5:15 PM - 7:15 PM
[PPS07-P12] Petrology and mineralogy of calcium-aluminium-rich inclusions from the CH/CB chondrite Isheyevo
Keywords:CAI, meteorite, early solar system, electron microscope
Introduction: Ca-Al-rich inclusions (CAIs) are the oldest objects formed in the Solar System [1]. Most CAIs contained live 26Al at the time of their formation [2]. CAIs in CH and CH/CB chondrites often contain highly refractory oxide minerals such as hibonite, grossite, krotite, spinel and perovskite and have a wide range of O-isotopic compositions (from Δ17O ~ –35‰ to –5‰) [3–5]. Only ~5% of the grossite-bearing CAIs in CH chondrites show 26Al-rich signatures with (26Al/27Al)0 of > 3 × 10–5 and the remaining 95% are 26Al-poor with (26Al/27Al)0 of < 1 × 10–5 [5]. The 26Al-poor CAIs are suggested to have formed at the earliest stage of the evolution of the solar protoplanetary disk, presumably prior to the addition of stellar 26Al to the disk [5, 6]. On the other hand, stable isotope compositions other than O and Al-Mg for CH and CH/CB CAIs have been poorly understood [7]. In order to perform multi-element isotope analyses of O, Al-Mg, Ca, and Ti using secondary ion mass spectrometry (SIMS), we are investigating petrological and mineralogical studies of CAIs in the CH/CB chondrite Isheyevo as a preparation using the FE-SEM-EDS system.
Results and Discussion: We observed 40 CAIs with sufficient mineral sizes for the SIMS isotope analyses from Isheyevo. Most of them are rounded shape with diameters ranging from ~50 to ~200 µm. 32 of the 40 CAIs contain hibonite or grossite and their bulk compositions are enriched in Al compared to igneous, coarse-grained CAIs and fine-grained CAIs in CV chondrites. The hibonite/grossite-bearing CAIs also contain perovskite, melilite, spinel, and diopside with different abundances and textures. The mineralogical textures indicate that the hibonite crystallized prior to the other constituent minerals, which is inconsistent with crystallization from the complete melt [8]. This suggests that they are primarily condensates from the solar nebular gas, some of which may have experienced later partial melting.
Remaining CAIs without hibonite and grossite have bulk compositions similar to CAIs in CV chondrites. They contain spinel, perovskite, melilite, anorthite, and diopside with different abundances and textures, and most are texturally very likely fragments of igneous, coarse-grained CAIs and fine-grained CAIs. They were formed by melt crystallization and condensation, respectively.
References: [1] Connelly J. N. et al. (2012) Science 338, 651–655. [2] MacPherson et al. (1995) Meteoritics 30, 365–386. [3] Krot A. N. et al. (2008) MaPS 43, 1531–1550. [4] Gounelle et al. (2009) ApJ 698, 18–22. [5] Krot A. N. et al. (2020) Sci. Adv. 6, eaay2724. [6] Krot et al. (2012) MaPS 47, 1948–1979. [7] Kimura M. et al. (1993) GCA 57, 2329–2359. [8] MacPherson G. J. and Huss G. R. (2005) GCA 69, 3099–3127.
Results and Discussion: We observed 40 CAIs with sufficient mineral sizes for the SIMS isotope analyses from Isheyevo. Most of them are rounded shape with diameters ranging from ~50 to ~200 µm. 32 of the 40 CAIs contain hibonite or grossite and their bulk compositions are enriched in Al compared to igneous, coarse-grained CAIs and fine-grained CAIs in CV chondrites. The hibonite/grossite-bearing CAIs also contain perovskite, melilite, spinel, and diopside with different abundances and textures. The mineralogical textures indicate that the hibonite crystallized prior to the other constituent minerals, which is inconsistent with crystallization from the complete melt [8]. This suggests that they are primarily condensates from the solar nebular gas, some of which may have experienced later partial melting.
Remaining CAIs without hibonite and grossite have bulk compositions similar to CAIs in CV chondrites. They contain spinel, perovskite, melilite, anorthite, and diopside with different abundances and textures, and most are texturally very likely fragments of igneous, coarse-grained CAIs and fine-grained CAIs. They were formed by melt crystallization and condensation, respectively.
References: [1] Connelly J. N. et al. (2012) Science 338, 651–655. [2] MacPherson et al. (1995) Meteoritics 30, 365–386. [3] Krot A. N. et al. (2008) MaPS 43, 1531–1550. [4] Gounelle et al. (2009) ApJ 698, 18–22. [5] Krot A. N. et al. (2020) Sci. Adv. 6, eaay2724. [6] Krot et al. (2012) MaPS 47, 1948–1979. [7] Kimura M. et al. (1993) GCA 57, 2329–2359. [8] MacPherson G. J. and Huss G. R. (2005) GCA 69, 3099–3127.