Rare earth element identification and quantification in millimetre-sized Ryugu rock fragments from the Hayabusa2 space mission

Abstract

Millimetre-sized primordial rock fragments originating from asteroid Ryugu were investigated using high energy X-ray fluorescence spectroscopy, providing 2D and 3D elemental distribution and quantitative composition information on the microscopic level. Samples were collected in two phases from two sites on asteroid Ryugu and safely returned to Earth by JAXA's asteroid explorer Hayabusa2, during which time the collected material was stored and maintained free from terrestrial influences, including exposure to Earth's atmosphere. Several grains of interest were identified and further characterised to obtain quantitative information on the rare earth element (REE) content within said grains, following a reference-based and computed-tomography-assisted fundamental parameters quantification approach. Several orders of magnitude REE enrichments compared to the mean CI chondrite composition were found within grains that could be identified as apatite phase. Small enrichment of LREE was found for dolomite grains and slight enrichment or depletion for the general matrices within the Ryugu rock fragments A0055 and C0076, respectively.

Supplementary information: The online version contains supplementary material available at 10.1186/s40623-022-01705-3.

Keywords: Fundamental parameter quantification; Hayabusa2; REE; Ryugu; X-ray fluorescence spectroscopy.

Fig. 1

Instrumental overview photograph of the setup as applied at beamline ID15A (ESRF, Grenoble, France) during the experiment

Fig. 2

XRF overview elemental distribution images (5 µm step size, 0.2 s/pt) for Hayabusa2 return sample A0055 (A) and XRF–CT cross-sectional RGB image (B) presenting a virtual horizontal cut through point 2 displaying the Ce, Sr and Compton scatter distribution within the sample (5 µm × 0.4° steps, 0.2 s/pt). Yellow marks and numbers indicate the points of interest that were further investigated

Fig. 3

XRF spectra corresponding to the measurements in points 1 to 4 in rock fragment A0055, as indicated in Fig. 2A (600 s/pt). A clear enhancement in the REE region is observed for points 1 and 2. XRF spectra were normalised for the Ta–K_α signal intensity to provide more straightforward comparison. Magnified inserts of two select energy ranges marked by dashed bounding boxes in light blue and red are displayed in parts B and C, respectively

Fig. 4

XRF yields for the different reference materials that were investigated, as calculated by Eq. 2. Error bars indicate three standard error margins (99.7% confidence interval)

Fig. 5

CT slices that were obtained at SPring-8 beamline 20XU (Nakamura et al. 2022b) show the positions of the points of interest indicated in Fig. 2A. A red arrow marks the primary X-ray beam path and direction, fluorescence detector was positioned at the left of the CT image. Yellow circles indicate the Ca-rich grains (points 1–3) from which REE information is primarily obtained. Point 4 is a matrix measurement and as such has no distinct Ca-rich grains. Separate grains along the beam path are indicated by their respective size along the beam path in orange, along with an estimate of the mineralogical phase for the larger grains

Fig. 6

REE trend of different phases within the Ryugu A0055 and C0076 rock fragments. Error bars indicate three standard error margins (99.7% confidence interval)