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

摘要

Deep-sea ferromanganese nodules occur over alarge area and on many different sediment types of the CentralIndian Basin, Indian Ocean. Selected samples were studied todetermine their chemical and mineralogical compositions andmicrostructural features. Repeated laminations of variablethickness, alternately dominated by todorokite and vernadite, arecharacteristic of these nodules. These laminae show, on electronmicroprobe line scans, corresponding interlaminar partitioning ofMn-Cu-Ni and Fe-Co. The bulk chemical compositions of thesenodules plot in both the hydrogenetic and early diagenetic fields onthe Fe-Mn-(Ni+Cu+Co) ×10 ternary diagram. The binarydiagram depicting the covariation of Mn+Ni+Cu against Fe+Coshows two distinct parallel regression lines, one delineated bynodules 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. Anincreasing diagenetic influence in the nodules with the nature ofthe host sediment types was observed in the sequence: terrigenous→ siliceous ooze and red clay → siliceous/calcareous ooze-redclay. A negative correlation between Mn/Fe ratio and Co and apositive correlation between Mn/Fe ratio and (Ni+Cu) wasestablished. The nodules show dendritic, laminated, and globularmicrostructures formed by primary growth of Fe-Mn oxidelaminae. Depositional hiatuses in the primary microstructuresindicate that the growth of these nodules was episodic. The oxidelamintions show extremely complex growth patterns. Scatteredbiogenic remains and mineral grains acted as accessory 'seeds' forgrowth of oxide layers in addition to the main nuclei. Nine of theprimary microstructures can be uniquely linked to a particulargrowth process or growth rate. Radial cracks, cutting acrossprimary microstructures, are often filled by todorokite of a latergeneration. Post-depositional modifications of the nodules werelargely controlled by accreted biogenic remains as indicated bytheir progressive dissolution with increasing depth from nodulesurfaces, their pseudomorphic replacement by todorokite and thelater growth of phillipsite and todorokite in the microfossil molds.The growth patterns of the in-filled oxides are often controlledentirely by the cavity-walls and are discordant with the primarygrowth fabric. Primary todorokite was recrystallized to coarsergrains of different chemical composition. Later generation veins oftodorokite cut across and chaotically disrupted primary laminae.1999 Elsevier Science B.V. All rights reserved.