EDS: energy dispersive X-ray spectroscopy
EPMA: electron probe micro analysis
FFTC: facility-to-facility transfer container
FIB: focused ion beam processing
INAA: instrumental neutron activation analysis
NanoSIMS: nano-scale secondary ion mass spectrometry
SEM: scanning electron microscopy
SR-XCT: synchrotron radiation-based computed tomography
SR-XRD-CT: synchrotron radiation-based X-ray diffractometry - computed tomography analysis
STXM-NEXAFS: scanning transmission X-ray microscope - near-edge X-ray absorption fine structure
TEM: transmission electron microscopy
XRD: X-ray diffraction
Designs of analysis for Ryugu particles
To obtain acquire complex micro-texture and chemical characteristics of the sample in sub-micrometer scale, we conducted coordinated micro analysis utilizing an SR-XCT & XRD – FIB-XCT & XRD – FIB – STXM-NEXAFS – NanoSIMS – TEM analysis without degradation, contamination due to the terrestrial atmosphere and small particles, or mechanical sample damage. In parallel, we carried out systematic bulk analysis with SEM-EDS, EPMA, XRD, INAA, and a laser fluorination oxygen isotope facility. The analytical procedure is shown in Fig. S3, and each analysis was described in the following sections.
Sample transportation and handling processes
The Ryugu asteroid particles were recovered from the Hayabusa2 reentry capsule and transported to the JAXA Curation Facility at Sagamihara Japan without terrestrial atmospheric contamination (Yada et al., 2021). After initial and non-destructive characterizations at the JAXA Curation Facility, an airtight sample transport vessel (FFTC) and a sample capsule pack (made of sapphire glass and stainless steel with 10 mmϕ or 15 mmϕ depending on sample size) were used to avoid chemical reactions with the surrounding environment and/or terrestrial contaminants (e.g., water vapor, hydrocarbon, atmospheric gases, and small particles) and cross-contamination between samples during sample preparation and transportation among institutes and universities (Ito et al., 2020). To avoid degradation and contamination due to interaction with the terrestrial atmosphere (water vapor and oxygen gas), all the sample preparations including chipping by a tantalum chisel, cutting by a counter balanced diamond wire saw (Meiwa Fosis Corp. DWS 3400), and epoxy mount preparation) were conducted in a glove box in an atmosphere of pure, dry N2 (Dew point: -80 to -60˚C, O2 ~50 to 100 ppm). All items used here were cleaned by a combination of an ultra-pure water and ethanol under ultrasonication with different frequencies.
We studied meteorite collections (Orgueil, Yamato (Y)-791198, Y-82162, and Y 980115) of Antarctic meteorite center at the NIPR in this study.
For a transfer between instruments of SR-XCT, NanoSIMS, STXM-NEXAFS, and TEM, we used the universal holders for ultrathin film samples that described in previous studies (Ito et al., 2020; Uesugi et al., 2020; Shirai et al., 2020).
A synchrotron radiation-based computed tomography analysis
SR-CT analyses for Ryugu samples were performed with integrated computed tomography (CT) system at BL20XU/Spring-8. The integrated CT system consists of different measurement modes: wide-field and low-resolution (WL) mode to capture the entire structure of the sample, narrow-field and high-resolution (NH) mode for precise measurement of the region of interest, and X-ray diffraction (XRD) mode to acquire the diffraction pattern of bulk volume of the sample and perform XRD-CT to perform 2D mineral phase mapping of a horizontal plane in the sample. The X-ray detector for the WL mode (BM AA40P; Hamamatsu Photonics) is equipped with a complementary metal oxide semiconductor (CMOS) camera, which has 4608 × 4608 pixels (C14120-20P; Hamamatsu Photonics), a scintillator consisting of a 10-μm-thick Lutetium Aluminum Garnet (Lu3Al5O12: Ce) foil, and relay lenses. The pixel size of the WL mode is approximately 0.848 µm. Thus, the field-of-view (FOV) of the WL mode is ~ 6 mm. The X-ray detector for the NH mode (BM AA50; Hamamatsu Photonics) is equipped with a scintillator consisting of Gadolinium Aluminum Gallium Garnet (Gd3Al2Ga3O12) that is 200 µm thick, the CMOS camera which has 2048 × 2048 pixels (C11440-22CU; Hamamatsu Photonics), and a 20x lens. The pixel size of the NH mode is ~0.25 µm and the FOV is ~0.5 mm. The detector for XRD mode (BM AA60; Hamamatsu Photonics) is equipped with a scintillator consisting of a P43 (Gd2O2S: Tb) powder screen that was 50 µm thick, the CMOS camera which has 2304 × 2304 pixels (C15440-20UP; Hamamatsu Photonics), and relay lenses. Pixel size of the detector is 19.05 µm and the FOV was 39 mm2. In order to increase the FOV, we applied the offset CT procedure in the WL mode. Transmitted light image for CT reconstruction was composed by combining images of 180 to 360 degrees which horizontally reflected around the rotation axis, with the images from 0 to 180 degrees.
