• Korean Journal of Mineralogy and Petrology
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• v.37 no.2
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• pp.59-66
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• 2024

This study aimed to understand the mineral characteristics of the sediment from the summit of Magellan Seamount KC-7. In December 2023, approximately 20 years after the Korea Institute of Ocean Science and Technology conducted an initial field investigation of the sediment from the summit of seamount KC-7 in 2004, were obtained and subjected to mineralogical analysis to determine the essential sedimentary environmental characteristics of the seamount summit. For this purpose, smear slide analysis through a polarized microscope and X-ray diffraction analysis were conducted. The total length of the obtained sediments was 672.7 cm, and they were generally composed of calcareous ooze. In the lower part of the sediments (283.7-672.7 cm), Discoaster groups and coccoliths were predominantly observed. However, in the upper part above 283.7 cm, both coccoliths and foraminifera began to appear together, and in the uppermost sediments (0-151.7 cm), foraminifera became dominant, and the grain size of the sediments increased. This reflects a shift in the marine environment from high temperature and eutrophic conditions to low temperature and oligotrophic conditions as the seamount moved to its current position, aligning with the present oligotrophic environment of the western Pacific. The sediments comprised clay minerals, quartz, feldspar, and calcite. Examining the content changes of the four major mineral groups with depth, calcite was the most predominant, averaging 89.8 wt%. However, towards the upper layers, there was a trend of increasing clay content (up to 12.1 wt%) and decreasing calcite content (down to 85.1 wt%). This indicates that while the sedimentation mechanism remained unchanged, there has been a gradual influx of aeolian sediments.

• Korean Journal of Mineralogy and Petrology
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• v.37 no.2
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• pp.35-46
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• 2024

This study investigated the origin and mineralogical-geochemical characteristics of a small ferromanganese (Fe-Mn) nodule sample obtained from the abyssal seafloor of the Magellan Seamount cluster. To this end, X-ray diffraction and X-ray fluorescence analyses were conducted. The dark brown Fe-Mn oxides constituting the nodule had a homogeneous texture without distinguishable layers, forming around three distinct nuclei. The oxides had a low average Mn/Fe ratio of 0.73 (0.24-1.10) and were characterized by high Co content (0.41-0.85 wt.%, average = 0.58 wt.%) as well as low Ni (0.06-1.24 wt.%, average = 0.55 wt.%) and Cu (0.27-1.02 wt.%, average = 0.59 wt.%) concentrations. The maximum age of the nodule was estimated at 0.52 Ma, suggesting that it began forming during the transition from the glacial to the interglacial periods in the middle Pleistocene. The Fe-Mn oxide layer comprised vernadite, smectite, quartz, and feldspar, while the nuclei were composed of soft sediments. The presence of vernadite, a typical hydrogenetic Fe-Mn oxide mineral, along with the low Mn/Fe ratio, high Co content, and low Ni and Cu concentrations, indicates that Fe-Mn nodules on the abyssal seafloor of the Magellan Seamount cluster in the western Pacific Ocean formed through hydrogenetic processes.

• Economic and Environmental Geology
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• v.56 no.1
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• pp.55-63
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• 2023

Variations in geochemical and mineralogical properties of the ferromanganese(Fe-Mn) crust reflect environmental changes. In the present study, geochemical and mineralogical analyses, including micro X-ray fluorescence and X-ray diffraction, were utilized to reconstruct the paleo-ocean environment of western Pacific Magellan seamount cluster. Samples of the Fe-Mn crust were collected using an epibenthic sledge from the open seamount XX (151° 51.12' 7.2" E and 16° 8.16' 9.6" N, 1557 meters below sea level) in the Western Pacific Magellan Seamount. According to the structure and phosphating status, the Fe-Mn crust of the OSM-XX can be divided into the following: phosphatizated (L4-L5), massive non-phosphatizated (L3), and porous non-phosphatizated (L1-L2) portions. All ferromanganese layers contain vernadite, and owing to the presence of carbonate fluorapatite (CFA), the phosphatizated portion (L4-L5) is rich in Ca and P. The massive non-phosphatizated section (L3) contains high Mn, Ni, and Co, whereas the porous non-phosphatizated portion (L1-L2), which comprises detrital quartz and feldspar, is rich in Fe. Variations in properties of the Fe-Mn crust from the OSM-XX reflect changes in the nearby marine environment. The formation of this crust started at approximately 51.87 Ma, and precipitation of the CFA during the global phosphatization event that occurred at approximately 36-32 Ma highlights an elevated sea level and low temperature during the associated period. The high Mn, Ni, and Co concentrations and elevated Mn/Fe ratios of samples from the massive phosphatizated portion indicate that the oxygen minimum zone (OMZ) was enhanced, and reducing conditions prevailed during the crust formation. The high Fe and low Mn/Fe ratios in the porous portion indicate a weak OMZ and dominantly oxidizing conditions. These data reflect environmental changes following the end of the Mi-1 glacial period in the Miocene-Oligocene boundary. Subsequently, Mn/Fe and Co/Mn ratios increased slightly in the outermost part of Fe-Mn crust because of the enhanced bottom current and OMZ associated with the continued cooling from approximately 9 Ma. However, the reduced carbonate dissolution rate in the Pacific Ocean from approximately 6 Ma decreased the growth rate of the Fe-Mn crust.