DIAGENESIS OF CORAL REEFS: AN IN-SITU GEOCHEMICAL STUDY OF CORAL REEFS AT THE YONGXING ISLAND, SOUTH CHINA SEA
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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。
Abstract: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, high-magnesium 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.
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Key words:
- Yongxing Island /
- coral reef /
- diagenesis /
- geochemical characteristics
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表 1 中国南海西沙群岛永兴岛珊瑚礁SSZK1井岩心样品的电子探针分析结果
Table 1. The results of core samples by electron probe analysis for well SSZK1 in the coral reef of Yongxing Island，Xisha Islands，South China Sea
/% 样品编号 MgO K2O FeO Al2O3 CaO MnO SiO2 P2O5 Cr2O3 Na2O TiO2 SrO CO2 Total S01-1 − 0.029 0.016 − 54.666 0.002 − 0.071 0.286 0.18 − 1.082 43.75 100.082 S01-2 0.065 0.051 − − 54.326 − 0.011 0.077 − 0.165 − 1.113 43.868 99.676 S01-3 0.784 0.027 0.032 0.036 52.708 0.042 0.057 0.016 − 0.141 0.027 0.154 44.537 98.561 S10-1 0.054 0.022 − − 54.845 − − 0.038 0.032 0.322 0.024 0.908 43.785 100.03 S10-3 0.148 − − 0.019 54.421 − − 0.005 0.024 0.287 − 0.865 43.907 99.676 S10-4 0.112 0.015 0.002 − 53.774 0.015 − 0.06 − 0.302 − 0.816 44.076 99.172 S10-5 0.026 0.101 − 0.033 54.345 − 0.059 0.054 0.037 0.386 0.006 0.186 44.142 99.375 S10-6 0.019 0.027 − 0.001 54.734 0.006 0.024 0.022 − 0.268 0.023 0.154 44.123 99.401 S13-1 0.024 0.034 0.014 − 54.524 − − 0.005 0.008 0.214 − 1.023 43.84 99.686 S13-2 0.04 0.029 − − 55.491 − 0.006 − 0.011 0.095 − 1.022 43.655 100.349 S18-1 0.354 0.114 − − 54.362 − 0.041 0.011 − 0.266 0.027 0.059 44.182 99.416 S18-2 − 0.078 0.016 0.028 50.447 − 0.050 0.022 0.048 0.228 − 0.839 44.228 96.625 S21-1 0.141 0.036 0.034 − 54.543 − − 0.038 0.019 0.365 − 0.845 43.848 99.869 S21-2 0.15 0.126 0.018 − 52.221 0.004 0.033 − 0.037 0.425 − 0.791 44.362 98.167 S21-3 0.121 0.041 0.004 0.004 52.825 − 0.008 0.016 − 0.383 − 0.798 44.275 98.475 S21-4 0.089 0.038 0.028 0.008 51.855 − − 0.088 0.013 0.281 − 0.773 44.547 97.720 S34-1 0.695 0.043 0.03 0.011 55.51 0.001 0.004 0.109 0.024 0.179 0.016 0.051 43.93 100.603 S34-2 0.686 − 0.028 0.022 54.862 0.001 0.011 − 0.179 0.021 0.023 0.172 44.049 100.054 S34-3 0.685 0.017 0.01 0.017 56.036 − 0.02 − 0.059 0.042 − 0.155 43.807 100.848 S39-1 1.104 0.099 0.042 − 53.02 0.05 0.015 0.022 − 0.095 − 0.219 44.385 99.051 S39-2 1.397 0.112 0.022 0.043 52.821 0.002 0.018 0.066 0.013 0.102 − 0.159 44.44 99.195 S39-3 0.129 0.057 − 0.007 54.605 0.012 − 0.033 − 0.249 − 0.775 43.894 99.761 S54-1 3.027 0.025 0.006 0.006 53.409 − 0.028 0.038 − 0.303 − 0.215 44.102 101.159 S54-2 1.625 0.024 − 0.003 51.468 − 0.011 0.022 − 0.046 − 0.256 44.758 98.213 注：表中编号相应位置具体见图6蓝色标记；−表示未检测出。 
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表 2 LA-ICP-MS测定SSZK1井岩心样品中不同组分的地球化学组成
Table 2. Geochemical composition of different components by LA-ICP-MS for core samples of well SSZK1
样品 Sr/Ca/
(mmol/mol)Mg/Ca/
(mmol/mol)U/Ca/
(μmol/mol)珊瑚 S10 10.24 2.82 1.61 S21 9.08 3.20 0.97 S39 1.83 45.91 0.23 绿藻 S01 12.47 0.61 1.04 S13 11.98 0.71 1.07 S18 7.82 5.70 1.06 S39 1.99 27.92 0.37 红藻 S01 0.76 31.88 0.44 S10 0.36 19.52 0.26 S13 0.56 22.80 0.30 有孔虫 S10 3.98 1.62 1.07 S18 7.72 1.83 1.31 S34 1.10 25.07 0.31 棘皮类 S54 5.05 20.04 0.63 方解石胶结物 S01 0.97 65.18 0.45 S13 1.14 24.00 0.59 S34 1.77 30.32 0.29 S39 5.04 35.37 0.38
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