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  

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

作　　者：吴佳庆 刘刚[3] 韩孝辉 赵彦彦 魏浩天 杨俊 WU Jiaqing;LIU Gang;HAN Xiaohui;ZHAO Yanyan;WEI Haotian;YANG Jun(Key Laboratory of Submarine Geosciences and Prospecting Technique,Ocean University of China,Ministry of Education,Qingdao 266100,China;College of Marine Geosciences,Ocean University of China,Qingdao 266100,China;Marine Geological Survey Institute of Hainan Province,Key Laboratory of Marine Geological Resources and Environment of Hainan Province,Haikou 570206,China;Laboratory for Marine Mineral Resources,Qingdao National Laboratory for Marine Science and Technology,Qingdao 266237,China)

机构地区：[1]中国海洋大学海底科学与探测技术教育部重点实验室,青岛266100 [2]中国海洋大学海洋地球科学学院,青岛266100 [3]海南省海洋地质资源与环境重点实验室,海南省海洋地质调查研究院,海口5702063 [4]青岛海洋科学与技术国家实验室海洋地质过程与环境功能实验室,青岛266235

出　　处：《海洋地质前沿》2021年第1期31-44,共14页Marine Geology Frontiers

基　　金：“海底透视”创新团队建设项目“南海全新世珊瑚礁高分辨率地球化学研究”(MGQNLM-TD201703);海南省自然科学基金“三沙市永兴岛东部海底滑坡类型分布和成因探讨”(418QN306);国家自然科学基金“华南新元古代盖帽白云岩沉积微相的镁硅同位素研究”(41873006)。

摘　　要：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。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.

关 键 词：永兴岛 珊瑚礁 成岩作用 地球化学特征 

分 类 号：P736.4[天文地球—海洋地质]