Thick hydrogenetic ferromanganese (FeMn) crusts from the northwest and central Pacific seamounts often show a distinct dual structure composed of a typical hydrogenetic porous, friable upper part of FeMn oxides (Layer 1) and the underlying dense, hard phosphatized growth generation of FeMn oxides (Layer 2 in this study). Layer 2 always appears above the substrate rock and composes the lower part of the crust; it is never found as the upper crust layer in contact with seawater. The chemical composition of Layer 2 clearly differs from the younger Layer 1 hydrogenetic FeMn oxides, and is depleted in Fe, Al, Ti, and Co, and enriched in Ni, Cu, and Zn relative to Layer 1. The Be isotope age models of the crusts were refined with paleomagnetic and paleontological information, and applied to selected crust samples. The age model indicates fairly continuous growth from the substrate to the surface and fairly constant growth rates during the past 17Ma. The growth rate from the Miocene to the present has varied by a factor of two, about 2–4mm/Myr in Layer 1, while Layer 2 has similar but more variable growth rates than Layer 1. The calculated age for the base of Layer 1, and possibly the age of termination of phosphatization, is never younger than the late Miocene. The age seems to vary with water depth, shallower-water crusts (between 991 and 1575m) showing a younger age of about 10Ma whereas the deeper-water (2262m) crusts have extrapolated ages for the base of Layer 1 of be 17.1±2.5Ma. This trend indicates that phosphatization took place in a less-oxidizing environment during growth of Layer 2, followed by a weakened oxygen-minimum zone or intensified AABW during growth of Layer 1. Display omitted •Phosphatized older layer was found from hydrogenetic Fe–Mn crusts from more than 60 samples in the NW and central Pacific.•The ages of the base of non-phosphatized layer seem to vary from 17.1Ma to 7.8Ma accordingly with water depth.•The chemical composition of phosphatized layer is of characteristic nature of early-diagnenetic buserite.