11:00 〜 13:00
[SVC32-P06] Surface Observation of the pele’s tears
キーワード：ペレーの涙、表面観察、微細結晶
Pele's tears are spherical or tear-shaped glassy pyroclasts formed by the rapid cooling of droplets from lava fountains of basaltic magma. The microstructure of the surface has not been described in detail, although some previous studies have focused on surface deposits and internal bubbles. In this study, we observed the microstructure on the surface of Pele's tears in detail. As a result, we found curious patterns formed by microcrystals, their distribution, and the chemical heterogeneity of glass. In this presentation, we report the method, description, and classification of the patterns.
The samples were Pele's tears from the PuuPuai 1959 eruption, collected near the trail surrounding PuuPai cone at the rim of the Kilauea Iki crater, Hawaii. After coating with osmium for surface observation and carbon for cross-sectional observation, we used SEM-EDS, FE-SEM-EDS, and FE-EPMA (accelerating voltages of 5 and 15 keV) for microstructural observation and chemical composition analysis.
Pele's tears have a beautiful deformed spherical surface and there are many bubbles like craters, so the BSEI mosaic images (low magnification) of whole Pele's tears look like some planets. Further detailed observation under high magnification reveals dendritic microcrystals with various distribution patterns. The individual crystals are 3-5 μm in size.
Through surface observation, the distribution patterns of microcrystals commonly observed in most of the samples were found. The distribution patterns of microcrystals can be classified into "Ladder structure", "Circle", "Cluster", "Mesh", "Ridge", and "Underground". The occurrence of each distribution pattern was found to be independent of the size of the sample. In "Ladder structure", crystals are distributed on a brighter (in BSEI) ladder-like pattern with ladder spacing of approximately 5-15 µm. In the "Circle" pattern, crystals are arranged in a circle with a diameter of approximately 30-100 µm, and crystals are also distributed in the center of the circle. “Cluster" is a domain of crystals, where the crystal size increases as the positions become closer to the rim of the domain. “Mesh" is a pattern of crystals arranged in a mesh about 10 µm in spacing. “Ridge" is a pattern in which microcrystals are distributed along the narrow linear area corresponding to the edges of the crystals such as olivine, which exist in the interior of tears. "Underground" is a pattern that is observed on the chipped cross-sectional area.
By reducing the acceleration voltage under which condition, the penetration depth of incident electrons becomes shallower, we found that most of the crystals were covered by a thin film of glass. The brightness of BSEI indicates darker at the glass surrounding the crystals.
We made polished sections of cross sections of Pele’s tears to analyze the composition of the crystals. Since the diameters of the microcrystals are smaller than the EPMA beam diameter, we semi-quantitatively found that they have a high concentration in iron content.
From the results of cross-sectional observation and EPMA analysis, it was found that the bright region of the Ladder structure extended from the surface to a depth of 1-5 µm, and was richer in Fe, Ca, and Mg and poorer in Si and Al than the surrounding glass.
Based on the results of compositional analysis, the microcrystals are most likely magnetite (FeFe3+2O4). In addition, there are Fe, Ca, and Mg-rich patterns in glass compositions, which are correlated with crystal distribution patterns like a ladder on the surface of Pele's tears. The fact that microcrystals are distributed on Ladder structure should provide a clue to understand the formation process of microcrystals and Ladder structure.
The samples were Pele's tears from the PuuPuai 1959 eruption, collected near the trail surrounding PuuPai cone at the rim of the Kilauea Iki crater, Hawaii. After coating with osmium for surface observation and carbon for cross-sectional observation, we used SEM-EDS, FE-SEM-EDS, and FE-EPMA (accelerating voltages of 5 and 15 keV) for microstructural observation and chemical composition analysis.
Pele's tears have a beautiful deformed spherical surface and there are many bubbles like craters, so the BSEI mosaic images (low magnification) of whole Pele's tears look like some planets. Further detailed observation under high magnification reveals dendritic microcrystals with various distribution patterns. The individual crystals are 3-5 μm in size.
Through surface observation, the distribution patterns of microcrystals commonly observed in most of the samples were found. The distribution patterns of microcrystals can be classified into "Ladder structure", "Circle", "Cluster", "Mesh", "Ridge", and "Underground". The occurrence of each distribution pattern was found to be independent of the size of the sample. In "Ladder structure", crystals are distributed on a brighter (in BSEI) ladder-like pattern with ladder spacing of approximately 5-15 µm. In the "Circle" pattern, crystals are arranged in a circle with a diameter of approximately 30-100 µm, and crystals are also distributed in the center of the circle. “Cluster" is a domain of crystals, where the crystal size increases as the positions become closer to the rim of the domain. “Mesh" is a pattern of crystals arranged in a mesh about 10 µm in spacing. “Ridge" is a pattern in which microcrystals are distributed along the narrow linear area corresponding to the edges of the crystals such as olivine, which exist in the interior of tears. "Underground" is a pattern that is observed on the chipped cross-sectional area.
By reducing the acceleration voltage under which condition, the penetration depth of incident electrons becomes shallower, we found that most of the crystals were covered by a thin film of glass. The brightness of BSEI indicates darker at the glass surrounding the crystals.
We made polished sections of cross sections of Pele’s tears to analyze the composition of the crystals. Since the diameters of the microcrystals are smaller than the EPMA beam diameter, we semi-quantitatively found that they have a high concentration in iron content.
From the results of cross-sectional observation and EPMA analysis, it was found that the bright region of the Ladder structure extended from the surface to a depth of 1-5 µm, and was richer in Fe, Ca, and Mg and poorer in Si and Al than the surrounding glass.
Based on the results of compositional analysis, the microcrystals are most likely magnetite (FeFe3+2O4). In addition, there are Fe, Ca, and Mg-rich patterns in glass compositions, which are correlated with crystal distribution patterns like a ladder on the surface of Pele's tears. The fact that microcrystals are distributed on Ladder structure should provide a clue to understand the formation process of microcrystals and Ladder structure.