Secondary minerals of basalt substrate associated with ferromanganese crusts from the Magellan seamount cluster, Northwest Pacific ocean

Abstract

The R/V Isabu expedition CR1302 by the Korea Institute of Ocean Science and Technology (KIOST) recovered ferromanganese crust sample DGCR1314 U01 using dredge from OSM7 (152° 00´E, 11° 42´ N) in the Magellan seamount cluster, Northwest pacific ocean. The seamounts of Magellan seamount cluster formed as submarine shields volcanoes during the Cretaceous. After guyot formation between 59 and 45 Ma, proto manganese crusts had been formed in Paleocene-Eocene boundary. Before being covered with manganese oxides, slopes of the seamounts would be undergone low temperature basalt alteration by ambient seawater. Mineralogy of secondary minerals reflects formation environments. Thus, paleoceanographic environments could be reconstructed by mineralogical and geochemical analyses of basalt substrate of ferromanganese crusts. For identifying the secondary minerals which are products of low temperature alteration, X-Ray Diffraction (XRD) analysis, Scanning Electron Microscope-Energy dispersive spectrometer (SEM-EDS) and Transmission Electron Microscope (TEM)-EDS were performed. Vernadite was observed which is major mineralogy of hydrogenous ferromanganese crust at the interface between basalt substrate and manganese oxide layer. Alteration rims, enriched K, Mg and Fe, were observed at the basalt substrate. Iron-rich smectite appears to major mineralogy of secondary minerals. The formation of iron-rich smectite is known that implications of basalt weathering by basalt-seawater interactions without vigorous fluid circulation.