1989 Volume 39 Issue 215 Pages 205-218
Manganese nodules from the northern Central Pacific Basin are subjected to chemical and mineralogical analyses as well as mode of occurrence. Under the ore microscope, three different reflectivity zones (high, moderate and low on arbitrary scale) are intermingled in the single ferromanganese oxide layer around the margins of older nodule fragments. The fragments consist mainly of ∂-MnO 2 and amorphous Fe oxyhydroxides referred to as DAFO.
The characteristic reflectivity is attributed to micro-scale variations in chemical composition (Mn, Fe, Cu, Ni, Co, Ca, Mg, Ti, Si, K and Al) and mineral assemblage as detected by EPMA, SEM, and EDX. The results of EPMA and microscopic investigations suggest that the high-reflectivity zone consists of secondary todorokite replacing the original mixtures of DAFO (low-reflectivity) although primary todorokite layers have been commonly deposited from sediment interstitial waters in the younger encrustations. The moderate-reflectivity zone thus corresponds to an alteration front associated with great variation in Fe and Ti contents while the low-reflectivity zone represents the unaltered DAFO zone. These altered zones are usually in contact with surface sediments where the alteration takes place.
These data suggest that the alteration is related to micro-scale variations of physico-chemical conditions probably controlled by the diagenesis of surface siliceous sediments, bottom current fluctuations, and benthic activities in local bottom environments.
