2024 Volume 74 Issue 2 Pages 95-102
Japan Organization for Metals and Energy Security (JOGMEC) contracted with the International Seabed Authority (ISA) to explore Cobalt-rich ferromanganese crusts (CRC) for six guyots (Lamont, Arnold, Maloney, Xufu, Zhinyu, and Scripps) in the international waters 600 km southeast of Minami-Torishima Island (Marcus Island) in 2014. Until the selection to one third of the contracted area (3,000 km2) in January 2024 on the basis of the contract with ISA, JOGMEC has conducted total 573 drills by the benthic multi-coring system (BMS). Through the course of the survey, characteristics of occurrences and chemical compositions about CRC were understood as follows.
The CRC can be divided into a younger non-phosphatized outer layer (Layer 1) and an older phosphatized inner layer (Layer 2) based on their mineral and chemical characteristics and microstratigraphy. Furthermore, these two layers can be divided into two sub-layers each based on the microstratigraphy in descending order as follows: Layer 1a, which is black, dense, and weakly laminated structure; Layer 1b, which is dark brown, porous, and columnar to mottled structure; Layer 2a, which is submetallic, black, dense, and columnar structure; and Layer 2b, which is submetallic, black, dense, and laminated structure. Basically, these 4 sublayers are widely correlated through the flat tops of the six guyots of similar water depth and age. However, the CRC also appears locally variable in thickness and the microstratigraphy, suggesting that foraminiferal sand deposition and instability of the basement rocks have affected the growth process of the CRC. Contrarily, ferromanganese nodules with small diameters are locally observed on the foraminiferal sand.
Elemental mapping by the X-ray fluorescence (XRF) for CRC indicates that Ca, P, Cu, and Ni are rich in inner layer and Si, Al, Fe, Ti, and Co are rich in outer layer, although Mn is almost thoroughly contained in both layers. Besides, Ti-Vrelation diagram by geochemical data of quantitative analyses indicates that basement basalt of CRC is classified into ocean island alkaline basalt, and the data show clearly different values between CRC and basement basalt.
