16:15 〜 16:30
[PPS07-22] 小惑星リュウグウの年代学
キーワード：Ryugu、Al-Mg systematics、Mn-Cr systematics、SIMS
Samples collected from asteroid Ryugu are chemically and mineralogically similar to CI chondrites [1]. Ryugu samples and CI chondrites are dominated by hydrous phyllosilicates and contain coarser grains of aqueously formed minerals including carbonates (dolomite, magnesite, and calcite), magnetite, and sulfides, whereas anhydrous primary minerals, such as olivine, low-Ca pyroxene, spinel, and hibonite, are rarely present [1–3]. Chronology of the primary and secondary minerals in Ryugu and CI chondrites is key for understanding their building blocks and the origin of carbonaceous planetesimals. Using secondary ion mass spectrometry, we investigated 26Al–26Mg systematics for refractory inclusions (CAIs and AOAs) and 53Mn–53Cr systematics for dolomite in the Ryugu samples and the Ivuna CI chondrite. The data were corrected for the relative sensitivity factors and instrumental mass fractionation using terrestrial and synthetic standards [4–6].
26Al–26Mg systematics of refractory inclusions: Little is known about the Al–Mg isotopic compositions for refractory inclusions in Ryugu and CI chondrites [7]. Spinel and hibonite in CAIs from Ryugu and Ivuna that we measured are all 16O-rich (Δ17O ~ –24‰), similar to refractory inclusions from Ivuna and other carbonaceous chondrites [4, 9]. We measured Al–Mg isotopic compositions for hibonites from the CAIs, one from Ryugu and one from Ivuna, and for olivine in an AOA from Ivuna. We defined isochrons of the CAI hibonites using the Ivuna AOA olivine data, because the Mg-isotope composition of the gaseous reservoir where refractory inclusions in Ryugu and Ivuna formed is unknown. Isochrons for the CAI hibonites from Ryugu and Ivuna give initial 26Al/27Al ratios of (5.1 ± 0.6) × 10–5 and (4.2 ± 0.7) × 10–5, respectively. These are in agreement with those for most CAIs [4]. These CAIs formed within ~0.2 Ma after the formation of the canonical CAIs [9].
53Mn–53Cr systematics of dolomite: We have determined Mn–Cr isotopic compositions for dolomite in Ryugu and Ivuna using appropriate standards that has been a long-standing problem in estimating the aqueous alteration age of parent planetesimals of Ryugu and CI chondrites [6]. Dolomite grains in Ivuna and the Ryugu samples A0058 and C0002 show initial 53Mn/55Mn ratios of (3.95 ± 0.49) × 10−6 [6], (3.17 ± 0.49) × 10−6, and (4.69 ± 0.51) × 10−6, respectively. Combined with the O-isotope thermometry for dolomite and magnetite [1, 10], the Ivuna dolomite formed at 76 ± 19°C and 2.9 or 3.8 (+0.7/−0.6) Ma, the A0058 dolomite formed at 37 ± 10°C and 4.1 or 5.0 (+0.9/−0.8) Ma, and the C0002 dolomite formed at 92 ± 21°C and 2.0 or 2.9 (+0.6/−0.6) Ma, after the formation of the canonical CAIs. Note that the relative ages calculated from the initial 53Mn/55Mn ratios depend on the age anchors, which remain controversial [e.g., 11]. Thermal modeling to satisfy the temperature rise to ~90°C for the C0002 dolomite indicates that the Ryugu’s parent planetesimal accreted earlier than 2.0 Ma, which is older than the most chondrules in carbonaceous chondrites [e.g., 12]. This old accretion age of Ryugu’s parent planetesimal provides an explanation for the near absence of chondrules or their pseudomorphs in their samples [1–3], and implies that the parent planetesimals of Ryugu and CI chondrites formed earlier than those of other carbonaceous chondrite groups. The asteroid Ryugu and CI chondrites are likely remnants of earlier generations of carbonaceous planetesimals that formed in a chondrule-free region of the disk.
