2:15 PM - 2:30 PM
[PPS08-15] Formation process of melilite mantle of type B CAI: Evaporation and crystallization experiments
Keywords:CAIs, melting, evaporation, crystal growth, X-ray CT, 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 from melt occurred (Grossman et al., 2000; Richter et al., 2002; Richter et al., 2007; Mendybaev et al., 2021). The type B CAIs 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)), while type B2 CAIs lack a melilite mantle. Experimental investigations on type B CAI analogue 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 timing of formation and growth of the melilite mantle and its compositional zoning profile have not yet been fully understood. Here we present the preliminary results of the investigation of the melilite mantles of experimentally synthesized CAI analogues and discuss the melilite mantle formation processes, induced by evaporation during melting.
Experiments at PH2 of 10 Pa with the composition CAIχ and CAIδ melt, which are the compositions on the equilibrium condensation path from the solar gas (Grossman et al., 2002), were conducted using a high-temperature vacuum furnace (Takigawa et al., 2009; Mendybaev et al., 2021; Kamibayashi et al., 2021). The samples were heated at 1420°C for 1 hour and then cooled at a rate of 20°C hr-1. The samples were quenched at different temperatures during experiments to track the melilite mantle formation process. The sample weights were measured before and after the experiments to evaluate the evaporative weight loss of the melt. Three-dimensional textures of experimental samples were observed using microscopic X-ray computed tomography (microCT, Bruker SkyScan 1272) at ISAS/JAXA. The internal textures, chemical compositions and crystallography of their cross sections were observed and analyzed by using SEM, EDS, and EBSD. For comparison, a natural type B1 CAI HKV02 in Vigarano (CV) was also observed and analyzed by using SEM, EDS, and EBSD.
The evaporative weight loss of the samples increased in the first few hours and halted at about 1400 °C during cooling. X-ray microCT images of the sample that was cooled down to 1400°C and then quenched show that melilite mantle covers the whole surface of the sample. This suggests that the evaporation of Mg and Si from the melt may be suppressed due to the melilite mantle formation after heating for a few hours at PH2 of 10 Pa.
The zoning profile of the melilite mantle of the experimental sample are divided into three regions: a gehlenite-rich region near the sample rim (åk < 10 mol%), a homogeneous middle region (åk ~ 12 mol%), and the region with elemental zoning where the åkermanite content increases up to åk ~ 70 mol% towards the core of the sample. The homogeneous region is likely to have crystallized in the early stage of mantle formation. The selective evaporation of åkermanite component from the rim of the melilite mantle may have formed the gehlenite-rich region near the rim. The zoning region towards the core represents the crystallization during cooling. Although the melilite mantle of the natural CAI shows the similar trend of zoning profile of the experimental sample, the thickness of the melilite mantle of the experimental product is thinner than that in the natural CAI, which may be derived from insufficient growth of melilite mantle in the experiment and/or the sample size effect. The melilite mantle should have detailed information on the type B CAI formation occurred in the earliest stage of the Solar System evolution. We plan to make more systematic experiments with various conditions to constrain the physics and chemistry of the CAI forming environment.
Experiments at PH2 of 10 Pa with the composition CAIχ and CAIδ melt, which are the compositions on the equilibrium condensation path from the solar gas (Grossman et al., 2002), were conducted using a high-temperature vacuum furnace (Takigawa et al., 2009; Mendybaev et al., 2021; Kamibayashi et al., 2021). The samples were heated at 1420°C for 1 hour and then cooled at a rate of 20°C hr-1. The samples were quenched at different temperatures during experiments to track the melilite mantle formation process. The sample weights were measured before and after the experiments to evaluate the evaporative weight loss of the melt. Three-dimensional textures of experimental samples were observed using microscopic X-ray computed tomography (microCT, Bruker SkyScan 1272) at ISAS/JAXA. The internal textures, chemical compositions and crystallography of their cross sections were observed and analyzed by using SEM, EDS, and EBSD. For comparison, a natural type B1 CAI HKV02 in Vigarano (CV) was also observed and analyzed by using SEM, EDS, and EBSD.
The evaporative weight loss of the samples increased in the first few hours and halted at about 1400 °C during cooling. X-ray microCT images of the sample that was cooled down to 1400°C and then quenched show that melilite mantle covers the whole surface of the sample. This suggests that the evaporation of Mg and Si from the melt may be suppressed due to the melilite mantle formation after heating for a few hours at PH2 of 10 Pa.
The zoning profile of the melilite mantle of the experimental sample are divided into three regions: a gehlenite-rich region near the sample rim (åk < 10 mol%), a homogeneous middle region (åk ~ 12 mol%), and the region with elemental zoning where the åkermanite content increases up to åk ~ 70 mol% towards the core of the sample. The homogeneous region is likely to have crystallized in the early stage of mantle formation. The selective evaporation of åkermanite component from the rim of the melilite mantle may have formed the gehlenite-rich region near the rim. The zoning region towards the core represents the crystallization during cooling. Although the melilite mantle of the natural CAI shows the similar trend of zoning profile of the experimental sample, the thickness of the melilite mantle of the experimental product is thinner than that in the natural CAI, which may be derived from insufficient growth of melilite mantle in the experiment and/or the sample size effect. The melilite mantle should have detailed information on the type B CAI formation occurred in the earliest stage of the Solar System evolution. We plan to make more systematic experiments with various conditions to constrain the physics and chemistry of the CAI forming environment.