| Seminar: | CPS Seminar |
| date: | 2025 October 6 (Mon) 13:30-15:00 |
| Room: | CPS Conference Room and Online (Hybrid meeting) |
| Speaker1: | USHIKUBO Takayuki (Senior Researcher, JAMSTEC, Kochi Institute for Core Sample Research) |
| Title: | "Constraints on chondrule-forming environments based on oxygen isotope systematics of chondrules"
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| Abstract: |
Chondrules are sub-mm size igneous silicate spherules which mostly consist of olivine and pyroxene. They are considered to have formed by transient high temperature processes in the protosolar disk. Chondrules are abundant in chondrites (20 to 80 vol%), and they are also recognized in the returned sample from Comet 81P/Wild [1]. Chondrules would have formed in a broad area of the protosolar disk. Because chondrules preserve the 26Al-26Mg ages, oxygen isotopic compositions and redox conditions of their forming environments, their isotopic compositions are useful constraints on the evolution of the protosolar disk before formations of chondritic parent bodies.
The characteristics of oxygen isotopic systematics of chondrules can be summarized as follows:
1) The oxygen isotopic composition of individual chondrules is, in most cases, homogeneous from olivine, pyroxene to the glass [2]. This indicates that the oxygen isotope ratio of chondrule-forming melt became homogeneous, and that it did not change significantly from the onset of the crystallization to the solidification of the residual glass. 2) Crystals with distinct oxygen isotope ratios are sometimes found in chondrules. They are interpreted as “relict grains” which did not melt during the last chondrule-forming event and thus retained their original isotope signatures. In some meteorites, nearly 50% of chondrules have relict grains [2], suggesting that many chondrules experienced multiple melting episodes. 3) Chondrules in Non-carbonaceous Chondrites (NC) tend to have similar Δ17O values (defined as δ17O - 0.52×δ18O), regardless of the redox conditions of their formation environment. The Δ17O values of chondrules in ordinary chondrites (OC) and enstatite chondrites (EC) are typically about +0.5‰ and 0‰, respectively [3,4]. These values probably reflect the characteristic Δ17O values of the dust in the regions where chondrules formed. The variation in redox conditions of chondrule-forming environment was likely caused by the dust enrichment. 4) In contrast, the Δ17O values of chondrules in carbonaceous chondrites (CC) vary in correlation with the redox conditions of their formation environment. Chondrules formed under reducing conditions show the Δ17O values around -5‰, whereas those formed under oxidizing conditions show the Δ17O values of about -2‰ or even +1‰ [2,5]. The elevated Δ17O values of chondrules formed in oxidizing environments are considered the result of the enrichment of dust containing H2O ice with high Δ17O value, leading to the oxidizing and high Δ17O chondrule-forming environment [4]. Because chondrules with Δ17O values of +1‰ are found in parent bodies accreted in distant regions from the Sun, such as CR chondrite, Tagish Lake-like meteorites, and Comet 81P/Wild 2, these chondrules probably formed in the outermost part of the chondrule-forming regions [6,7]. Systematic differences in oxygen isotopic compositions among chondrite types indicate that the oxygen isotopic composition of the protoplanetary disk spatially varied at the time of chondrule formations. By examining the distribution of oxygen isotope ratios of chondrules among different chondrite types, it may be possible to investigate the transport of the sub-mm size solid particles in the protoplanetary disk. References: [1] Nakamura et al. (2008) Science 321, 1664-1667. [2] Ushikubo et al. (2012) GCA 242-264. [3] Siron et al. (2022) GCA 324, 312-345. [4] Weisberg et al. (2021) GCA 300, 279-295. [5] Tenner et al. (2015) GCA 148, 228-250. [6] Nakashima et al. (2012) EPSL 357-358, 355-365. [7] Zhang et al. (2024) GCA 371, 214-227. |
| Keywords: | Chondrule, Oxygen isotope ratio |
| Speaker2: | FUKUDA Kohei (Assistant Professor, Graduate School of Science, The University of Osaka) |
| Title: | "Spatiotemporal distribution of chondrules in the solar protoplanetary disk"
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| Abstract: | Chondrules, which are mm-sized silicate spherules in undifferentiated meteorites, are considered the primary constituent material of chondrite meteorite parent bodies. While the formation process of chondrules has long been debated, it is clear that they formed during the transient heating of solid material within the protoplanetary disk, and such transient heating processes are thought to be related to the growth of protoplanets. Therefore, by estimating the formation regions and timing (i.e., spatiotemporal distribution) of chondrules, and combining this with their constituent mineral chemistry and various isotope systematics, we can elucidate the evolution of the physicochemical environment of the protoplanetary disk during planet formation. Since many chondrules contain plagioclase rich in Al and poor in Mg, the formation age of individual chondrules can be determined using the 26Al-26Mg chronometer, which utilizes the decay of the short-lived radionuclide 26Al. With the recent improvements in Al-Mg dating techniques, systematic differences in chondrule ages have been observed among different meteorite groups. Meanwhile, discussions regarding the formation regions of individual chondrules have also been developed based on the systematic differences in oxygen isotopic compositions and degrees of nucleosynthetic isotopic anomalies. This presentation introduces the spatiotemporal distribution of chondrules within the protoplanetary disk, as inferred from high-precision 26Al-26Mg ages and O (Cr, and Ti) isotopic compositions. |
| Keywords: | Chondrule, Al-Mg age |
| Organizer: | George L. Hashimoto |