5:15 PM - 6:45 PM
[PPS08-P13] Al–Mg systematics of spinel-hibonite inclusions in Ryugu and Ivuna
Keywords:CAI, Al-Mg, Ryugu, SIMS
Introduction: Samples returned from asteroid (162173) Ryugu and CI chondrites are similar to each other and rarely contain high-temperature anhydrous primary minerals derived from chondrules and refractory inclusions (CAIs and AOAs) [1, 2]. CAIs are recognized as the oldest objects formed in the Solar System [3]. The Al–Mg mineral isochron studies using SIMS revealed variations in initial 26Al/27Al ratios, (26Al/27Al)0, for CAIs in carbonaceous chondrites ranging from ~5.2 to ~3.4 × 10–5 [4], corresponding to a formation age spread of ~0.5 Myr if 26Al was once homogeneously distributed in the CAI-forming region at the canonical level [5]. On the other hand, some kinds of rare CAIs such as FUN objects show significantly lower (26Al/27Al)0, < ~1 × 10–5 [6]. The presence of such 26Al-poor CAIs suggests the heterogeneous distribution of 26Al in the early solar system. From the Ivuna CI chondrite, a melilite-bearing CAI was found that also shows 26Al-poor signature, (26Al/27Al)0 of < 1.5 × 10–5 [7]. In this study, we measured Al–Mg isotopic compositions of refractory inclusions composed of spinel and hibonite in Ryugu and Ivuna. These minerals are representative refractory minerals in Ryugu and CI chondrites [1], but have not been measured for (26Al/27Al)0.
Methods: We obtained BSE and X-ray elemental maps with pixel size of 0.48 μm using SEM-EDS to find out CAI minerals with sizes of > 2 μm from polished sections of Ivuna and Ryugu C0006 samples. O- and Al–Mg isotopic compositions for CAI minerals were measured using SIMS (Cameca ims-1280HR).
Results and Discussion: We found refractory inclusions composed of Mg-Al spinel and hibonite, one from Ryugu and one from Ivuna. They are contained in less-altered clasts, where abundant anhydrous primary minerals are present, consistent with the previous work [2]. On the O three-isotope diagram, the compositions of spinel and hibonite in the two inclusions plot along the CCAM line and consistently show Δ17O ~ –24‰, consistent with refractory inclusion-like minerals in Ryugu and Ivuna [2] and those minerals in CAIs in carbonaceous chondrites [6]. The hibonites in the two inclusions both exhibit resolvable excess 26Mg. Defining a model isochron of the inclusions gives (26Al/27Al)0 of ~4.4 × 10–5, consistent with the (26Al/27Al)0 for most CAIs [4]. The refractory inclusions in Ryugu and Ivuna have similar isotopic characteristics to those in other carbonaceous chondrites.
References: [1] Nakamura et al. (2022) Science 379, abn8671. [2] Kawasaki et al. (2022) Sci. Adv. 8 (50), eade2067. [3] Connelly et al. (2012) Science 338, 651−655. [4] Kawasaki et al. (2020) GCA 279, 1−15. [5] Larsen et al. (2011) ApJL 735, L37–43. [6] Krot (2019) MaPS 54, 1647–1691. [7] Frank et al. (2023) MaPS 58, 1495–1511.
Methods: We obtained BSE and X-ray elemental maps with pixel size of 0.48 μm using SEM-EDS to find out CAI minerals with sizes of > 2 μm from polished sections of Ivuna and Ryugu C0006 samples. O- and Al–Mg isotopic compositions for CAI minerals were measured using SIMS (Cameca ims-1280HR).
Results and Discussion: We found refractory inclusions composed of Mg-Al spinel and hibonite, one from Ryugu and one from Ivuna. They are contained in less-altered clasts, where abundant anhydrous primary minerals are present, consistent with the previous work [2]. On the O three-isotope diagram, the compositions of spinel and hibonite in the two inclusions plot along the CCAM line and consistently show Δ17O ~ –24‰, consistent with refractory inclusion-like minerals in Ryugu and Ivuna [2] and those minerals in CAIs in carbonaceous chondrites [6]. The hibonites in the two inclusions both exhibit resolvable excess 26Mg. Defining a model isochron of the inclusions gives (26Al/27Al)0 of ~4.4 × 10–5, consistent with the (26Al/27Al)0 for most CAIs [4]. The refractory inclusions in Ryugu and Ivuna have similar isotopic characteristics to those in other carbonaceous chondrites.
References: [1] Nakamura et al. (2022) Science 379, abn8671. [2] Kawasaki et al. (2022) Sci. Adv. 8 (50), eade2067. [3] Connelly et al. (2012) Science 338, 651−655. [4] Kawasaki et al. (2020) GCA 279, 1−15. [5] Larsen et al. (2011) ApJL 735, L37–43. [6] Krot (2019) MaPS 54, 1647–1691. [7] Frank et al. (2023) MaPS 58, 1495–1511.