12:00 PM - 12:15 PM
[PCG21-12] First Identification of Presolar Metastable Alumina by In-situ Analysis of Carbonaceous Chondrite DOM 08006
Keywords:presolar grains, circumstellar dust, alumina, meteorite, silicates, AGB star
Presolar grains, survivors of circumstellar dust, are identified in primitive meteorites and interplanetary dust particles based on their isotopic anomalies [1]. In addition to amorphous silicates, corundum, and spinel, metastable (transitional/amorphous) alumina is expected to be abundant dust around oxygen-rich AGB stars, which are the main sources of presolar oxides/silicates [2]. However, there are few studies of metastable alumina as presolar oxides in meteorites. Metastable alumina is easily soluble in acid and may be missed during acid treatment analysis [3]. Although in-situ analysis is effective for the identification of metastable alumina, the abundance of presolar oxides in meteorites is very low [4]. In this study, we established an efficient identification method of presolar oxides in chondrites by using elemental composition MAP of the matrix surface and estimated the formation and alteration processes of circumstellar dust by analyzing the microstructure of the identified presolar grains using FIB-TEM.
Based on isotopic measurement in 2023[5], smooth matrix areas containing Al-rich grains in a thin section of Dominion Range (DOM) 08006 (CO3.0) were selected based on SEM-EDS maps (SU-6600; HITACHI). Isotopic imaging of secondary ions (12C, 13C, 16O, 17O, 18O, 28Si, 27Al16O) was performed with a CAMECA NanoSIMS 50L (ARIM, U.Tokyo). The charge neutralization electron gun was also used. We analyzed the oxygen isotope ratios per pixel (17O/16O, 18O/16O) and identified presolar grains using L'IMAGE software [6]. Some of the identified presolar grains were extracted by FIB (Versa3D Dualbeam; FEI, Helios5 Dualbeam; TF), and (S)TEM analysis (JEM-2800; JEOL) was performed to determine the chemical compositions and the mineral phases.
We identified 27 presolar oxides and presolar silicates (2 oxides and 25 silicates, 15 in Group 1, 6 in Group 3, and 6 in Group 4) from ~5800 μm2 area. The abundance of presolar grains is approximately 330 ppm. This value is much higher than that obtained in 2023 (about 120 ppm) [5] and about 1.3-1.5 times higher than those obtained in previous studies [6, 7], where isotopic measurements were performed in the matrix of the same meteorite. This is because the isotope measurement was performed on the selected region of the matrix surface where the surface is smooth and the grain boundaries are unclear. The charge neutralization electron gun kept the relatively large grains from being charged up, leading to the identification of micronized grains. FIB-TEM analysis showed that one of the identified presolar silicates (Group 3, low mass, low metallicity AGB star origin) was the largest (1350 × 540 nm2) presolar silicate identified in meteorites so far. The SEM-EDS analysis of the grain indicates that it has an olivine composition with (Mg + Fe)/Mg ~ 0.9. We identified one presolar oxide (Group 1, low to intermediate mass AGB star origin) of a crystalline transition alumina of about 500 nm in size containing Ca, with an outer rim of amorphous alumina of about 100 nm in thickness. This is the first in-situ and undoubted identification of both presolar transition alumina and amorphous alumina. The fact that the grain contains Ca is consistent with the experimental result that the spectra of Al oxides with small amounts of Ca reproduce those of circumstellar dust well [8], which supports that most of the metastable alumina observed in AGB stars may have been lost during the acid treatment of the meteorites. The transition alumina containing Ca with the amorphous alumina rim without Ca may indicate the grain growth in the pulsating atmosphere of an AGB star [9]. We will also report the results of FIB-TEM analyses of other identified presolar grains.
References
[1] Nittler, L. R. and Ciesla F. (2016) ARAA 54, 53.
[2] Takigawa, A., et al. (2019) ApJL 878, L7.
[3] Takigawa, A., et al. (2014) GCA 124, 309.
[4] Leitner, J., et al. (2012) ApJL 745, 38
[5] Hashizume, H., et al. (2024) MAPS, #6306.
[6] Nittler, L. R., et al. (2018) GCA 226, 107.
[7] Haenecour, P., et al. (2018) GCA 221, 379.
[8] Takigawa, A., et al. (2023) LPSC LIV, #2136.
[9] Gobrecht, D. et al. (2016), A&A, 585 A6.
Based on isotopic measurement in 2023[5], smooth matrix areas containing Al-rich grains in a thin section of Dominion Range (DOM) 08006 (CO3.0) were selected based on SEM-EDS maps (SU-6600; HITACHI). Isotopic imaging of secondary ions (12C, 13C, 16O, 17O, 18O, 28Si, 27Al16O) was performed with a CAMECA NanoSIMS 50L (ARIM, U.Tokyo). The charge neutralization electron gun was also used. We analyzed the oxygen isotope ratios per pixel (17O/16O, 18O/16O) and identified presolar grains using L'IMAGE software [6]. Some of the identified presolar grains were extracted by FIB (Versa3D Dualbeam; FEI, Helios5 Dualbeam; TF), and (S)TEM analysis (JEM-2800; JEOL) was performed to determine the chemical compositions and the mineral phases.
We identified 27 presolar oxides and presolar silicates (2 oxides and 25 silicates, 15 in Group 1, 6 in Group 3, and 6 in Group 4) from ~5800 μm2 area. The abundance of presolar grains is approximately 330 ppm. This value is much higher than that obtained in 2023 (about 120 ppm) [5] and about 1.3-1.5 times higher than those obtained in previous studies [6, 7], where isotopic measurements were performed in the matrix of the same meteorite. This is because the isotope measurement was performed on the selected region of the matrix surface where the surface is smooth and the grain boundaries are unclear. The charge neutralization electron gun kept the relatively large grains from being charged up, leading to the identification of micronized grains. FIB-TEM analysis showed that one of the identified presolar silicates (Group 3, low mass, low metallicity AGB star origin) was the largest (1350 × 540 nm2) presolar silicate identified in meteorites so far. The SEM-EDS analysis of the grain indicates that it has an olivine composition with (Mg + Fe)/Mg ~ 0.9. We identified one presolar oxide (Group 1, low to intermediate mass AGB star origin) of a crystalline transition alumina of about 500 nm in size containing Ca, with an outer rim of amorphous alumina of about 100 nm in thickness. This is the first in-situ and undoubted identification of both presolar transition alumina and amorphous alumina. The fact that the grain contains Ca is consistent with the experimental result that the spectra of Al oxides with small amounts of Ca reproduce those of circumstellar dust well [8], which supports that most of the metastable alumina observed in AGB stars may have been lost during the acid treatment of the meteorites. The transition alumina containing Ca with the amorphous alumina rim without Ca may indicate the grain growth in the pulsating atmosphere of an AGB star [9]. We will also report the results of FIB-TEM analyses of other identified presolar grains.
References
[1] Nittler, L. R. and Ciesla F. (2016) ARAA 54, 53.
[2] Takigawa, A., et al. (2019) ApJL 878, L7.
[3] Takigawa, A., et al. (2014) GCA 124, 309.
[4] Leitner, J., et al. (2012) ApJL 745, 38
[5] Hashizume, H., et al. (2024) MAPS, #6306.
[6] Nittler, L. R., et al. (2018) GCA 226, 107.
[7] Haenecour, P., et al. (2018) GCA 221, 379.
[8] Takigawa, A., et al. (2023) LPSC LIV, #2136.
[9] Gobrecht, D. et al. (2016), A&A, 585 A6.