Formation age of the dual structure and environmental change recorded in hydrogenetic ferromanganese crusts from Northwest and Central Pacific seamounts |
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| Institution: | 1. Department of Geology, Graduate School of Integrated Arts and Sciences, Kochi University, Japan;2. Department of Geology, Kochi University, Japan;3. Sumiko Resources Exploration and Development, Co., Japan;4. Center for Advanced Marine Core Research, Kochi University, Japan;1. Ifremer – Unité de Recherche Géosciences Marines, F-29280 Plouzané, France;2. CNRS, Ifremer c/Brest, F-29280 Plouzane, France;3. Institut Universitaire Européen de la Mer, UMS 3113, Plouzané, France;1. Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan;2. Geological Sciences, The University of Texas at El Paso, 500 West University Ave., El Paso, TX 79968, USA;3. Faculty of Science, Kochi University, 2-5-1 Akebono-cho, Kochi 780-8520, Japan;1. Saint-Petersburg State University, St. Petersburg 199034, Russia;2. Institute for Geology and Mineral Resources of the Ocean (VNIIOkeangeologia), St. Petersburg 190121, Russia;3. U.S. Geological Survey, Santa Cruz, CA 95060, USA;4. The Laboratory HERCULES, University of Evora, Évora 7000-809, Portugal;5. Estrutura de Missão para a Extensão da Plataforma Continental, Paçod''Arcos 2770-047, Portugal;6. Universidade de Évora, Instituto de Ciências da Terra, Dep. de Geociências, Évora 7000-671, Portugal;1. Ifremer, Geochemistry and Metallogeny Laboratory, Plouzané, France;2. HydrISE, Institut Polytechnique LaSalle Beauvais, 60026 Beauvais cedex, France;3. Institut Universitaire Européen de la Mer, UMS 3113, Plouzané, France;4. University of Southampton, National Oceanography Centre, European Way, Southampton S014 3ZH, UK |
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| Abstract: | Thick hydrogenetic ferromanganese (Fe Mn) 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 17 Ma. The growth rate from the Miocene to the present has varied by a factor of two, about 2–4 mm/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 1575 m) showing a younger age of about 10 Ma whereas the deeper-water (2262 m) crusts have extrapolated ages for the base of Layer 1 of be 17.1 ± 2.5 Ma. 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. |
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