Electron microprobe analysis of selected manganese nodules from the Southwestern Pacific Basin

Four manganese nodules collected south and southwest of Rarotonga one of the Cook Islands were subjected to microprobe analysis in order to determine variations in chemical composition between the concentric laminations making up the nodules. Nodules of maximum diameter of about 2.5 cm or less were selected so that the entire nodule could be sectioned for one probe mount. Probe mounts were prepared by vacuum embedding in cold-setting, clear-casting resin. Cutting and polishing were then performed using standard polished section techniques. The electron microprobe mounts were examined with a reflected light microscope in the microprobe, and 2 micrometer spots were selected for analysis along a line of traverse going through the centre (nucleus) of the nodule. Due to the fact that optically-reflective metal-rich layers were selected for analysis and darker detrital inclusions and silicate-rich layers were not analysed here the microprobe values are generally higher for Mn, Ni, Cu and Co, and lower for Fe, than the bulk chemical analyses.