A geochemical constraint on the formation process of a manganese carbonate nodule in the siliceous mudstone of the Jurassic accretionary complex in the Mino Belt, Japan

摘要

Manganese (Mn) carbonate nodules, which differ from seafloor Mn nodules mainly composed of MnO_2, are occasionally embedded in the form of a lens shape in the Jurassic accretionary complexes, such as the Mino Belt in Japan. The interpretation of the formation process of Mn carbonate is still controversial, particularly concerning whether the Mn carbonate was formed primarily or secondarily. In this study, a fresh Mn carbonate nodule incorporated into the red siliceous mudstone was collected for geochemical and sedimentological analysis. The optical observation of thin sections indicates that the Mn carbonate nodules are composed of abraded grains of rhodochrosite spherule with radiolarians and are sedimentary embedded in siliceous mudstone. Microfossil radiolarians from the Mn carbonate nodules and the host red siliceous mudstone are dated as the Bajocian, but the radiolarians in the nodules are somewhat older than those in the host red siliceous mudstone. Geochemical analysis using the X-ray absorption near-edge structure on Ce indicates the dominance of trivalent Ce at present, despite the observation of a positive Ce anomaly in the PAAS-normalized REE pattern of Mn carbonate. The REE adsorption experiment on synthesized MnCO_3 does not show any distinctive positive Ce anomaly, and a thermodynamic calculation suggests the possible coexistence of rhodochrosite and spontaneous oxidation of Ce. A leaching experiment that can selectively decompose the carbonate phase demonstrated no Ce anomaly in the carbonate phase of the Mn carbonate and a poor contribution to the bulk REE concentration. The carbon isotope data of Mn carbonate nodule implied the dominance of inorganic marine carbonate origin with small contribution of organic decomposition. The most plausible account of all of the observational and geochemical results is that the rhodochrosite grains were primarily formed on the depositional site and subsequently transferred to a different site, where siliceous mudstone was deposited.