Presentation Information
[PPS12-15]Evaporation and crystallization experiments of type B CAI melt in Mg-, Si-bearing gases
*Yasuaki Tsuruoka1, Shogo Tachibana1 (1.The University of Tokyo)
Keywords:
CAIs,melting,evaporation,oxygen fugacity,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, the evaporation of Mg and Si at PH2 >~1 Pa in an open system for a few days, which is necessary for the oxygen isotope exchange reaction (Yamamoto et al., 2021), could result in the excessive depletion of Mg and Si. Grossite (CaAl4O7), which is rare in natural type B CAIs, has also formed via incongruent evaporation of melilite in H2 gas (Mendybaev et al., 2006; Kamibayashi, Ph.D thesis; this study). Since most of Mg and Si did not condense into the solid phase in the CAI forming region, Mg-, Si-bearing gases, such as Mg and SiO, were likely present in the gas phase. The evaporation of Mg and Si is expected to be suppressed in the presence of Mg-, Si-bearing gases (e.g., Richter et al., 2002). Here we experimentally investigated the effects of ambient Mg-, Si-bearing gases on the CAI formation.
Experiments on the CAIχ-composition melt (Grossman et al., 2002) were conducted at PH2 of 1 Pa using a high-temperature vacuum furnace (Takigawa et al., 2009; Mendybaev et al., 2021; Kamibayashi et al., 2021). Synthetic orthoenstatite (MgSiO3) (Tachibana et al., 2002) was placed ~5 mm below the melt to produce ambient Mg-, Si-bearing gases. The samples were heated at 1420 °C for 1 hr and then cooled at a rate of 5 °C hr–1. Isothermal experiments at 1420 °C for 1 hr were also conducted. Experiments without enstatite (open system) 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 and chemical compositions of their cross sections were observed and analyzed by using SEM, EDS, and EPMA.
The samples showed the melilite mantles, irrespective of the presence of enstatite. However, most of the outermost part of the melilite mantle in the sample heated in an open system was covered with grossite, while those in the sample heated with enstatite showed little grossite, which is more consistent with the observations of natural CAIs. The samples heated with enstatite showed smaller weight losses than those in an open system. Mg-, Si-bearing gases from enstatite likely suppressed the evaporation of the melts and melilite mantles. Based on the evaporation rates in isothermal experiments in the presence of enstatite, it is estimated that partial pressures of Mg, SiO, and H2O around the melt were ~40 % of their equilibrium vapor pressures of the melt at 1420 °C. This corresponds to enrichment of Mg-, Si-bearing gases by several tens of times the solar abundance. On the other hand, the Ti3+/Ti4+ ratio of the melt when pyroxene began crystallizing (~1200 °C; Stolper & Paque, 1986) suggests that oxygen fugacities around the melt were similar to those of natural CAIs (Grossman et al., 2008), which were lower than at 1420°C. This is likely due to suppressed evaporation of enstatite at lower temperatures.
This study suggests that the decrease in oxygen fugacity with cooling could explain the mineralogical and chemical characteristics of natural CAIs. Further understanding of the formation of CAIs in Mg-, Si-bearing gases would put stronger constraints on the CAI-forming region in the protosolar disk.
Experiments on the CAIχ-composition melt (Grossman et al., 2002) were conducted at PH2 of 1 Pa using a high-temperature vacuum furnace (Takigawa et al., 2009; Mendybaev et al., 2021; Kamibayashi et al., 2021). Synthetic orthoenstatite (MgSiO3) (Tachibana et al., 2002) was placed ~5 mm below the melt to produce ambient Mg-, Si-bearing gases. The samples were heated at 1420 °C for 1 hr and then cooled at a rate of 5 °C hr–1. Isothermal experiments at 1420 °C for 1 hr were also conducted. Experiments without enstatite (open system) 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 and chemical compositions of their cross sections were observed and analyzed by using SEM, EDS, and EPMA.
The samples showed the melilite mantles, irrespective of the presence of enstatite. However, most of the outermost part of the melilite mantle in the sample heated in an open system was covered with grossite, while those in the sample heated with enstatite showed little grossite, which is more consistent with the observations of natural CAIs. The samples heated with enstatite showed smaller weight losses than those in an open system. Mg-, Si-bearing gases from enstatite likely suppressed the evaporation of the melts and melilite mantles. Based on the evaporation rates in isothermal experiments in the presence of enstatite, it is estimated that partial pressures of Mg, SiO, and H2O around the melt were ~40 % of their equilibrium vapor pressures of the melt at 1420 °C. This corresponds to enrichment of Mg-, Si-bearing gases by several tens of times the solar abundance. On the other hand, the Ti3+/Ti4+ ratio of the melt when pyroxene began crystallizing (~1200 °C; Stolper & Paque, 1986) suggests that oxygen fugacities around the melt were similar to those of natural CAIs (Grossman et al., 2008), which were lower than at 1420°C. This is likely due to suppressed evaporation of enstatite at lower temperatures.
This study suggests that the decrease in oxygen fugacity with cooling could explain the mineralogical and chemical characteristics of natural CAIs. Further understanding of the formation of CAIs in Mg-, Si-bearing gases would put stronger constraints on the CAI-forming region in the protosolar disk.