Observation of manganese deposits on Kaikata Seamount, Izu-Ogasawara (Bonin) Arc by the Shikai 2000 submersible

During Dive 408 of Shinkai 2000 on the flank of the Kaikata Seamount located on the volcanic front of the Izu-Ogasawara (Bonin) Arc, possibly recent hydrothermal manganese deposits were observed on sandy volcanic sediments at water depths of between 800 and 1200 m. The deposits cover almost the entire surface sediment across the manganese belt on the northwestern flank on the seamount on a scale of kilometers. The occurrence of the deposits, seafloor morphology and temperature of the sediments, together with previous shipboard results, lead to a possible model for the formation of the manganese deposits: A low-temperature hydrothermal solution ascends through fractures or faults in the volcanic substrate into permeable volcanic sands and precipitates manganate minerals which cement several centimeters of surface sand as hardpan. More intense and probably intermittent discharge of hydrothermal solution follows and forms many typical ridge-like vents and mounds on the seafloor. Subsequently, pure dense manganate layers developed beneath the semi-consolidated volcanic sandstone and act as a cap to the ascending fluids. The proposed mechanism may be the dominant type of low-temperature hydrothermal mineralization in submarine island-arc volcanoes and rifts. Similar hydrothermal manganese deopsits most probably occur in other active areas of the modern seafloors and in inactive island arcs.

在伊豆-小笠原（博宁，Izu-Ogasawara (Bonin)）火山弧火山前锋带的甲斐方海山（Kaikata Seamount）侧翼开展的新海2000（Shinkai 2000）号潜水器第408次下潜作业中，科研人员于水深800~1200 m的砂质火山沉积物内发现了疑似新近形成的热液锰矿床（hydrothermal manganese deposits）。该矿床以千米级尺度覆盖了该海山西北翼锰矿带内几乎全部表层沉积物。结合矿床产出特征、海底地貌与沉积物温度数据，以及此前的船载调查结果，研究人员提出了该锰矿床的可能成因模型：低温热液流体通过火山基底的裂隙或断层向上运移，进入渗透性良好的火山砂层中，析出锰酸盐矿物，将数厘米厚的表层砂胶结为硬质结壳。后续更为强烈且大概率呈间歇性的热液流体喷溢，在海底塑造了大量典型的脊状喷口与热液丘。此后，半固结火山砂质岩层下方形成了致密纯净的锰酸盐层，作为盖层阻挡上升流体的运移。该成因机制或为海底岛弧火山与裂谷带内低温热液成矿的主导类型。类似的热液锰矿床极有可能存在于现代海底其他活动区域以及休眠岛弧带中。