2:45 PM - 3:00 PM
[PPS08-05] An experimental reproduction of Fe-Ni metal grain in radial pyroxene chondrules
Keywords:Chondrite, Radial pyroxene chondrule, Fe-Ni metal grain
Chondrules, the majority components of primitive chondritic meteorites, are mainly composed of silicate minerals, glass, and opaque inclusions such as Fe-Ni metals and sulfides. The Fe-Ni metals inclusions have various elemental compositions and textures, suggesting that they may have been condensed directly from nebular gas [1], formed by reduction reactions during chondrule formation [2], or record metamorphism in the parent body [3].
In this study, we performed experiments to understand the effects of heating temperature and redox state on the formation of the metal inclusions during the chondrule formation process.
The starting material was prepared by adding NiO (Ni/Fe = 5.48 x 10-2 atom%: solar abundance) to six oxides (Na2O, MgO, Al2O3, SiO2, CaO, and FeO), excluding trace elements, from the composition of natural radial pyroxene chondrule. Two types of experiments were conducted: (1) the starting materials were completely melted, kept high temperature for a designated duration, and then quenched to investigate the dependence of the melting time on the composition of the metallic spheres formed by reduction, and (2) the starting materials were completely melted, cooled at a designated rate, and crystallized. These experiments were conducted in a non-contact state between the sample melt and the container using a gas-jet levitation apparatus, which was developed in Nagashima et al.[4] In-situ observation using a high-speed camera was performed during the experiment, and microstructural observation and compositional analysis were performed on the recovered samples by SEM/EDS.
In the experiment to investigate the dependence of the melting time on the metallic spheres formation, the reduction of nickel oxide and iron oxide by the surrounding hydrogen gas began immediately after the melting of the starting material, and almost all of the nickel oxide in the chondrule melt was reduced and concentrated into metallic spheres and was completely absent in the glass. The composition of the first metallic spheres was rich in Ni (Ni/Fe>1), but after heating for several minutes, the Ni/Fe ratio of the metallic spheres was lowered and reached a constant value. The metallic particles were initially a few µm in size and were scattered throughout the sample spheres, but during the heating and melting process, they moved to the surface of the melt and grew larger. However, the number of metallic particles decreased with continued heating due to evaporation of the metallic spheres.
In the experiments in which crystallization was performed, metallic spheres with various Ni contents were observed, and some of the spheres were Ni-free. In samples where one hemisphere of the sample crystallized and the other hemisphere solidified as glass, the metallic particles surrounded by crystals had low-Ni compositions, whereas the metallic spheres contained in the glass showed always high Ni contents. Since the metallic particles formed during the total melting contain Ni, the Ni-free metallic particles are considered to have formed after crystallization because NiO was already taken by the earlier formed metallic spheres. The Ni/Fe ratios of the metallic spheres that produced before crystallization were relatively homogeneous, while the metallic spheres formed after crystallization have various Ni/Fe ratios, depend on the availability of FeO in the local regions in the glass. It is suggested that the diversity in Ni/Fe ratios of metal spheres in natural chondrules is due to reduced reactions after crystallization.
In this study, we performed experiments to understand the effects of heating temperature and redox state on the formation of the metal inclusions during the chondrule formation process.
The starting material was prepared by adding NiO (Ni/Fe = 5.48 x 10-2 atom%: solar abundance) to six oxides (Na2O, MgO, Al2O3, SiO2, CaO, and FeO), excluding trace elements, from the composition of natural radial pyroxene chondrule. Two types of experiments were conducted: (1) the starting materials were completely melted, kept high temperature for a designated duration, and then quenched to investigate the dependence of the melting time on the composition of the metallic spheres formed by reduction, and (2) the starting materials were completely melted, cooled at a designated rate, and crystallized. These experiments were conducted in a non-contact state between the sample melt and the container using a gas-jet levitation apparatus, which was developed in Nagashima et al.[4] In-situ observation using a high-speed camera was performed during the experiment, and microstructural observation and compositional analysis were performed on the recovered samples by SEM/EDS.
In the experiment to investigate the dependence of the melting time on the metallic spheres formation, the reduction of nickel oxide and iron oxide by the surrounding hydrogen gas began immediately after the melting of the starting material, and almost all of the nickel oxide in the chondrule melt was reduced and concentrated into metallic spheres and was completely absent in the glass. The composition of the first metallic spheres was rich in Ni (Ni/Fe>1), but after heating for several minutes, the Ni/Fe ratio of the metallic spheres was lowered and reached a constant value. The metallic particles were initially a few µm in size and were scattered throughout the sample spheres, but during the heating and melting process, they moved to the surface of the melt and grew larger. However, the number of metallic particles decreased with continued heating due to evaporation of the metallic spheres.
In the experiments in which crystallization was performed, metallic spheres with various Ni contents were observed, and some of the spheres were Ni-free. In samples where one hemisphere of the sample crystallized and the other hemisphere solidified as glass, the metallic particles surrounded by crystals had low-Ni compositions, whereas the metallic spheres contained in the glass showed always high Ni contents. Since the metallic particles formed during the total melting contain Ni, the Ni-free metallic particles are considered to have formed after crystallization because NiO was already taken by the earlier formed metallic spheres. The Ni/Fe ratios of the metallic spheres that produced before crystallization were relatively homogeneous, while the metallic spheres formed after crystallization have various Ni/Fe ratios, depend on the availability of FeO in the local regions in the glass. It is suggested that the diversity in Ni/Fe ratios of metal spheres in natural chondrules is due to reduced reactions after crystallization.