For the XRD mode, an X-ray beam was focused by a Fresnel Zone Plate. The detector for the XRD mode was placed 110 mm behind the sample, with a 3 mm beam stop just in front of the detector. Diffraction images from 2θ = 1.43° to 18.00° were obtained by the detector. Samples were vertically translated with a certain interval, and a half-rotated for each vertical scan step. Diffraction of mineral grains in a horizontal plane can be obtained if the mineral grains meet the Bragg condition in the half rotation. Diffraction images were then integrated into one image for each vertical scan step (Uesugi et al., 2020). The SR-XRD-CT procedure is almost the same as for SR-XRD, except for the direction of scanning. The sample was scanned horizontally with half rotating of the sample. The SR-XRD-CT image was reconstructed using peak intensity of minerals as pixel value. Typically, samples were scanned with 500 to 1,000 steps for a horizontal scan.
The X-ray energy was fixed to 30 keV for all experiments because it is the lower limit for X-ray penetration of meteorites that are ~6 mm in diameter (Uesugi et al., 2010; Uesugi et al., 2013). The number of images acquired for all CT measurements during the half rotation was 1800 (3600 for offset CT procedure), and the exposure time for an image was 100 ms for the WL mode, 300 ms for the NH mode, 500 ms for XRD, and 50 ms for XRD-CT. The typical scanning time for one sample in WL mode was ~10 min, in NH mode was ~15 min, ~ 3 hours for XRD, and 8 hours for SR-XRD-CT.
CT images were reconstructed by convolution-backprojection (CBP) method and normalized for 0 to 80 cm-1 of linear attenuation coefficient. Slice software was applied for the analysis of 3D data, and muXRD software was used for the analysis of XRD data.
Optical, SEM–EDS and EPMA analyses
The epoxy mount Ryugu particles (A0029, A0037, C0009, C0014, and C0068) were polished on a surface in a stepwise manner down to the level of a 0.5 µm diamond lapping film (3M company) under dry condition avoiding elution of any materials from a surface during polishing. A polished surface of each sample was examined first by an optical microscope, and then by a JEOL JSM-7100F scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) (AZtec energy) to obtain an overview of the mineralogy and textures of the samples by high-resolution imaging and qualitative chemical mapping at NIPR. Major and minor elemental abundance analysis of each sample were conducted with an electron probe microanalyzer (EPMA, JEOL JXA-8200). A series of natural and synthetic mineral standards were used for external calibration of EPMA performed at typical operating conditions of accelerating voltage of 15 kV and beam current of 5 nA (carbonates, phosphates, olivine, pyroxene, magnetite, and sulfide) or 30 nA (natural and synthetic standards).
High precision oxygen isotopic analysis
Oxygen isotopic analysis was undertaken at the Open University (Milton Keynes, UK) using an infrared laser-assisted fluorination system (Miller et al., 1999; Greenwood et al., 2017). Four distinct Hayabusa2 samples were transported to the Open University in two sealed, nitrogen-filled “FTTC: facility-to-facility transport container” (Ito et al., 2020). One of the two FFTC contained grains from the initial Hayabusa2 touchdown collection (particle A0098,2: 5 grains), the other FFTC contained three sets of particles from the second, post impactor collection: C0014,2 1 particle 5.5 mg; C0068,2 1 particle 0.5 mg; C0087,2 approximately 10 grains, 0.8 mg. Both holders were stored at the Open University in a dedicated cabinet with a continuously purged nitrogen atmosphere. The cabinet was housed within a secure class 100 cleanroom.
