16:30 〜 16:45
[SCG47-11] 南鳥島東方沖に分布するMn団塊の記載およびOs層序年代
キーワード：マンガン団塊、Os年代、薄片、北西太平洋、南鳥島、ハイエタス
Ferromanganese (Fe-Mn) nodule is one of the seafloor mineral resources widely distributed on the abyssal plain [1]. During cruises YK10-05, YK16-01 and YK17-11C by HOV Shinkai 6500 and R/V Yokosuka, densely and widely distributed Fe-Mn nodule fields were discovered from the eastern and southeastern offshore of Minamitorishima Island [2-6]. Here, we report petrographic signatures using microscopy and electron probe micro analyzer (EPMA) as well as bulk and Re-Os geochemistry to determine the stratigraphic Os isotope age (= depositional age) of Fe-Mn nodules collected from the eastern offshore of Minamitorishima Island during the dive 6K#1207 (observer: Teruaki Ishii), cruise YK10-05.
Under the microscope, the Fe-Mn nodules samples can be divided into several (sub)layers of L0, L1 and L2 from surface to the bottom parts like as previous studies [2–6]. L0 is mainly composed of intercalated layers of vernadite and ferrihydrite. Ferrihydrite often contained a high TiO2 concentration up to 9.71 wt%. L1 is dominated by columnar-shaped vernadite. Outer part of L2 is also dominated by columnar-shaped vernadite, but the mode of occurrence of clay minerals derived from altered seamount basalt is higher than that of L1. In the middle and inner parts of L2, the size of columnar-shaped vernadite becomes smaller than that of outer part of L2 and continuous growth of columnar-shaped vernadite seems to be disturbed by intermittent supply of seamount basaltic fragments. Mn/Fe ratios of vernadite continuously increases from surface to bottom parts (from L0 to L2), except for the innermost part of L2, periphery of altered core, having the low Mn/Fe ratios like as L0. These variations of Mn/Fe ratios fundamentally control the bulk geochemistry of this Fe-Mn nodule sample. Todorokite and vernadite-todorokite mixed layers characterized by high NiO and MgO concentrations up to 4.08 and 8.32 wt%, respectively, are only observed at the periphery of altered core and layer boundary between L1 and L2. One spherule grain was observed from the inner part of L2. This spherule is composed of inner Fe-Ni-Co alloy and outer Fe-Mg spinel parts.
Fe-Mn nodules sample was cut into 3-mm thick slices and totally 13 slices were pulverized and analyzed. Re concentration, Os concentration, 187Re/188Os and 187Os/188Os of Fe-Mn nodule sample are ranging from 34.3–84.8 ppt, 276–5,739 ppt, 0.063–1.53 and 0.138–0.957, respectively. 187Re/188Os is low enough to ignore the internal decay of 187Re to 187Os from the time of deposition. Os concentration and 187Os/188Os decrease from surface to the inner parts, except for the sample at 33–36 mm depth at the inner part of L2, having the highest Os concentration (5,739 ppt) and most unradiogenic 187Os/188Os value (0.138). This highest Os concentration and the most unradiogenic 187Os/188Os value could be only produced by mixing with ultramafic rock or extraterrestrial material. The stratigraphic Os isotope age (relative age) can be determined by fitting with the secular change curve of marine 187Os/188Os record [7–10]. The Os isotope age was clustered into three groups; (1) 0–3, (2) 7–13 and (3) 28–36 Ma. The boundary and age gap of these three clusters correspond to the layer boundaries of L0, L1 and L2, indicating that there were growth hiatuses of Fe-Mn nodule.
[1] Hein et al. (2013) Ore Geol. Rev., 51, 1–14. [2] Machida et al. (2016) Geochem. J., 50, 539–555. [3] Machida et al. (2021) Mar. Georesour. Geotec., 39, 267–279. [4] Machida et al. (2021) Island Arc, 30, e12395. [5] Nakamura et al. (2021) Minerals, 11, 1100. [6] Machida et al. (2021) Minerals, 11, 1246. [7–10] Klemm et al. (2005) Earth Planet. Sci. Lett., 238, 42–48. [8] Klemm et al. (2008) Earth Planet. Sci. Lett., 273, 175–183. [9] Nozaki et al. (2019) Sci. Rep., 9, 16111. [10] Ohta et al. (2020) Sci. Rep., 10, 9896.
Under the microscope, the Fe-Mn nodules samples can be divided into several (sub)layers of L0, L1 and L2 from surface to the bottom parts like as previous studies [2–6]. L0 is mainly composed of intercalated layers of vernadite and ferrihydrite. Ferrihydrite often contained a high TiO2 concentration up to 9.71 wt%. L1 is dominated by columnar-shaped vernadite. Outer part of L2 is also dominated by columnar-shaped vernadite, but the mode of occurrence of clay minerals derived from altered seamount basalt is higher than that of L1. In the middle and inner parts of L2, the size of columnar-shaped vernadite becomes smaller than that of outer part of L2 and continuous growth of columnar-shaped vernadite seems to be disturbed by intermittent supply of seamount basaltic fragments. Mn/Fe ratios of vernadite continuously increases from surface to bottom parts (from L0 to L2), except for the innermost part of L2, periphery of altered core, having the low Mn/Fe ratios like as L0. These variations of Mn/Fe ratios fundamentally control the bulk geochemistry of this Fe-Mn nodule sample. Todorokite and vernadite-todorokite mixed layers characterized by high NiO and MgO concentrations up to 4.08 and 8.32 wt%, respectively, are only observed at the periphery of altered core and layer boundary between L1 and L2. One spherule grain was observed from the inner part of L2. This spherule is composed of inner Fe-Ni-Co alloy and outer Fe-Mg spinel parts.
Fe-Mn nodules sample was cut into 3-mm thick slices and totally 13 slices were pulverized and analyzed. Re concentration, Os concentration, 187Re/188Os and 187Os/188Os of Fe-Mn nodule sample are ranging from 34.3–84.8 ppt, 276–5,739 ppt, 0.063–1.53 and 0.138–0.957, respectively. 187Re/188Os is low enough to ignore the internal decay of 187Re to 187Os from the time of deposition. Os concentration and 187Os/188Os decrease from surface to the inner parts, except for the sample at 33–36 mm depth at the inner part of L2, having the highest Os concentration (5,739 ppt) and most unradiogenic 187Os/188Os value (0.138). This highest Os concentration and the most unradiogenic 187Os/188Os value could be only produced by mixing with ultramafic rock or extraterrestrial material. The stratigraphic Os isotope age (relative age) can be determined by fitting with the secular change curve of marine 187Os/188Os record [7–10]. The Os isotope age was clustered into three groups; (1) 0–3, (2) 7–13 and (3) 28–36 Ma. The boundary and age gap of these three clusters correspond to the layer boundaries of L0, L1 and L2, indicating that there were growth hiatuses of Fe-Mn nodule.
[1] Hein et al. (2013) Ore Geol. Rev., 51, 1–14. [2] Machida et al. (2016) Geochem. J., 50, 539–555. [3] Machida et al. (2021) Mar. Georesour. Geotec., 39, 267–279. [4] Machida et al. (2021) Island Arc, 30, e12395. [5] Nakamura et al. (2021) Minerals, 11, 1100. [6] Machida et al. (2021) Minerals, 11, 1246. [7–10] Klemm et al. (2005) Earth Planet. Sci. Lett., 238, 42–48. [8] Klemm et al. (2008) Earth Planet. Sci. Lett., 273, 175–183. [9] Nozaki et al. (2019) Sci. Rep., 9, 16111. [10] Ohta et al. (2020) Sci. Rep., 10, 9896.