[M-ZZ44] 海底マンガン鉱床の生成環境と探査･開発

[MZZ44-P03] Reconstruction of the Paleo-Ocean Environment Using Mineralogical and Geochemical Analyses of Mixed-Type Ferromanganese Nodules from the Tabletop of Western Pacific Magellan Seamount

*Jinsub Park¹、Jaewoo Jung³、Kiho Yang^1,2 (1.Department of Oceanography, Pusan National University, Busan, Republic of Korea、2.Department of Oceanography and Marine Research Institute, Pusan National University, Busan Republic of Korea、3.Global Ocean Research Center, Korea Institute of Ocean Science & Technology, Busan, Republic of Korea )

キーワード：mixed-type Ferromanganese nodules, micro X-ray fluorescence, Synchrotron X-ray powder diffraction

Variations in the geochemistry and mineralogical properties of ferromanganese (Fe-Mn) nodules are closely related to variations in environmental parameters. Therefore, analysis of Fe-Mn nodules can reconstruct the paleo-ocean environment. Here, three differently shaped Fe-Mn nodules were collected from the OSM 9-1 of the Western Pacific Magellan Seamount. Geochemical and mineralogical analyses were conducted using micro X-ray fluorescence (μ-XRF) and Synchrotron X-ray powder diffraction (Sy-XRD) to reconstruct the paleo-ocean environment. We found that the shape of the three nodules was different; however, the variations in their geochemical and mineralogical properties were similar. For all three nodules, the μ-XRF elemental distribution revealed a distinct alternation between the Mn-rich and Fe-rich layers. The Mn-rich and Fe-rich layers surrounded the nuclei and appeared as concentric circles. Fe-rich and Mn-rich layers exhibited low Mn/Fe ratios (2.5<) and were dominated by vernadite, whereas layers with high Mn/Fe ratios (>2.5) were dominated by todorokite, respectively. These results suggest an early diagenesis process at tabletop of Magellan Seamount. The high contents of Ca and P and the existence of carbonate fluorapatite in the center of Fe-Mn nodules indicate that the formation of these nodules began during the Miocene phosphatization event (19–16 Ma). In the early stages of nodule formation, a diagenetic process affected Fe-Mn nodule formation. Because of global cooling from 9 Ma, early diagenesis was weakened, after which, the formation of hydrogenetic Fe-Mn nodules became dominant. As a result, variations of redox conditions such as oxygen minimum zones and oxic-suboxic fronts were recorded in Fe-Mn nodules from tabletop of seamount.