[MGI34-P04] 北西太平洋産マンガンノジュールの切断面に対するマイクロ蛍光X線マッピング
キーワード：マンガンノジュール、マイクロX線分析、南鳥島EEZ
A dense field of hydrogenous ferromanganese nodules was discovered on a small seamount approximately 300 km east of Minamitorishima (Marcus) Island, in the western North Pacific [1]. Previous study [1] defined two fundamental features on structure and geochemistry of nodules, as follows. The large nodules generally consist of three concentric layers: the outermost mottled (sediment-filled) layer L0; the massive black layer L1; and the innermost porous (sediment-filled) layer L2. Elements including Fe, Ti, and Co are concentrated in the nodule rim rather than the center. In contrast, Mn, Al, P, Ca, Ni, and Zn are concentrated in the center, and decrease toward the rim.
To further investigate the relationship between structure of oxide layers and its geochemical features, we conducted mapping analysis for polished slab of nodules including nuclei and whole layers of oxide by a microfocus X-ray fluorescence analytical microscope (XGT-7000; HORIBA) at Kyoto University. Analysis was conducted in vacuo, and the intensity of Si, Al, P, Ca, K, Mn, Fe, Ti, Ni, Cu, and Y were determined by an energy-dispersive spectrometer at an accelerating voltage of 50 kV, 100 µm beam, and intervals of approximately 160 µm.
We identified that L2 is further sub-divided into three layers, which correspond to layers obtained by X-ray CT [2]. The most inner part of L2 (around nuclei) is characterized by high intensity of Ti and Fe. In contrast, outer part of L2 is characterized by higher intensity of Mn than that of the inner part. Many spots of high intensity of P, Ca, and Y, indicating particles of calcium phosphate including highly amount of Y (and probably rare-earth elements), are observed in the entire area of L2. We found that L1 is clearly characterized by low Si. Furthermore, we observed thin layers showing high intensity of Cu and Ni at the sediment-filled boundaries between L1 and L2, and L1 and L0. Existence of such thin layers suggests that nodules were fully covered by pelagic sediment at the intervals before formation of L1 and L0.
References: [1] Machida et al. (2016) Geochem. J.50, 539-555. [2] Shimomura et al. (2018) Goldschmidt Conference 2018.
To further investigate the relationship between structure of oxide layers and its geochemical features, we conducted mapping analysis for polished slab of nodules including nuclei and whole layers of oxide by a microfocus X-ray fluorescence analytical microscope (XGT-7000; HORIBA) at Kyoto University. Analysis was conducted in vacuo, and the intensity of Si, Al, P, Ca, K, Mn, Fe, Ti, Ni, Cu, and Y were determined by an energy-dispersive spectrometer at an accelerating voltage of 50 kV, 100 µm beam, and intervals of approximately 160 µm.
We identified that L2 is further sub-divided into three layers, which correspond to layers obtained by X-ray CT [2]. The most inner part of L2 (around nuclei) is characterized by high intensity of Ti and Fe. In contrast, outer part of L2 is characterized by higher intensity of Mn than that of the inner part. Many spots of high intensity of P, Ca, and Y, indicating particles of calcium phosphate including highly amount of Y (and probably rare-earth elements), are observed in the entire area of L2. We found that L1 is clearly characterized by low Si. Furthermore, we observed thin layers showing high intensity of Cu and Ni at the sediment-filled boundaries between L1 and L2, and L1 and L0. Existence of such thin layers suggests that nodules were fully covered by pelagic sediment at the intervals before formation of L1 and L0.
References: [1] Machida et al. (2016) Geochem. J.50, 539-555. [2] Shimomura et al. (2018) Goldschmidt Conference 2018.