16:45 〜 17:00
[PPS07-24] 低圧H2, H2-H2O混合ガス中でのファッサイトチタン酸化還元状態に関する実験的研究
キーワード：難揮発性包有物 (CAI)、ファッサイト (輝石)、チタン価数、メルト-ガス相互作用、酸化還元状態、原始太陽系円盤
Coarse-grained, calcium-aluminum-rich inclusions (CAIs) experienced high-temperature melting events in the protosolar disk (e.g., Grossman, 1972; Mendybaev et al., 2006). They contain Ti-rich pyroxene with a quaternary solid solution, called fassaite (e.g., Simon et al., 1991). The high proportion of trivalent Ti in fassaite of Type B CAI (Ti3+/Titot < ~0.6–0.8) is suggestive of crystallization from the melt exposed to a reducing disk gas (e.g., Grossman et al., 2006). The valence state distribution of Ti in Type B CAI fassaite could be controlled by various factors including the disk gas redox condition, total pressure, and cooling rate, but little is known about the effect of these factors.
To investigate the effect of these factors on the fassaite composition, we carried out crystallization experiments on CAI analog melt under low-pressure H2 gas and H2-H2O gas mixture (H2/H2O ratios (H/W) of ~410–4550) with a total pressure (Ptot) of 2.5 and 10 Pa using a vacuum furnace. The values of H/W were estimated based on the amount of liquid H2O (used as a water vapor source) consumed and the H2 flow rate measured by a mass flow controller. The starting materials’ compositions are similar to the (CAIχ) and ETEG in Grossman et al. (2006). The CAIχ samples were heated at 1450°C for 1–3 h and then cooled down to ~1100°C with the rates (Rc) of 5–50°C h–1. The ETEG samples were first heated at 1270°C (~40°C above the fassaite liquidus temperature; Stolper, 1982) for 2–8 h and then annealed at 1210°C for 10–13 h. We determined fassaite compositions and Ti valence state (Beckett, 1986) using an FE-EPMA instrument. EMPA analyses were also made for the CAIχ samples synthesized through heating at 0.1, 1, and 10 Pa of H2 gas followed by cooling with Rc of 5 and 50°C h–1 (Kamibayashi et al., 2021).
For the run products of CAIχ cooled with slower Rc (e.g., 5°C h–1), we found a correlation between Ti3+/Titot ratios of early-crystallized fassaite and the gas redox conditions; for instance, Ti3+/Titot ratios are ~0.8 when heated in H2, ~0.6 in a H2-H2O gas with H/W of ~4550, and ~0.2 in a H2-H2O gas with H/W of ~410.
For the samples heated with H2, Ti3+/Titot ratios and their distributions in the samples cooled at 5 and 50°C h–1 are almost identical each other at PH2 = 10 Pa, while the ratios for the samples cooled at 50°C h–1 are distinctly smaller than those in the samples cooled at 5°C h–1 at PH2 = 1–2.5 Pa. This behavior was also evident in the samples heated with H2-H2O gas at Ptot of 2.5 Pa. These results suggest that the rate of the redox reaction of Ti through the gas-melt interaction is slower than expected in a previous study where the equilibrium was achieved in an hour (Schreiber et al., 1978). This discrepancy is probably attributed to the effect of total pressure and little communication between melt and gas due to the formation of melilite mantle (Simon & Grossman, 2006). The ETEG samples heated in a H2-H2O gas with H/W of ~410 showed Ti3+/Titotratios of ~0.2, close to those of early-crystallized fassaite in the samples crystallized with Rc = 5°C h–1 under the same gaseous conditions.
