Structure, composition, and formation of ferromanganese micronodules at the eastern southwest Indian ridge

Abstract

This study evaluated the structure, mineralogy, and chemical composition of ferromanganese micronodules in gravity core GC01 obtained from the eastern southwest Indian ridge using micro-Raman spectroscopy, WITec Alpha 300R confocal Raman spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, TESCAN integrated mineral analysis and electron microprobe analysis. The results revealed vernadite as the major crystalline phase in the ferromanganese micronodules. In addition, the micronodules contained small amounts of Fe oxyhydroxides and silicate minerals. All micronodules had low Mn/Fe ratios (~1), and their internal structures included concentric, clastic, dendritic, and foraminifera-like structures, among which the concentric and clastic structures were predominant. These characteristics suggest that micronodules are formed in an oxidizing environment via hydrogenetic processes. Volcanic materials and biological debris may act as “seeds” for micronodules, among which volcanic materials may serve as material sources. During water-rock interaction, volcanic clastics and biological debris react with seawater, and some cations (such as Ca2+, Na+, Mg2+) are leached out; Fe2+ and Mn2+ remain and generate Fe oxides or oxyhydroxides and Mn oxides which forms ferromanganese micronodules. Conclusively, the formation of ferromanganese micronodules in this study was controlled by a coupled dissolution-reprecipitation mechanism.