11:45 〜 12:00
[MIS14-05] タギッシュレイク隕石コンドルールの酸素同位体比組成から見えてくる原始惑星系円盤の揮発性物質分布
キーワード：タギッシュレイク隕石、コンドルール、酸素同位体、二次イオン質量分析計
Previous studies on oxygen isotope systematics of chondrules show that individual chondrules preserve information of oxygen isotopic composition and redox state of chondrule-forming environments in the protoplanetary disk [e.g., 1, 2]. Highly MgO-rich (Mg# ≧ 96; Mg# ≡ mol.% of MgO/(MgO + FeO)) and 16O-rich ( Δ17O ~ −5‰; Δ17O ≡ δ17OVSMOW – 0.52 × δ18OVSMOW) chondrules are abundant in CM, CO, and CV chondrites. Minor 16O-poor (Δ17O ~ −2‰) and FeO-rich (Mg# < 90) chondrules are also found in them. Increase of Δ17O values of FeO-rich chondrules indicates that oxidizing environment to form ferroan chondrules was caused by enrichment of an 16O-poor oxidizing agent (most likely 16O-poor H2O ice) [2]. Assuming that CM, CO, and CV chondrites are derived from C- or K-type asteroids, reducing environment was common and enrichment of 16O-poor H2O ice occurred in middle to outer asteroid main belt region of the protoplanetary disk.
In addition to these two types chondrules, FeO-rich chondrules with higher Δ17O values (Δ17O ≧ 0‰) were found in Tagish Lake meteorite [3]. Such chondrules have been found in Tagish Lake-like meteorites [4], CR chondrites [2 and references therein] and the comet Wild 2 returned samples [e.g., 5, 6]. Since comet Wild 2 is originally a Kuiper belt object and Tagish Lake meteorite is possibly derived from a D-type asteroid [7], the occurrence of FeO-rich chondrules with higher Δ17O values in Tagish Lake and comet Wild 2 suggests existence of a more 16O-depleted oxidizing agent further out than the outer main belt. Recently, Fujiya et al. [8] found the evidence for accretion of CO2 ice as well as H2O ice into the parent body of Tagish Lake meteorite. Higher Δ17O values of FeO-rich chondrules in Tagish Lake and comet Wild 2 may represent higher Δ17O values of the chemically distinct icy components existed in the outermost part of chondrule-forming environment.
The 26Al-26Mg systematics of chondrules show that the initial 26Al/27Al values of FeO-rich chondrules from comet Wild 2 and CR chondrites (< 3×10-6) are significantly lower than those of typical chondrules in CO and CV chondrites (~5×10-6) [e.g., 9, 10], suggesting that the former group of chondrules formed more than one million years later than the formation of the latter group of chondrules. The occurrence of more 16O-depleted oxidizing agent can be results of temporal isotopic evolution of icy components in the protoplanetary disk.
Although we cannot tell either spatial heterogeneity or temporal change of the oxygen isotopic composition of the oxidizing agents formed distinct FeO-rich chondrule groups (Δ17O ~ −2‰ or ≧ 0‰), we suggest that oxygen isotopic compositions of chondrules would be important constrains for better understanding of volatile components in the protoplanetary disk.
References:
[1] Ushikubo T. et al. (2012) GCA, 90, 242-240.
[2] Tenner T. J. et al. (2018) Chapter 8 in Chondrules (Cambridge Univ. press).
[3] Ushikubo T. and Kimura M. (2021) GCA, 293, 328-343.
[4] Yamanobe M. et al. (2018) Polar Science, 15, 29-38.
[5] Nakamura T. et al. (2008) Science, 321, 1664-1667.
[6] Nakashima D. et al. (2012) EPSL, 358, 355-365.
[7] Hiroi T. et al. (2001) Science, 293, 2234-2236.
[8] Fujiya W. et al. (2019) Nature Astornomy, 3, 910-915.
[9] Ogliore R. C. et al. (2012) Astrophys. J. Lett., 745, L19.
[10] Tenner T. J. et al. (2019) GCA, 260, 133-160.
In addition to these two types chondrules, FeO-rich chondrules with higher Δ17O values (Δ17O ≧ 0‰) were found in Tagish Lake meteorite [3]. Such chondrules have been found in Tagish Lake-like meteorites [4], CR chondrites [2 and references therein] and the comet Wild 2 returned samples [e.g., 5, 6]. Since comet Wild 2 is originally a Kuiper belt object and Tagish Lake meteorite is possibly derived from a D-type asteroid [7], the occurrence of FeO-rich chondrules with higher Δ17O values in Tagish Lake and comet Wild 2 suggests existence of a more 16O-depleted oxidizing agent further out than the outer main belt. Recently, Fujiya et al. [8] found the evidence for accretion of CO2 ice as well as H2O ice into the parent body of Tagish Lake meteorite. Higher Δ17O values of FeO-rich chondrules in Tagish Lake and comet Wild 2 may represent higher Δ17O values of the chemically distinct icy components existed in the outermost part of chondrule-forming environment.
The 26Al-26Mg systematics of chondrules show that the initial 26Al/27Al values of FeO-rich chondrules from comet Wild 2 and CR chondrites (< 3×10-6) are significantly lower than those of typical chondrules in CO and CV chondrites (~5×10-6) [e.g., 9, 10], suggesting that the former group of chondrules formed more than one million years later than the formation of the latter group of chondrules. The occurrence of more 16O-depleted oxidizing agent can be results of temporal isotopic evolution of icy components in the protoplanetary disk.
Although we cannot tell either spatial heterogeneity or temporal change of the oxygen isotopic composition of the oxidizing agents formed distinct FeO-rich chondrule groups (Δ17O ~ −2‰ or ≧ 0‰), we suggest that oxygen isotopic compositions of chondrules would be important constrains for better understanding of volatile components in the protoplanetary disk.
References:
[1] Ushikubo T. et al. (2012) GCA, 90, 242-240.
[2] Tenner T. J. et al. (2018) Chapter 8 in Chondrules (Cambridge Univ. press).
[3] Ushikubo T. and Kimura M. (2021) GCA, 293, 328-343.
[4] Yamanobe M. et al. (2018) Polar Science, 15, 29-38.
[5] Nakamura T. et al. (2008) Science, 321, 1664-1667.
[6] Nakashima D. et al. (2012) EPSL, 358, 355-365.
[7] Hiroi T. et al. (2001) Science, 293, 2234-2236.
[8] Fujiya W. et al. (2019) Nature Astornomy, 3, 910-915.
[9] Ogliore R. C. et al. (2012) Astrophys. J. Lett., 745, L19.
[10] Tenner T. J. et al. (2019) GCA, 260, 133-160.