16:00 〜 18:30
[R22-P-3] （エントリー）Formation of modern iron-ooidal sands in a shallow-marine hydrothermal environment at Nagahama Bay, Satsuma Iwo-Jima Island, Japan
キーワード：Ooids、Iron Ooidal Sands、Granular Iron Formation、Modern Analogy、Satsuma Iwo-Jima
Modern iron-ooidal sands have been considered as one of the most important tools for investigating the history of the earth as its capacity to provides the analogy of the granular iron sedimentation in the ancient ocean. Interestingly, we found that such an ongoing formation process of iron-ooidal sands can be observed at Nagahama Bay of Satsuma Iwo-Jima Island, 38 km south of Kyushu Island, Japan. The minimum influence from seawater (e.g. waves) due to the occurrence of breakwaters and the intense hydrothermal activities causes the water to become orange-brown in color with acidic (low pH), hot (high temperature), and containing a large amount of dissolved iron (generated by mixing volcanic fluids and seawater). These conditions lead to the precipitation of Fe-oxyhydroxide sediments on the floor of the bay; and iron-coated sands on the wharf and along the beach. Therefore, in order to understand the characteristics and formation of iron-ooidal sands, we collect unlithified sand samples from 15 locations: west and east site of the fishing port, original river, the mouth of the river, 9 samples from the sandy beach, and 2 samples from the rocky beach. Sand grains are spheroidal in shape (ooids) with a rust black-brown color, vary from 0.2 to 2 mm in diameter, and exhibit roundness from sub-angular to sub-rounded. Based on our petrography and FE-SEM/EDS data, we found that ooidal sands are consist of volcanic rock fragments (basaltic and rhyolitic rocks) and free crystals (plagioclase, quartz, biotite) on the nucleus; and covered by concentric amorphous Fe-oxide cortex. Three types of Fe-oxide were observed: (1) covering the granules, (2) fracture filling, and (3) void filling. EDS analysis indicates that most of the amorphous Fe-oxide particles are mainly composed of Fe (~58-64%) and Si (~6-9%). Element mapping using FE-EPMA shows that the cortex is composed of Fe-rich which is almost uniform in each layer. Furthermore, high-Ca mineral inclusions in one layer can be a key layer for correlation. The microbial community is well documented in the form of cocci, rod, and filamentous morphology, which responsible for triggering the chemical precipitation of Fe-oxide through their metabolic activity hence results in the formation of iron-ooidal sands.