冲绳海槽表层沉积物元素地球化学及其对物源和热液活动的指示

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

对采自冲绳海槽北部和南部唐印、第四与那国热液区附近的表层沉积物进行了主、微量和稀土元素分析。结果表明冲绳海槽北部和南部表层沉积物元素组成具有很大差异。北部沉积物中Ca、Sr、Na元素含量较高,大部分微量和总稀土元素含量较南部沉积物低。北部沉积物中有钙质生物组分及火山物质的加入,由于这些物质稀土含量较低,对沉积物中的稀土元素造成了稀释作用。北美页岩标准化稀土元素配分模式整体较平坦,轻稀土略富集,重稀土相对亏损,弱Ce和Eu异常,表明冲绳海槽北部表层沉积物陆源物质主要来自黄河和台湾河流输入的部分物质。冲绳海槽南部表层沉积物Si、Fe、Ba、Cu、Pb、Zn和稀土元素含量较高,轻重稀土分馏减弱,重稀土含量升高。由于受热液活动影响,一定量的黄铁矿、磁黄铁矿、重晶石及热液Fe-Mn氧化物的加入,使得南部沉积物的Fe以及部分微量元素含量升高,并表现出类似热液流体正Eu异常的稀土特征。这些Fe-Mn氧化物不仅从热液流体中清扫稀土元素,而且可以从海水中清扫稀土元素,使得沉积物负Ce异常减弱,总稀土含量升高。此外,样品中有一定量的过剩Si,来自热液活动。稀土元素配分模式及(La/Sm)N-(Gd/Yb)N比值表明冲绳海槽南部沉积物主要陆源物质来自台湾。

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 havethe 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）Nratio indicate that terrigenous materials of surface sediments in the SOT are mainly derived from Taiwan.