THE ORIGIN AND GEOCHEMICAL CHARACTERISTICS OF FE-SI-MN OXYHYDROXIDES AT PACMANUS HYDROTHERMAL FIELD

Four Fe-Si-Mn oxyhydroxides dredged from the area between hydrothermal sites of Roger's Ruins and Roman Ruins at the PACMANUS hydrothermal field in the Eastern Manus Basin were analyzed for major, trace and rare earth elements. The low trace metal and REE contents, low Co/Zn ratio and high growth rate indicate a hydrothermal origin. Most of the samples show obvious positive Ce anomalies, of which the ranges (lg 3Ce/(2La + Nd),0.008~0.229) are lower than the range of typical hydrogenic oxyhydroxides (lg 3Ce/(2La + Nd),0.352~0.637). Therefore, the contribution of seawater is very little. The samples with positive Ce anomalies are mainly composed of Fe-oxyhydroxides. It means that the positive Ce anomalies were mainly controlled by the adsorption by Fe-oxyhydroxides and the diagenesis in the later period. The content of TiO₂, and Al/(Al +Fe + Mn) ratio were lower than the ratio of volcanic materials, which implies that the contribution of volcanic materials is little. The high Ba contents may be derived from the todorokite and barite scattering in the samples. The high Pb contents reflect that Pb was not incorporated into the sulfides formed in the early period. On the contrary, Pb was incorporated into Fe-Si-Mn oxyhydroxides along with the hydrothermal fluid.