11:30 〜 11:45
[PPS07-10] 微量元素の価数決定によるRyugu母天体における水質変成過程の推定
キーワード：リュウグウ、小惑星、水質変成、XAFS
Ryugu is one of the first C-type asteroids that were directly observed and sampled by a spacecraft. Ryugu and CI chondrite such as Orgueil meteorite are believed to have experienced aqueous alteration at low temperature, whose environment is estimated to have been reductive and alkaline. Terrestrial organic matters and life today may have originated from abiotically synthesized organic matters in such settings. Determining the Eh and pH in the environment is crucial to understanding which organic synthesis reactions could have occurred.
Indirect evidence such as iron (Fe) valence ratio in phyllosilicate and arsenic valence in apatite have been used in aqueous environment estimation (Nakamura et al., 2023). However, using the valence state of trace elements distributed throughout the phyllosilicate matrix will allow the discussion based on direct evidence from the sample. Some trace elements change their valence in response to the redox environment, and determining these changes can constrain the Eh-pH environment during the aqueous alteration. In this study, seven trace and major elements, chromium (Cr), vanadium (V), titanium (Ti), iron (Fe), arsenic (As), selenium (Se) and sulfur (S), from Ryugu and Orgueil meteorite samples were investigated. Resin-embedded and polished Ryugu specimens were prepared under air-tight glovebox conditions to prevent sample oxidation. In order to determine their chemical species, bulk scale and micron-scale X-ray Absorption Near Edge Structure (XANES) spectra were measured using synchrotron radiation in dedicated facilities. Micron-scale XANES spectra were collected with the image stack method, which provides three-dimensional (2D-area and energy dimension) data and exploited to discuss both concentration and elemental chemical composition.
Divalent Cr was detected in the Ryugu sample locally (Cr-spot), while trivalent Cr was detected in the rest of phyllosilicate matrix. The μ-XRD results indicated that pyrrhotite (Fe7S8) was one of the main crystalline phases in the Cr-spot, suggesting that phyllosilicate minerals in Cr-spot were also formed in highly reductive, non-equilibrium environment.
2D-XRF analysis of V suggested that it was widely distributed throughout the sample. Its bulk valence was calculated using a way proposed in a previous study (Sutton et al., 2005), based on the pre-edge peak height. Consequently, it was calculated that V was trivalent in the Ryugu sample, and tetravalent in the Orgueil meteorite.
XANES spectra of Se in the Ryugu sample showed to be consistent with iron selenide, supporting the indication of a reductive environment during aqueous alteration.
Analysis of Ti in the Ryugu and Orgueil samples revealed that most of the Ti-rich phases are tetravalent, these have been identified as ilmenite and titanite. As(V) was detected in the phyllosilicate matrix in Ryugu sample, which is not the dominant species in the aqueous environment of Ryugu’s parent body. As(V) species are also reported in terrestrial phyllosilicate formed in reductive condition (Hattori et al., 2005), suggesting that As species in phyllosilicates are regulated by the structure of host phases. These two elements are considered to reflect structural constraint of host phases, rather than the aqueous environmental record per se.
The presence of Cr(II) species found locally in the Ryugu sample suggests that the aqueous environment experienced extremely reductive and non-equilibrium state when phyllosilicate minerals in Cr-spot formed. The widely distributed V(III) chemical species throughout the Ryugu sample imply that the Eh environment evolved during the aqueous alteration and that the chemical species record in the matrix area may be overwritten, except for Cr. Considering the mineral formation sequence inside Ryugu’s parent body (Tsuchiyama et al., 2024), the Eh-pH record in saponite and apatite is reflecting the late stage of the aqueous alteration environment.
Indirect evidence such as iron (Fe) valence ratio in phyllosilicate and arsenic valence in apatite have been used in aqueous environment estimation (Nakamura et al., 2023). However, using the valence state of trace elements distributed throughout the phyllosilicate matrix will allow the discussion based on direct evidence from the sample. Some trace elements change their valence in response to the redox environment, and determining these changes can constrain the Eh-pH environment during the aqueous alteration. In this study, seven trace and major elements, chromium (Cr), vanadium (V), titanium (Ti), iron (Fe), arsenic (As), selenium (Se) and sulfur (S), from Ryugu and Orgueil meteorite samples were investigated. Resin-embedded and polished Ryugu specimens were prepared under air-tight glovebox conditions to prevent sample oxidation. In order to determine their chemical species, bulk scale and micron-scale X-ray Absorption Near Edge Structure (XANES) spectra were measured using synchrotron radiation in dedicated facilities. Micron-scale XANES spectra were collected with the image stack method, which provides three-dimensional (2D-area and energy dimension) data and exploited to discuss both concentration and elemental chemical composition.
Divalent Cr was detected in the Ryugu sample locally (Cr-spot), while trivalent Cr was detected in the rest of phyllosilicate matrix. The μ-XRD results indicated that pyrrhotite (Fe7S8) was one of the main crystalline phases in the Cr-spot, suggesting that phyllosilicate minerals in Cr-spot were also formed in highly reductive, non-equilibrium environment.
2D-XRF analysis of V suggested that it was widely distributed throughout the sample. Its bulk valence was calculated using a way proposed in a previous study (Sutton et al., 2005), based on the pre-edge peak height. Consequently, it was calculated that V was trivalent in the Ryugu sample, and tetravalent in the Orgueil meteorite.
XANES spectra of Se in the Ryugu sample showed to be consistent with iron selenide, supporting the indication of a reductive environment during aqueous alteration.
Analysis of Ti in the Ryugu and Orgueil samples revealed that most of the Ti-rich phases are tetravalent, these have been identified as ilmenite and titanite. As(V) was detected in the phyllosilicate matrix in Ryugu sample, which is not the dominant species in the aqueous environment of Ryugu’s parent body. As(V) species are also reported in terrestrial phyllosilicate formed in reductive condition (Hattori et al., 2005), suggesting that As species in phyllosilicates are regulated by the structure of host phases. These two elements are considered to reflect structural constraint of host phases, rather than the aqueous environmental record per se.
The presence of Cr(II) species found locally in the Ryugu sample suggests that the aqueous environment experienced extremely reductive and non-equilibrium state when phyllosilicate minerals in Cr-spot formed. The widely distributed V(III) chemical species throughout the Ryugu sample imply that the Eh environment evolved during the aqueous alteration and that the chemical species record in the matrix area may be overwritten, except for Cr. Considering the mineral formation sequence inside Ryugu’s parent body (Tsuchiyama et al., 2024), the Eh-pH record in saponite and apatite is reflecting the late stage of the aqueous alteration environment.