Sample loading was undertaken in a nitrogen “glove box” with monitored oxygen levels below 0.1 %. A new Ni sample holder was fabricated for the Hayabusa2 analysis work that consisted of just two sample wells (2.5 mm diameter, 5 mm depth), one for the Hayabusa2 particle and the other for the internal obsidian standard. During analysis, the sample well containing the Hayabusa2 material was overlain by a ~1 mm thick, 3 mm diameter internal BaF2 window to retain the sample during laser reaction. The flow of BrF5 to the sample was maintained by gas mixing channels scribed into the Ni sample holder. The sample chamber configuration was also modified so that it could be removed from the fluorination line under vacuum and then opened within the nitrogen-filled glove box. The two-part chamber is made vacuum tight using a compression seal with a copper gasket and quick-release KFX clamp (Miller et al., 1999; Greenwood et al., 2017). A 3 mm thick BaF2 window at the top of the chamber allows simultaneous viewing and laser heating of samples. Following sample loading the chamber was then reclamped and reattached to the fluorination line. Prior to analysis the sample chamber was heated overnight under vacuum to a temperature of about 95°C to remove any adsorbed moisture. Following overnight heating, the chamber was allowed to cool to room temperature and then the section that had been brought up to atmosphere during the sample transfer process was purged using three aliquots of BrF5 to remove any moisture. These procedures ensured that the Hayabusa2 samples were never opened to the atmosphere or contaminated with moisture from those parts of the fluorination line that had been brought up to atmosphere during the sample loading procedure.
All Hayabusa2 samples were run in modified “single shot” mode (Schrader et al., 2014). Sample heating in the presence of BrF5 was carried out using a Photon Machines Inc. 50 W infrared CO2 laser (10.6 μm) mounted on an X-Y-Z gantry. Reaction progress was monitored by means of an integrated video system. After fluorination, the released O2 was purified by passing it through two cryogenic nitrogen traps and over a bed of heated KBr to remove any excess fluorine. The isotopic composition of the purified oxygen gas was analyzed using a Thermo Fisher MAT 253 dual inlet mass spectrometer with a mass resolving power of approximately 200.
In most cases the amount of O2 gas liberated during sample reaction was less than 140 µg, the approximate limit for using the bellows facility on the MAT 253 mass spectrometer. In these cases, analysis was undertaken using the microvolume. Following analysis of the Hayabusa2 particle, the internal obsidian standard was fluorinated, and its oxygen isotope composition determined.
The NF+ fragment ion of NF3+ can cause interference with the mass 33 beam (16O17O). In order to eliminate this potential problem all samples were treated using a cryogenic separation procedure. This was either done in the forward sense prior to analysis on the MAT 253 or as a second analysis with the already analyzed gas pulled back onto a dedicated molecular sieve and then rerun after cryogenic separation. Cryogenic separation involved taking the gas onto the molecular sieve at liquid nitrogen temperature and then releasing it to the main molecular sieve at a temperature of -130°C. Extensive tests have shown that NF+ is retained on the first molecular sieve and that no significant fractionation results from the use of this technique.
Overall system precision in bellows mode, as defined by replicate analyses of our internal obsidian standard, is: ± 0.053 ‰ for δ17O; ± 0.095 ‰ for δ18O; ± 0.018 ‰ for Δ17O (2σ) (Starkey et al., 2016). Oxygen isotopic analyses are reported in standard δ notation, where δ18O has been calculated as:
δ18O = [(18O /16O)sample/(18O /16O)VSMOW -1] 1000 (‰)
and similarly for δ17O using the 17O/16O ratio. VSMOW is the international standard Vienna Standard Mean Ocean Water. Δ17O, which represents the deviation from the terrestrial fractionation line, has been calculated as: Δ17O = δ17O-0.52δ18O
Sample preparation using a focused ion beam (FIB)
Approximately 150 to 200 nm-thick sections were extracted from Ryugu particles using a Hitachi High Tech SMI4050 focused ion beam (FIB) instrument at the Kochi Institute for Core Sample Research (Kochi), JAMSTEC. After deposition of tungsten protection layers, regions-of-interest (up to 25 × 25 µm2) were cut out and thinned using a Ga+ ion beam at an accelerating voltage of 30 kV and then finalized at 5 kV and probe current of 40 pA to minimize surface damage layers. Subsequently, the ultrathin sections were mounted on scaled-up Cu grids (Kochi grid, Ito et al., 2020) using a micromanipulator equipped with the FIB.
