西沙石岛西科1井生物礁碳酸盐岩地球化学特征

Geochemical Characteristics of Reef Carbonate Rocks in well Xike-1 of Shidao Island,Xisha Area

对西科1井生物礁碳酸盐岩样品进行了地球化学(常量组分、微量及稀土元素含量等)分析,旨在探讨生物礁的形成演化以及所记录的环境变化信息.结果表明:西科1井岩心在井深36m附近存在地球化学参数的显著变化,暗示该深度是一处重要的地层界面或环境突变界面.岩心中的常量组分可以分为3类组合:以CaO为主,辅以K2O的原生碳酸岩组分,该组分对应的主要造岩矿物是方解石,代表了未经白云岩化的生物礁原生碳酸盐沉积;以MgO为主,辅以Na2O和P2O5,代表了白云岩化作用中的富集组分,反映了相对封闭的泻湖环境;以SiO2为主,加以Al2O3、TiO2、Fe2O3和MnO,代表造礁生物对这些组分的富集作用,但不能排除火山组分少量混入的可能性.相对于全球第四纪碳酸盐岩、上陆壳及页岩的平均值,岩心碳酸盐岩中大部分微量元素和稀土元素含量都较低.岩心中氧化还原敏感性微量元素(RSE)含量较低,指示岛礁发育过程中大部分时期处于氧化环境条件下,而RSE含量在岩心中的大幅波动体现出氧化/还原环境的交替变化.微量元素含量、∑REE、∑LREE、∑HREE、LREE/HREE、δCe、δEu等地球化学指标在岩心中的分布与岩心样品的矿物组成无明显的相关性,说明成岩作用和白云岩化作用并没有造成微量和稀土元素含量及特征指标的明显变化.

Reef carbonate rocks of well Xike-1 are analyzed for major constituents, trace and rare earth elements in this study, aimed to explore the formation, evolution and environmental change recorded in reef carbonates. The results show that there is an important interface of formation or environmental mutation at depth of 36 m, where geochemical parameters change signifi- cantly. The major constituents can be divided into following three components： the lithogenic carbonate components, consisting of CaO and K2O, corresponding to calcites; the enriched components by dolomitization, including MgO, Na2O and P2O5, which indicates restricted lagoon environment; the enriched components by reef-builders, composed of SiO2, Al2O3, TiO2, Fe2O3 and MnO, potentially including a small amount of volcanic components. Contents of most trace and rare earth elements in well Xike-1 are low compared to those of other endmembers such as carbonate rocks of Quaternary around the world, upper crustal, and shales. The contents of redox-sensitive trace elements （RSE） in well Xike-1 are low, indicating oxic environment through most of development process of reefs. There are alternations of oxidation and reduction environment, with wide fluctu- ations of RSE contents. In addition, there is no obvious correlation between mineral compositions and geochemical parameter distribution, such as trace elements, REEs and their characteristic parameters, indicating that dolomitization and diagenesis did not result in the significant changes of trace elements, REEs, and their eigenvalues.