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
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.