[S-CG48] Ocean Floor Geoscience

[SCG48-05] Verification of metal particles reportedly contained in hydrothermal ferromanganese oxides and igneous rocks in the Sea of Japan

*Keishiro Azami¹, Tokio Doshita², Kanta Koyama², Shiki Machida¹, Kentaro Nakamura^3,1, Kato Yasuhiro^4,1, Estuo Uchida² (1.Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 2.Faculty of Science and Engineering, Waseda University, 3.Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo, 4.Department of Systems Innovation, School of Engineering, The University of Tokyo)

Keywords:Ferromanganese oxides, Hydrothermal activity, Sea of Japan

Submarine hydrothermal ferromanganese (Fe–Mn) oxides are generally valueless as a mineral resource due to their low rare metal content. However, hydrothermal Fe–Mn oxides that are rich in Co, Ni, and Cu (up to 4.79%) have been recognized (e.g., Pelleter et al., 2017). It has been suggested that the lack of sulfide deposition in subseafloor and the formation of the cap layer of Fe–Mn oxides are involved with rare metal enrichment in hydrothermal Fe–Mn oxides. However, the enrichment process of rare metals is still unclear, as rare metal-rich hydrothermal Fe–Mn oxides have only been recognized from limited regions. The Sea of Japan may be one of these limited regions. Hydrothermal Fe–Mn oxides and associated igneous rocks containing rare metal particles (~10 µm), such as native elements and sulfides, have been reported in seamounts in the Sea of Japan (e.g., Astakhova et al., 2014) although their bulk content of rare metal is low. These metal particles have a potential to deepen our understanding of the behavior of metals in hydrothermal systems. However, many of them are difficult to coexist thermodynamically with Mn oxide. Therefore, in order to verify their existence, we performed SEM observation and geochemical analysis for Fe–Mn oxides and volcanic rocks (basaltic andesite) dredged from the Daini-Nishi-Yamato Seamount in the Yamato Basin, Sea of Japan.
The Nd-Sr isotope ratios and trace element content of the volcanic rock sample are similar to those of seamounts formed after the cessation of back-arc spreading in the Sea of Japan. The Fe–Mn oxide samples showed a typical chemical composition of hydrothermal Fe–Mn oxides, which is low elemental content other than Fe and Mn. The mineral composition and growth structure of the Fe–Mn oxide samples suggest that they were resulted from low-temperature hydrothermal activity. Because the Ar-Ar ages of the Yamato Seamount Chain, which was formed in the Yamato Basin after the cessation of back-arc spreading, is 10–17 Ma (Kaneoka et al., 1992), the Fe–Mn oxide samples would have been formed by low-temperature hydrothermal activity associated with the formation of the Daini-Nishi-Yamato Seamount in about 10–17 Ma.
Many of the Fe–Mn oxide samples showed bigger Eu anomalies than that of the volcanic rock sample. Europium is readily leached from host rock (igneous rock) by water–rock interactions at above about 200℃ (Bau, 1991). Therefore, the hydrothermal fluids that precipitated the Fe–Mn oxide samples would result from water–rock interactions at above about 200℃. Additionally, Ti-rich Fe–Mn oxides precipitated earlier than Ti-poor Fe–Mn oxides. Since the solubility of Ti is highly dependent on temperature and pH (Jiang et al., 2005), Ti-rich hydrothermal fluids would have been produced under higher temperature and lower pH conditions in the early stages of hydrothermal activity. In the late stages of hydrothermal activity, the Ti content in hydrothermal fluids decreased likely due to the depletion of Ti in the host rocks and a decrease in temperature.
In addition to silicates and Mn oxides, barite was observed in the Fe–Mn oxide samples and the volcanic rock sample, and pyrite and ilmenite were observed in the volcanic rock sample. These minerals are commonly formed through hydrothermal and magmatic activities. While many of the metal particles reported in previous studies were found in the cracks and material boundaries of samples, they were not observed in our samples which were impregnated with resin in a vacuum to prevent contamination during polishing process. Therefore, the metal particles reported in previous are strongly suggested to be contaminants, indicating that hydrothermal Fe–Mn oxides in the Sea of Japan would be formed by typical hydrothermal activity as revealed by this study.

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Jiang et al., 2005. Physics and Chemistry of the Earth, Parts A/B/C 30, 1020–1029.
Kaneoka et al., 1992. Sea. Proc., scientific results, ODP, Legs 127/128, Japan Sea 127, 819–836.
Pelleter et al., 2017. Ore Geology Reviews 87, 126–146.