Trace-element analysis of mineral grains in Ryugu rock fragment sections by synchrotron-based confocal X-ray fluorescence

https://doi.org/10.1186/s40623-022-01726-y

“A fundamental parameter-based quantification scheme for confocal XRF was applied to sub-micron synchrotron radiation X-ray fluorescence (SR-XRF) data obtained at the beamline P06 of the Deutsches Elektronen-Synchrotron (DESY, Hamburg, Germany) from two sections C0033-01 and C0033-04 that were wet cut from rock fragment C0033 collected from Cb-type asteroid (162173) Ryugu by JAXA’s Hayabusa2 mission. Trace-element quantifications show that C0033 bulk matrix is CI-like, whereas individual mineral grains (i.e., magnetite, pyrrhotite, dolomite, apatite and breunnerite) show, depending on the respective phase, minor to strong deviations. The non-destructive nature of SR-XRF coupled with a new PyMca (a Python toolkit for XRF data analysis)-based quantification approach, performed in parallel with the synchrotron experiments, proves to be an attractive tool for the initial analysis of samples from return missions, such as Hayabusa2 and OSIRIS-REx, the latter returning material from a B-type asteroid (101955) Bennu in 2023.”

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Trace-element concentrations of individual minerals, i.e., apatite, dolomite, breunnerite, (framboidal) magnetite and pyrrhotite, in the studied C0033 sections were derived from SR-XRF spectra obtained at the PETRA III facility of the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, with the energy-dispersive micro-XRF setup (Fig. 1) at the Hard X-ray Micro/Nano-Probe beamline P06 (Schroer et al., 2010). The experiment made use of a sub-micron beam of X-rays monochromatized by a Si(111) double-crystal monochromator and subsequently focused by a Kirkpatrick-Baez (KB) mirror pair. Confocal data sets were acquired using an XOS focusing optic (Fig. 1) with a focal distance of 2.2 mm and an enclosure length of 27.5 mm, coupled with a Vortex-EM silicon drift detector (SDD) (model No: 578-VTX-EM-300). This polycapillary optic consists of an array of many glass fibres with micron-sized internal channels, oriented to only accept X-ray photons originating from a micron-sized volume. Polycapillaries guide the X-rays based on repeated total reflections inside the glass channels when the X-rays hit a surface with a lower angle than the critical angle of total reflection θC(mrad)≈30/E(keV)θC(mrad)≈30/E(keV), where E is the X-ray photon energy. For a more detailed explanation of detecting fluorescent X-rays of a selected area with a polycapillary optic, the reader is referred to Vincze et al. (2004). The single-element SDD has a 50 mm² active area collimated on chip, having a 350 µm crystal thickness, and a 12 µm Be window combined with an Xspress 3 readout system by Quantum Detectors Limited. After confocal alignment with a 10 mμμ GoodFellow stainless steel wire, the detector optic showed a FWHM acceptance of 16.4 µm at Fe-KαK�, defining the maximum probed dimension within the sample along the X-ray beam at 6.4 keV energy. The confocal depth resolution at different energies E can be approximated by 16.4μm∗6.4keV/E(keV)16.4�m∗6.4keV/E(keV) (Vincze et al., 2004). Imaging data sets were acquired using a 4-element Vortex-ME4 SDD (model No: 865-VTX-ME4-300) with a total collimated active area of 170 mm², a 350 µm crystal, a 12.5 µm Be window and an Xspress 3 Mini readout system. The latter used an Al collimator to minimize the spectral contribution of background scatter and undesired signals from the sample environment. For the SR-XRF measurements, the samples were probed in a position characterised by a 240 nm (H) × 200 nm (V) beam size at an excitation energy of 20.5 keV. As the Ryugu material was found to be rich in Fe, the incident flux was attenuated accordingly to minimize detector pile-up. For confocal point measurements, the beam was 90% attenuated. For overview scans and point measurements with the 4-element detector, the beam was attenuated by 90% and 99%, respectively. Although most of the flux had to be attenuated, the P06 beamline was chosen for its small beamsize, which was needed to investigate the micron-sized mineral grains. In the experimental setup, the incident X-ray beam impinged on the sample at an angle of 90° and the confocal and multi-element detectors were positioned, respectively, under opposite scatter angles of 47° and 72° with respect to the primary beam, as shown in Fig. 1.

XRF detection setup used at beamline P06 to analyse a Ryugu sections C0033-01 and C0033-04 fixed in epoxy disks and b the geological glass reference material ATHO-G. Confocal SDD detector on the left, 4-element SDD detector on the right. Both are positioned, respectively, under opposite angles of 47° and 72°. The incident SR beam is indicated with a red arrow

Section C0033-01 is dominated by a prominent magnetite rich vein that cross-cuts the complete section (Fig. 2a). A high-resolution SR-XRF scan of the vein area displayed as an RGB (Ca–Fe–S) image (Fig. 2b) confirms that the vein is largely composed of magnetite (green) grains but also includes some rare pyrrhotite (blue–green) grains and laths. Also present within the vein are several Ca-rich grains 5–10 µm in size.

a BSE image of C0033-01 after epoxy fixation. The magnetite vein is highlighted in yellow. b High-resolution SR-XRF overview scan (200 nm step size) of the magnetite region presented in a. Unlike the BSE image that includes only surface information, the SR-XRF image includes additional SR-XRF signals received from subsurface layers of C0033-01, which extends to a broader area beneath the surface. The SR-XRF overview scan is displayed as an RGB image showing the K-line intensities of Ca (Red), Fe (Green), and S (Blue). Confocal point analyses #1, #2 and #3 are indicated with a cross

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Section C0033-04 is a cross-sectional slice of a Ca-rich part of rock fragment C0033 (Fig. 3) and shows a widespread abundance of magnetite grains and framboids, pyrrhotite laths and fragments, and a light grey matrix in which darker areas of less dense material can be observed. Four areas (Fig. 3, yellow boxes) were selected for the performance of high-resolution SR-XRF scans (maps 1, 2, 3 and 4). The high-resolution SR-XRF scans (Fig. 4b, d, f and g) reveal the distribution of Ca-rich phases (such as apatite and dolomite) and Fe-rich phases (such as magnetite and pyrrhotite).

BSE image of C0033-04 after epoxy fixation. SR-XRF overview scans are highlighted in yellow

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BSE images (a, c, e, g) of maps 1, 2, 3 and 4, respectively (indicated in Fig. 3). High-resolution non-confocal SR-XRF scans displayed as RGB images showing the K-line intensities of Ca (R), Fe (G), and S (B) (b, 250 nm step size), Ca (R), Fe (G), and Cr (B) (d and h, 100 nm step size) and Ca (R), Fe (G), and Mn (B) (f, 200 nm step size). Red/white crosses mark the points, where confocal point analyses were performed, the results of which are shown in Tables 2 and 3. Due to difficult sample positioning, there is a slight misalignment between the BSE and SR-XRF images

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