Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
Volume 69 , Issue 5
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  • Atsuyuki Ohta
    2018 Volume 69 Issue 5 Pages 233-263
    Published: November 30, 2018
    Released: December 12, 2018

    This paper describes high-density geochemical mapping of isolated islands in southwest Japan. A total of 193 stream sediments and three volcanic ash deposits collected from isolated islands around Kyushu were analyzed to determine the content of 53 elements to supplement land and sea geochemical mapping of the Kyushu area and regional Sr isotope mapping. The relationship between the spatial distribution of elements in stream sediments and volcanic ash deposits and the geology was closely examined using geographical information system (GIS) software. Stream sediments derived from mafic volcanic and pyroclastic rocks and volcanic ash deposits were enriched with MgO, CaO, Sc, TiO2, V, T-Fe2O3, Co, and Sr. The presence of alkaline mafic volcanic rock increased the concentration of Cr, Ni, Nb, La, Ce, Pr, Nd, and Ta in stream sediments. Stream sediments originating from granitic rock were abundant in Be, Na2O, K2O, CaO, Sr, Y, Sn, Ln, Th, and U. Accretionary and non-accretionary sedimentary rocks caused an increase in Nb and Ta concentrations in stream sediments, and a reduction in Na2O, MgO, CaO, and Sr concentrations. These geochemical features could be explained by the relative abundance of major rock forming minerals (such as quartz, plagioclase, K-feldspar, and mafic minerals) and accessary minerals (such as apatite and monazite) derived from the host rocks. Also, Zn-Pb deposits increased Zn, Cd, and Pb concentrations in stream sediments on Tsushima Island, and Sb deposits enhanced the Sb concentration in stream sediments on Amakusa-Shimoshima Island.

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