 fs (Integra-C. USP, Quantronix).12 In this study, we upgraded the high-power Ti-sapphire fs laser (Astrella, Coherent, Inc.) to 6 mJ per 30 fs pulse to increase the post-ionization efficiency for He. The timing of laser irradiation for the sputtered neutral plume was also tuned based on Bajo et al.13 because the number of particles in the plume that interact with the pulsed laser beam depends on the timing between sputtering by the pulsed primary beam and laser irradiation. After modification, the ionization efficiency of He increased to 70%, primarily because of the higher laser power density.16In addition, the vacuum system was improved  to reduce the residual He in the sample chamber, which is the direct cause of background He for the LIMAS measurements, by replacing a 300 L s–1 ion pump and a 400 L s–1 non-evaporable getter (NEG) pump with a 410 L s–1 ion pump StarCell element (Vaclon Plus 500, Agilent) and a 1300 L s–1 NEG pump (CapaciTorr B1300, SAES getters). Consequently, the vacuum in the sample chamber improved from 2  10–8 to 4  10–9 Pa.MATERIAL AND METHODThe NWA 801 meteorite is known as one of the most noble gas-rich meteorites,7 where the matrix preserves a large amount of solar wind noble gases (e.g. ~1018 4He cm–3). However, the noble gas carrier phases in the matrix have not yet been determined. The chip of NWA 801 was sliced to prepare a polished section measuring 5 × 6 mm2. The sample was polished using an automatic polishing machine (MA-200e, Musashino Denshi) at Hokkaido University. The sample was attached to the holder of the polishing machine using a thermoplastic adhesive. Diamond slurry containing polycrystalline diamond particles of ~3 μm dissolved in ethylene glycol sprayed onto a copper polishing plate was used on the sample to obtain a flat surface. Then, ~1 μm diamond slurry sprayed onto a tin-antimony alloy polishing plate and that on a polishing cloth were used to perform final polishing. These procedures provided a sufficiently flat surface for the LIMAS measurements. The polished section was coated with a thin gold film of ~10 nm thickness using a vacuum evaporator (JEE-400, JEOL). Prior to isotope imaging using LIMAS, backscattered electron (BSE) images of the section were obtained using field-emission scanning electron microscope (FE-SEM: JSM-7000F, JEOL) at Hokkaido University. After isotope imaging with LIMAS, BSE images, secondary electron images and X-ray elemental maps were obtained for the isotope imaging areas to evaluate the texture and chemical composition. X-ray elemental mapping was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), achieved by installing an energy-dispersive X-ray spectrometer (X-Max 150, Oxford) on the FE-SEM instrument. A 15 keV electron beam probe with a current of 10 nA was used. The SEM-EDS system was also used to determine the quantitative Si concentrations in the minerals, as listed in Table 1. A polished section of San Carlos olivine grains with a forsterite content (= Mg / (Mg + Fe) × 100) of ~89 mol % was prepared and irradiated with 4 keV 4He at a fluence of 1 × 1015 cm–2 (hereafter referred to as He-Ol) to quantify the 4He concentration in the meteorite using a relative sensitivity factor (RSF). Although the fluence was a nominal value, we estimated the systematic uncertainty of the true value to be about 10%. To determine the background level of 4He in the depth profiling, we measured a polished section of San Carlos olivine without 4He ion implantation (blank-Ol). We used a 30 keV 69Ga+ probe with 30–45 nA as the primary ion beam of LIMAS. The primary beam was pulsed to widths of 200–400 ns. The beam was corrected for spherical aberration by the aberration-correcting optics,12,13,17 and focused to 2 µm on the sample surface, which is defined by the 16-84% criterion using line scans.17 The angle of incidence of the primary beam on the sample was set to 55°, which was formed by the optical axis of the primary beam and the sample surface. The neutrals sputtered from the surface were ionized using the fs laser. The laser beam was focused to 30 µm in diameter at ~50 µm above the point of primary beam irradiation. Ionized particles within the m/z ranges of 3.5 to 4.5, 11.5 to 13.5, and 27.5 to 28.5 were introduced into the multiturn TOF-MS system of LIMAS, MULTUM-II,12–15 by activating ion gate deflectors in order. This was done to measure the 4He+ signals and the major element signals of the meteorite (24Mg2+, 28Si+ and 56Fe2+) (Figure 1a). The flight time