2:00 PM - 2:15 PM
[PPS08-02] Effect of recondensation on type B CAI formation: Experiments in the semi-closed system
Keywords:CAIs, melting, evaporation, recondensation, dust enrichment, protosolar disk
Calcium–aluminum-rich inclusions (CAIs) are the oldest materials formed in the Solar System. Type B CAIs experienced melting and crystallization in the disk gas during which evaporation of Mg and Si would occur (Grossman et al., 2000; Richter et al., 2002; Richter et al., 2007; 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 (Ca2Al2SiO7 (gehlenite)–Ca2MgSi2O7 (åkermanite)) surrounding a core consisting of randomly distributed melilite, fassaite, anorthite, and spinel, while type B2 CAIs lack a melilite mantle. Experimental investigations on type B CAI analog melt in an open system under low PH2 conditions (Kamibayashi et al., 2021) have shown that type B1-like melilite mantle formed due to the evaporative loss of Mg and Si and enrichment of Al and Ca near the melt surface at PH2 >~1Pa. However, the recondensation (back-reaction) of evaporated species expected to occur in a dust-enrichment system (e.g., Tsuchiyama et al., 1999; Ozawa & Nagahara, 2001), which suppresses the evaporation (e.g., Richter et al., 2007) and may affect the evaporation-induced crystallization of melilite. Because the effects of recondensation on evaporation kinetics in protoplanetary disks have never been experimentally investigated under controlled conditions except for metallic iron (Tachibana et al., 2011), it is necessary to conduct experiments on CAI melt under controlled recondensation conditions. We experimentally investigated the effect of recondensation of evaporated species during the crystallization of type B CAIs.
Experiments with the composition CAIχ and CAIδ melt, on the equilibrium condensation path (Grossman et al., 2002), at PH2 of 1 and 10 Pa were conducted using a high-temperature vacuum furnace (Mendybaev et al., 2021; Kamibayashi et al., 2021; Takigawa et al., 2009). The sample was hung in a platinum tube (10-mm or 20-mm in height) to reproduce the recondensation of evaporated species (Tachibana et al., 2011). Some samples were heated in an open system for comparison. The maximum temperature was 1420 and 1430°C and cooling rates was 5 and 50°C hr-1. The sample weights were measured before and after the experiments to evaluate the evaporative weight loss of the melt. The internal textures and chemical compositions of minerals were observed and analyzed by a scanning electron microscope (JEOL JCM-7000) equipped with an energy dispersive X-ray spectrometer.
The texture of the samples differed depending on the cooling rates irrespective of the presence/absence of Pt tubes; The sample cooled at a rate of 5 and 50°C hr-1 showed a continuous mantle of melilite and incomplete melilite mantles, respectively. The chemical composition of melilite mantles in the sample heated in Pt tubes tends to have the higher åkermanite content than those in the open system. The samples heated inside the Pt tubes showed smaller weight losses than those in the open system. These results indicate suppressed evaporation of Mg and Si due to recondensation, resulting in crystallization of melilite with a higher åkermanite content from the melt surface.
In the present experiments, no textural difference was observed, but we confirmed that the recondensation affects crystallization conditions of melilite mantles. The vapor pressures of recondensing gas species were evaluated using the evaporation rate of the melt and the conductance of the tube (Tachibana et al., 2011), which correspond to the dust enrichment with a factor of ~100 from the system of solar abundance. We will conduct experiments under various degrees of recondensation to constrain the relation between the hydrogen pressure and the dust enrichment in the CAI forming region.
Experiments with the composition CAIχ and CAIδ melt, on the equilibrium condensation path (Grossman et al., 2002), at PH2 of 1 and 10 Pa were conducted using a high-temperature vacuum furnace (Mendybaev et al., 2021; Kamibayashi et al., 2021; Takigawa et al., 2009). The sample was hung in a platinum tube (10-mm or 20-mm in height) to reproduce the recondensation of evaporated species (Tachibana et al., 2011). Some samples were heated in an open system for comparison. The maximum temperature was 1420 and 1430°C and cooling rates was 5 and 50°C hr-1. The sample weights were measured before and after the experiments to evaluate the evaporative weight loss of the melt. The internal textures and chemical compositions of minerals were observed and analyzed by a scanning electron microscope (JEOL JCM-7000) equipped with an energy dispersive X-ray spectrometer.
The texture of the samples differed depending on the cooling rates irrespective of the presence/absence of Pt tubes; The sample cooled at a rate of 5 and 50°C hr-1 showed a continuous mantle of melilite and incomplete melilite mantles, respectively. The chemical composition of melilite mantles in the sample heated in Pt tubes tends to have the higher åkermanite content than those in the open system. The samples heated inside the Pt tubes showed smaller weight losses than those in the open system. These results indicate suppressed evaporation of Mg and Si due to recondensation, resulting in crystallization of melilite with a higher åkermanite content from the melt surface.
In the present experiments, no textural difference was observed, but we confirmed that the recondensation affects crystallization conditions of melilite mantles. The vapor pressures of recondensing gas species were evaluated using the evaporation rate of the melt and the conductance of the tube (Tachibana et al., 2011), which correspond to the dust enrichment with a factor of ~100 from the system of solar abundance. We will conduct experiments under various degrees of recondensation to constrain the relation between the hydrogen pressure and the dust enrichment in the CAI forming region.