9:15 AM - 9:30 AM
[PCG22-02] Abundance of presolar oxides in matrix with different micro lithologies by in-situ analysis of CO3 Chondrite Dominion Range 08006
Keywords:presolar grains, oxides, chondrite, TEM analysis, alumina
Presolar grains identified by their large isotopic compositions in primitive chondrites and interplanetary dust particles are thought to record information before the solar system formation [1, 2]. Most presolar oxide grains have been found in acid residues of ordinary chondrites. Presolar alumina (Al2O3) grains in acid residues were mainly crystalline corundum (α-Al2O3) [3]. On the other hand, MIR spectroscopy of circumstellar dust showed that amorphous or low-crystallinity alumina is abundant around low-mass evolved stars in addition to corundum [4]. This indicates that the analysis of acid treatment residues may overlook amorphous or low-crystallinity alumina grains due to their solubility in acid [3]. However, there are limited studies of the crystal structures of presolar oxide identified by in-situ isotopic measurements because the abundance of the presolar oxides in chondrites is at most tens of ppm [5]. In this study, we investigated the abundances of presolar grains in the matrix of a carbonaceous chondrite with different micro-lithologies for efficient identification of presolar oxides and analyzed the crystal structure of the identified presolar oxides.
EDS maps and SE images (SU-6600; HITACHI) were obtained for the matrix in a thin section of Dominion Range (DOM) 08006 (CO3.00). Several areas in the matrix including Al-rich grains with different micro-lithologies classified based on grain size and porosity were selected using the EDS maps and SE images, and isotopic imaging was performed with CAMECA NanoSIMS 50 (AORI, U. Tokyo) and CAMECA NanoSIMS 50L (ARIM, U. Tokyo). Each 10 μm × 10 μm area (256 × 256 pixels) was rastered by a focused Cs+ primary ion beam (~0.9 pA) in multicollection mode. Secondary ions (16O, 17O, 18O, 12C2, 12C13C, 28Si, and 27Al16O) and SE images were acquired simultaneously. The obtained images were analyzed with L'image software [6]. After image alignment, deadtime correction, and QSA effect correction, isotopic ratios per pixel (17O/16O, 18O/16O) were calculated after 3 × 3 pixel boxcar smoothing. Oxygen isotopes were internally normalized to the average isotopic composition of each image. Isotopically anomalous grains compared with surrounding areas above 4σ based on the Poisson distribution were identified as presolar grains. Some identified presolar grains were extracted by FIB (Versa3D Dualbeam; FEI), and TEM analysis (JEM-2800; JEOL) was performed to determine their chemical composition and crystal structure.
We identified 24 presolar oxides and silicates (4 oxides and 20 silicates, 19 in Group 1, 2 in Group 3, and 3 in Group 4) from ~7500 μm2 area. In terms of matrix microstructure, the abundance of presolar grains in relatively smooth areas consisting of fine grains reached ~270 ppm, a higher value compared to previous studies [e.g., 6]. On the other hand, the abundance of presolar grains in porous or coarse-grained (about 1 μm) areas was relatively low (~90 ppm). These results suggest a clear dependence of the presolar grain abundance on the matrix micro-lithology. The relative abundance of presolar to solar Al-rich grains was approximately 3%. This value is similar to that of presolar to solar alumina grains in acid treatment residue of ordinary chondrite [3], suggesting that the presolar fraction of refractory oxides in the matrix is high (several percent) both in ordinary and carbonaceous chondrites. One of the presolar oxides analyzed by FIB-TEM was identified as a crystalline γ-Al2O3 with a spinel inclusion. In the presentation, we will also discuss the results of crystal structure analysis of other presolar oxides to determine the crystallinity of Al-rich presolar oxides.
References
[1] Nittler L. R. and Ciesla F. (2016) ARAA 54, 53. [2] Floss C., and Haenecour P. (2016) GJ 50, 3. [3] Takigawa A., et al. (2014) GCA 124, 309. [4] Takigawa A., et al. (2019) ApJL 878, L7. [5] Zinner, E. (2014) Treatise on Geochemistry (2nd Ed.) 1, 181. [6] Nittler L. R., et al. (2018) GCA 226, 107.
EDS maps and SE images (SU-6600; HITACHI) were obtained for the matrix in a thin section of Dominion Range (DOM) 08006 (CO3.00). Several areas in the matrix including Al-rich grains with different micro-lithologies classified based on grain size and porosity were selected using the EDS maps and SE images, and isotopic imaging was performed with CAMECA NanoSIMS 50 (AORI, U. Tokyo) and CAMECA NanoSIMS 50L (ARIM, U. Tokyo). Each 10 μm × 10 μm area (256 × 256 pixels) was rastered by a focused Cs+ primary ion beam (~0.9 pA) in multicollection mode. Secondary ions (16O, 17O, 18O, 12C2, 12C13C, 28Si, and 27Al16O) and SE images were acquired simultaneously. The obtained images were analyzed with L'image software [6]. After image alignment, deadtime correction, and QSA effect correction, isotopic ratios per pixel (17O/16O, 18O/16O) were calculated after 3 × 3 pixel boxcar smoothing. Oxygen isotopes were internally normalized to the average isotopic composition of each image. Isotopically anomalous grains compared with surrounding areas above 4σ based on the Poisson distribution were identified as presolar grains. Some identified presolar grains were extracted by FIB (Versa3D Dualbeam; FEI), and TEM analysis (JEM-2800; JEOL) was performed to determine their chemical composition and crystal structure.
We identified 24 presolar oxides and silicates (4 oxides and 20 silicates, 19 in Group 1, 2 in Group 3, and 3 in Group 4) from ~7500 μm2 area. In terms of matrix microstructure, the abundance of presolar grains in relatively smooth areas consisting of fine grains reached ~270 ppm, a higher value compared to previous studies [e.g., 6]. On the other hand, the abundance of presolar grains in porous or coarse-grained (about 1 μm) areas was relatively low (~90 ppm). These results suggest a clear dependence of the presolar grain abundance on the matrix micro-lithology. The relative abundance of presolar to solar Al-rich grains was approximately 3%. This value is similar to that of presolar to solar alumina grains in acid treatment residue of ordinary chondrite [3], suggesting that the presolar fraction of refractory oxides in the matrix is high (several percent) both in ordinary and carbonaceous chondrites. One of the presolar oxides analyzed by FIB-TEM was identified as a crystalline γ-Al2O3 with a spinel inclusion. In the presentation, we will also discuss the results of crystal structure analysis of other presolar oxides to determine the crystallinity of Al-rich presolar oxides.
References
[1] Nittler L. R. and Ciesla F. (2016) ARAA 54, 53. [2] Floss C., and Haenecour P. (2016) GJ 50, 3. [3] Takigawa A., et al. (2014) GCA 124, 309. [4] Takigawa A., et al. (2019) ApJL 878, L7. [5] Zinner, E. (2014) Treatise on Geochemistry (2nd Ed.) 1, 181. [6] Nittler L. R., et al. (2018) GCA 226, 107.