Adopting Ti3+/Titot ratios for early-crystallized fassaite in the CAIχ samples cooled with 5°C h–1 and those for fassaite in the ETEG samples and considering residual PH2O in the furnace (~1 × 10–4 Pa) during the experiments with H2 gas, we found a relation between PH2O/PH2 and Ti3+/Ti4+ ratios. The relation is consistent with the prediction from the equilibrium reaction, 2CaTi4+Al2O6 (s) + 2SiO2 (l) + H2 (g) = 2CaTi3+AlSiO6 (s) + Al2O3 (l) + H2O (g). The present results indicate that, if Type B CAIs crystallized while the melt was in redox equilibrium with the protosolar disk gas, Type B CAIs crystallized in the disk gas whose H2/H2O ratios are ~1–2 orders of magnitude smaller than expected from the elemental abundance of Solar System.
To investigate the effect of these factors on the fassaite composition, we carried out crystallization experiments on CAI analog melt under low-pressure H2 gas and H2-H2O gas mixture (H2/H2O ratios (H/W) of ~410–4550) with a total pressure (Ptot) of 2.5 and 10 Pa using a vacuum furnace. The values of H/W were estimated based on the amount of liquid H2O (used as a water vapor source) consumed and the H2 flow rate measured by a mass flow controller. The starting materials’ compositions are similar to the (CAIχ) and ETEG in Grossman et al. (2006). The CAIχ samples were heated at 1450°C for 1–3 h and then cooled down to ~1100°C with the rates (Rc) of 5–50°C h–1. The ETEG samples were first heated at 1270°C (~40°C above the fassaite liquidus temperature; Stolper, 1982) for 2–8 h and then annealed at 1210°C for 10–13 h. We determined fassaite compositions and Ti valence state (Beckett, 1986) using an FE-EPMA instrument. EMPA analyses were also made for the CAIχ samples synthesized through heating at 0.1, 1, and 10 Pa of H2 gas followed by cooling with Rc of 5 and 50°C h–1 (Kamibayashi et al., 2021).
For the run products of CAIχ cooled with slower Rc (e.g., 5°C h–1), we found a correlation between Ti3+/Titot ratios of early-crystallized fassaite and the gas redox conditions; for instance, Ti3+/Titot ratios are ~0.8 when heated in H2, ~0.6 in a H2-H2O gas with H/W of ~4550, and ~0.2 in a H2-H2O gas with H/W of ~410.
For the samples heated with H2, Ti3+/Titot ratios and their distributions in the samples cooled at 5 and 50°C h–1 are almost identical each other at PH2 = 10 Pa, while the ratios for the samples cooled at 50°C h–1 are distinctly smaller than those in the samples cooled at 5°C h–1 at PH2 = 1–2.5 Pa. This behavior was also evident in the samples heated with H2-H2O gas at Ptot of 2.5 Pa. These results suggest that the rate of the redox reaction of Ti through the gas-melt interaction is slower than expected in a previous study where the equilibrium was achieved in an hour (Schreiber et al., 1978). This discrepancy is probably attributed to the effect of total pressure and little communication between melt and gas due to the formation of melilite mantle (Simon & Grossman, 2006). The ETEG samples heated in a H2-H2O gas with H/W of ~410 showed Ti3+/Titotratios of ~0.2, close to those of early-crystallized fassaite in the samples crystallized with Rc = 5°C h–1 under the same gaseous conditions.
Adopting Ti3+/Titot ratios for early-crystallized fassaite in the CAIχ samples cooled with 5°C h–1 and those for fassaite in the ETEG samples and considering residual PH2O in the furnace (~1 × 10–4 Pa) during the experiments with H2 gas, we found a relation between PH2O/PH2 and Ti3+/Ti4+ ratios. The relation is consistent with the prediction from the equilibrium reaction, 2CaTi4+Al2O6 (s) + 2SiO2 (l) + H2 (g) = 2CaTi3+AlSiO6 (s) + Al2O3 (l) + H2O (g). The present results indicate that, if Type B CAIs crystallized while the melt was in redox equilibrium with the protosolar disk gas, Type B CAIs crystallized in the disk gas whose H2/H2O ratios are ~1–2 orders of magnitude smaller than expected from the elemental abundance of Solar System.