09:15 〜 09:30
[PPS08-02] リュウグウ試料に含まれるナトリウム含有鉱物の記載
キーワード：リュウグウ、はやぶさ２、炭酸塩、水質変成
Introduction: Ryugu samples exhibit brecciated features that contain several different fragments [1, 2]. Most of the fragments have experienced extensive aqueous alteration, while some fragments show lesser degree of alteration. One of the most prominent chemical features that distinguish the lithologies is the concentration of sodium (Na) [1]. It has been shown that Na concentrations vary between the different fragments, including highly Na-rich lithologies [1,2]. Fragments with the least aqueous alteration have the high Na concentrations [1]. In addition, Na enrichment has been identified in hot water extracted from Ryugu grains [3]. The Na-rich phase in phyllosilicate matrix could not be identified under transmission electron microscopy (TEM) analysis It was suggested that the Na-rich phases correspond to sodium hydroxide (NaOH) that has disappeared before the analysis [2]. The Na-rich region reported in [2] consists mainly of serpentine and saponite, suggesting extensive aqueous alteration. Thus, Na-rich phases in Ryugu samples are important for understanding the early stages of interaction between silicate components and ices, or the late stage of alteration with various chemical conditions. However, mineral phase rich in sodium have been poorly restricted so far. In this study, we examined sodium-rich minerals in Ryugu grains recovered from the second touch-down site.
Methods: Ryugu particle (C0071) with a diameter of ~1.5 mm was examined by scanning electron microscopy (SEM) and X-ray tomography in a non-destructive condition without exposure to the air, and then analyzed by scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM). Additionally, fine grains in the hundreds of micrometers (C369) were examined by SEM and STEM.
Results and Discussions: Surface observation of the Ryugu grains by SEM revealed the presence of mineral veins rich in sodium with a width ranging from 6-20 µm, with a length extending up to 500 µm. The sodium-rich vein is poor in silicon, magnesium and iron, and is clearly distinct from the surrounding phyllosilicates matrix. Examination of the three-dimensional shape of the C0071 grain by X-ray CT confirmed that the vein develops along the edge of a flat surface that has several spots enriched in sodium. Three fine grains among ~200 grains we investigated have sodium-rich veins with < ~10 µm in width and <~140 µm in length. STEM analysis revealed that the sodium-rich veins consist of natron (Na2CO3·10H2O) and natrite (Na2CO3). Sodium carbonates can precipitate by evaporation or freezing conditions at the late stage of aqueous alteration [4]. The sodium carbonate veins found in this study may record either the final stage of aqueous alteration or post-alteration fluid activity. These minerals have been identified on icy dwarf planet Ceres and plumes from icy satellite Enceladus [5, 6], suggesting a similarity in characteristics of fluids on these icy bodies and Ryugu.
References:
[1] Nakamura et al (2022) Science, eabn8671. [2] Yamaguchi et al. (2023) Nature Astronomy, 1–8. [3] Yoshimura (2023) Nature Comuunications. 5284. [4] Zolotov et al. (2012) Icarus. 220.2. 713-729. [5] De Sanctis et al. (2016) Nature,536.7614. 54-57. [6] Postberg et al. (2009) Nature, 459. 7250. 1098-1101.
Methods: Ryugu particle (C0071) with a diameter of ~1.5 mm was examined by scanning electron microscopy (SEM) and X-ray tomography in a non-destructive condition without exposure to the air, and then analyzed by scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM). Additionally, fine grains in the hundreds of micrometers (C369) were examined by SEM and STEM.
Results and Discussions: Surface observation of the Ryugu grains by SEM revealed the presence of mineral veins rich in sodium with a width ranging from 6-20 µm, with a length extending up to 500 µm. The sodium-rich vein is poor in silicon, magnesium and iron, and is clearly distinct from the surrounding phyllosilicates matrix. Examination of the three-dimensional shape of the C0071 grain by X-ray CT confirmed that the vein develops along the edge of a flat surface that has several spots enriched in sodium. Three fine grains among ~200 grains we investigated have sodium-rich veins with < ~10 µm in width and <~140 µm in length. STEM analysis revealed that the sodium-rich veins consist of natron (Na2CO3·10H2O) and natrite (Na2CO3). Sodium carbonates can precipitate by evaporation or freezing conditions at the late stage of aqueous alteration [4]. The sodium carbonate veins found in this study may record either the final stage of aqueous alteration or post-alteration fluid activity. These minerals have been identified on icy dwarf planet Ceres and plumes from icy satellite Enceladus [5, 6], suggesting a similarity in characteristics of fluids on these icy bodies and Ryugu.
References:
[1] Nakamura et al (2022) Science, eabn8671. [2] Yamaguchi et al. (2023) Nature Astronomy, 1–8. [3] Yoshimura (2023) Nature Comuunications. 5284. [4] Zolotov et al. (2012) Icarus. 220.2. 713-729. [5] De Sanctis et al. (2016) Nature,536.7614. 54-57. [6] Postberg et al. (2009) Nature, 459. 7250. 1098-1101.