[S-CG54] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG54-P15] Early stage alteration of subseafloor pumice layers prior to replacement by sulfide at the hydrothermal fields in Okinawa Trough

* Song Zijun¹, Yagyu Shinsuke², Matsuzaki Takuya², Masafumi MURAYAMA², Naoya Kanna³, Hajime Obata³, Toshiro Yamanaka¹ (1.Tokyo University of Marine Science and Technology, 2.Marine Core Research Institute, Kochi University, 3.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Okinawa Trough, Hydrothermal Field, Buried Pumice, Water-Rock Interaction, Sulfide Replacement

The Okinawa Trough, located in the East China Sea, is a typical back-arc basin developed behind the Ryukyu arc-trench system, where the Philippine Sea Plate subducts beneath the Eurasia Plate. This region is covered by thick hemipelagic sediments supplied from the adjacent continent, and also partly by a significant thick of volcaniclastics, of which pumice is an important component. Its mineral assemblage and chemical composition of the pumice provide key information on magma evolution before eruption and alteration by water-rock interactions after eruption. Furthermore, due to its porous nature, its high permeability is expected to act as an aquifer below seafloor, therefore its chemistry of porewater plausibly provides a current snapshot of the physico-chemical condition of the subseafloor environment. Meanwhile, an intense hydrothermal activity is involved with magmatism in the trough, leading to the formation of abundant sulfide deposits below and above the seafloor. There, replacement of the subseafloor pumice layer by sulfide is expected to occur as one of the typical ore formation mechanisms. However, research on the geochemical condition of pumice in active hydrothermal areas remains relatively limited.
This study collected buried pumice samples from two hydrothermal fields, Izena Hole and North Iheya, in the middle of Okinawa Trough using a large-gravity corer and BMS. Mineralogical, morphological and chemical analyses of solid phase in the samples were performed using a X-ray diffraction (XRD), polarized light microscopy (PLM), and X?ray Fluorescence (XRF). The pore fluids extracted using pure water by ultrasonification were provided for chemical analysis of concentrations of dissolved ions including REEs and sulfur isotope ratios of dissolved sulfate.
Based on the visual and PLM observations, the pumice samples can be divided into three types by their colors: white, gray, and black, which correspond to significant differences in mineral assemblages and microstructures. Major elemental composition obtained by XRF reveals distinct chemical compositions among them, possibly reflecting degrees of alteration and/or weathering of the pumice. The differences in colors among the pumice samples implies primarily related to their internal structures. Sulfur isotope ratios of extracted porewater sulfate show significant depleted δ³⁴S values relative to that of seawater sulfate. It suggests that the sulfate originates in oxidation of authigenic pyrite involved with microbial sulfate reduction in the overlying sedimentary layers during oxygenated seawater recharging and flowing into pumice layers. The fluid flow may be driven by hydrothermal activity, that is hydrothermal circulation. The acidic conditions and released metal ions resulting from bacterial sulfate reduction and pyrite oxidation enhance alteration and/or weathering of pumice. Such situation is important as an early stage of replacement pumice by sulfide.