研究業績

We performed X-ray adsorption fine structure (XAFS) analyses of Y, Ce, Fe, and P for rare earth elements and yttrium-rich muds (REY-rich muds) from the two regions of the Pacific Ocean: the eastern South Pacific and the central North Pacific. Yttrium K-edge XAFS revealed that calcium phosphate (apatite) is the common host of REY even in the hydrothermal sediments near the East Pacific Rise (EPR), as it is in REY-rich muds in the other regions including the western North Pacific and the Indian Ocean. In contrast, Ce L3-edge XANES revealed a systematic difference in the major oxidation states of Ce between the central North Pacific, where Ce(III) is dominant (60?77%), and the eastern South Pacific, where Ce(IV) is dominant (55?83%). The dominance of Ce(IV) indicates a strong hydrothermal influence on REY behavior in the eastern South Pacific. Based on inter-elemental relationships, it has long been considered that hydrothermal Fe precipitates control the REY concentration in the sediments of the eastern South Pacific. However, on the basis of these spectroscopic findings, we propose that the initial uptake of REY by rapid precipitation of hydrothermal Fe-Mn (oxyhydr)oxides is followed by eventual fixation of REY to apatite via REY redistribution in the oxic sediments. In contrast, REY accumulation in the central North Pacific can be explained by primary seawater-mineral interactions with apatite and small contributions of Mn oxides. Phosphorous and Fe K-edge XANES for the eastern South Pacific sediments both show transitions in their major species with distance from the EPR, from P adsorbed on goethite to apatite and goethite to montmorillonite, respectively. In particular, apatite appears coincident with the occurrence of the REY-rich muds, even though the Fe species still suggests intensive deposition of hydrothermal goethite. The correlation between the mass accumulation rates (MAR) of Fe and P together with these results implies the existence of two types of the apatite: biogenic apatite (i.e., fish debris) and authigenic one derived from P species adsorbed on hydrothermal Fe-Mn (oxyhydr)oxides. This result implies that authigenic apatite forms uniquely in the geological setting around the EPR, and also that biogenic apatite is responsible for the widespread formation of the REY-rich muds. We found that apatite is of specific importance as the eventual host of REY within the REY-rich muds, but various geochemical processes account for the origin of apatite and its accumulation of REY.