珊瑚礁对热液流体的地球化学记录——来自南海西沙永兴岛珊瑚礁稀土元素的证据

Geochemical records of hydrothermal fluids in corals:Evidence of rare earth elements from coral reefs in the Yongxing Island,Xisha,South China Sea

碳酸盐岩中稀土元素的含量、配分模式及元素异常记录了周围沉积水体的特征,能够很好地指示古海洋及沉积环境.珊瑚具有的高分辨率和稀土元素的高稳定性的特点,能够忠实地记录周围海水的地球化学特征.本文以南海西沙宣德环礁永兴岛142~84 ka发育的珊瑚礁为研究对象,通过主微量元素含量,尤其是稀土元素含量及其配分图解,判断珊瑚礁形成时周围水体的特征.结果表明自142 ka以来,永兴岛大部分珊瑚礁具有正常海相碳酸盐岩的稀土配分特征,表现为LREE亏损,Ce负异常及高的Y/Ho比值,表明周围水体属于开阔的浅海,但是位于23 m处(年龄为114 ka)的滨珊瑚骨骼格架除了有正常海相碳酸盐岩的特征外,还具有明显的Eu正异常,这表明其形成时有热液流体的加入.经过模型计算,认为在滨珊瑚骨骼格架的生长阶段,至少有0.1%的热液加入周围的海水中.通过资料查询和年龄对比,认为这些热液可能与高尖石岛或海南岛火山活动有关.

The contents,distribution pattern and elemental anomalies of rare earth elements in carbonates are the records of surrounding water.Corals are characterized by high resolution and high stability of rare earth elements and may faithfully record the geochemical characteristics of the surrounding seawater.In this paper,we analyzed the coral reefs from 142 to 84 ka collected from the Yongxing Island of the Xuande Atoll of Xisha Islands,South China Sea.Trace element contents,especially the rare earth element contents and their distribution patterns are used in this paper to determine the characteristics of the sea water,in which the coral reefs grew.Results show that,since 142 ka,most of the coral reefs in the Yongxing Island has a normal rare earth element distribution pattern of marine carbonates,characterized by LREE depletion,negative Ce anomalies and high Y/Ho ratios,indicating an environment of open shallow sea.In contrast,the coral skeletons in depth of 23 m 114 ka have similar LREE depletion,negative Ce anomalies and high Y/Ho ratios,but positive Eu anomalies.This suggests that certain amount of hydrothermal fluid has been input during the growth of corals.Based on the model calculations,it is inferred that at least 0.1%of hydrothermal fluid has been added to the open seawater during that time.The hydrothermal fluids may be related to the volcanic activities observed at Gaojianshi island or Hainan island.