5:15 PM - 6:45 PM
[PPS08-P15] Experimental simulation on survivability of presolar SiC grains in the protosolar disk
Keywords:presolar grains, silicon carbide (SiC), evaporation, kinetics, protosolar disk
Presolar SiC is one of the most abundant presolar grains (e.g., Zinner et al., 2014), and their survivability would be an excellent tool for investigating physicochemical conditions of the protosolar disk because of their thermodynamically unstable nature in oxidative protosolar disk gas (Larimer & Bartholomay, 1979). Evaporation kinetics of SiC under various temperature-oxygen fugacity conditions at 1 atm has been investigated by Menydybaev et al. (2002). However, the kinetics obtained at 1 atm may not be applicable to the low-pressure disk conditions. Here, we conducted evaporation experiments of SiC in the low-pressure gas of an H2–H2O gas mixture to determine the reaction mechanism and kinetics of evaporation of SiC in the disk.
Evaporation experiments were conducted at 1506–1250°C and total pressures (Ptot.) of 0.5 and 2.5 Pa of an H2-H2O gas mixture (Ptot.) for 0.5–110.3 h in a vacuum furnace with a gas flow system. The starting material was a plate of polycrystalline β (3C)-SiC, which is a major polytype of presolar SiC grains (Daulton et al., 2003). Following three different gas conditions were achieved in this study; (1) Ptot. = 0.5 Pa and PH2O of 2.2 × 10–3 Pa, (2) Ptot. = 2.5 Pa and PH2O of 6.1 × 10–3 Pa, and (3) Ptot. = 2.5 Pa and PH2O of 1.5 × 10–2 Pa. These redox conditions are similar to or slightly oxidative than that in the canonical protosolar disk (Lodders, 2003). The sample weight before and after the experiments was measured with an ultra-micron balance. FIB lift-out sections of some samples were prepared with a FIB-SEM (Hitachi NX2000; Thermo Fisher Scientific Versa 3D), and analyses of the thin sections were carried out with a STEM-EDS (JEOL JEM-2800). Micro-Raman spectroscopic observations were also made for the phase characterization.
STEM-EDS analysis of the heated samples showed that there was no oxide layer on the sample surface, but porous carbon-rich layers were occasionally observed. Based on Raman spectroscopic observations suggest that graphite exists homogeneously or heterogeneously in all analyzed samples. These results suggest that evaporation of SiC would have proceeded via active oxidation (i.e., no oxide layers on the SiC surface) (e.g., Narushima et al., 1997) in this study. Under all heating conditions, the evaporation flux J estimated from the weight loss and original size of the sample (Takigawa et al., 2009) depends weakly on temperatures higher than ~1350–1400°C, while larger temperature dependence of J was observed at lower temperatures. The temperature-independent J suggests that the supply of gaseous species is a rate-limiting step (e.g., Yamamoto et al., 2021). At lower temperatures, the evaporation reaction would be rate-limited by the chemical reaction at the bare SiC surface. The obtained activation energies (~564–1160 kJ mol–1) increase with increasing PH2O and are broadly consistent with the range of those reported for the active oxidation of SiC in previous studies (~460–1130 kJ mol–1) (e.g., Rosner & Allendorf, 1970; Mendybaev et al., 2002).
The present results suggest that evaporation of presolar SiC grains would proceed via active oxidation in the canonical disk, suggesting that the oxide rim on the presolar SiC grains in primitive meteorites (Bernatowicz et al., 2006) would form during more oxidative events associated with chondrule formation and/or during aqueous alteration in the parent bodies. The lifetime of 1 µm-sized SiC grain in the protosolar disk with PH2O of 0.1 Pa would have little temperature dependence at temperatures higher than ~1400°C, while it would have large temperature dependence at lower temperatures. Presolar SiC could not escape complete evaporation under the conditions where original O-isotopic anomalies in presolar corundum were completely lost through isotopic exchange with disk gas (Prot & Monty, 1996), but could survive at lower temperatures where O-isotopic signatures of presolar silicate grains were erased (e.g., Yamamoto et al., 2020).
Evaporation experiments were conducted at 1506–1250°C and total pressures (Ptot.) of 0.5 and 2.5 Pa of an H2-H2O gas mixture (Ptot.) for 0.5–110.3 h in a vacuum furnace with a gas flow system. The starting material was a plate of polycrystalline β (3C)-SiC, which is a major polytype of presolar SiC grains (Daulton et al., 2003). Following three different gas conditions were achieved in this study; (1) Ptot. = 0.5 Pa and PH2O of 2.2 × 10–3 Pa, (2) Ptot. = 2.5 Pa and PH2O of 6.1 × 10–3 Pa, and (3) Ptot. = 2.5 Pa and PH2O of 1.5 × 10–2 Pa. These redox conditions are similar to or slightly oxidative than that in the canonical protosolar disk (Lodders, 2003). The sample weight before and after the experiments was measured with an ultra-micron balance. FIB lift-out sections of some samples were prepared with a FIB-SEM (Hitachi NX2000; Thermo Fisher Scientific Versa 3D), and analyses of the thin sections were carried out with a STEM-EDS (JEOL JEM-2800). Micro-Raman spectroscopic observations were also made for the phase characterization.
STEM-EDS analysis of the heated samples showed that there was no oxide layer on the sample surface, but porous carbon-rich layers were occasionally observed. Based on Raman spectroscopic observations suggest that graphite exists homogeneously or heterogeneously in all analyzed samples. These results suggest that evaporation of SiC would have proceeded via active oxidation (i.e., no oxide layers on the SiC surface) (e.g., Narushima et al., 1997) in this study. Under all heating conditions, the evaporation flux J estimated from the weight loss and original size of the sample (Takigawa et al., 2009) depends weakly on temperatures higher than ~1350–1400°C, while larger temperature dependence of J was observed at lower temperatures. The temperature-independent J suggests that the supply of gaseous species is a rate-limiting step (e.g., Yamamoto et al., 2021). At lower temperatures, the evaporation reaction would be rate-limited by the chemical reaction at the bare SiC surface. The obtained activation energies (~564–1160 kJ mol–1) increase with increasing PH2O and are broadly consistent with the range of those reported for the active oxidation of SiC in previous studies (~460–1130 kJ mol–1) (e.g., Rosner & Allendorf, 1970; Mendybaev et al., 2002).
The present results suggest that evaporation of presolar SiC grains would proceed via active oxidation in the canonical disk, suggesting that the oxide rim on the presolar SiC grains in primitive meteorites (Bernatowicz et al., 2006) would form during more oxidative events associated with chondrule formation and/or during aqueous alteration in the parent bodies. The lifetime of 1 µm-sized SiC grain in the protosolar disk with PH2O of 0.1 Pa would have little temperature dependence at temperatures higher than ~1400°C, while it would have large temperature dependence at lower temperatures. Presolar SiC could not escape complete evaporation under the conditions where original O-isotopic anomalies in presolar corundum were completely lost through isotopic exchange with disk gas (Prot & Monty, 1996), but could survive at lower temperatures where O-isotopic signatures of presolar silicate grains were erased (e.g., Yamamoto et al., 2020).