Elemental abundance by Instrumental Neutron Activation Analysis (INAA)
Ryugu particles A0098 (1.6303 mg) and C0068 (0.6483 mg) were doubly sealed in cleaned and high purity polyethylene sheets in a pure N2-filled glove box at SPring-8 without any interaction with terrestrial atmosphere. Sample preparation of JB-1 (a geological standard rock sample issued by the Geological Survey of Japan) was carried out at Tokyo Metropolitan University.
INAA was done at Institute for Integrated Radiation and Nuclear Science, Kyoto University. Samples were irradiated two times with different irradiation periods in consideration of half-lives of the nuclides used for elemental quantification. First, samples were irradiated for 30 s at the pn-3 with thermal and first neutron fluxes of 4.6 × 1012 and 9.6 × 1011 cm-2s-1, respectively, for the determination of Mg, Al, Ca, Ti, V and Mn. Chemical reagents such as MgO (99.99% purity; Soekawa Chemical, Tokyo, japan), Al (99.9% purity; Soekawa Chemical) and Si metals (99.999% purity; FUJIFILM Wako Pure Chemical, Osaka, Japan) were also irradiated to correct for interfering nuclear reactions such as (n,p). Sodium chloride (99.99% purity; MANAC, Tokyo, Japan) was also irradiated with the samples to correct for neutron flux variations.
After neutron irradiation, the outer polyethylene sheet was replaced with a new sheet and gamma rays emitted from the samples and reference standards were immediately measured using Ge detectors. The same samples were reirradiated for 4 hours at a pn-2 with thermal and first neutron fluxes of 5.6 × 1012 and 1.2 × 1012 cm-2s-1, respectively, for the determination of Na, K, Ca, Sc, Cr, Fe, Co, Ni, Zn, Ga, As, Se, Sb, Os, Ir and Au. Reference samples for Ga, As, Se, Sb, Os, Ir and Au were prepared by dropping a proper amount of concentration-known standard solutions of these elements onto the two sheets of filter papers which were then irradiated with the samples. Gamma-ray counting was carried out at Institute for Integrated Radiation and Nuclear Science, Kyoto University and RI Research Center, Tokyo Metropolitan University. For elemental quantification, a reference value of Cr for JB-1 was taken from Kong and Ebihara (1997), while literature values of Jochum et al. (2016) were used for the remaining elements. The analytical procedure of INAA is the same as that described by Shirai et al. (2020).
Bulk mineralogy by XRD
X-ray diffractometer (Rigaku SmartLab) was used to collect diffraction patterns of Ryugu samples A0029 (1<mg), A0037 (<<1 mg) and C0087 (<1 mg) at the National Institute of Polar Research. All the samples were ground to be fine powders using a sapphire glass plate on a silicon non-reflection plate, and then homogeneously placed onto the silicon non-reflection plate. The measurement conditions are as follows: Cu Ka X-ray was produced at 40 kV tube voltage and 40 mA tube current, the length of length limiting slit was 10 mm, the divergence angle was (1/6)º, in-plane rotation speed was 20 rpm, two theta range (two-fold Bragg angle) was 3–100º, and it took ~28 hours for single analysis. BraggBrentano optics was used. The detector was one-dimensional SSD (D/teX Ultra 250). Cu Kb X-ray was removed using a Ni filter. The peaks were identified from the comparison with the measured data using the available samples, synthesized Mg-saponite (JCSS-3501, Kunimine Industries CO., LTD.), serpentine (antigorite, Miyazu, Nichika #14-4-12-1), and pyrrhotite (monoclinic 4C, Chihuahua, Mexico), and using the PDF data from ICDD, dolomite (PDF 01-071-1662) and magnetite (PDF 00-019-0629). The diffraction data of Ryugu was also compared with those of hydrously altered carbonaceous chondrites, Orgueil CI, Y-791198 CM2.4, and Y 980115 CY (heating stage III). The comparison showed the similarity with Orgueil but not with Y-791198 and Y-980115.
