Elemental geochemistry of surface sediments in Okinawa Trough and its implications for provenance and hydrothermal activity

Surface samples taken from both the northern Okinawa Trough (NOT) and the South Okinawa Trough (SOT) proximal to the hydrothermal fields of Tangyin and Yonaguni Knoll IV are analyzed for major, trace and rare earth elements. Results show that the element composition of surface sediments in the NOT is quite different from that in the SOT. Except for Ca, Sr, and Na, most samples from the NOT are low in trace elements and rare earth elements (REE), possibly owing to the deposition of calcareous components and volcanic materials with low contents of REE. The North American Shale Composition normalized REE have a relatively flat pattern, and characterized by light rare earth elements (LREE) enrichment, heavy rare earth elements (HREE) depletion, weak Ce and Eu anomaly. REE composition indicates that terrigenous materials of surface sediments in the NOT are mainly derived from the Yellow River and Taiwan rivers. In the SOT, however, the contents of Si, Fe, Ba, Cu, Pb, Zn and REE in the surface sediments are relatively high, and the fractionation between LREE and HREE is weak while the HREE contents increase. As influenced by hydrothermal activity, a certain amount of pyrite, pyrrhotite, barite and Fe-Mn oxides are mixed into the sediments which causes the increase in Fe and some trace elements. The REE patterns are similar to the hydrothermal fluid with a positive Eu anomaly. The Fe-Mn oxides have the capability to scavenge REE from hydrothermal fluids and surrounding seawater, which can promote the decrease in negative Ce anomaly and increase in ∑REE contents. In addition, there is a certain amount of excess Si in the sediments derived from hydrothermal activity. The REE patterns and (La/Sm)_N-(Gd/Yb)_N ratio indicate that terrigenous materials of surface sediments in the SOT are mainly derived from Taiwan.