Abstract: A reddish-brown detrital material has been discovered,which is obviously younger than the Co-rich ferromanganese crust,from the seamount MP2 of the Line Islands.The material is fine in grain size,and occurs in the interstices between the columns of Fe-Mn oxides.Based on the observation of hand specimen and polished sections of Co-rich ferromanganese crusts from the Line Islands,it can be concluded that this reddish-brown detrital material can be used as a marker of a younger deposit in ferromanganese crusts.The reddish-brown detrital material and the Fe-Mn oxides around it were investigated geochemically by electron-probe microanalysis (EPMA).EPMA data suggest that the Co-rich ferromanganese crust is hydrogenetic in origin,and the chemical composition of the reddish-brown detrital material is characterized by Fe,Si and Al,and a lower Mn in comparison with the Fe-Mn oxides around it.Base on the geochemical characteristics and morphology of the reddish-brown detrital material,and with reference to the high primary productivity in the area around the seamount MP2,it is supposed that the reddish-brown detrital material is most probably a kind of authigenic Fe-Si-Al colloidal precipitation from seawater younger than the Co-rich ferromanganese crusts.