9:00 AM - 9:15 AM
[PPS08-01] Physicochemical conditions in the protosolar disk constrained by oxygen isotopic kinetics among CAI melt-CO-H2O system
Keywords:CAIs, CO, H2O, oxygen isotope exchange, kinetics, protosolar disk
Igneous calcium-aluminum-rich inclusions (CAIs) experienced partial melting events with the maximum temperatures of ~1400°C with subsequent cooling in the earliest Solar System (e.g., Grossman, 1972; Stolper & Paque, 1986). O-isotopic compositions of igneous CAIs exhibit mass-independent isotopic distribution among their constituent minerals that is considered to have formed through O-isotope exchange reaction among isotopically distinct CAI melt, CO gas, and H2O gas in the protosolar disk (e.g., Yurimoto & Kuramoto, 2004; Yurimoto et al., 1998; Kawasaki et al., 2018; Suzumura et al., 2021). We have shown the heating timescale for type B igneous CAIs during their partial melting events, based on O-isotope exchange kinetics between CAI melt and H2O (Yamamoto et al., 2021) and that between H2O and CO gas (Alexander, 2004). In this study, to put further constraints, we experimentally explored O-isotope exchange kinetics between CAI melt and CO gas, which has not yet been investigated.
O-isotope exchange experiments between type B CAI analogue melt and low pressure-18O-enriched CO gas (~98% 18O) were carried out at 1420 and 1460°C (above melilite liquidus of 1380°C; Yamamoto et al., 2021) and PCO of 0.1, 0.5, and 1 Pa for 52 min–22 h using a high temperature vacuum furnace equipped with a gas flow system. The chemical and O-isotopic composition of glass of the polished cross section of the samples were analyzed by EPMA (JEOL JXA-8900L) at the University of Tokyo and SIMS (Cameca ims-1280HR) at Hokkaido University.
All the isotopic profiles of glass in 2.5–2.7 mm-diameter spherical-shaped run products exhibited increase of O-isotope ratios (f18O = 18O/(16O + 18O)) toward their surface and gradual increase of f18O at the surface with heating duration, indicating that O-isotope exchange process at the melt-gas interface and diffusional transport process toward the melt interior took place simultaneously. The isotopic profiles were well fitted by a three-dimensional spherical diffusion model with a time-dependent surface concentration (Crank, 1975), yielding the diffusion coefficient D and the surface isotopic exchange efficiency alpha that expresses isotopic exchange efficiency of colliding gas species at the melt surface. The estimated D of 1.78 × 10–11 m2 sec–1 is consistent with that estimated by O-isotope exchange experiments of CAI melt–H2O (1.62 × 10–11 m2 sec–1; Yamamoto et al., 2021), suggesting that O2– anion is the dominant diffusion species (i.e., oxygen self-diffusion). We also found that the values of alpha for CO are in the orders of 10–3–10–4, and are 2–3 orders of magnitude smaller than alpha for H2O (~0.28; Yamamoto et al., 2021).
The reaction kinetic suggests the order of O-isotopic exchange rates (k) always obeys the relation of kCO-H2O > kCAI-H2O > kCAI-CO and type B CAIs should be heated for at least a few days at temperatures above melilite liquidus to form the homogeneous O-isotopic composition of melilite in type B CAIs (as discussed in Yamamoto et al., 2021) even if the effect of CO gas is taken into account. Considering the open-system evaporation experimental data (e.g., Mendybaev et al., 2021; Kamibayashi et al., 2021; Kamibayashi, 2021 PhD thesis), recondensation of evaporated materials to the melt in addition to melt evaporation should be taken into account to avoid excess amounts of evaporation and associated large isotopic fractionation for Mg and Si during heating at ~1400°C for 2–3 days. The amount of Mg evaporated from the melt at 1420°C and PH2 = 10 Pa for 2–3 days (Grossman et al., 2000; Kamibayashi, 2021 PhD thesis) suggests the evaporation flux/condensation flux ratio of ~1.1–1.5. Therefore, the type B CAI formation would take place in a (semi-)closed system caused by surrounding high partial gas pressure of hydrogen and evaporated materials (Richter et al., 2002; Tsuchiyama et al., 1999) and/or under circumstance with high PH2O/PH2 ratio relative to the Solar value (Aléon et al., 2022).
O-isotope exchange experiments between type B CAI analogue melt and low pressure-18O-enriched CO gas (~98% 18O) were carried out at 1420 and 1460°C (above melilite liquidus of 1380°C; Yamamoto et al., 2021) and PCO of 0.1, 0.5, and 1 Pa for 52 min–22 h using a high temperature vacuum furnace equipped with a gas flow system. The chemical and O-isotopic composition of glass of the polished cross section of the samples were analyzed by EPMA (JEOL JXA-8900L) at the University of Tokyo and SIMS (Cameca ims-1280HR) at Hokkaido University.
All the isotopic profiles of glass in 2.5–2.7 mm-diameter spherical-shaped run products exhibited increase of O-isotope ratios (f18O = 18O/(16O + 18O)) toward their surface and gradual increase of f18O at the surface with heating duration, indicating that O-isotope exchange process at the melt-gas interface and diffusional transport process toward the melt interior took place simultaneously. The isotopic profiles were well fitted by a three-dimensional spherical diffusion model with a time-dependent surface concentration (Crank, 1975), yielding the diffusion coefficient D and the surface isotopic exchange efficiency alpha that expresses isotopic exchange efficiency of colliding gas species at the melt surface. The estimated D of 1.78 × 10–11 m2 sec–1 is consistent with that estimated by O-isotope exchange experiments of CAI melt–H2O (1.62 × 10–11 m2 sec–1; Yamamoto et al., 2021), suggesting that O2– anion is the dominant diffusion species (i.e., oxygen self-diffusion). We also found that the values of alpha for CO are in the orders of 10–3–10–4, and are 2–3 orders of magnitude smaller than alpha for H2O (~0.28; Yamamoto et al., 2021).
The reaction kinetic suggests the order of O-isotopic exchange rates (k) always obeys the relation of kCO-H2O > kCAI-H2O > kCAI-CO and type B CAIs should be heated for at least a few days at temperatures above melilite liquidus to form the homogeneous O-isotopic composition of melilite in type B CAIs (as discussed in Yamamoto et al., 2021) even if the effect of CO gas is taken into account. Considering the open-system evaporation experimental data (e.g., Mendybaev et al., 2021; Kamibayashi et al., 2021; Kamibayashi, 2021 PhD thesis), recondensation of evaporated materials to the melt in addition to melt evaporation should be taken into account to avoid excess amounts of evaporation and associated large isotopic fractionation for Mg and Si during heating at ~1400°C for 2–3 days. The amount of Mg evaporated from the melt at 1420°C and PH2 = 10 Pa for 2–3 days (Grossman et al., 2000; Kamibayashi, 2021 PhD thesis) suggests the evaporation flux/condensation flux ratio of ~1.1–1.5. Therefore, the type B CAI formation would take place in a (semi-)closed system caused by surrounding high partial gas pressure of hydrogen and evaporated materials (Richter et al., 2002; Tsuchiyama et al., 1999) and/or under circumstance with high PH2O/PH2 ratio relative to the Solar value (Aléon et al., 2022).