講演情報
[PPS12-P05]RyuguとBennu試料に含まれる難揮発性包有物
*川崎 教行1、松本 徹2、荒川 創太3、坂本 直哉1、山本 大貴4、馬上 謙一1、Russell Sara5、Barnes Jessica6、Nguyen Ann7、McCoy Timothy8、Haenecour Pierre6、圦本 尚義1、Connolly Harold9、Laurreta Dante6 (1.北大、2.京大、3.JAMSTEC、4.九大、5.NHM、6.University of Arizona、7.NASA、8.Smithsonian Institution、9.Rowan Univ.)
キーワード：
Ryugu、Bennu、難揮発性包有物、SIMS
Samples collected from the carbonaceous asteroids Ryugu and Bennu and CI chondrites exhibit chemical and petrographic similarities [1–3]. Refractory inclusions, including Ca-Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), which formed either by condensation from the solar nebular gas or by the remelting of condensate precursor solids, have been identified in Ryugu and Bennu samples, as well as the Ivuna CI chondrite [1,4–6]. In this study, we conducted mineralogical observations, O-isotope measurements, and 26Al–26Mg systematics of refractory inclusions in Ryugu and Bennu samples and Ivuna.
We found one CAI from Ryugu [4], three CAIs and six AOAs from Bennu [5], and one CAI and one AOA from Ivuna [4,6]. Such inclusions are small (~10–70 μm in size) and rare. They are mainly found in less-altered clasts or particles where calcite is the dominant carbonate phase. Their mineralogical textures and chemical compositions of the constituent minerals resemble those for refractory inclusions in chondrites other than CI. The refractory inclusions show evidence of in situ aqueous alteration. For example, magnetite plaquettes are enclosed in a CAI composed of hibonite and spinel in Ryugu [4], and phyllosilicates replacing olivine grains are observed in an AOA in Bennu [5]. These inclusions were incorporated into the parent planetesimal(s) as part of the primary building blocks prior to the onset of aqueous alteration.
The minerals in the refractory inclusions, except for a Bennu CAI, exhibit 16O-rich compositions with Δ17O ~ –24‰, consistent with many pristine minerals in refractory inclusions in carbonaceous chondrites and an unequilibrated ordinary chondrite [7]. These observations indicate that these inclusions formed in a 16O-rich nebular gas. In contrast, a CAI composed of Al-Ti diopside and spinel in Bennu exhibit 16O-rich compositions with Δ17O ~ –4‰. Together with the chemical composition of the Al-Ti diopside, this CAI resembles coarse-grained Type B and Type C CAIs found in CV chondrites [8,9], suggesting that it is a fragment of such an inclusion. Its 16O-poor composition likely reflects melting in a 16O-poor nebular gas followed by O-isotope exchange with the ambient gas. The CAIs composed of hibonite and spinel in Ryugu, Bennu, and Ivuna exhibit initial 26Al/27Al ratios of (5.1 ± 0.6) × 10−5, (5.4 ± 0.4) × 10−5, and (4.2 ± 0.7) × 10−5, respectively. They formed within ~0.2 Ma of the formation of the first solids in the Solar System. These initial values are consistent with those reported for CAIs in carbonaceous chondrites, which range from ~5.2 × 10−5 to ~3.4 × 10−5 [10], and with those in ordinary chondrites [11].
These mineralogical, isotopic, and chronological similarities among refractory inclusions in Bennu, Ryugu, and Ivuna provide evidence that their parent planetesimal(s) accreted a common suite of high-temperature refractory inclusions and their fragments. This suggests a close affinity in the primary building blocks of their parent planetesimal(s). These inclusions are also mineralogically, isotopically, and chronologically similar to those in other carbonaceous and ordinary chondrites, demonstrating the ubiquitous presence of refractory inclusions across chondritic astromaterials. Thus, all chondritic parent planetesimals shared at least some building blocks in common within the reservoirs from which they accreted, though in different relative proportions. In contrast, larger (> submillimeter) CAIs, which are common in other carbonaceous chondrites and thought to have been retained by pressure bump(s) in the disk [12], have not been identified in Bennu, Ryugu, or CI chondrites. This absence implies that their parent planetesimal(s) formed at a greater heliocentric distance, beyond the influence of a pressure bump created by proto-Jupiter, accreting only small CAIs that evaded radial drift toward the Sun [4,5].
