Calcium-aluminum-rich inclusions (CAIs) are known as the oldest high-temperature mineral assemblages of the Solar System. The CAIs record thermal events that occurred during the earliest epochs of the Solar System formation in the form of heterogeneous oxygen isotopic distributions between and within their constituent minerals. Here, we explored the kinetics of oxygen isotope exchange during partial melting events of CAIs by conducting oxygen isotope exchange experiments between type B CAI-like silicate melt and ¹⁸O-enriched water vapor (P_H2O = 5 × 10⁻² Pa) at 1420 °C. We found that the oxygen isotope exchange between CAI melt and water vapor proceeds at competing rates with surface isotope exchange and self-diffusion of oxygen in the melt under the experimental conditions. The ¹⁸O concentration profiles were well fitted with the three-dimensional spherical diffusion model with a time-dependent surface concentration. We determined the self-diffusion coefficient of oxygen to be ∼1.62 × 10⁻¹¹ m² s⁻¹, and the oxygen isotope exchange efficiency on the melt surface was found to be ∼0.28 in colliding water molecules. These kinetic parameters suggest that oxygen isotope exchange rate between cm-sized CAI melt droplets and water vapor is dominantly controlled by the supply of water molecules to the melt surface at P_H2O < ∼10⁻² Pa and by self-diffusion of oxygen in the melt at P_H2O > ∼1 Pa at temperatures above the melilite liquidus (1420–1540 °C). To form type B CAIs containing ¹⁶O-poor melilite by oxygen isotope exchange between CAI melt and disk water vapor, the CAIs should have been heated for at least a few days at P_H2O > 10⁻² Pa above temperatures of the melilite liquidus in the protosolar disk. The larger timescale of oxygen isotopic equilibrium between CAI melt and H₂O compared to that between H₂O and CO in the gas phase suggests that the bulk oxygen isotopic compositions of ambient gas at ∼1400 °C in the type B CAI-forming region is preserved in the oxygen isotopic compositions of type B CAI melilite. Based on the observed oxygen isotopic composition, we suggest that a typical type B1 CAI (TS34) from Allende was cooled at a rate of ∼0.1–0.5 K h⁻¹ during fassaite crystallization.

富含钙铝的包裹体 (CAI) 被称为太阳系中最古老的高温矿物组合。CAIs 记录了在太阳系形成的最早时期发生的热事件，以它们的组成矿物之间和内部的异质氧同位素分布的形式。在这里，我们通过在 B 型类 CAI 硅酸盐熔体和¹⁸ O 富集水蒸气（P _H2O  = 5 × 10 ^-2Pa) 在 1420 °C。我们发现，在实验条件下，CAI 熔体和水蒸气之间的氧同位素交换与表面同位素交换和熔体中氧的自扩散以竞争速率进行。在^18条Ô浓度曲线以及分别装配有与时间相关的表面浓度的三维球形扩散模型。我们确定氧的自扩散系数为∼1.62 × 10 ^-11 m ² s ^-1，并且发现熔体表面的氧同位素交换效率在碰撞水分子中为~0.28。这些动力学参数表明，cm 大小的 CAI 熔体液滴和水蒸气之间的氧同位素交换率主要受P _H2O < ∼10 ^-2 Pa 时向熔体表面供应水分子和氧在熔体中的自扩散控制。在高于黄晶石液相线 (1420–1540 °C) 的温度下，在P _H2O > ∼1 Pa 下熔化。为了通过 CAI 熔体和圆盘水蒸气之间的氧同位素交换形成含有¹⁶个贫氧黄长石的B 型CAI，CAI 应该在P _H2O  > 10 ^{-2 下}加热至少几天Pa 高于原太阳盘中黄长石液相线的温度。与气相中的H ₂ O 和 CO之间的氧同位素平衡相比，CAI 熔体和 H ₂ O之间的氧同位素平衡时间尺度更大，这表明在 B 型 CAI 形成区，环境气体在 1400 °C 时的本体氧同位素组成保存在 B 型 CAI 黄长石的氧同位素组成中。根据观察到的氧同位素组成，我们建议来自阿连德的典型 B1 CAI (TS34)在钙钛矿结晶过程中以 ~0.1–0.5 K h ^-1的速率冷却。