TERRAPUB Geochemical Journal
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Geochemical Journal, Vol. 50 (No. 6), pp. 591-603, 2016
doi:10.2343/geochemj.2.0435

Geological factors responsible for REY-rich mud in the western North Pacific Ocean: Implications from mineralogy and grain size distributions

Junichiro Ohta,1 Kazutaka Yasukawa,2,3 Shiki Machida,4 Koichiro Fujinaga,3 Kentaro Nakamura,2 Yutaro Takaya,5 Koichi Iijima,4 Katsuhiko Suzuki4 and Yasuhiro Kato2,3,4,6*

1Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan
2Department of Systems Innovation, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
3Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
4Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan
5Department of Resources and Environmental Engineering, School of Creative Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjyuku-ku, Tokyo 169-8555, Japan
6Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

(Received March 26, 2015; Accepted July 15, 2016)

Abstract: Seven piston cores were collected from the seafloor ~250 km south of Minamitorishima Island in the western North Pacific Ocean during the cruise KR13-02 of R/V Kairei; in some portions of cores PC04 and PC05, the total contents of rare-earth elements and yttrium (ΣREY) exceeded 4,000 ppm. Microscopic observations showed that the highly REY-enriched layers (ΣREY > ~2,000 ppm) contained significant amounts of calcium phosphate and phillipsite. We conducted microscopic observations and grain size distribution analyses of bulk sediments and distinctive components (calcium phosphate and phillipsite) from cores PC04 and PC05 to elucidate the mechanism of the anomalous REY enrichment. The shapes of the calcium phosphate grains suggest that they were mostly biogenic in origin. The grain size distributions of bulk sediments from the REY-enriched layers of cores PC04 (7.88.6 mbsf) and PC05 (2.63.6 mbsf) were bimodal, with peaks at fine (~4 μ) and coarse (~4080 μ) sizes. Calcium phosphate and phillipsite were the major components of the coarse-grained portions of these REY-enriched layers. The bulk ΣREY content was mainly controlled by the amount of biogenic calcium phosphate, which is well known to concentrate REY. Thus, increased accumulation of biogenic calcium phosphate was responsible for the REY enrichment. The volume-based cumulative median diameters of calcium phosphate and phillipsite grains appeared to be proportional to the contents of both calcium phosphate and ΣREY. Increased phillipsite grain size suggests a low sedimentation rate, which may have allowed biogenic calcium phosphate to accumulate without dilution by low-ΣREY components. In addition, increased grain sizes of calcium phosphate and phillipsite suggest that sorting by a bottom current allowed coarse-grained biogenic calcium phosphate to become concentrated in sediments by removing fine-grained particles, including low-ΣREY components. Multiple factors should be considered to explain the overall features of the highly REY-enriched layers.
Key words: REY-rich mud, biogenic calcium phosphate, phillipsite, pelagic sediment, grain size distribution


*Corresponding author E-mail: ykato@sys.t.u-tokyo.ac.jp


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