References: [1] Nakamura T. et al. (2023) Science 379, eabn8671. [2] Yokoyama T. et al. (2023) Science 379, eabn7850. [3] Lauretta D. S. et al. (2024) MaPS 59, 2453–2486. [4] Kawasaki N. et al. (2025) Commun. Earth Environ. 6, 537. [5] Kawasaki N. et al. submitted. [6] Kawasaki N. et al. (2022) Sci. Adv. 8, eade2067. [7] Yurimoto H. et al. (2008) Rev. Min. Geochem. 68, 141–186. [8] Krot A. N. et al. (2019) GCA 72, 2534–2555. [9] Kawasaki N. et al. (2018) GCA 221, 318–341. [10] Kawasaki N. et al. (2020) GCA 279, 1–15. [11] Russell S. S. et al. (1996) Science 273, 757–762. [12] Desch S. J. et al. (2018) Astrophys. J. Suppl. Ser. 238, 11.
We found one CAI from Ryugu [4], three CAIs and six AOAs from Bennu [5], and one CAI and one AOA from Ivuna [4,6]. Such inclusions are small (~10–70 μm in size) and rare. They are mainly found in less-altered clasts or particles where calcite is the dominant carbonate phase. Their mineralogical textures and chemical compositions of the constituent minerals resemble those for refractory inclusions in chondrites other than CI. The refractory inclusions show evidence of in situ aqueous alteration. For example, magnetite plaquettes are enclosed in a CAI composed of hibonite and spinel in Ryugu [4], and phyllosilicates replacing olivine grains are observed in an AOA in Bennu [5]. These inclusions were incorporated into the parent planetesimal(s) as part of the primary building blocks prior to the onset of aqueous alteration.
The minerals in the refractory inclusions, except for a Bennu CAI, exhibit 16O-rich compositions with Δ17O ~ –24‰, consistent with many pristine minerals in refractory inclusions in carbonaceous chondrites and an unequilibrated ordinary chondrite [7]. These observations indicate that these inclusions formed in a 16O-rich nebular gas. In contrast, a CAI composed of Al-Ti diopside and spinel in Bennu exhibit 16O-rich compositions with Δ17O ~ –4‰. Together with the chemical composition of the Al-Ti diopside, this CAI resembles coarse-grained Type B and Type C CAIs found in CV chondrites [8,9], suggesting that it is a fragment of such an inclusion. Its 16O-poor composition likely reflects melting in a 16O-poor nebular gas followed by O-isotope exchange with the ambient gas. The CAIs composed of hibonite and spinel in Ryugu, Bennu, and Ivuna exhibit initial 26Al/27Al ratios of (5.1 ± 0.6) × 10−5, (5.4 ± 0.4) × 10−5, and (4.2 ± 0.7) × 10−5, respectively. They formed within ~0.2 Ma of the formation of the first solids in the Solar System. These initial values are consistent with those reported for CAIs in carbonaceous chondrites, which range from ~5.2 × 10−5 to ~3.4 × 10−5 [10], and with those in ordinary chondrites [11].
These mineralogical, isotopic, and chronological similarities among refractory inclusions in Bennu, Ryugu, and Ivuna provide evidence that their parent planetesimal(s) accreted a common suite of high-temperature refractory inclusions and their fragments. This suggests a close affinity in the primary building blocks of their parent planetesimal(s). These inclusions are also mineralogically, isotopically, and chronologically similar to those in other carbonaceous and ordinary chondrites, demonstrating the ubiquitous presence of refractory inclusions across chondritic astromaterials. Thus, all chondritic parent planetesimals shared at least some building blocks in common within the reservoirs from which they accreted, though in different relative proportions. In contrast, larger (> submillimeter) CAIs, which are common in other carbonaceous chondrites and thought to have been retained by pressure bump(s) in the disk [12], have not been identified in Bennu, Ryugu, or CI chondrites. This absence implies that their parent planetesimal(s) formed at a greater heliocentric distance, beyond the influence of a pressure bump created by proto-Jupiter, accreting only small CAIs that evaded radial drift toward the Sun [4,5].
References: [1] Nakamura T. et al. (2023) Science 379, eabn8671. [2] Yokoyama T. et al. (2023) Science 379, eabn7850. [3] Lauretta D. S. et al. (2024) MaPS 59, 2453–2486. [4] Kawasaki N. et al. (2025) Commun. Earth Environ. 6, 537. [5] Kawasaki N. et al. submitted. [6] Kawasaki N. et al. (2022) Sci. Adv. 8, eade2067. [7] Yurimoto H. et al. (2008) Rev. Min. Geochem. 68, 141–186. [8] Krot A. N. et al. (2019) GCA 72, 2534–2555. [9] Kawasaki N. et al. (2018) GCA 221, 318–341. [10] Kawasaki N. et al. (2020) GCA 279, 1–15. [11] Russell S. S. et al. (1996) Science 273, 757–762. [12] Desch S. J. et al. (2018) Astrophys. J. Suppl. Ser. 238, 11.