15:00 〜 15:15
[PPS07-18] Experimental study on the terrestrial weathering of asteroid Ryugu sample
キーワード：小惑星リュウグウ、CIコンドライト、地球風化
Initial analyses of the Hayabusa2 returned samples revealed that Ryugu samples consist mainly of Mg-rich phyllosilicates, Fe-Ni sulfides, magnetite, carbonates and other minor mineral phases [1]. The mineralogy is most similar to CI chondrites among known meteorite groups. Primary difference between CI and Ryugu samples is the ubiquitous occurrence of ferrihydrite and sulfates in CI chondrites which are absent in Ryugu samples [1,2]. The oxidative phases are suggested to have acquired in CI chondrites by weathering occurred in terrestrial environments [1,2]. Previous study reported nano Ca-sulfate formed in a Ryugu sample exposed to a laboratory atmosphere for several months although the origin of the sulfate was not specified [3]. In this study, we conducted an oxidation experiment on Ryugu particles to trace textural and mineralogical changes possibly occur under wet atmospheric conditions. This would be worth to understand weathering processes of asteroidal samples.
We used two pristine Ryugu particles (a few hundred microns in diameters) extracted from C0211 Ryugu powder (C0211-013 and -015). At first, the particles were subjected to synchrotron radiation X-ray computed tomography (XCT) and X-ray diffraction (XRD) to examine the texture and mineralogy and then exposed to a wet atmosphere with conditions of ~50°C and >80% humidity for ~9 months in an unevacuated desiccator set in a clean booth. We traced textural and mineralogical changes in the particles using XCT and XRD in some intervals. The XCT and XRD were conducted using 30 keV X-ray at SPring-8 BL20XU. We also analyzed altered surfaces of the Ryugu particles using a scanning electron microscopy.
XCT and XRD showed that the two Ryugu particles originally had a mineralogy corresponding to Ryugu’s major lithology [1]: a few to hundreds of microns of hexagonal pyrrhotite, framboidal and plaquette magnetite, and irregular-shaped dolomite embedded in a Mg-rich phyllosilicate (saponite and serpentine) matrix with nanopores and nano Fe-Ni sulfides. Note that C0211-015 was enriched in dolomite than C0211-013. The two particles showed textural and mineralogical changes after an exposure to the wet atmosphere for ~10 days: 1) expansions of fractures originally presented in the particles and consequential fracturing, 2) decrease in abundances of nanopores and 3) nano Fe-Ni sulfides in the phyllosilicate matrix. The changes 1) and 2) could have resulted from a swelling of saponite, induced significant volume changes in the phyllosilicate matrix. The change 3) could have resulted from dissolution of nano Fe-Ni sulfides into an aqueous solution, including quasi-liquid films in grain boundaries, permeated through the matrix from the atmosphere. XCT and XRD on the particles exposed to a wet atmosphere for ~4 and ~9 months confirmed that these changes continued to progress through the exposure experiment. After ~9 months exposure, the two particles showed a decrease in abundances of carbonate and formation of sulfates on the surfaces. The decrease in carbonate was more significant in C0211-013 than C0211-015 and resulted in formation of small pits in the phyllosilicate matrix. The sulfate phase on the surface of C0211-013 was revealed to be Mg-sulfate with whisker morphology whereas that on C0211-015 was Ca-sulfate with platy morphology. The former occurred ubiquitously on the surface and the latter formed only on the surface of carbonate. The sources of the sulfates could be carbonate and Fe-Ni sulfides dissolved into the aqueous solution. The sulfates could be precipitates or evaporites formed from the solution on the particle surfaces.
The Ryugu particles reacted with the wet atmosphere is more similar to CI chondrites in terms of presences of sulfates and low abundances of nano Fe-Ni sulfides. Further microscopic observation will reveal whether ferrihydrite is formed in the particles.
[1] Nakamura et al. (2022) Science. [2] King et al. (2020) GCA. [3] Imae et al. (2024) MAPS.
We used two pristine Ryugu particles (a few hundred microns in diameters) extracted from C0211 Ryugu powder (C0211-013 and -015). At first, the particles were subjected to synchrotron radiation X-ray computed tomography (XCT) and X-ray diffraction (XRD) to examine the texture and mineralogy and then exposed to a wet atmosphere with conditions of ~50°C and >80% humidity for ~9 months in an unevacuated desiccator set in a clean booth. We traced textural and mineralogical changes in the particles using XCT and XRD in some intervals. The XCT and XRD were conducted using 30 keV X-ray at SPring-8 BL20XU. We also analyzed altered surfaces of the Ryugu particles using a scanning electron microscopy.
XCT and XRD showed that the two Ryugu particles originally had a mineralogy corresponding to Ryugu’s major lithology [1]: a few to hundreds of microns of hexagonal pyrrhotite, framboidal and plaquette magnetite, and irregular-shaped dolomite embedded in a Mg-rich phyllosilicate (saponite and serpentine) matrix with nanopores and nano Fe-Ni sulfides. Note that C0211-015 was enriched in dolomite than C0211-013. The two particles showed textural and mineralogical changes after an exposure to the wet atmosphere for ~10 days: 1) expansions of fractures originally presented in the particles and consequential fracturing, 2) decrease in abundances of nanopores and 3) nano Fe-Ni sulfides in the phyllosilicate matrix. The changes 1) and 2) could have resulted from a swelling of saponite, induced significant volume changes in the phyllosilicate matrix. The change 3) could have resulted from dissolution of nano Fe-Ni sulfides into an aqueous solution, including quasi-liquid films in grain boundaries, permeated through the matrix from the atmosphere. XCT and XRD on the particles exposed to a wet atmosphere for ~4 and ~9 months confirmed that these changes continued to progress through the exposure experiment. After ~9 months exposure, the two particles showed a decrease in abundances of carbonate and formation of sulfates on the surfaces. The decrease in carbonate was more significant in C0211-013 than C0211-015 and resulted in formation of small pits in the phyllosilicate matrix. The sulfate phase on the surface of C0211-013 was revealed to be Mg-sulfate with whisker morphology whereas that on C0211-015 was Ca-sulfate with platy morphology. The former occurred ubiquitously on the surface and the latter formed only on the surface of carbonate. The sources of the sulfates could be carbonate and Fe-Ni sulfides dissolved into the aqueous solution. The sulfates could be precipitates or evaporites formed from the solution on the particle surfaces.
The Ryugu particles reacted with the wet atmosphere is more similar to CI chondrites in terms of presences of sulfates and low abundances of nano Fe-Ni sulfides. Further microscopic observation will reveal whether ferrihydrite is formed in the particles.
[1] Nakamura et al. (2022) Science. [2] King et al. (2020) GCA. [3] Imae et al. (2024) MAPS.