珊瑚礁的成岩作用:来自南海永兴岛珊瑚礁的原位地球化学研究

珊瑚礁的地球化学特征记录了其形成时周围海水的状况,能够反映古海洋、古气候和古环境变化;然而珊瑚礁形成过程中及其形成后,容易受到成岩作用的影响,导致其矿物组成和地球化学特征发生变化;因此,在对珊瑚礁的研究中,首先要识别出保存原始沉积特征的组分,并排除后期成岩改造的影响。以西沙群岛永兴岛的SSZK1珊瑚礁钻孔岩心为研究对象,通过矿物学、岩相学和地球化学相结合的研究方法,对不同层位的生物化石、碳酸盐胶结物进行原位地球化学分析,探索成岩作用对不同形成阶段矿物的改造。SSZK1井岩心的岩石类型主要为骨架灰岩和生物碎屑灰岩两大类;岩心礁相碳酸盐岩沉积后主要受控于早期大气成岩作用,成岩层段揭示的主要成岩作用类型有胶结作用、新生变形作用和溶解作用。电子探针和LA-ICP-MS的原位分析结果表明,不同阶段的珊瑚礁碳酸盐岩的矿物成分较为单一,主要是由方解石组成,仅在局部的生物化石中保存了原始形成的文石。后期形成的碳酸盐胶结物(低Sr/Ca、低Sr、高Mg/Ca)和原始的生物化石(高Sr/Ca、高Sr、低Mg/Ca)具有明显不同的地球化学特征,表明不同阶段的碳酸盐矿物受不同来源流体的制约。

DIAGENESIS OF CORAL REEFS:AN IN-SITU GEOCHEMICAL STUDY OF CORAL REEFS AT THE YONGXING ISLAND,SOUTH CHINA SEA

The geochemistry of coral reefs provides evidence for surrounding seawater conditions and may reflect the paleoceanographic,paleoclimatic and paleoenvironmental variations at the time of deposition.However,coral reefs are apt to be altered by diagenesis during and after their formation,which can lead to substantial changes in mineralogy,geochemistry and biological characteristics.In such a circumstance,the geochemistry of coral reefs is not able to reflect the characteristics of the surrounding water.Therefore,the influences of diagenesis must be excluded before the geochemical indicators are applied to coral reef research if the surrounding water conditions are studied.Our study this time focuses on the cores from the well SSZK1 drilled at the Yongxing Island,Xisha Islands.Microfacies with different fossils and carbonate cements are studied in-situ based on mineralogy,petrography and geochemistry of the reef so as to explore the history of diagenesis.Observation of hand specimens and thin sections suggests that,the carbonate rock types from the well SSZK1 core are mainly composed of branching coral skeleton,bioclast and others fossils,such as calcareous algae,gastropod and foraminifera,dominated by aragonite,highmagnesium calcite and low-magnesium calcite.Early atmospheric diagenesis,which includes cementation,neomorphism and dissolution,play critical roles in determination of the composition of reef rocks.The in-situ analysis with electronic probe and LA-ICP-MS suggests that the diagenetic minerals in different diagenetic stages are extensively dominated by low-magnesium calcite.Diagenetic aragonite is only observed in some local fossils.The primary fossils are characterized by high Sr/Ca and Sr,with low Mg/Ca,while the diagenetic cements characterized by low Sr/Ca and Sr,with high Mg/Ca.It indicates that the coral reefs have been reformed by the diagenetic fluids of different origin,which controlled the mineralogy and geochemical characteristics of the coral reef in different stages.