Petrogenesis of ferromanganese nodules from east of the Chagos Archipelago, Central Indian Basin, Indian Ocean

Deep-sea ferromanganese nodules occur over a large area and on many different sediment types of the Central Indian Basin, Indian Ocean. Selected samples were studied to determine their chemical and mineralogical compositions and microstructural features. Repeated laminations of variable thickness, alternately dominated by todorokite and vernadite, are characteristic of these nodules. These laminae show, on electron microprobe Line scans, corresponding interlaminar partitioning of Mn-Cu-Ni and Fe-Co. The bulk chemical compositions of these nodules plot in both the hydrogenetic and early diagenetic fields on the Fe-Mn-(Ni + Cu + Co) x 10 ternary diagram. The binary diagram depicting the covariation of Mn + Ni + Cu against Fe + Co shows two distinct parallel regression lines, one delineated by nodules from terrigenous, siliceous ooze and siliceous ooze-terrigenous sediments and the other by nodules from red clay, siliceous ooze-red clay and calcareous ooze-red clay. An increasing diagenetic influence in the nodules with the nature of the host sediment types was observed in the sequence: terrigenous --> siliceous ooze and red clay --> siliceous/calcareous ooze-red clay. A negative correlation between Mn/Fe ratio and Co and a positive correlation between Mn/Fe ratio and (Ni + Cu) was established. The nodules show dendritic, laminated, and globular microstructures formed by primary growth of Fe-Mn oxide laminae. Depositional hiatuses in the primary microstructures indicate that the growth of these nodules was episodic. The oxide laminations show extremely complex growth patterns. Scattered biogenic remains and mineral grains acted as accessory 'seeds' for growth of oxide layers in addition to the main nuclei. None of the primary microstructures can be uniquely linked to a particular growth process or growth rate. Radial cracks, cutting across primary microstructures, are often filled by todorokite of a later generation. Post-depositional modifications of the nodules were largely controlled by accreted biogenic remains as indicated by their progressive dissolution with increasing depth from nodule surfaces, their pseudomorphic replacement by todorokite and the later growth of phillipsite and todorokite in the microfossil molds. The growth patterns of the in-filled oxides are often controlled entirely by the cavity-walls and are discordant with the primary growth fabric. Primary todorokite was recrystallized to coarser grains of different chemical composition. Later generation veins of todorokite cut across and chaotically disrupted primary laminae. (C) 1999 Elsevier Science B.V. All rights reserved.