[PPS10-08] Thermal history of coarse-grained CAIs in the protosolar disk constrained by O-isotope exchange kinetics between CAI melt and water vapor
キーワード：CAI、酸素、同位体交換、速度論、原始太陽系円盤
Calcium-aluminum-rich refractory inclusions (CAIs) are the sub-millimeter to centimeter-sized oldest objects, consisting of high temperature mineral assemblages, in chondrites (e.g., MacPherson, 2003; Connelly et al., 2012). CAIs would preserve information on high temperature events in the earliest Solar System. Coarse-grained CAIs are considered to experience multiple heating stages after the condensation of their precursors, and exhibit mass-independent O-isotopic variation among their constituent minerals such as spinel, melilite, anorthite and fassaite (e.g., Yurimoto et al., 1998; Kawasaki et al., 2018). An explanation of the O-isotopic variations in coarse-grained CAIs is the partial melting and associated O-isotope exchange between CAI melt and protosolar disk gas (e.g., Yurimoto et al., 1998; Kawasaki et al., 2018). Melilite is the first phase crystallizing from spinel-bearing typical type B CAI melt (Stolper, 1982; Stolper & Paque, 1986) and has typically 16O-poor isotope composition with a limited isotopic variation (e.g., Yurimoto et al., 2008; Kawasaki et al., 2018). If the precursors of CAIs had 16O-rich isotopic compositions close to that of spinel, those observations indicate that melilite crystallized from the melt that was isotopically equilibrated with 16O-poor disk gas above the melilite liquidus. However, because of the lack of quantitative data on O-isotope exchange kinetics between CAI melt and disk gas, the thermal history of CAIs cannot be constrained from O-isotope compositions of constituent minerals. In this study, we conducted O-isotope exchange experiments between type B CAI analogue melt and water vapor.
O-isotope exchange experiments between ~2.5 mm-sized synthetic spherical CAI droplet with the average type B composition (Wark et al., 1979; CAIB hereafter) and 18O-enriched water vapor (~97% 18O) were carried out at 1390°C (10°C above the melilite liquidus) and a disk-like low water vapor pressure of 5 × 10-2 Pa using a newly developed high temperature vacuum furnace with a gas flow system. The low PH2O condition was achieved by the supply of water vapor from water ice in the gas flow system (Yamamoto et al., 2018, 2019). The run products were analyzed with FE-SEM (JEOL JSM-7000F) with an EDS. The O-isotope measurements were conducted with a secondary ion mass spectrometry (Cameca-ims 1280HR) at Hokkaido University.
The run products were composed of glass and spinel. The 18O fraction in the melt increased toward the surface of the sphere, and that at the surface of the sphere gradually increases with time. This suggests that both the gas-melt isotope exchange at the surface and the diffusive transport of O-isotopes into the melt interior occurred simultaneously. The obtained O-isotope profiles were explained by a three-dimensional spherical diffusion equation with a time-dependent surface concentration (Crank, 1975), yielding the O-self diffusion coefficient D of 1.87 × 10–7 cm2 sec–1 and the surface O-isotope exchange efficiency of ~0.25. The surface isotope exchange efficiency of 0.25 requires that one of four colliding water vapor molecules exchanges O-isotopes with the melt surface. The obtained D in the CAIB melt is consistent with that estimated in basaltic melt at 1390°C (Canil and Muehlenbachs, 1990; Lesher et al., 1996). This is likely because the diffusivity of oxygen would be related to the degree of melt polymerization (NBO/T) (Liang et al., 1996), and the CAIB melt has a similar NBO/T value to that of basaltic melt.
The reaction kinetic suggests that the timescale for O-isotopic equilibration between a 1 cm-sized spinel-bearing CAIB melt droplet and water vapor should be at least a dozen days at plausible range of PH2O in the protosolar disk (10–9 < PH2O (bar) <10–6). Homogeneous O-isotope compositions of melilite in most type B CAIs would be explained by heating of CAI melt above the melilite liquidus for at least a dozen days during the reheating events in the early Solar System.
O-isotope exchange experiments between ~2.5 mm-sized synthetic spherical CAI droplet with the average type B composition (Wark et al., 1979; CAIB hereafter) and 18O-enriched water vapor (~97% 18O) were carried out at 1390°C (10°C above the melilite liquidus) and a disk-like low water vapor pressure of 5 × 10-2 Pa using a newly developed high temperature vacuum furnace with a gas flow system. The low PH2O condition was achieved by the supply of water vapor from water ice in the gas flow system (Yamamoto et al., 2018, 2019). The run products were analyzed with FE-SEM (JEOL JSM-7000F) with an EDS. The O-isotope measurements were conducted with a secondary ion mass spectrometry (Cameca-ims 1280HR) at Hokkaido University.
The run products were composed of glass and spinel. The 18O fraction in the melt increased toward the surface of the sphere, and that at the surface of the sphere gradually increases with time. This suggests that both the gas-melt isotope exchange at the surface and the diffusive transport of O-isotopes into the melt interior occurred simultaneously. The obtained O-isotope profiles were explained by a three-dimensional spherical diffusion equation with a time-dependent surface concentration (Crank, 1975), yielding the O-self diffusion coefficient D of 1.87 × 10–7 cm2 sec–1 and the surface O-isotope exchange efficiency of ~0.25. The surface isotope exchange efficiency of 0.25 requires that one of four colliding water vapor molecules exchanges O-isotopes with the melt surface. The obtained D in the CAIB melt is consistent with that estimated in basaltic melt at 1390°C (Canil and Muehlenbachs, 1990; Lesher et al., 1996). This is likely because the diffusivity of oxygen would be related to the degree of melt polymerization (NBO/T) (Liang et al., 1996), and the CAIB melt has a similar NBO/T value to that of basaltic melt.
The reaction kinetic suggests that the timescale for O-isotopic equilibration between a 1 cm-sized spinel-bearing CAIB melt droplet and water vapor should be at least a dozen days at plausible range of PH2O in the protosolar disk (10–9 < PH2O (bar) <10–6). Homogeneous O-isotope compositions of melilite in most type B CAIs would be explained by heating of CAI melt above the melilite liquidus for at least a dozen days during the reheating events in the early Solar System.