Copper‐nickel‐rich, amalgamated ferromanganese crust‐nodule deposits from Shatsky Rise, NW Pacific

Abstract

[1] A unique set of ferromanganese crusts and nodules collected from Shatsky Rise (SR), NW Pacific, were analyzed for mineralogical and chemical compositions, and dated using Be isotopes and cobalt chronometry. The composition of these midlatitude, deep-water deposits is markedly different from northwest-equatorial Pacific (PCZ) crusts, where most studies have been conducted. Crusts and nodules on SR formed in close proximity and some nodule deposits were cemented and overgrown by crusts, forming amalgamated deposits. The deep-water SR crusts are high in Cu, Li, and Th and low in Co, Te, and Tl concentrations compared to PCZ crusts. Thorium concentrations (ppm) are especially striking with a high of 152 (mean 56), compared to PCZ crusts (mean 11). The deep-water SR crusts show a diagenetic chemical signal, but not a diagenetic mineralogy, which together constrain the redox conditions to early oxic diagenesis. Diagenetic input to crusts is rare, but unequivocal in these deep-water crusts. Copper, Ni, and Li are strongly enriched in SR deep-water deposits, but only in layers older than about 3.4 Ma. Diagenetic reactions in the sediment and dissolution of biogenic calcite in the water column are the likely sources of these metals. The highest concentrations of Li are in crust layers that formed near the calcite compensation depth. The onset of Ni, Cu, and Li enrichment in the middle Miocene and cessation at about 3.4 Ma were accompanied by changes in the deep-water environment, especially composition and flow rates of water masses, and location of the carbonate compensation depth.

Arctic Deep Water Ferromanganese‐Oxide Deposits Reflect the Unique Characteristics of the Arctic Ocean

“…A key characteristic of the Arctic Fe‐Mn deposits is the uniquely high Fe contents relative to Mn (Fe/Mn ratio), consistent with the mineralogy (supporting information Tables S1 and S5); the high Fe/Mn ratios are due to anomalously low Mn contents, compared to crusts from elsewhere in the global ocean (Hein et al, a). The mean Fe/Mn for all samples is 2.67 (range 1.17–5.69), which is higher than other crusts (means 0.74–1.44) from the global ocean (Hein et al, a; Hein & Koschinsky, ). Another key finding is that mean As, Hg, Li, Sc, Th, and V contents are enriched in the Arctic hydrogenetic Fe‐Mn crusts much more so than in hydrogenetic crusts from open‐ocean sites or from the California margin.…”

“…Another key finding is that mean As, Hg, Li, Sc, Th, and V contents are enriched in the Arctic hydrogenetic Fe‐Mn crusts much more so than in hydrogenetic crusts from open‐ocean sites or from the California margin. The mean contents for the entire data set are: As 559 ppm, Li 89 ppm, Sc 47 ppm, Th 62 ppm, and V 936 ppm, compared to mean data for crusts from other areas in the global ocean of As 207–393 ppm, Li 2.9–33 ppm, Sc 6.6–16 ppm, Th 11–56 ppm, and V 613–849 ppm (Hein & Koschinsky, ; Hein et al, a). Sc is highly enriched relative to the average content of the Earth's crust (14–16 ppm) and is enriched only in Arctic crust and nodule samples (Figure ).…”

Molybdenum isotopes in hydrothermal manganese crust from the Ryukyu arc system: Implications for the source of molybdenum

“…In contrast, hydrogenous Fe-Mn crusts and Mn nodules are characterized by high concentrations of both Fe and Mn and have high contents of manganophile elements (e.g. Wen et al, 1997;Glasby, 2006;Hein et al, 2012). Accordingly, the chemical compositions of hydrothermal Mn deposits and hydrogenous Mn deposits show different patterns when normalized by Mn contents (Fig.…”

“…These characteristics contrast with those of other modern deep-ocean Mn oxides, such as hydrogenous ferromanganese crusts (Fe-Mn crusts) and Mn nodules, which have higher Fe/Mn ratios and are enriched in manganophile elements (e.g. Wen et al, 1997;Glasby, 2006;Hein et al, 2012).…”

“…Taken as a whole, location (see Section 2), growth rate (see Section 5.1), and chemical compositions of the analyzed Mn crust Banakar et al (1997), Wen et al (1997), Rajani et al (2005), Jiang et al (2009), andHein et al (2012), and the data for hydrothermal Mn crusts are from Scott et al (1974), Moore and Vogt (1976), Moorby et al (1984), Usui et al (1986), Hodkinson et al (1994), Hein et al (1996), Eckhardt et al (1997), Rogers et al (2001), and Kuhn et al (2003). (Levasseur et al, 1998;Woodhouse et al, 1999).…”