17:15 〜 18:45
[MZZ45-P06] Elemental abundance variations in Ryugu grains
キーワード：asteroid Ryugu, carbonaceous chondrites, bulk chemical composition, aqueous alteration
Samples from the carbonaceous asteroid Ryugu, retrieved by the Hayabusa2 mission [1], consist mainly of phyllosilicates with iron sulfides, carbonates, magnetite, and phosphates, showing mineralogical and chemical compositions similar to those of Ivuna-type carbonaceous (CI) chondrites [2, 3]. Furthermore, like CIs [4, 5], Ryugu samples exhibit chemical heterogeneity at the mm scale, which seems to stem from aqueous alteration in its parent body [3, 6].
Variations in elemental abundances, along with nucleosynthetic isotope anomalies in various elements including Cr and Ti [6], reflect not only the conditions and processes that the precursor materials underwent prior to accretion, but also the secondary processes in their accreted bodies [4-6]. Therefore, determining the variability in elemental and isotopic compositions, as well as attempting to elucidate the processes behind the observed data in materials such as Ryugu and CI chondrites will provide important clues for understanding the history within the protoplanetary disk.
In this study, we aim to investigate the small-scale chemical heterogeneity and the variation of Cr-Ti isotopic compositions against elemental abundances in Ryugu and CI chondrites to gain better insights into the conditions and processes that these materials have undergone during their formation and evolution.
Eight Ryugu grains (A0066, A0238, A0247, A0256, A0259, A0268, A0301, A0313) provided in the second Announcement of Opportunity by JAXA and fifteen CI chondrite samples (ten Orgueil, five Ivuna), with masses between 1-4 mg, were individually weighed into Teflon vials for acid digestion. The digested samples were then dissolved in 0.5 M HNO3, from which 10-20% of the solution was removed for the measurement of 54 elements using ICP-MS. In future work, the isotopic compositions of Cr and Ti of the samples will be measured using TIMS and MC-ICP-MS.
The bulk chemical compositions of the eight Ryugu grains show a wide variation in the abundance of most elements. In particular, P, Ca, Mn, Sr, and rare earth elements (REEs) show a broader range (P: 0.048-0.235 wt.%, Ca: 0.40-3.55 wt.%, Mn: 0.075-0.691 wt.%, Sr: 3.16-24.1 ppm, Nd: 0.341-1.03 ppm) (Fig.1a) compared to the CI chondrites analyzed in this study (Fig.1b), while similar to other Ryugu samples from the first touchdown [2,6] (Fig.1c).
The variation in abundance of these elements can be related to the small-scale heterogeneity between the grains due to the presence of secondary minerals produced during aqueous alteration in the parent body, as previously reported [2,3,6]. The occurrence of carbonates is evident in samples A0066, A0247, A0301, and A0313, which are characterized by elevated abundances of Ca, Mn, and Sr compared to CIs. The presence of phosphates can be seen in samples A0259, A0268, A0301, and A0313, which have higher P abundances than CI chondrites. Additionally, higher REE abundances than CIs in samples A0247, A0259, A0301, and A0313 indicate the fractionation of these elements during the formation of carbonates and phosphates (Fig.1a).
While the enrichments or depletions of P, Ca, Mn, Sr, and REEs in most of the samples can be related to the fractionation during the formation of secondary minerals, sample A0256 exhibits significant depletions in all these elements (Fig.1a), compared to the mean CI composition (element/CI ratios between 0.41-0.77). This depletion could indicate either a low modal abundance of carbonates and phosphates or that this grain underwent some other alteration event causing their loss at a bulk sample scale.
References: [1] Yada T. et al. (2022), Nat. Astron., 6(2). [2] Nakamura T. et al. (2022), Proc. Jpn. Acad., Ser. B, 98(6). [3] Yokoyama T. et al. (2023), Science, 379(6634). [4] Morlok A. et al., Geochim. Cosmochim. Acta, 70(21). [5] Barrat J. A. et al. (2012), Geochim. Cosmochim. Acta, 83. [6] Yokoyama T. et al. (2023), Sci. Adv., 9(45). [7] Lodders K. (2021), Space Sci. Rev., 217.
Variations in elemental abundances, along with nucleosynthetic isotope anomalies in various elements including Cr and Ti [6], reflect not only the conditions and processes that the precursor materials underwent prior to accretion, but also the secondary processes in their accreted bodies [4-6]. Therefore, determining the variability in elemental and isotopic compositions, as well as attempting to elucidate the processes behind the observed data in materials such as Ryugu and CI chondrites will provide important clues for understanding the history within the protoplanetary disk.
In this study, we aim to investigate the small-scale chemical heterogeneity and the variation of Cr-Ti isotopic compositions against elemental abundances in Ryugu and CI chondrites to gain better insights into the conditions and processes that these materials have undergone during their formation and evolution.
Eight Ryugu grains (A0066, A0238, A0247, A0256, A0259, A0268, A0301, A0313) provided in the second Announcement of Opportunity by JAXA and fifteen CI chondrite samples (ten Orgueil, five Ivuna), with masses between 1-4 mg, were individually weighed into Teflon vials for acid digestion. The digested samples were then dissolved in 0.5 M HNO3, from which 10-20% of the solution was removed for the measurement of 54 elements using ICP-MS. In future work, the isotopic compositions of Cr and Ti of the samples will be measured using TIMS and MC-ICP-MS.
The bulk chemical compositions of the eight Ryugu grains show a wide variation in the abundance of most elements. In particular, P, Ca, Mn, Sr, and rare earth elements (REEs) show a broader range (P: 0.048-0.235 wt.%, Ca: 0.40-3.55 wt.%, Mn: 0.075-0.691 wt.%, Sr: 3.16-24.1 ppm, Nd: 0.341-1.03 ppm) (Fig.1a) compared to the CI chondrites analyzed in this study (Fig.1b), while similar to other Ryugu samples from the first touchdown [2,6] (Fig.1c).
The variation in abundance of these elements can be related to the small-scale heterogeneity between the grains due to the presence of secondary minerals produced during aqueous alteration in the parent body, as previously reported [2,3,6]. The occurrence of carbonates is evident in samples A0066, A0247, A0301, and A0313, which are characterized by elevated abundances of Ca, Mn, and Sr compared to CIs. The presence of phosphates can be seen in samples A0259, A0268, A0301, and A0313, which have higher P abundances than CI chondrites. Additionally, higher REE abundances than CIs in samples A0247, A0259, A0301, and A0313 indicate the fractionation of these elements during the formation of carbonates and phosphates (Fig.1a).
While the enrichments or depletions of P, Ca, Mn, Sr, and REEs in most of the samples can be related to the fractionation during the formation of secondary minerals, sample A0256 exhibits significant depletions in all these elements (Fig.1a), compared to the mean CI composition (element/CI ratios between 0.41-0.77). This depletion could indicate either a low modal abundance of carbonates and phosphates or that this grain underwent some other alteration event causing their loss at a bulk sample scale.
References: [1] Yada T. et al. (2022), Nat. Astron., 6(2). [2] Nakamura T. et al. (2022), Proc. Jpn. Acad., Ser. B, 98(6). [3] Yokoyama T. et al. (2023), Science, 379(6634). [4] Morlok A. et al., Geochim. Cosmochim. Acta, 70(21). [5] Barrat J. A. et al. (2012), Geochim. Cosmochim. Acta, 83. [6] Yokoyama T. et al. (2023), Sci. Adv., 9(45). [7] Lodders K. (2021), Space Sci. Rev., 217.