5:15 PM - 7:15 PM
[SVC32-P34] Attempts to visualize and characterize the chemical composition of volcanic gases
Keywords:Volcanic gas, Chemical composition, visualize, magma-hydrothermal system
Preface)
One of the purposes of collecting and analyzing volcanic gas from volcanoes associating phreatic eruptions is to extract information about the magma-hydrothermal system present in the lower part of the volcano from the chemical composition and isotope ratio of the volcanic gas, and to contribute to the evaluation of volcanic activity. Volcanic gas is a homogeneous system consisting of a single gas phase, mainly composed of water vapor, but since it contains many gas components of different origins, there are multiple indicators that indicate the characteristics of its chemical composition, and the interpretation of the chemical composition may depend on the researcher's perspective. In addition, because there are multiple indicators, there are some research cases where only certain indicators are interpreted and the rest are not interpreted, which is a waste. In light of this situation, we attempted to create a diagram that visualizes the chemical composition of volcanic gas and makes it possible to interpret the relationship between the characteristics of volcanic gas composition and the structure of the magma-hydrothermal system.
Results)
A rectangular diagram was set up with the gas component number on the x-axis and the logarithm of the gas component ratio on the y-axis. The gas components CH4, CO2, He, H2S, SO2, and H2 were assigned x-axis values of 1, 2, 3, 4, 5, and 6, respectively. The mole fraction of each component in the volcanic gas was designated Xi. For components x=1 to 5, Log(Xi/XCO2) – Log(Pi/PCO2) was calculated and used as the y-axis value. Here, Pi is the mole fraction of component i of a hypothetical magmatic gas set based on the composition of the high-temperature fumaroles of Mount Unzen. For X=6, Log(XH2/H2O) (=RH) was calculated and used as the y-axis value.
Discussion)
In the rectangular diagram above, the composition of volcanic gas is expressed as a curve connecting the points x=1 to 6 with straight lines. Naturally, magmatic gas is expressed as a curve that is close to horizontal.
The composition of volcanic gas at Hakone volcano differed depending on the sampling point. The volcanic gas from Owakudani 15-2 fumarole (c), which contains a lot of SO2, has a downward sloping curve, and the RH value at x=6 is -6, indicating that it was generated in an oxidizing environment controlled by the gas buffer. The volcanic gas collected from the natural fumarole (n) close to the 15-2 fumarole and the natural fumarole (s) at Kamiyuba, more than 500 m away, has a lower SO2/CO2 ratio at x=4 than c, indicating the influence of passing through the cap rock. The influence of the cap rock is also supported by the fact that the CH4/CO2 ratio at x=1 is higher than c.
The three volcanic gases collected from the northern slope of the summit of Kusatsu-Shirane volcano were almost horizontal from x=2 to 4, and were expressed as a curve that sloped downward to the right from x=4 to 6. In particular, the RH value of x=6 was -7, indicating that it was generated in a strongly oxidizing environment controlled by a fluid buffer. Although the three fumaroles for sampling were distributed at a distance of only about 500 m, a clear difference was observed in the CH4/CO2 ratio of x=1. This suggests that the CH4 contained in the volcanic gases originated from a relatively shallow underground.
The volcanic gas (h) sampled from Ebinokogen-Iwoyama volcano, which has a high discharge pressure, showed an almost horizontal curve from x=1 to 5, indicating a characteristic of magmatic origin. The curve of the volcanic gas with low discharge pressure (c), which is about 220 m away from fumarole (h), showed a decrease in the H2S/CO2 and SO2/CO2 ratios at x=4 and 5 compared to h, suggesting the involvement of a hydrothermal system. The RH of both volcanic gases at x=6 was in the range of -4.5 to -4.0, suggesting the formation of an oxidizing environment controlled by the gas buffer.
One of the purposes of collecting and analyzing volcanic gas from volcanoes associating phreatic eruptions is to extract information about the magma-hydrothermal system present in the lower part of the volcano from the chemical composition and isotope ratio of the volcanic gas, and to contribute to the evaluation of volcanic activity. Volcanic gas is a homogeneous system consisting of a single gas phase, mainly composed of water vapor, but since it contains many gas components of different origins, there are multiple indicators that indicate the characteristics of its chemical composition, and the interpretation of the chemical composition may depend on the researcher's perspective. In addition, because there are multiple indicators, there are some research cases where only certain indicators are interpreted and the rest are not interpreted, which is a waste. In light of this situation, we attempted to create a diagram that visualizes the chemical composition of volcanic gas and makes it possible to interpret the relationship between the characteristics of volcanic gas composition and the structure of the magma-hydrothermal system.
Results)
A rectangular diagram was set up with the gas component number on the x-axis and the logarithm of the gas component ratio on the y-axis. The gas components CH4, CO2, He, H2S, SO2, and H2 were assigned x-axis values of 1, 2, 3, 4, 5, and 6, respectively. The mole fraction of each component in the volcanic gas was designated Xi. For components x=1 to 5, Log(Xi/XCO2) – Log(Pi/PCO2) was calculated and used as the y-axis value. Here, Pi is the mole fraction of component i of a hypothetical magmatic gas set based on the composition of the high-temperature fumaroles of Mount Unzen. For X=6, Log(XH2/H2O) (=RH) was calculated and used as the y-axis value.
Discussion)
In the rectangular diagram above, the composition of volcanic gas is expressed as a curve connecting the points x=1 to 6 with straight lines. Naturally, magmatic gas is expressed as a curve that is close to horizontal.
The composition of volcanic gas at Hakone volcano differed depending on the sampling point. The volcanic gas from Owakudani 15-2 fumarole (c), which contains a lot of SO2, has a downward sloping curve, and the RH value at x=6 is -6, indicating that it was generated in an oxidizing environment controlled by the gas buffer. The volcanic gas collected from the natural fumarole (n) close to the 15-2 fumarole and the natural fumarole (s) at Kamiyuba, more than 500 m away, has a lower SO2/CO2 ratio at x=4 than c, indicating the influence of passing through the cap rock. The influence of the cap rock is also supported by the fact that the CH4/CO2 ratio at x=1 is higher than c.
The three volcanic gases collected from the northern slope of the summit of Kusatsu-Shirane volcano were almost horizontal from x=2 to 4, and were expressed as a curve that sloped downward to the right from x=4 to 6. In particular, the RH value of x=6 was -7, indicating that it was generated in a strongly oxidizing environment controlled by a fluid buffer. Although the three fumaroles for sampling were distributed at a distance of only about 500 m, a clear difference was observed in the CH4/CO2 ratio of x=1. This suggests that the CH4 contained in the volcanic gases originated from a relatively shallow underground.
The volcanic gas (h) sampled from Ebinokogen-Iwoyama volcano, which has a high discharge pressure, showed an almost horizontal curve from x=1 to 5, indicating a characteristic of magmatic origin. The curve of the volcanic gas with low discharge pressure (c), which is about 220 m away from fumarole (h), showed a decrease in the H2S/CO2 and SO2/CO2 ratios at x=4 and 5 compared to h, suggesting the involvement of a hydrothermal system. The RH of both volcanic gases at x=6 was in the range of -4.5 to -4.0, suggesting the formation of an oxidizing environment controlled by the gas buffer.