in MULTUM-II was set such that the mass-resolving power was calculated to be ~13,000 in a 1% valley for m/z 4 with 93 laps of 4He+. Under these conditions, 12C3+ and 16O4+ ions were in flight as interferences before 4He+. The interference was removed using the ion gate deflector within the ion trajectory of the MULTUM-II. Therefore, no interference is observed in the 4He+ peak (Figure 1b). These conditions were applied to the imaging and depth profiling analyses. The background signal of 4He is produced by the ionization of the residual 4He gas (hereafter, blank 4He) in the ultrahigh vacuum sample chamber. Thus, we measured the blank 4He after each measurement with irradiation of the fs laser pulse without primary beam irradiation. The average blank 4He signal was defined as the detection limit for the measurements. A total of 400,000 TOF spectrum scans were carried out for blank 4He measurements. The blank 4He was (5 ± 2) × 10–5 cpms (ion counts per mass spectrum scan), of which the error is expressed as the standard deviation (1SD) of 7 blank measurements. The standard deviation was approximately equal to the statistical error of ion counting.Depth profilingThe RSF for meteorite analysis was obtained by depth profiling of He-Ol. The imaging area of He-Ol was 18 × 25 μm2 consisting of 30 × 30 pixels. The pulse width of the primary beam was set to 400 ns. The longer pulse width compared with the imaging analysis for meteorites accelerated the sputtering rate per pulse for cratering. The depth profile was obtained from the center of an 8 × 8 pixel to avoid crater edge effects. A total of 200 TOF mass spectra were collected and stored to construct a single pixel. The imaging analysis was repeated continuously on the same imaging area to obtain three-dimensional images by sputtering. After depth profiling, crater depths were measured using a confocal laser microscope (VK-X200, Keyence). The measurement conditions for blank-Ol were identical to those for He-Ol.Isotope imagingTwo areas in the NWA 801 section were imaged using scanning with the primary beam of LIMAS. This primary beam with a pulse width of 200 ns was utilized on the sample surface to perform 2D analysis. The primary beam had a diameter of 2 µm and was scanned on the sample using a pattern of 80 × 60 pixels with the scan interval of 580 nm horizontally and 820 nm vertically. As a result, the imaging area measured 46 × 49 μm2. A total of 200 TOF mass spectra were collected for each pixel. After the imaging analysis, blank 4He was measured.Long-term measurements of small areas were performed for selected regions of interest (ROI) in isotope imaging to obtain precise 4He abundances. The ROIs were set to 10 × 10 pixels, with a scan interval of 100 nm horizontally and 160 nm vertically. The data accumulation sequence for the ROI was a 10 × 10 pixel image repeated 20 times. As the primary beam was 2 µm and the angle of the incidence was 55°, the area of the ROI was an ellipse of 3 × 5 μm2. After the ROI analysis, blank 4He was measured. RESULTS AND DISCUSSIONQuantification of 4He concentrationThe depth profiles of 4He+ and 28Si+ for He-Ol are shown in Figure 2. The crater depth was 238 nm for 160 measurement layers. Assuming a constant sputtering rate during the measurement, each measurement layer was converted to a depth from surface. The implantation profile of 4He is observed from the surface to a depth of ~100 nm. The background signal of 4He+, 4He+BG, is to be 1.6 × 10–4 cpms. To convert 4He intensities to 4He concentrations, ion intensity ratios are typically used for the calibration method.5 Assuming that the RSF of 4He+ against 28Si+, RSFHe,Si, is independent of its concentration, the RSF is defined as: RSFHe,Si  = ([4He]/[28Si])/(4He+/28Si+)meas,  (1)where [4He] and [28Si] are the concentrations of 4He and 28Si in the sample, respectively, and (4He+/28Si+)meas is the corresponding 4He+/28Si+ intensity ratio.For the He-Ol standard, the integrated value of the 4He+/28Si+ ratios from the surface to a given depth, Σ(4He+/28Si+)meas, is given by the following equation:,  (2)where 4Hei+ and 28Sii+ are the ion intensities of 4He and 28Si in the measurement layers, respectively, and  i is the number of measurement layers on the surface. If the n is sufficiently large, the [4He] of the integration region corresponds to Φ/d, where Φ is the fluence of 4He implantation into the He-Ol and d is the depth corresponding to the n. Therefore, Equation (1) is transformed into the following equation:.  (3)The [28Si] is 1.24 × 1022 cm–3 for olivine. Φ is the 4He fluence of 1 × 1015 cm–2. As the integration depth appears sufficient up to the depth of 100 nm (Figure 2), the RSFHe,Si is calculated to be 0.79 ± 0.01 (1σ) for olivine. The uncertainty of the RSFHe,Si was calculated by counting statistics for 4He+ and 28Si+. Therefore, the 4He concentration ([4He]He,Si (cm–3)) of the olivine was estimated using the following equation:[4He]He,Si  (cm–3) = RSFHe,Si × [Si] × (4Hei+/28Sii+)meas. (4)To apply Equation (4), the Si concentration at each measurement point must be determined. However, meteorites consist of various minerals with different Si concentrations, ranging from 0% (e.g., Fe metal) to ~50% (e.g., silicates) in the imaging areas in this study. It is difficult to accurately determine the Si concentrations for all pixels in a 2D analysis at a resolution of one pixel. Post-ionization by a focused fs laser suppresses the matrix effect for element quantification because the ionization efficiency is nearly 100% for all ions.12 If the sputtering rate is not very different between minerals, we can directly estimate the 4He concentration from 4Hei+ as follows. Removing normalization terms from Equation (3), .   (5)Then, [4He]He (cm–3) = SFHe × 4Hei+.   (6)The sensitivity factor (SF) is calculated to be 1.03 ± 0.02 × 1022 (cm–3 cpms–1) by duplicate measurements of the He-Ol standard.Using Equations (4) and (6), the ion intensities of the depth profiles were converted into 4He concentrations (Figure 2).Detection limit of 4HeThe blank 4He value of (6 ± 1) × 10–5 cpms, which is calculated to be (2.1 ± 0.3)  1017 cm–3 using Equation (6). The detection limit was improved by approximately one order of magnitude compared with that reported in previous study (3  1018 cm–3).5The detection limit for depth profiling (Figure 2) corresponds to the concentration at the bottom of the crater (> 150 nm), which is (5.5 ± 0.4)  1017 cm–3 ((1.6 ± 0.1) × 10–4 cpms). The ion intensity of 4He+ for blank-Ol is (1.5 ± 0.2) × 10–4 cpms, which is the same within the error to that of the He-Ol measurement and ~2.6 times greater than that of the blank 4He. This indicates that the 4He intensity was increased by the irradiation of the sample by the primary beam. The 4He from the crater bottom in the depth profiling was likely generated from the adsorbed 4He on the sample surface during the intervals between the primary beam pulses, in addition to the residual He gas in the sample chamber. The detected adsorbed 4He was comparable to the residual 4He under these measurement conditions. Texture and isotope imaging of NWA 801The NWA 801 meteorite consists of millimeter-sized chondrules and millimeter-to-sub-millimeter- sized grains of olivine, low-Ca pyroxene, and Fe-Ni metal (Figure 3). The chondrules and Fe-Ni metals are embedded in a fine-grained matrix composed mainly of sub-micrometer-sized phyllosilicates.18 We obtained ion images of 4He, 24Mg2+, 28Si+, and 56Fe2+ from matrix areas A and B, as shown in Figures 4 and 5, respectively. The 4He concentrations in the 4He images were calculated using Equation (6) with SFHe. Olivine, pyroxene, metal fragments, and a fine-grained matrix can be distinguished in the 24Mg2+, 28Si+ and 56Fe2+ images shown in Figures 4 and 5.Comparison between RSF and SF for He quantificationWe measured four small areas (spots) for the silicate phases, as shown in Figure 4a, to compare the two [4He] conversion methods using Equations (4) and (6) because the ion image cannot accurately determine Si concentrations for all pixels. Table 1 summarizes the ion intensities of the spot analysis, the Si concentrations for the spots with SEM-EDS analysis, and 4He concentrations obtained by the two conversion methods. Spots #1 and #2 were dominated by phyllosilicates enclosing the mineral particles of metals, sulfides, and pyroxene. Spots #3 and #4 consisted of phyllosilicates and monomineralic olivine, respectively. Figure 6 shows the correlations between [4He]He, and [4He]He,Si. Both concentrations are plotted on the slope-1 line, suggesting agreement between the two quantification methods, regardless of the differences in the matrix phases. This also suggests that the sputtering rates were similar among these minerals.4He concentration maps of NWA 801Helium is highly concentrated and sparsely scattered (Figures 4a and 5a). Helium-4 signals above 1019 cm–3 are observed from the phyllosilicates and in the matrix, and are not detected in