講演情報
[R5-14]A highly primitive asteroid Bennu particle preserving dust from the early solar system
*松本 恵1、土`山 明2,3、安武 正展4、松本 徹5、森田 朋代1、中嶋 大輔1、大野 遥5、橘 省吾6、Connolly Harold 7,8,9、Lauretta Dante8 (1. 東北大学、2. 立命館大学、3. 中国科学院広州地球科学研究所、4. 高輝度光科学研究センター、5. 京都大学、6. 東京大学、7. Rowan Univ.、8. Univ. Arizona、9. American Museum of Natural History)
キーワード：
小惑星ベンヌ、GEMS、水質変成
Initial analysis of samples from asteroid Bennu, returned by the OSIRIS-REx mission, indicated the presence of Mg-rich phyllosilicates, magnetite, Fe-Ni sulfides, carbonates, Mg,Na -rich phosphates, and organic matter1. This mineralogy resembles that of samples from asteroid Ryugu2 and represents extensive secondary alteration by aqueous processes. However, in a synchrotron X-ray diffraction (XRD) and computed tomography (XCT) survey of more than 50 Bennu particles3 , we found one (OREX-803191-0, ~800 µm ) containing abundant Fe-Ni metal and anhydrous silicate grains, suggesting that it did not experience significant alteration. Here we present the results of XRD, XCT, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses of this particle. XRD and XCT revealed that Bennu particle OREX-803191-0 consists mainly of a fined-grained, porous matrix material with embedded, relatively coarse (up to ~100 µm) Mg-rich olivine, pyroxene, and kamacite grains. SEM analysis of cross sections of the particle show that fragmental grains consisting of Mg-rich olivine and probable Ca,Al-bearing silicate glass are widely distributed. These grains exhibit a barred olivine texture and are probably fragments of chondrules. The kamacite grains have thin Fe-oxide (probable magnetite) rims, suggesting a minor degree of aqueous alteration. TEM observations showed that the fine-grained, porous matrix consists mainly of GEMS (glass with embedded metal and sulfide)–like grains entangled with spongy organic matter. The GEMS-like grains do not contain metal inclusions and show no evidence of replacement by hydrous silicate phases. Overall, our findings suggest that this Bennu particle escaped the significant aqueous alteration typical of most of the Bennu samples and preserves primitive dust accreted to the parent body. 1Lauretta & Connolly et al. (2024) MAPS 59, 2453–2486. 2Nakamura et al. (2022) Science 379, eabn8671. 3Tsuchiyama et al. (2025) 3S Symposium 2025.