4:30 PM - 4:45 PM
[PPS07-23] Evaporation of type B CAI with increasing temperature: Evaporation and crystallization experiments
Keywords:CAIs, melting, evaporation, crystal growth, protosolar disk
Calcium–aluminum-rich inclusions (CAIs) are the oldest materials formed in the Solar System. Type B CAIs consist of melilite (Ca2Al2SiO7 (gehlenite)–Ca2MgSi2O7 (åkermanite)), Al-Ca-rich pyroxene, spinel, and anorthite. The type B CAIs experienced melting and crystallization in the disk gas, during which evaporation of Mg and Si from melt occurred (e.g., Grossman et al., 2000; Richter et al., 2002; Mendybaev et al., 2021). They are textually subdivided into type B1s and type B2s (e.g., Wark & Lovering, 1982). Type B1 CAIs have a continuous mantle of melilite, while type B2 CAIs lack a melilite mantle. Experimental investigations on type B CAI analog melt under low PH2 conditions (Kamibayashi et al., 2021) showed that type B1-like melilite mantle formed due to the depletion of Mg and Si near the melt surface at PH2 >~1Pa. However, Kamibayashi et al. (2021) focused on the evaporation during cooling from the Tmax. If the timescales for heating and cooling are comparable (e.g., McNally et al., 2014), evaporation during temperature increase may cause the depletion of Mg and Si before the temperature reaches Tmax, affecting the crystallization of melilite. Here we experimentally investigated the effects of evaporation with increasing temperature on the CAI formation and discuss the CAI forming conditions.
Experiments on the CAIχ-composition melt (Grossman et al., 2002) were conducted at PH2 of 0.1, 1, 10 Pa using a high-temperature vacuum furnace (Takigawa et al., 2009; Mendybaev et al., 2021; Kamibayashi et al., 2021). The samples were heated from 1100°C to 1420°C at a rate of 20°C hr-1, heated at 1420°C for 1 hr and then cooled to 1100°C at a rate of 20°C hr-1. Experiments under the same condition as Kamibayashi et al. (2021) were also conducted. The sample weights were measured before and after the experiments to evaluate the evaporative weight loss of the melt. The internal textures, chemical compositions and crystallography of their cross sections were observed and analyzed by using SEM, EDS, and EBSD.
The samples showed the melilite mantles with gradual increase in åkermanite content (Åk) from the rim to the interior, irrespective of PH2, which are different from those with the homogeneous Åk region in the samples heated under the same condition as in the previous study (Kamibayashi et al., 2021). The core melilites with Åk as low as that of the mantle rim were observed in the samples heated at PH2 of 0.1 and 10 Pa.
The difference in the zoning profiles of the mantles with and without hydrogen gas during temperature increase can be attributed to the timing of crystallization of the melilite. The evaporation of Mg and Si in hydrogen gas with increasing temperature would cause the compositional change of the melt and crystal growth, resulting in the earlier formation of melilite mantle, which suppressed further evaporation. This may have prevented the sufficient growth of homogeneous region at Tmax, resulting in the mantle with gradual increase in Åk with cooling. Since the melilite mantles with similar characteristics are found in natural CAIs (e.g., Simon & Grossman, 2006; Bullock et al., 2013), type B CAIs may have undergone various degrees of evaporation during temperature increase.
The center of the samples heated at PH2 of 0.1 and 10 Pa was likely depleted in Mg and Si due to their evaporation from the surface and diffusion in the melt during the temperature increase, resulting in the Ak-poor core melilites. Since core melilites in natural CAIs tend to have higher Åk than mantle melilites (Simon & Grossman, 2006), the CAI formation would require heating conditions with smaller degrees of evaporation than the present experiments: rapid heating (> 20°C hr-1) and/or recondensation of Mg and Si. Further understanding of the evaporation and crystallization of CAIs, including recondensation effects, would put stronger constraints on the CAI formation in the protosolar disk.
Experiments on the CAIχ-composition melt (Grossman et al., 2002) were conducted at PH2 of 0.1, 1, 10 Pa using a high-temperature vacuum furnace (Takigawa et al., 2009; Mendybaev et al., 2021; Kamibayashi et al., 2021). The samples were heated from 1100°C to 1420°C at a rate of 20°C hr-1, heated at 1420°C for 1 hr and then cooled to 1100°C at a rate of 20°C hr-1. Experiments under the same condition as Kamibayashi et al. (2021) were also conducted. The sample weights were measured before and after the experiments to evaluate the evaporative weight loss of the melt. The internal textures, chemical compositions and crystallography of their cross sections were observed and analyzed by using SEM, EDS, and EBSD.
The samples showed the melilite mantles with gradual increase in åkermanite content (Åk) from the rim to the interior, irrespective of PH2, which are different from those with the homogeneous Åk region in the samples heated under the same condition as in the previous study (Kamibayashi et al., 2021). The core melilites with Åk as low as that of the mantle rim were observed in the samples heated at PH2 of 0.1 and 10 Pa.
The difference in the zoning profiles of the mantles with and without hydrogen gas during temperature increase can be attributed to the timing of crystallization of the melilite. The evaporation of Mg and Si in hydrogen gas with increasing temperature would cause the compositional change of the melt and crystal growth, resulting in the earlier formation of melilite mantle, which suppressed further evaporation. This may have prevented the sufficient growth of homogeneous region at Tmax, resulting in the mantle with gradual increase in Åk with cooling. Since the melilite mantles with similar characteristics are found in natural CAIs (e.g., Simon & Grossman, 2006; Bullock et al., 2013), type B CAIs may have undergone various degrees of evaporation during temperature increase.
The center of the samples heated at PH2 of 0.1 and 10 Pa was likely depleted in Mg and Si due to their evaporation from the surface and diffusion in the melt during the temperature increase, resulting in the Ak-poor core melilites. Since core melilites in natural CAIs tend to have higher Åk than mantle melilites (Simon & Grossman, 2006), the CAI formation would require heating conditions with smaller degrees of evaporation than the present experiments: rapid heating (> 20°C hr-1) and/or recondensation of Mg and Si. Further understanding of the evaporation and crystallization of CAIs, including recondensation effects, would put stronger constraints on the CAI formation in the protosolar disk.