Atypical ferromanganese nodules from pelagic areas of the Central Indian Basin, equatorial Indian Ocean

Two box core samples, one each from red clay and siliceous ooze sediment provinces from the Central Indian Basin (Indian Ocean) were studied. Both cores contained ferromanganese nodules at their tops. The red clay sediment also hosts buried nodules at a depth of 82 cm. Chemical characterization of the sediments indicates that early diagenetic remobilization of transitional metals is pronounced in the siliceous ooze sediment, while in the red clay no such effect is discernible. This is further corroborated by the abundance and dissolution patterns of microorganisms and ferromanganese micronodules occurring in the two sediment columns. Bulk chemical analyses indicate that all the nodules are of the hydrogenetic B-type. Vernadite and todorokite are the major minerals, both of which constitute laminated, nondirectional colloform and cuspate microstructures. Petrographic studies yield evidence of recrystallization of todorokite and partial transformation of vernadite to todorokite, the former process being more advanced in buried nodules. Microprobe analyses revealed that primary todorokite has a lower Mn/Fe ratio and higher Ni content than recrystallized todorokite in the same sample. The mineralogical reconstitution in the nodules is attributed to postdepositional intranodule diagenesis. The present study does not reveal the correlation between mineralogy and microstructure in the nodules which has been suggested for the nodules from the Pacific, neither can any mineral or microstructure be considered specific for any particular genetic model. The studied nodules are, therefore, atypical. The mineralogical and chemical reordering in these ferromanganese nodules raises the question of the viability of radiometric methods in determining growth rates of such nodules