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One of the objectives of Leg 55 was to investigate the Tertiary history of sedimentation and environment on the Emperor Seamounts after their volcanic activity. For the three first sites, 430, 431, and 432, drilled on Ojin, Nintoku, and Yömei Seamounts, the Neogene sedimentary deposits are not well represented and are not typical pelagic sediments. Except for two holes (430A and 432), where we found calcareous oozes, the sediments are heterogeneous sands, gravels, and pebbly mudstones with a wide range in grain size and composition. Two phenomena characterize these deposits: the inheritance of volcaniclastic material and its alteration, and the authigenesis of secondary minerals including silicates, phosphates, and ferromanganese oxides formed under volcanic influence in a marine environment. The three first sites are located in the central part of the Emperor Chain (Figure 1) at Ojin (37°59 'N, 170°35 'E), Nintoku (41°20'N, 170°22'E), and Yömei (42°25'N, 170°22 'E) seamounts. The six holes studied appear to be in a lagoonal depression, the top of a reef flat, and a faulted terrace, respectively (Greene et al., this volume). The lithologic composition of the cored formations described in the site reports is summarized in Figure 2, which also shows the positions of the samples studied. The ages of the deposits are established by Koizumi, Takayama, Ling, and Butt (this volume). Because of the heterogeneity of the sediments, the sampling was very discontinuous. The objectives of this study are to establish the nature of the different components from the sedimentary deposits and to examine their evolution and relationships.

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Mineralogy and Geochemistry of Sedimentary Deposits on Emperor Seamounts, Sites 430, 431, and 432: Authigenesis of Silicates, Phosphates, and Ferromanganese Oxides