1:45 PM - 3:15 PM
[SVC34-P09] Geochemical study of seafloor hydrothermal activity in the summit caldera of Nikko Seamount, North Mariana Arc
Keywords:Seafloor hydrothermal activity, Nikko seamount, North Mariana arc, water-rock interaction
Nikko Seamount is a submarine arc volcano located at the northern end of the Mariana Arc that is one of the typical intra-oceanic arc. The seamount is a large old caldera in which several cone-shaped peaks developed. One of the peaks, called ‘main cone’, located on the southwest rim of the caldera wall has a small summit caldera. During the expeditions in 2005 and 2006, active hydrothermal venting sites were found in the summit caldera of the main cone. The fluid samples collected during the expedition in 2005 suggest that the dissolved gases in the samples were contained significant amounts of magma volatiles. In addition, pools of molten sulfur were observed on the caldera floor in 2006. Geochemical studies of dissolved gases in the fluid samples and rocks obtained from the main cone have been published, while geochemical study of the hydrothermal fluids have not been reported. The purpose of this study is to discuss the origin of the hydrothermal fluid emitting from the caldera floor as part of the research to evaluate the volcanic activity of Nikko Seamount. The fluids and rock samples composed mainly of elemental sulfur were collected during the expedition KM-20-10c using the R/V Kaimei and ROV/KM-ROV belonging to JAMSTEC in December 2020.
Potassium and calcium concentrations of the endmember hydrothermal fluid were high relative to those of other hydrothermal systems, suggesting that water-rock interaction occurred significantly. The lowest pH value of 2.2 among the fluid samples is quite low as seafloor hydrothermal fluids. Such low pH value is considered due to disproportionation reaction of sulfur dioxide degassed from magma and dissolved in the hydrothermal fluid. Therefore, the chemical characteristics of hydrothermal fluid emitting from the main cone of Nikko seamount is reflected not only vigorous water-rock interaction but also magmatic input.
The δ34S values of elemental sulfur in the rock samples were ranging from -7.7 to -6.5 ‰, which values were similar to previously reported values for elemental sulfur fragments (-7.3 to -6.2‰) obtained from the molten sulfur pools. It suggests that the elemental sulfur in the rock samples used in this study were remnant of the pools. Furthermore, the negative δ34S values for elemental sulfur suggests that the sulfur in originated from the disproportionation reaction of sulfur dioxide, because the resultant product, elemental sulfur, is enriched in 32S relative to sulfur dioxide. The δ34S value of hydrogen sulfide in the fluid sample that has the highest hydrogen sulfide concentration is -4.3 ‰, suggesting that the sulfide is also originated from the disproportionation reaction.
Consequently, the hydrothermal fluid emitting from Nikko Seamount is characterized by not only vigorous water-rock interaction but also magmatic input. Volcanic activity of Nikko Seamount is still active after having observed in 2006.
Potassium and calcium concentrations of the endmember hydrothermal fluid were high relative to those of other hydrothermal systems, suggesting that water-rock interaction occurred significantly. The lowest pH value of 2.2 among the fluid samples is quite low as seafloor hydrothermal fluids. Such low pH value is considered due to disproportionation reaction of sulfur dioxide degassed from magma and dissolved in the hydrothermal fluid. Therefore, the chemical characteristics of hydrothermal fluid emitting from the main cone of Nikko seamount is reflected not only vigorous water-rock interaction but also magmatic input.
The δ34S values of elemental sulfur in the rock samples were ranging from -7.7 to -6.5 ‰, which values were similar to previously reported values for elemental sulfur fragments (-7.3 to -6.2‰) obtained from the molten sulfur pools. It suggests that the elemental sulfur in the rock samples used in this study were remnant of the pools. Furthermore, the negative δ34S values for elemental sulfur suggests that the sulfur in originated from the disproportionation reaction of sulfur dioxide, because the resultant product, elemental sulfur, is enriched in 32S relative to sulfur dioxide. The δ34S value of hydrogen sulfide in the fluid sample that has the highest hydrogen sulfide concentration is -4.3 ‰, suggesting that the sulfide is also originated from the disproportionation reaction.
Consequently, the hydrothermal fluid emitting from Nikko Seamount is characterized by not only vigorous water-rock interaction but also magmatic input. Volcanic activity of Nikko Seamount is still active after having observed in 2006.