14:15 〜 14:30
[PPS06-18] コンドルールの同位体比データから導かれる原始太陽系円盤の進化過程への制約
キーワード：コンドルール、コンドライト、微惑星、26Al-26Mg年代、彗星
Chondrules are abundant igneous silicate spherules in chondrite and are considered to form by transient high temperature processes in the protosolar disk. Because chondrules preserve the 26Al-26Mg ages, oxygen isotopic compositions and redox conditions of their forming environments [1-4], their isotopic compositions are useful constraints on evolution of the protosolar disk before formations of chondritic parent bodies.
The upper limit of the initial 26Al/27Al ratios of Ca-, Al-rich Inclusions (CAIs) is ~5.2×10-5 (ranges: (5.2 ~ 4.0)×10-5)) [e.g., 5] The initial 26Al/27Al ratios of chondrules are ~8×10-6 (~2 million years after CAIs) for ordinary chondrites, ~5×10-6 (~2.5 million years after CAIs) for carbonaceous chondrites, and ~5×10-6 to no detectable 26Mg-excess (<2×10-6) (~2.5 to more than 3.5 million years after CAIs) for CR chondrites, respectively [e.g., 1]. Chondrules from Comet 81P/Wild 2 also show no 26Mg-excess (26Al/27Al<3×10-6) [6,7]. These data indicate: (i) There is a temporal gap of ~2 million years between CAIs and chondrule formation events, (ii) Considering the distance of the accretion regions of chondrite parent bodies (Ordinary Chondrites < Carbonaceous Chondrites < CR chondrites < Comets), the chondrule 26Al-26Mg age data indicate that locations of chondrule formation moved outwardly with time, although it is not clear if this outward motion was gradually or discretely.
Both type I chondrules (consisting of MgO-rich silicates) and type II chondrules (consisting of FeO-rich silicates) are found in individual chondrites. This indicates diverse redox conditions existed in individual accretion regions of chondrite parent bodies. The occurrence of an oxidizing chondrule-forming environment is very likely correlated with an increase of dust-to-gas ratio in each region. No apparent temporal difference of the chondrule 26Al-26Mg ages is recognized between type I (reducing environment) and type II (oxidizing environment) chondrules [e.g., 8]. In Carbonaceous Chondrites (CC), type II chondrules show more 16O-poor oxygen isotopic compositions (higher Δ17O values) relative to type I chondrules, indicating enrichment of 16O-poor H2O ice in type II chondrule-forming regions [e.g., 3,4]. This is distinct from the consistent Δ17O values (~+1‰) of type I and type II chondrules in Non-Carbonaceous (NC) chondrites (ordinary, enstatite, R, K chondrites) [e.g., 2].
Typical Δ17O values of chondrules in carbonaceous chondrites are -5‰ and -2‰ for type I and type II chondrules [e.g., 3], respectively. In addition, type II chondrules with Δ17O>0‰ are found in CR and Tagish Lake(-like) chondrites [9,10], and type II chondrules with Δ17O~-2‰ are abundant in CR chondrites [e.g., 4]. Oxygen isotopic compositions of chondrules and anhydrous mineral fragments from Comets (81P/Wild 2 and Interplanetary Dust Particles) are similar to those of chondrules from CR chondrites, but the abundance of type II chondrules in comets is significantly higher than that in CR chondrites [11,12]. Similar oxygen isotopic characteristics as well as similar 26Al-26Mg ages between cometary and CR chondrules indicate that most of cometary chondrules are derived from the outermost part of chondrule forming region (probably farther than CR chondrite accretion region from the Sun) [13]. In addition, this indicates that chondrules were transported through inside of the protosolar disk to the Kuiper belt region. A ballistic transportation by the outflow near the inner edge of the disk is not the (at least, major) process of chondrules in comets.
References:
[1] Fukuda et al. (2022) GCA
[2] Kita et al. (2010) GCA
[3] Ushikubo et al. (2012) GCA
[4] Tenner et al. (2015) GCA
[5] MacPherson et al. (2017) GCA
[6] Ogliore et al. (2012) ApJ
[7] Zhang et al. (2024) ApJ
[8] Hertwig et al. (2019) GCA
[9] Yamanobe et al. (2018) Polar Res.
