Speaker
Description
Presolar grains are survivors of circumstellar dust preserved in primitive meteorites. They enable laboratory analyses of circumstellar dust formation and evolution [1]. Astronomical observations and circumstellar models suggest that circumstellar dust of oxygen-rich asymptotic giant branch (AGB) stars comprises diverse mineral phases, including predominantly amorphous, Mg-rich silicates, corundum, and metastable (transition or amorphous) alumina [2]. However, microstructural studies have shown that nearly half of presolar silicates characterized by TEM are crystalline, whereas the remaining amorphous grains exhibit a broad range of Mg/(Mg+Fe) ratios [3]. Similar discrepancies are also observed for Al-rich oxides, where circumstellar dust is inferred to contain corundum and metastable alumina, whereas presolar oxides are dominated by corundum and spinel [4]. These observations raise the possibility that the discrepancy between circumstellar silicate dust and presolar silicates may also reflect post-condensation dust evolution [5]. To investigate the relationship between the mineralogical and compositional diversity of presolar silicates and circumstellar dust evolution, we conducted coordinated isotopic and microstructural analyses of newly identified presolar silicates.
Smooth matrix areas, which are expected to contain abundant presolar grains [5], were selected from a thin section of Dominion Range (DOM) 08006 (CO3.0). Isotopic imaging of secondary ions (12C, 13C, 16O, 17O, 18O, 28Si, 27Al16O) was performed with a CAMECA NanoSIMS 50L (ARIM, U.Tokyo). We analyzed the oxygen isotope ratios per pixel (17O/16O, 18O/16O) and identified presolar grains using L'IMAGE software. Some of the identified presolar grains and surrounding matrix were extracted by FIB (Helios5 Dualbeam; TF), and (S)TEM analysis (JEM-2800, JEM 2100F; JEOL) was performed to determine the chemical compositions and the mineral phases.
We identified 79 presolar oxides and silicates (8 oxides and 71 silicates) from an area of ~19,670 μm². Five presolar silicates were further characterized by TEM. One grain (AREA06_20_Si_PG47) belongs to Group 3, indicating an origin in a low-mass, low-metallicity AGB star. This is the first in-situ TEM analysis of a Group 3 presolar silicate from a meteorite. The grain is a ~1400 nm single-crystalline olivine with Mg/(Mg+Fe) ~ 0.9. The remaining four grains belong to Group 1 (low- to intermediate-mass AGB stars): three are olivine with Mg/(Mg+Fe) > 0.7, whereas one (LCM01b_Si_PG07) is an amorphous, Ca-bearing Fe-rich grain with a composition of (Mg0.50Fe0.83Ca0.28)Al0.09SiO3.75.
The high fraction of crystalline grains among TEM-characterized presolar silicates in this study may reflect secondary crystallization in the circumstellar region or protoplanetary disk, or a sampling bias resulting from the easier recognition of crystalline grains during FIB preparation for TEM analyses. The amorphous grain (LCM01b_Si_PG07) is distinguished by its unusual Fe-rich, Ca-bearing, but Al-poor composition. The enrichment of Ca, together with abundant Fe, without corresponding Al enrichment is not readily explained by direct condensation of the present composition. The formation of a Ca-bearing amorphous silicate precursor followed by chemical modification involving Fe enrichment through interaction with the surrounding environment.
[1] Nittler, L. R. and Ciesla F. (2016) ARAA 54, 53., [2] Takigawa, A., et al. (2019) ApJL 878, L7., [3] Seifert et al. (2022) MAPS 57, 1119., [4] Takigawa, A., et al. (2014) GCA 124, 309., [5] Hashizume, H. (2026) M.Sc. thesis, U-Tokyo.
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