Abstract: 17 Co-rich ferromanganese crusts from 9 sites at the Seamount M in the northwest of Magellan Seamounts were investigated geochemically with inductively coupled plasma atomic emission spectroscopy (ICP-AES) and weight method. Geochemical composition of those samples suggests that those Co-rich Fe-Mn crusts are hydrogenetic in origin and most of them were not phosphatized during formation. This paper extends the usage of Co chronology to bulk samples. The ages dated by the Co chronology suggest that those Co-rich Fe-Mn crusts mainly formed since Miocene, and one of them were phosphatized around 15 Ma when the oxygen minimum zone (OMZ), where the Co-rich Fe-Mn crust formed, immerged to the present water depth of 2 000~2 700 m.