Synchrotron X-ray spectroscopic perspective on the formation mechanism of REY-rich muds in the Pacific Ocean

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.