10:45 〜 12:15
[SVC29-P12] ペレーの涙の表面構造を再現するための実験的研究
キーワード：ペレーの涙、キラウエア
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 e.g., Kilauea volcano in Hawaii. Pele’s tears are typical pyroclasts formed in lava fountains. To constrain the formation condition will lead to understand the behavior of low-viscosity magma near the Earth’s surface.
Surface observations of Pele's tears produced by Kilauea 1959 eruption (Kanazawa and Toramaru, JpGU 2022) described that there are dendritic crystals of magnetite (Mt, ~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". EPMA analysis revealed that ladder structure is rich in FeO, CaO, and MgO and poor in SiO2 and Al2O3, representing, the heterogeneous structure of glass composition. Mt crystals and ladder structure are assumed to be formed by rapid oxidation and certain range of temperature(T). If we can reproduce the formation condition of these observation results through experiment, we can constraint the physical conditions of lava fountains which are difficult to observe, such as T, 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. 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.
We examined Experimental conditions based on the phase equilibrium conditions estimated from the calculation results by MELTS (Gualda and Ghiorso, 2015) using the whole-rock composition from XRF analysis of the scoria powder. In experiments, we varied the initial melting T, annealing T (First phase), and each holding time. The initial melting condition was set to 1450℃ and 1h, because olivine (Ol) and chromite, still remain below this T and the retention time. Experiment A was conducted with various annealing T (1300–1400℃) and duration (1–2 h) at fO2=NNO (Ni-Ni oxide buffer). In Experiment B, after keeping at 1400°C 1 h as First phase, the gas flow was stopped and the furnace was left open for 10 min. to reproduce oxidized conditions.
FE-SEM observation of all of the samples of Exp.A have the area that dendritic Ol (~10 µm) formed in areas with an elliptic shape (~100 µm). In this area, Mt (< 1 µm) crystallizes in a linear pattern surrounding single Ol crystals. The sample kept at 1300℃ 1h in First phase has almost Mt-dominant area lacking glass area outside of Ol dominant area, while the samples kept higher than 1350℃ have uniform glass area. One of the reasons for the formation of glass area and Ol dominant area is that the lower part cooled more rapidly than the upper part while dropping. These results suggest that the difference in T and cooling rate in First phase makes a difference in the amount of Mt crystals and crystal distribution.
In Exp.A, the composition of the glass part was uniform but in Exp.B the heterogeneous structure of glass composition was formed in the surface glass area like Ladder structure on the Pele’s tears. BSEI show the ladder-like pattern transitions the mesh-like pattern. This fact suggests that oxidation induces the heterogeneous structure of glass and the elongation direction of the ladder structure is affected by external forces.
In summary, we could reproduce very similar structure such as ladder structure of glass, although we couldn’t reproduce dendritic Mt. These experimental results and the observation of the natural samples will give us a clue to understand the detail of interaction between air, magmas and volcanic gases in fire fountaining eruptions.
Surface observations of Pele's tears produced by Kilauea 1959 eruption (Kanazawa and Toramaru, JpGU 2022) described that there are dendritic crystals of magnetite (Mt, ~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". EPMA analysis revealed that ladder structure is rich in FeO, CaO, and MgO and poor in SiO2 and Al2O3, representing, the heterogeneous structure of glass composition. Mt crystals and ladder structure are assumed to be formed by rapid oxidation and certain range of temperature(T). If we can reproduce the formation condition of these observation results through experiment, we can constraint the physical conditions of lava fountains which are difficult to observe, such as T, 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. 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.
We examined Experimental conditions based on the phase equilibrium conditions estimated from the calculation results by MELTS (Gualda and Ghiorso, 2015) using the whole-rock composition from XRF analysis of the scoria powder. In experiments, we varied the initial melting T, annealing T (First phase), and each holding time. The initial melting condition was set to 1450℃ and 1h, because olivine (Ol) and chromite, still remain below this T and the retention time. Experiment A was conducted with various annealing T (1300–1400℃) and duration (1–2 h) at fO2=NNO (Ni-Ni oxide buffer). In Experiment B, after keeping at 1400°C 1 h as First phase, the gas flow was stopped and the furnace was left open for 10 min. to reproduce oxidized conditions.
FE-SEM observation of all of the samples of Exp.A have the area that dendritic Ol (~10 µm) formed in areas with an elliptic shape (~100 µm). In this area, Mt (< 1 µm) crystallizes in a linear pattern surrounding single Ol crystals. The sample kept at 1300℃ 1h in First phase has almost Mt-dominant area lacking glass area outside of Ol dominant area, while the samples kept higher than 1350℃ have uniform glass area. One of the reasons for the formation of glass area and Ol dominant area is that the lower part cooled more rapidly than the upper part while dropping. These results suggest that the difference in T and cooling rate in First phase makes a difference in the amount of Mt crystals and crystal distribution.
In Exp.A, the composition of the glass part was uniform but in Exp.B the heterogeneous structure of glass composition was formed in the surface glass area like Ladder structure on the Pele’s tears. BSEI show the ladder-like pattern transitions the mesh-like pattern. This fact suggests that oxidation induces the heterogeneous structure of glass and the elongation direction of the ladder structure is affected by external forces.
In summary, we could reproduce very similar structure such as ladder structure of glass, although we couldn’t reproduce dendritic Mt. These experimental results and the observation of the natural samples will give us a clue to understand the detail of interaction between air, magmas and volcanic gases in fire fountaining eruptions.