coarser mineral fragments such as olivine and pyroxene. The highest 4He concentrations shown in Figures 4a and 5a are ~1 × 1021 cm–3. This is identical to the saturation concentration of the silicates.16 Because almost all the 4He in NWA 801 is solar wind He,7 the high 4He concentration can be explained by solar wind ion implantation. In the black colored pixels in Figures 4a and 5a, the 4He concentrations are lower, with concentrations of ~1018 cm–3 or less. The 4He isotope imaging technique revealed a sparsely scattered 4He distribution in the NWA 801 matrix. The 4He-enriched portions are composed of fine particles of sub-micrometer phyllosilicates enclosing mineral particles of metal, sulfide, and pyroxene. The presence of solar wind 4He in the fine particles indicates that these particles were exposed on the surface of the parent body for a longer period than the 4He-poor fine particles.The 4He images in Figure 7 were generated from Figures 4 and 5 to compare the changes in spatial resolution between the previous (~20 μm) and this study (2 μm). The generation method is that a window consisting of the 40 × 25 pixels (23 × 20 μm2) in the original images was prepared. The window was scanned in the x- and y-directions with steps of 6.2 and 6.5 μm, respectively. The intensities in the window were averaged to a single pixel to generate a 5 × 6 pixel image. As a result, the spatial resolution of the images in Figure 7 becomes equivalent to the spatial resolution of images taken with a ~20 μm primary ion beam, which is the highest resolution of previous studies. This process increases the count intensity by a factor of approximately 1000 and lowers the detection limit of 4He to approximately 1/30. Therefore, the color scale in Figure 7 was set to lower than in Figures 4b and 5b. The low spatial resolution images in Figure 7 do not show the distribution of concentrated and sparsely scattered 4He, as shown in Figures 4a and 5a. This study is the first to demonstrate distribution of solar wind 4He implanted in individual minerals (phases), which has not been determined by previous studies.1–4 Using this imaging technique, we can explore the processes through which the solar wind is incorporated into asteroids and how its components become distributed into their asteroidal interiors.CONCLUSIONSThe fs laser and vacuum system of LIMAS were improved, resulting in an increase in the 4He ion count rate and a reduction in the 4He background. The revision of the vacuum resulted in an improvement of the blank 4He from 3  1018 cm–3 to 2  1017 cm–3. The detection limit for 4He was reduced by one order of magnitude. We succeeded in obtaining in situ microscale images of the 4He distribution in a solar wind gas-rich meteorite. The 4He isotope images showed the active movement of fine particles on the asteroid surface.AUTHOR INFORMATIONCorresponding Author*Ken-ichi Bajo, Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan; e-mail: k-bajo@eis.hokudai.ac.jp orcid.org/0000-0002-9013-2730AuthorsNoriyuki Kawasaki, Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan. orcid.org/0000-0001-6502-6488Isao Sakaguchi, National Institute for Materials Science, Tsukuba 305-0044, Japan. Taku T. Suzuki, National Institute for Materials Science, Tsukuba 305-0044, Japan.Satoru Itose, JEOL Ltd., Akishima 196-8558, JapanMiyuki Matsuya, JEOL Ltd., Akishima 196-8558, JapanMorio Ishihara, Department of Physics, Graduate School of Science, Osaka University, Toyonaka 560-0043, Japan.Kiichiro Uchino, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan.Hisayoshi Yurimoto, Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan. orcid.org/0000-0003-0702-0533AUTHOR CONTRIBUTIONSThe manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.ACKNOWLEDGEMENTSWe thank associate editor Dr. Scott A. McLuckey for editorial handling and three anonymous reviewers for their constructive comments. We thank Drs. Kosuke Nagata and Azusa Tonotani for their assistance in modifying the LIMAS system. We would like to thank Editage (www.editage.jp) for English language editing.REFERENCES(1) Nakamura, T.; Nagao, K.; Takaoka, N. Microdistribution of primordial noble gases in CM chondrites determined by in situ laser microprobe analysis: Decipherment of nebular processes. Geochem. Cosmochem. Acta. 1999, 63, 241–255. DOI: 10.1016/S0016-7037(98)00275-0(2) Okazaki, R.; Takaoka, N.; Nagao, K.; Sekiya, M.; Nakamura, T. Noble-gas-rich chondrules in an enstatite meteorite. Nature 2001, 412, 795–798. DOI: 10.1038/35090520(3) Sumino, H.; Ikehata, K.; Shimizu, A.; Nagao, K.; Nakada, S. Magmatic processes of Unzen volcano revealed by excess argon distribution inzero-age plagioclase phenocrysts. J. Volcanol. Geotherm. Res. 2008, 175, 189–207. DOI: 10.1016/j.jvolgeores.2008.03.027(4) Gnaser, H.; Oechsner, H. Novel Detection Scheme for the Analysis of Hydrogen and Helium by Secondary Ion Mass Spectrometry. Surf. Interface Anal. 1991, 17, 646–649. DOI: 10.1002/sia.740170906(5) Bajo, K.; Olinger, C. 