References: [1] Yokoyama et al. (2023) Science 379, eabn7850. [2] Nakamura et al. (2023) Science 379, eabn8671. [3] Kawasaki et al. (2022) Sci. Adv. 8, eade2067. [4] Kawasaki et al. (2020) GCA 279, 1–15. [5] Kawasaki et al. (2024) MaPS 59, 630–639. [6] Sugawara et al. (2024) GCA 382, 40–50. [7] Frank et al. (2023) MaPS 58, 1495–1511. [8] Krot (2019) MaPS 54, 1647–1691. [9] Larsen et al. (2011) ApJL 735, L37−L43. [10] Nagashima et al. (2022) Hayabusa Symposium. [11] Desch et al. (2023) Icarus 402, 115611. [12] Fukuda et al. (2022) GCA 322, 194–226.
26Al–26Mg systematics of refractory inclusions: Little is known about the Al–Mg isotopic compositions for refractory inclusions in Ryugu and CI chondrites [7]. Spinel and hibonite in CAIs from Ryugu and Ivuna that we measured are all 16O-rich (Δ17O ~ –24‰), similar to refractory inclusions from Ivuna and other carbonaceous chondrites [4, 9]. We measured Al–Mg isotopic compositions for hibonites from the CAIs, one from Ryugu and one from Ivuna, and for olivine in an AOA from Ivuna. We defined isochrons of the CAI hibonites using the Ivuna AOA olivine data, because the Mg-isotope composition of the gaseous reservoir where refractory inclusions in Ryugu and Ivuna formed is unknown. Isochrons for the CAI hibonites from Ryugu and Ivuna give initial 26Al/27Al ratios of (5.1 ± 0.6) × 10–5 and (4.2 ± 0.7) × 10–5, respectively. These are in agreement with those for most CAIs [4]. These CAIs formed within ~0.2 Ma after the formation of the canonical CAIs [9].
53Mn–53Cr systematics of dolomite: We have determined Mn–Cr isotopic compositions for dolomite in Ryugu and Ivuna using appropriate standards that has been a long-standing problem in estimating the aqueous alteration age of parent planetesimals of Ryugu and CI chondrites [6]. Dolomite grains in Ivuna and the Ryugu samples A0058 and C0002 show initial 53Mn/55Mn ratios of (3.95 ± 0.49) × 10−6 [6], (3.17 ± 0.49) × 10−6, and (4.69 ± 0.51) × 10−6, respectively. Combined with the O-isotope thermometry for dolomite and magnetite [1, 10], the Ivuna dolomite formed at 76 ± 19°C and 2.9 or 3.8 (+0.7/−0.6) Ma, the A0058 dolomite formed at 37 ± 10°C and 4.1 or 5.0 (+0.9/−0.8) Ma, and the C0002 dolomite formed at 92 ± 21°C and 2.0 or 2.9 (+0.6/−0.6) Ma, after the formation of the canonical CAIs. Note that the relative ages calculated from the initial 53Mn/55Mn ratios depend on the age anchors, which remain controversial [e.g., 11]. Thermal modeling to satisfy the temperature rise to ~90°C for the C0002 dolomite indicates that the Ryugu’s parent planetesimal accreted earlier than 2.0 Ma, which is older than the most chondrules in carbonaceous chondrites [e.g., 12]. This old accretion age of Ryugu’s parent planetesimal provides an explanation for the near absence of chondrules or their pseudomorphs in their samples [1–3], and implies that the parent planetesimals of Ryugu and CI chondrites formed earlier than those of other carbonaceous chondrite groups. The asteroid Ryugu and CI chondrites are likely remnants of earlier generations of carbonaceous planetesimals that formed in a chondrule-free region of the disk.
References: [1] Yokoyama et al. (2023) Science 379, eabn7850. [2] Nakamura et al. (2023) Science 379, eabn8671. [3] Kawasaki et al. (2022) Sci. Adv. 8, eade2067. [4] Kawasaki et al. (2020) GCA 279, 1–15. [5] Kawasaki et al. (2024) MaPS 59, 630–639. [6] Sugawara et al. (2024) GCA 382, 40–50. [7] Frank et al. (2023) MaPS 58, 1495–1511. [8] Krot (2019) MaPS 54, 1647–1691. [9] Larsen et al. (2011) ApJL 735, L37−L43. [10] Nagashima et al. (2022) Hayabusa Symposium. [11] Desch et al. (2023) Icarus 402, 115611. [12] Fukuda et al. (2022) GCA 322, 194–226.