15:30 〜 15:45
[SCG55-07] Geochemistry and detrital debris uptake fluxes of ferromanganese crusts in the western North Pacific Ocean
キーワード：鉄マンガンクラスト、北西太平洋、風成塵、海洋Os同位体比
Ferromanganese (Fe–Mn) crusts are chemical sedimentary rocks that form on substrate rocks in deep-sea environments and are of interest as mineral resources of critical metals such as Co, Ni, and Pt1. Fe–Mn crusts can also serve as a paleoceanographic archive because they maintain the original chemical composition reflecting their growth environments. Growth rates of Fe–Mn crusts increase in regions with a large supply flux of detrital debris2, and the formation of Fe–Mn crusts is hindered by excessively large supply of debris. However, the limitation on debris supply at which Fe–Mn crusts can grow remains unclear. Therefore, this study aimed to constrain the formation environments of Fe–Mn crusts based on the debris uptake flux of debris-rich and fast-growing Fe–Mn crusts in the western North Pacific Ocean. In addition, this study also analyzed the relationship between metal contents and growth rate in Fe–Mn crusts to reveal the formation conditions of Fe–Mn crusts that are valuable as metal resources.
The Fe–Mn crust samples were collected from offshore northeastern Japan and the Philippine Sea during the YK14-05 cruise and the KT-95-9 cruise, respectively. We conducted elemental mapping, Os isotopic analysis, and leaching experiments on the samples. Based on their chemical composition, the samples from offshore northeastern Japan and the sample from the Philippine Sea were revealed to be of hydrogenetic origin and hydrogenetic origin with slight hydrothermal influence, respectively3. The age-depth models of the samples were constructed based on Os isotope stratigraphic ages using Bayesian estimation4. The estimated growth rates of the samples were 20.0 –100 mm/Myr, indicating that these samples exhibit the fastest growth rate among hydrogenetic Fe–Mn crusts reported so far. The debris uptake flux of the samples, which is defined as the product of growth rate, detritus content, and density, was 7.44 –71.9 mg/m2/year. Therefore, Fe–Mn crusts can form under conditions where debris flux is less than approximately 70 mg/m2/year, even in the absence of debris removal by currents. The aeolian flux in a part of central Pacific Ocean has been estimated to be lower than 70 mg/m2/year5. The growth rates of Fe–Mn crusts in this and previous studies negatively correlate with critical metals content. Based on this relationship, the upper limit of debris uptake flux to form Fe–Mn crusts with >0.5% Co content (called “cobalt rich crust”) was estimated to be <0.92 mg/m2/year.
1. Hein et al., Ore Geol. Rev. 51, 1–14 (2013).
2. Conrad et al., Ore Geol. Rev. 87, 25–40 (2017).
3. Josso et al., Ore Geol. Rev. 87, 3–15 (2017).
4. Blaauw & Christen, Bayesian Anal. 6, 457–474 (2011).
5. Maher et al., Earth-Sci. Rev. 99, 61–97 (2010).
The Fe–Mn crust samples were collected from offshore northeastern Japan and the Philippine Sea during the YK14-05 cruise and the KT-95-9 cruise, respectively. We conducted elemental mapping, Os isotopic analysis, and leaching experiments on the samples. Based on their chemical composition, the samples from offshore northeastern Japan and the sample from the Philippine Sea were revealed to be of hydrogenetic origin and hydrogenetic origin with slight hydrothermal influence, respectively3. The age-depth models of the samples were constructed based on Os isotope stratigraphic ages using Bayesian estimation4. The estimated growth rates of the samples were 20.0 –100 mm/Myr, indicating that these samples exhibit the fastest growth rate among hydrogenetic Fe–Mn crusts reported so far. The debris uptake flux of the samples, which is defined as the product of growth rate, detritus content, and density, was 7.44 –71.9 mg/m2/year. Therefore, Fe–Mn crusts can form under conditions where debris flux is less than approximately 70 mg/m2/year, even in the absence of debris removal by currents. The aeolian flux in a part of central Pacific Ocean has been estimated to be lower than 70 mg/m2/year5. The growth rates of Fe–Mn crusts in this and previous studies negatively correlate with critical metals content. Based on this relationship, the upper limit of debris uptake flux to form Fe–Mn crusts with >0.5% Co content (called “cobalt rich crust”) was estimated to be <0.92 mg/m2/year.
1. Hein et al., Ore Geol. Rev. 51, 1–14 (2013).
2. Conrad et al., Ore Geol. Rev. 87, 25–40 (2017).
3. Josso et al., Ore Geol. Rev. 87, 3–15 (2017).
4. Blaauw & Christen, Bayesian Anal. 6, 457–474 (2011).
5. Maher et al., Earth-Sci. Rev. 99, 61–97 (2010).