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Interface Anal. 2016, 48, 1181–1184. DOI: 10.1002/sia.6119(17) Nagata, K.; Bajo, K.; Itose, S.; Matsuya, M.; Ishihara, M.; Uchino, K.; Yurimoto H. Aberration-corrected focused ion beam for time-of-flight secondary neutral mass spectrometry. Appl. Phys. Exp. 2019, 12, 085005. DOI: 10.7567/1882-0786/ab30e4(18) Connolly, H.C., Jr.; Smith, C., Benedix, G., Folco, L., Righter, K., Zipfel, J., Yamaguchi, A. and Aoudjehane, H.C. The Meteoritical Bulletin, No. 92, 2007 September. Meteorit. Planet. Sci. 2007, 42, 1647–1694. DOI: 10.1111/j.1945-5100.2007.tb00596.xFIGURES AND TABLEFigure 1. (a) TOF spectrum of point #1 of NWA 801 sample shown in Figure 4a. The arrows are assigned to the measured ion species. The numbers in parentheses indicate the numbers of laps for each ion in MULTUM-II. (b) TOF spectrum of m/z 4 of the point #1. The peak positions of 16O4+ and 12C3+ which are removed by the ion gate are indicated by arrows.Figure 2. (a) Depth profiles of 4He+ and 28Si+ from the He-Ol sample. Silicon intensity is shown in tenths. The 4He concentration calculated by Equation (4) is shown on the right axis. (b) Depth profiles of 4He+/28Si+ from the He-Ol sample. The 4He concentration on the right axis is calculated using Equation (6).Figure 3. (a) BSE image of NWA 801. (b) Combined X-ray elemental map of (a). Mg: Red, Si: Green, and Fe: Blue. The scale bar represents 100 μm for all images. The area contains chondrules, lithic and mineral fragments, metal particles, and fine-grained minerals. Matrix is defined as aggregates of solids of smaller than micrometer size. The two matrix areas of A and B are measured by isotope imaging as shown in Figures 4 and 5, respectively.Figure 4. Isotope images from area A. (a) 4He image. Four yellow ellipses (#1–4) indicate points measured by the spot analysis. The white arrow in #1 indicates the data point used in Figure 1. (b) Color scale of 4He concentration. (c) 24Mg2+, (d) 28Si+, and (e) 56Fe2+ images. (f) BSE image for comparison. The scale bar represents 10 μm for all images. Labels px, ol, met, and mtx indicate pyroxene, olivine, metal, and fine-grained matrix, respectively.Figure 5. Isotope images of area B. (a) 4He image. (b) Color scale of 4He concentration. (c) 24Mg2+, (d) 28Si+, and (e) 56Fe2+ images. (f) BSE image for comparison. The scale bar represents 10 μm for all images. Labels px, ol, and mtx indicate pyroxene, olivine, and fine-grained matrix, respectively. Figure 6. Correlation between 4He concentrations using SFHe ([4He]He) and RSFHe,Si ([4He]He,Si) for the spot analysis. The dotted line represents the slope-1 line.Figure 7. Computer-simulated 4He isotope images. (a) Low spatial resolution image of Figure 4a.  (b) Low spatial resolution image of Figure 5a. The color scale is set to one-tenth from Figure 4b and 5b. A window consisting of the 40 × 25 pixel (23 × 20 μm2) in the original images are averaged to a single pixel to generate a 5 × 6 pixel images. The scale bars represent 10 μm.Table 1. Blank-corrected 4He+, 24Mg2+, 28Si+, 56Fe2+ intensities, Si, and 4He concentrations for the spot analysis.  4He+ 24Mg2+ 28Si+ 56Fe2+ 4He+/28Si+ Si* [4He]He,Si [4He] He  cpms 10-3 mol% 1019 cm-3 #1 0.0222 (2) 5.15 0.65 2.56 34 (2) 11.3 26.4 (3) 22.7 (2) #2 0.0065 (1) 8.37 0.80 2.53 8.1 (4) 11.3 6.31 (14) 6.62 (13) #3 0.00012 (3) 6.77 1.01 3.72 0.10 (3) 13.1 0.10 (2) 0.12 (3) #4 0.00007 (2) 22.15 1.18 0.42 0.06 (1) 14.0 0.06 (2) 0.07 (2)Uncertainties in the last decimal places of listed numbers are enclosed in parentheses: for example, 0.0222 (2) indicates 0.0222 ± 0.0002. The uncertainty in the intensity of 4He+ represents a statistical error in the overall 4He+ count. [4He]He,Si and [4He]He denote 4He concentrations calculated using Equations (4) and (6), respectively. * Si concentrations were calculated using Oxford AZtec software with the SEM-EDS system.14image1.pngimage2.pngimage3.pngimage4.tiffimage5.tiffimage6.jpegimage7.jpegimage8.jpegimage9.tiffimage10.tiff