Distribution of hydrothermal fluid around the ore body in the subseafloor of the Izena hydrothermal field

From 16th November to 15th December 2016, D/V Chikyu drilled the sea bottom around hydrothermal fields at HAKUREI site in the Izena Hole, Okinawa Trough. Site C9025, C9026, C9027, C9028, and C9032 are located along the transect line from the top of the northern mound of HAKUREI site to the eastward, and Site C9030 for the control site is located about 500 m northwest of the mound. Mg concentrations have generally been used to estimate mixing ratios between hydrothermal end-member and seawater in samples from hydrothermal vents. Higher Mg concentrations, however, were detected in the interstitial water than that of seawater, which could be due to artificially dissolution of Mg-bearing minerals that had formed in in-situ environments, when the cored sediments had become cool after their recovery on ship. Similar features were observed with regard to sulfate concentrations, and it suggests that these chemical species are not suitable to estimate quantitatively the contribution of hydrothermally-derived components. In some layers, chloride concentrations were different from that of seawater, indicating that hydrothermal fluids that had been suffered from phase separation flowed into the layers. The deviation, however, was positive or negative relative to that of seawater for an influence of brine or vapor phase, respectively. Therefore chloride concentrations are also not suitable to evaluate a quantitative contribution of hydrothermal end-member. On the other hand, K and B showed only enrichments relative to the seawater, and their highest concentrations are consistent with the reported hydrothermal end-members of each species at HAKUREI site. Using the concentrations of K and B can be evaluated for an influence of hydrothermal components. Furthermore, the headspace gas data are useful in the layers of sulfide minerals and silicified rocks, even though the interstitial waters could not be obtained because of their hardness. Based on these indices, hydrothermal fluids would be distributed in the upper and lower layers of the ore body. The hydrothermally altered sediment layers above the ore body contain relatively unstable minerals that dissociate immediately in a room temperature, which could play a role as a boundary between hydrothermal fluids and intruded seawater in in-situ environments.