講演情報
[PPS12-10]Microscale Distribution of Be and B in Ryugu: Mineralogical Constraints from In Situ Ion Imaging
*国広 卓也1、ミクルシチャク ノア1 (1.岡山大学 惑星物質研究所)
キーワード：
リュウグウ、ベリリウム、ホウ素、二次イオン質量分析法
Bulk analyses of samples returned from the C-type asteroid Ryugu revealed a pronounced negative correlation between Be and B concentrations [1]. This anticorrelation cannot be explained by volatility-driven fractionation or simple parent-body aqueous redistribution and instead implies the presence of chemically distinct host phases. Identifying the mineralogical carriers of Be and B is therefore essential for understanding the origin of this systematics and its implications for precursor heterogeneity in the early solar system. However, the microscale residence of these elements in Ryugu particles has not yet been resolved.
To address this issue, we performed in situ secondary ion mass spectrometry (SIMS) ion imaging of a polished section of Ryugu particle C0027 in order to determine the spatial distributions of Be and B at the micrometer scale. The Be ion map reveals discrete, sharply bounded domains a few micrometers in size that are localized within a Be-poor background. These Be-rich domains are heterogeneously distributed and are not associated with corresponding enrichments in B. In contrast, B exhibits a comparatively diffuse distribution and shows no spatial correlation with the Be-rich regions identified in the same analytical area.
Correlative electron microscopic observations demonstrate that the Be-rich domains correspond to euhedral Cr-rich oxide grains identified as chromite. These oxide grains are systematically associated with dense phyllosilicate nodules (DPNs) and commonly occur near their margins. Despite their extremely low modal abundance (~0.05 vol%), the Cr-rich oxides host a substantial fraction of the total Be budget in the analyzed particle. Boron, in contrast, is predominantly hosted by phyllosilicates forming both the matrix and the interiors of DPNs, indicating fundamentally different mineral reservoirs for the two elements.
The clear spatial decoupling of Be and B at the micrometer scale demonstrates that their bulk anticorrelation in Ryugu cannot be reproduced solely by parent-body aqueous alteration processes operating within the present mineral assemblage. Instead, the observed Be–B systematics most plausibly reflect inheritance from chemically heterogeneous precursor materials that experienced Be–B fractionation prior to accretion onto the Ryugu parent body. The presence of Be-rich Cr-spinel phases provides a mechanism for efficient sequestration of Be during high-temperature processing, preserving precursor signatures through subsequent aqueous alteration.
References: Nakamura E. et al. (2022) Proc. Jpn. Acad. Ser. B, 98, 227–282.
To address this issue, we performed in situ secondary ion mass spectrometry (SIMS) ion imaging of a polished section of Ryugu particle C0027 in order to determine the spatial distributions of Be and B at the micrometer scale. The Be ion map reveals discrete, sharply bounded domains a few micrometers in size that are localized within a Be-poor background. These Be-rich domains are heterogeneously distributed and are not associated with corresponding enrichments in B. In contrast, B exhibits a comparatively diffuse distribution and shows no spatial correlation with the Be-rich regions identified in the same analytical area.
Correlative electron microscopic observations demonstrate that the Be-rich domains correspond to euhedral Cr-rich oxide grains identified as chromite. These oxide grains are systematically associated with dense phyllosilicate nodules (DPNs) and commonly occur near their margins. Despite their extremely low modal abundance (~0.05 vol%), the Cr-rich oxides host a substantial fraction of the total Be budget in the analyzed particle. Boron, in contrast, is predominantly hosted by phyllosilicates forming both the matrix and the interiors of DPNs, indicating fundamentally different mineral reservoirs for the two elements.
The clear spatial decoupling of Be and B at the micrometer scale demonstrates that their bulk anticorrelation in Ryugu cannot be reproduced solely by parent-body aqueous alteration processes operating within the present mineral assemblage. Instead, the observed Be–B systematics most plausibly reflect inheritance from chemically heterogeneous precursor materials that experienced Be–B fractionation prior to accretion onto the Ryugu parent body. The presence of Be-rich Cr-spinel phases provides a mechanism for efficient sequestration of Be during high-temperature processing, preserving precursor signatures through subsequent aqueous alteration.
References: Nakamura E. et al. (2022) Proc. Jpn. Acad. Ser. B, 98, 227–282.