Carbon functional groups by STXM–NEXAFS
The carbon K-edge NEXAFS spectra of ultra-thin section samples made by FIB were measured using the STXM beam line, BL4U, at the UVSOR Synchrotron Facility, Institute for Molecular Science (Okazaki, Japan). The beam spot size focused with Fresnel zone-plate optics was about 50 nm. The energy step size was 0.1 eV in the fine-structure portions of the near-edge region (283.6–292.0 eV), and 0.5 eV in the pre-edge and post-edge regions (280.0–283.5 and 292.5–300.0 eV). The acquisition time per image pixel for each energy step was set to be 2 ms. Helium gas of ~20 mbar was backfilled with the STXM analysis chamber after evacuation. This helps to minimize thermal drift of X-ray optics-related apparatus in the chamber and the sample holder, and to reduce sample damage and/or oxidation (Wang et al., 2009). The carbon K-edge NEXAFS spectra were obtained from stack data using the aXis2000 software and the in-house customized software for STXM data reduction. Note that an in-house sample transfer vessel and a glove box were used to avoid oxidization and contamination on the sample.
Hydrogen, carbon, and nitrogen isotopic imaging analysis using NanoSIMS
The hydrogen, carbon, and nitrogen isotopic compositions of the Ryugu FIB sections were analyzed using isotopic imaging with the JAMSTEC NanoSIMS 50L after STXM-NEXAFS analysis. A focused primary Cs+ beam of approximately ~2 pA was used for carbon, and nitrogen isotopic analyses, and approximately ~13 pA was used for hydrogen isotopic analyses, rastered over approximately 24 × 24 to 30 × 30 μm2 areas on the samples. Each analysis was initiated after stabilization of the secondary-ion beam intensity following three minutes of pre-sputtering with a relatively strong primary-ion beam current. For carbon and nitrogen isotopic analysis, images of 12C–, 13C–, 16O–, 12C14N– and 12C15N– were acquired simultaneously in multidetection with seven electron multipliers (EMs) at a mass resolving power of approximately 9000, sufficient to separate all relevant isobaric interferences (that is, 12C1H on 13C and 13C14N on 12C15N). For hydrogen isotopic analysis, images of 1H–, 2D–, and 12C– were acquired using three EMs in multidetection at amass resolving power of approximately 3000. Each analysis consisted of 30 scanned images of the same area, with individual images consisting of 256 × 256 pixels for the carbon and nitrogen isotopic analyses and 128 × 128 pixels for the hydrogen isotopic analysis. The dwell times were 3,000 μs/pixel for the carbon and nitrogen isotopic analyses and 5000 μs/pixel for the hydrogen isotopic analysis. We used 1-hydroxybenzotriazole hydrate as the hydrogen, carbon, and nitrogen isotopic standards to correct for instrumental mass fractionations (Ito et al., 2014).
For silicon isotopic compositions of the presolar graphite in the C0068-25 FIB section, we acquired secondary ion images of 12C–, 13C–, 16O–, 28Si–, 29Si– and 30Si– simultaneously in multidetection with six electron EMs at a mass resolving power of approximately 9000. These images consist of 256 × 256 pixels with a dwell time of 3,000 µs/pixel. We used a silicon wafer as the hydrogen, carbon, and silicon isotopic standard to correct for instrumental mass fractionation.
The isotopic images were processed using the custom written software ‘NASA JSC imaging software for NanoSIMS’ (Ito and Messenger, 2008). Data were corrected for EM dead time (44 ns), and the QSA effect (Slodzian et al., 2004). Different scans of each image were aligned to correct image drift during acquisition. Final isotopic images were generated by adding the secondary ions of each image from each pixel over the scans.
Micron to submicron scale mineralogical observations by TEM
After STXM-NEXAFS and NanoSIMS analysis, the same FIB sections were studied using a transmission electron microscope (JEOL JEM-ARM200F) operated at an accelerating voltage of 200 kV at Kochi, JAMSTEC. Microtextural observations were performed by bright-field TEM (BF-TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging. Mineral phases were identified using selected-area electron diffraction and lattice-fringe imaging, and chemical analyses using energy-dispersive X-ray spectrometry (EDS) with a 100 mm2 silicon drift detector and JEOL Analysis Station 4.30 software. For quantitative analyses, the intensities of the characteristic X-rays of each element were measured using a fixed acquisition time of 30 s, beam scan area of ~100 × 100 nm2, and beam current of 50 pA in scanning TEM mode. The (Si+Al)-Mg-Fe ratios of phyllosilicates were determined using experimental thickness-corrected k-factors obtained from a natural pyrope-almandine garnet standard.
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