9:45 AM - 10:00 AM
[PPS07-08] Chemical and isotopic fractionation of Type B CAI caused by evaporation: An experimental study
Keywords:CAI, evaporation, protosolar disk
CAIs are the oldest objects formed in the early Solar System, and some subsets of these inclusions show chemical and isotopic features that indicate their formation through high temperature events that resulted in partial melting and evaporation of relatively volatile elements such as Mg and Si. Many laboratory and theoretical studies have discussed evaporation and crystallization of CAI melts (e.g., Stolper and Paque, 1986; Grossman et al., 2002), but the circumstances that made their chemical and isotopic features have not yet been fully understood. These characteristics can be used as a cosmobarometry in the early solar system when appropriate interpretation is given. In this study, crystallization experiments of CAI-like composition melts in nebula-like low pressure hydrogen gas were conducted to investigate the effect of evaporation during crystallization of Type B CAIs and to constrain the nebular pressure during the CAI formation.
The starting material was prepared by mixing SiO2, TiO2, Al2O3, MgO, and CaCO3 powders at a composition aiming at a precursor composition given on the condensation trajectory of Grossman et al. (2002). Premelted glassy spherical samples on Ir wire loop (2.5 mm in diameter) were used as starting materials. Cooling experiments were conducted using a vacuum furnace equipped with a hydrogen gas flow system. The experiments were conducted from 1420°C to 1100-1277°C at controlled cooling rates of 5-50°C/hr and under three different pressures (PH2=10, 1, 0.1 Pa) to examine the pressure dependence of compositional and textural change. Textural observation and quantitative analysis were conducted by FE-SEM-EDS. Magnesium isotopic analysis of melilite and pyroxene was conducted by SIMS (Cameca ims-1280HR at Hokkaido University).
All of the run products presented significant weight losses which can be attributed to the evaporation of volatile components, Mg and Si. Some of the experimental residues from experiments at PH2 = 1 and 10 Pa show their outer regions are enriched in melilite, which resembles the melilite mantle observed in natural Type B1 CAIs. The chemical composition of melilite is Mg-poor at the sample rim and becomes Mg-rich toward the inside, suggesting that these melilites crystallized inward from the melt droplet surface, as inferred from petrological studies of Type B1 (Wark and Lovering, 1982). Melilite composition at the rim is Mg-poor and as low as ~Åk10, which is more depleted in Mg than that expected for the melt of starting composition to crystallize (~Åk20). Such Mg-poor melilite can only crystallize from a melt which has lower Mg and Si concentrations.
For samples which exhibit a well-developed melilite mantle, elevated d25Mg values at the sample rim compared to the interior are observed. This can also be attributed to diffusion-limited evaporation of Mg and Si from the melt surface. Isotope fractionation in a vaporizing melt is controlled by the relative rates of evaporation and diffusion. At higher PH2 (e.g., 10 Pa), evaporation could occur faster than diffusive homogenization of the melt, because evaporation is enhanced by the presence of hydrogen (Mendybaev et al., 2021), while diffusion is not. The resulting consequence is preferential crystallization of melilite from the melt droplet surface enriched in Ca and Al. Despite many of the CAIs show lighter isotopic composition at the margin or show homogeneous isotopic composition throughout the inclusion (e.g., Bullock et al., 2013), such increase in isotope fractionation at the margin of the inclusion is consistent with some of the natural existing CAIs (Goswami, 1994). Although one or more steps needs to be taken into account to explain the isotopic fractionation of the majority of natural CAIs, Type B1-like melilite mantle can indeed crystallize as a result of evaporation in relatively high PH2 condition.
The starting material was prepared by mixing SiO2, TiO2, Al2O3, MgO, and CaCO3 powders at a composition aiming at a precursor composition given on the condensation trajectory of Grossman et al. (2002). Premelted glassy spherical samples on Ir wire loop (2.5 mm in diameter) were used as starting materials. Cooling experiments were conducted using a vacuum furnace equipped with a hydrogen gas flow system. The experiments were conducted from 1420°C to 1100-1277°C at controlled cooling rates of 5-50°C/hr and under three different pressures (PH2=10, 1, 0.1 Pa) to examine the pressure dependence of compositional and textural change. Textural observation and quantitative analysis were conducted by FE-SEM-EDS. Magnesium isotopic analysis of melilite and pyroxene was conducted by SIMS (Cameca ims-1280HR at Hokkaido University).
All of the run products presented significant weight losses which can be attributed to the evaporation of volatile components, Mg and Si. Some of the experimental residues from experiments at PH2 = 1 and 10 Pa show their outer regions are enriched in melilite, which resembles the melilite mantle observed in natural Type B1 CAIs. The chemical composition of melilite is Mg-poor at the sample rim and becomes Mg-rich toward the inside, suggesting that these melilites crystallized inward from the melt droplet surface, as inferred from petrological studies of Type B1 (Wark and Lovering, 1982). Melilite composition at the rim is Mg-poor and as low as ~Åk10, which is more depleted in Mg than that expected for the melt of starting composition to crystallize (~Åk20). Such Mg-poor melilite can only crystallize from a melt which has lower Mg and Si concentrations.
For samples which exhibit a well-developed melilite mantle, elevated d25Mg values at the sample rim compared to the interior are observed. This can also be attributed to diffusion-limited evaporation of Mg and Si from the melt surface. Isotope fractionation in a vaporizing melt is controlled by the relative rates of evaporation and diffusion. At higher PH2 (e.g., 10 Pa), evaporation could occur faster than diffusive homogenization of the melt, because evaporation is enhanced by the presence of hydrogen (Mendybaev et al., 2021), while diffusion is not. The resulting consequence is preferential crystallization of melilite from the melt droplet surface enriched in Ca and Al. Despite many of the CAIs show lighter isotopic composition at the margin or show homogeneous isotopic composition throughout the inclusion (e.g., Bullock et al., 2013), such increase in isotope fractionation at the margin of the inclusion is consistent with some of the natural existing CAIs (Goswami, 1994). Although one or more steps needs to be taken into account to explain the isotopic fractionation of the majority of natural CAIs, Type B1-like melilite mantle can indeed crystallize as a result of evaporation in relatively high PH2 condition.