[10] Ushikubo & Kimura (2021) GCA
[11] Nakashima et al. (2012) EPSL
[12] Zhang et al. (2024) GCA
[13] Kita et al. (2025) LPSC abst#1566
The upper limit of the initial 26Al/27Al ratios of Ca-, Al-rich Inclusions (CAIs) is ~5.2×10-5 (ranges: (5.2 ~ 4.0)×10-5)) [e.g., 5] The initial 26Al/27Al ratios of chondrules are ~8×10-6 (~2 million years after CAIs) for ordinary chondrites, ~5×10-6 (~2.5 million years after CAIs) for carbonaceous chondrites, and ~5×10-6 to no detectable 26Mg-excess (<2×10-6) (~2.5 to more than 3.5 million years after CAIs) for CR chondrites, respectively [e.g., 1]. Chondrules from Comet 81P/Wild 2 also show no 26Mg-excess (26Al/27Al<3×10-6) [6,7]. These data indicate: (i) There is a temporal gap of ~2 million years between CAIs and chondrule formation events, (ii) Considering the distance of the accretion regions of chondrite parent bodies (Ordinary Chondrites < Carbonaceous Chondrites < CR chondrites < Comets), the chondrule 26Al-26Mg age data indicate that locations of chondrule formation moved outwardly with time, although it is not clear if this outward motion was gradually or discretely.
Both type I chondrules (consisting of MgO-rich silicates) and type II chondrules (consisting of FeO-rich silicates) are found in individual chondrites. This indicates diverse redox conditions existed in individual accretion regions of chondrite parent bodies. The occurrence of an oxidizing chondrule-forming environment is very likely correlated with an increase of dust-to-gas ratio in each region. No apparent temporal difference of the chondrule 26Al-26Mg ages is recognized between type I (reducing environment) and type II (oxidizing environment) chondrules [e.g., 8]. In Carbonaceous Chondrites (CC), type II chondrules show more 16O-poor oxygen isotopic compositions (higher Δ17O values) relative to type I chondrules, indicating enrichment of 16O-poor H2O ice in type II chondrule-forming regions [e.g., 3,4]. This is distinct from the consistent Δ17O values (~+1‰) of type I and type II chondrules in Non-Carbonaceous (NC) chondrites (ordinary, enstatite, R, K chondrites) [e.g., 2].
Typical Δ17O values of chondrules in carbonaceous chondrites are -5‰ and -2‰ for type I and type II chondrules [e.g., 3], respectively. In addition, type II chondrules with Δ17O>0‰ are found in CR and Tagish Lake(-like) chondrites [9,10], and type II chondrules with Δ17O~-2‰ are abundant in CR chondrites [e.g., 4]. Oxygen isotopic compositions of chondrules and anhydrous mineral fragments from Comets (81P/Wild 2 and Interplanetary Dust Particles) are similar to those of chondrules from CR chondrites, but the abundance of type II chondrules in comets is significantly higher than that in CR chondrites [11,12]. Similar oxygen isotopic characteristics as well as similar 26Al-26Mg ages between cometary and CR chondrules indicate that most of cometary chondrules are derived from the outermost part of chondrule forming region (probably farther than CR chondrite accretion region from the Sun) [13]. In addition, this indicates that chondrules were transported through inside of the protosolar disk to the Kuiper belt region. A ballistic transportation by the outflow near the inner edge of the disk is not the (at least, major) process of chondrules in comets.
References:
[1] Fukuda et al. (2022) GCA
[2] Kita et al. (2010) GCA
[3] Ushikubo et al. (2012) GCA
[4] Tenner et al. (2015) GCA
[5] MacPherson et al. (2017) GCA
[6] Ogliore et al. (2012) ApJ
[7] Zhang et al. (2024) ApJ
[8] Hertwig et al. (2019) GCA
[9] Yamanobe et al. (2018) Polar Res.
[10] Ushikubo & Kimura (2021) GCA
[11] Nakashima et al. (2012) EPSL
[12] Zhang et al. (2024) GCA
[13] Kita et al. (2025) LPSC abst#1566