Post-depositional metal enrichment processes inside manganese nodules from the north equatorial Pacific |
| |
| Authors: | Virginia Mee Burns Roger G Burns |
| |
| Institution: | Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 U.S.A. |
| |
| Abstract: | Interiors of manganese nodules from siliceous ooze beneath the Pacific equatorial high-productivity region, when examined by scanning electron microscopy (SEM) and electron microprobe, display post-depositional recrystallization textures and metalliferous oxide bands (diameter 1–10 μm, 30–40 wt.% Mn, 4–5% Ni, 3–4% Cu). SEM has revealed biogenic siliceous matter in all stages of degradation and dissolution within nodule interiors, creating cavities and voids. Often these miniature vugs contain authigenic phillipsite crystallites which are coated with delicate clusters of crystalline Mn-Fe oxides (todorokite) containing significant amounts of Ni and Cu. We postulate the following diagenetic processes and mechanism of uptake of transition metals inside manganese nodules: (1) palagonite + biogenic silica + pelagic clay → phillipsite + montmorillonite; (2) biogenic matter + amorphous FeOOH or δ-MnO2 → Feaq2+ and/or MnIIMnIV oxide (todorokite); (3) aerated seawater or δ-MnO2 + Feaq2+ → FeOOH and/or todorokite (deposited on phillipsite); (4) (NiII and CuII) organic chelates (adsorbed on clays, etc.) + amorphous FeOOH or δ-MnO2 → Ni-Cu-todorokite + phillipsite, etc.This mechanism explains the well-known positive Mn-Ni-Cu and negative Fe-Ni, Fe-Cu correlations in nodules. By analogy with terrestrial todorokites, which require about 8 wt.% Mn to be in the divalent state to stabilize the crystal structure, as much as 8 wt.% (Ni + Cu) could be accommodated in todorokite-bearing deep-sea manganese nodules. However, although such nodules beneficiate Ni and Cu with respect to marine sediments and seawater, they remain undersaturated in these divalent cations. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
