[P-PS03] 太陽系小天体：太陽系の形成と進化における最新成果と今後の展望

[PPS03-03] An accurate aqueous alteration age for Ryugu, the least thermally processed asteroid

*田中 亮吏¹、ラトナヤケ ディラン¹、太田 努¹、ミクルシック ノア¹、国広 卓也¹、ポティシェル クリスチャン¹、坂口 千恵¹、小林 桂¹、北川 宙¹、山中 正博¹、中村 栄三¹ (1.岡山大学惑星物質研究所)

キーワード：リュウグウ、C型小惑星、炭素質コンドライト、水質変質、Mn-Cr年代

The analysis of returned samples from the C-type asteroid Ryugu has drastically advanced our knowledge of the evolution of early solar system materials. Accurate chronological data of the progenitor body of the asteroid Ryugu is an important parameter for understanding the evolution of planetesimals in the outer protoplanetary disk. Three papers have so far reported aqueous alteration ages for the Ryugu progenitor body via the Mn-Cr method for carbonates using SIMS as 5.2 Myr [1], 2.6 Myr [2], and 0.0 - 0.5 Myr [3] after the formation of CAI. The difference in ages reported could be due to the different matrix effect correction procedures used for SIMS analysis. Whether the age of aqueous alteration was <1 or >5 Myr after CAI formation has major implications for estimating the age of accretion, the size, and the thermal history of the Ryugu progenitor body. Therefore, it is important to accurately determine the aqueous alteration age of the Ryugu samples. In this study, we determined the aqueous alteration age of Ryugu particles using the Mn-Cr system of bulk samples [4]. Aliquots of approximately 0.2 to 0.5 mg each separated from nine Ryugu particles distributed to the Phase-2 curation facility at the Pheasant Memorial Laboratory were acid digested, subjected to appropriate pretreatment, and measured for Mn/Cr and Cr isotopic compositions by ICP-MS and TIMS, respectively. Except for one sample which contained a significant amount of phyllosilicate nodule, all other eight samples gave a positive correlation between ⁵⁵Mn/⁵²Cr and ε⁵³Cr, yielding an age of 4.13 +0.62/-0.55 Myr (MSWD = 0.81) after the formation of CAI. The ε⁵⁴Cr values of the measured samples range from 1.4 and 1.9, with a mean value of 1.57 ± 0.13 (2SE). The mean ⁵⁵Mn/⁵²Cr and ε⁵⁴Cr values and initial ε⁵³Cr values of the Ryugu samples are higher than any carbonaceous chondrite samples, implying that its progenitor body formed from the least thermally processed precursors in the outermost region of the protoplanetary disk. Despite accreting at different distances from the Sun, the hydrous asteroids (Ryugu and the parent bodies of CI, CM, CR, and ungrouped C2 meteorites) underwent aqueous alteration during a limited duration (3.8 ± 1.8 Myr after CAI). These ages are identical to the crystallization age of the carbonaceous achondirtes NWA6704/6693 within the error. The ε⁵⁴Cr and initial ε⁵³Cr values of Ryugu and NWA6704/6693 are also identical, while they show distinct Δ¹⁷O values. This suggests that the precursors that formed the progenitor bodies of Ryugu and NWA6703/6693 were formed in close proximity and experienced a similar degree of thermal processing in the protosolar nebula. Comparisons of Ryugu and these carbonaceous achondrites indicate the existence of large variations in ice/dust ratios in the outer protoplanetary disk over a few Myr after CAI formation.

References: [1] Yokoyama et al., 2023, Sci, 379, eabn7850. [2] Nakamura, E. et al., 2022, PJAB, 98, 227. [3] McCain, K. A. et al., 2023, NatAs, 7, 309. [4] Tanaka et al., 2024, ApJ (in press).