High energy synchrotron X-ray fluorescence trace element study of a millimeter-sized asteroidal particle in preparation for the Hayabusa2 return sample analyses

Abstract

The trace element content and distribution - including rare Earth elements (REEs) - measured in mm-sized asteroidal samples returned by JAXA's Hayabusa2 mission are important chemical parameters to decipher asteroid Ryugu's chronology of formation linked to early Solar System processes. In order to identify and analyze ancient Solar Nebula components, such as calcium‑aluminum-rich inclusions (CAIs), by their trace element and REE content, a synchrotron beam with an incident energy of 90 keV is used to optimize the XRF signal and increase the information depth in the sample. The application of a (sub-)microscopic X-ray beam with such a high excitation energy not only allows for the detection of a wide range of heavy elements, but also to study their 3-dimensional distribution in mm-sized samples by means of X-ray fluorescence computed tomography (XRF-CT). The experiment was performed in anticipation of the initial analysis of the Hayabusa2 return samples at beamline ID15a of the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The samples were analyzed with a focused beam of 0.5 × 0.5 μm2, achieving limit of detection values as low as 0.5 ppm with an acquisition time of 1 s. Here we present results of scanning XRF(-CT) analysis of a mm-sized sample of the Murchison meteorite, a Mighei type chondrite (CM2), wherein a 9.6 × 11.5 × 8.2 μm3 CAI phase was detected and analyzed. The CAI grain is shown to be detectable throughout the entire sample volume (~700 μm) during an XRF-CT scan over a full 360° angular range, thus proving the applicability of the method to study the microscopic distribution of high-Z elements at trace level concentration within millimeter-sized asteroidal particles.