11:30 〜 11:45
[SVC31-04] ペレーの涙の表面組織の形成過程に関する実験的研究
キーワード：ペレーの涙、スコリア、キラウエア、ワイヤーループ法
Pele's tears are spherical or teardrop-shaped volcanic glasses formed by the rapid cooling of droplets from lava fountains of basaltic magma. They can be seen at volcanoes such as Kilauea in Hawaii. Understanding of the formation condition will constrain the behavior of disrupted low-viscosity magma in atmosphere near the Earth’s surface.
Surface observation of Pele’s tears revealed that there are dendritic crystals of Fe-oxides (~3–5 µm) and their unique distribution morphology. We also found a ladder-like bright line on the surface in BSE images, which we named "Ladder structure". From EPMA analysis, it is found that ladder structure is rich in FeO, CaO, and MgO and poor in SiO2 and Al2O3, compared with other parts. Fe-oxides and ladder structure are thought to be formed by rapid oxidation and certain range of temperature. If we can reproduce the formation condition of these textures by experiment, we can constraint the physical conditions of the interior of lava fountains, such as temperature [T], oxugen fugacity [fO2] and cooling rate.
To reproduce these observed texture, we conducted experiments using the wire-loop method, which can make quenched glass under controlled fO2 and T at atmospheric pressure. We used the Siliconit furnace at AIST. We controlled the fO2 by H2–CO2 mixed gas. As starting material, we used powder of scoria produced by Kilauea 1959 eruption. We conducted three types of cooling and oxygen paths: Experiment (1) was conducted under NNO gas atmosphere at 1450℃ for 1hr → 1400℃ for 1hr → quench, Experiment (2) was conducted under dry air conditions at 1450℃ for 1hr → 1400℃ for 1hr → quench, and Experiment (3) was conducted under NNO gas atmosphere at 1450℃ for 1hr → 1400℃ for 1hr, followed by dry air inflow [t2 = 35s, 1min, 5min, 10min, 20min] → quench (the inflow time of dry air is defined as t2). In the wire-loop method, we hung the sample with Pt wire in the furnace, kept samples for definite period of time, and then finally the Pt wire was cut instantly and the sample was fallen. Prior to observation, surfaces of Pele's tears and the experimental products were coated with osmium and with carbon for cross-sectional observation. Microstructure of surface and cross-section of the products were observed by SEM-EDS, FE-SEM-EDS, and FE-EPMA.
The surface of all samples is divided into nearly two hemispheres: a crystalline region with an aggregation of dendritic olivine crystals [Ol] and a glassy region with almost no crystals. In Exp.(1), the crystalline region has only Ol crystals and the glass regions were homogeneous, while in Exp.(2) and (3), the crystalline region has Ol and Fe-oxides and the glass regions exhibited a mesh-like structure similar to a ladder structure.
The surface of the sample with dry air flow time t2=35s has the inhomogeneity in glass compositions with the cell size of the mesh-like structure. X-ray CT analysis (SKY SCAN 1272, Bruker) revealed that the presence of subsurface platinum wires influences the cell pattern. In addition, when the sample was quenched, the cells on the landing surface had smaller size cell than those on the upper surface. The cell area of the mesh-like structure becomes smaller at higher cooling rates. The mesh-like structure exhibits a hierarchical pattern in which large cells are filled by smaller cells. Furthermore, mesh-like pattern is characterized by local transitions to a ladder-like pattern.
Experimental results show that the surface organization of Pele's tears can be successfully reproduced and indicate that the Fe-oxides and mesh-like structure are formed when the high-temperature melt comes into contact with oxygen. Based on the inference that the mesh-like structure has undergone more oxidation than the surrounding glass and the fact that Fe-oxides are frequently distributed on the Ladder structure of Pele’s tears, we propose a mechanism for the formation of the mesh-like structure caused by rapid cooling and contraction of the surface layer.
Surface observation of Pele’s tears revealed that there are dendritic crystals of Fe-oxides (~3–5 µm) and their unique distribution morphology. We also found a ladder-like bright line on the surface in BSE images, which we named "Ladder structure". From EPMA analysis, it is found that ladder structure is rich in FeO, CaO, and MgO and poor in SiO2 and Al2O3, compared with other parts. Fe-oxides and ladder structure are thought to be formed by rapid oxidation and certain range of temperature. If we can reproduce the formation condition of these textures by experiment, we can constraint the physical conditions of the interior of lava fountains, such as temperature [T], oxugen fugacity [fO2] and cooling rate.
To reproduce these observed texture, we conducted experiments using the wire-loop method, which can make quenched glass under controlled fO2 and T at atmospheric pressure. We used the Siliconit furnace at AIST. We controlled the fO2 by H2–CO2 mixed gas. As starting material, we used powder of scoria produced by Kilauea 1959 eruption. We conducted three types of cooling and oxygen paths: Experiment (1) was conducted under NNO gas atmosphere at 1450℃ for 1hr → 1400℃ for 1hr → quench, Experiment (2) was conducted under dry air conditions at 1450℃ for 1hr → 1400℃ for 1hr → quench, and Experiment (3) was conducted under NNO gas atmosphere at 1450℃ for 1hr → 1400℃ for 1hr, followed by dry air inflow [t2 = 35s, 1min, 5min, 10min, 20min] → quench (the inflow time of dry air is defined as t2). In the wire-loop method, we hung the sample with Pt wire in the furnace, kept samples for definite period of time, and then finally the Pt wire was cut instantly and the sample was fallen. Prior to observation, surfaces of Pele's tears and the experimental products were coated with osmium and with carbon for cross-sectional observation. Microstructure of surface and cross-section of the products were observed by SEM-EDS, FE-SEM-EDS, and FE-EPMA.
The surface of all samples is divided into nearly two hemispheres: a crystalline region with an aggregation of dendritic olivine crystals [Ol] and a glassy region with almost no crystals. In Exp.(1), the crystalline region has only Ol crystals and the glass regions were homogeneous, while in Exp.(2) and (3), the crystalline region has Ol and Fe-oxides and the glass regions exhibited a mesh-like structure similar to a ladder structure.
The surface of the sample with dry air flow time t2=35s has the inhomogeneity in glass compositions with the cell size of the mesh-like structure. X-ray CT analysis (SKY SCAN 1272, Bruker) revealed that the presence of subsurface platinum wires influences the cell pattern. In addition, when the sample was quenched, the cells on the landing surface had smaller size cell than those on the upper surface. The cell area of the mesh-like structure becomes smaller at higher cooling rates. The mesh-like structure exhibits a hierarchical pattern in which large cells are filled by smaller cells. Furthermore, mesh-like pattern is characterized by local transitions to a ladder-like pattern.
Experimental results show that the surface organization of Pele's tears can be successfully reproduced and indicate that the Fe-oxides and mesh-like structure are formed when the high-temperature melt comes into contact with oxygen. Based on the inference that the mesh-like structure has undergone more oxidation than the surrounding glass and the fact that Fe-oxides are frequently distributed on the Ladder structure of Pele’s tears, we propose a mechanism for the formation of the mesh-like structure caused by rapid cooling and contraction of the surface layer.