Clumped isotope geochemistry of island carbonates in the South China Sea: Implications for early diagenesis and dolomitization
Clumped isotope geochemistry of island carbonates in the South China Sea: Implications for early diagenesis and dolomitization
复制标题
南海岛礁碳酸盐岩的团簇同位素地球化学：对早期成岩作用和白云石化作用的启示
DOI：
10.1016/j.margeo.2021.106513
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发表时间：
2021-07
期刊：
影响因子：
2.9
通讯作者：
Yangrui Guo;W. Deng;Xi Liu;K. Kong;Wenjie Yan;G. Wei
中科院分区：
文献类型：
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作者：
Yangrui Guo;W. Deng;Xi Liu;K. Kong;Wenjie Yan;G. Wei
研究岛屿碳酸盐岩的成岩作用可能会受到难以精确限制矿物形成温度的阻碍。碳酸盐聚集同位素组成（以Δ 47值测量）是基于13 C与18 O的温度依赖性结合，并提供了一种有前途的计算地层温度的方法。本文报道了南海美济礁岛状碳酸盐岩的Δ 47值，并结合岩相学、碳（δ 13 C）、氧（δ 18 O）同位素组成和微量元素含量对成岩环境进行了分析。我们发现，大多数的原始碳酸盐岩的深成蚀变可能发生在水深约200米以上的海平面以下，温度在15和28 °C之间，和流体在成岩方解石和方解石的形成过程中，有类似的δ 18 O值现代海水。考虑到海水δ18O的估计长期变化，白云岩可能是在晚中新世通过盐水回流机制由蒸发流体源形成的，并具有高对流速率。尽管礁碳酸盐岩的同位素组成被早期成岩蚀变所覆盖，但如果成岩作用发生在与沉积深度相似的深度，Δ 47值可以反映其沉积环境的温度。这些结果有助于完善现有的热带岛屿岩化模型，并提高浅埋环境下成岩礁碳酸盐岩Δ 47值作为浅水和气候条件有价值记录的实用性。
Studying the diagenesis of island carbonates can be hampered by the difficulty of accurately constraining the mineral formation temperature. The carbonate clumped isotope composition (measured as the Δ47value) is based on the temperature-dependent binding of13C to18O, and offers a promising means of calculating the formation temperature. In this study, we report the Δ47values of island carbonates from a ~1 km-long drill core from the Meiji Reef in the South China Sea, which are integrated with petrography, carbon (δ13C) and oxygen isotope (δ18O) compositions, and trace element contents to constrain the diagenetic environment. We find that most of the digenetic alteration of the initial carbonates likely occurred above a water depth of ~200 m below sea level at temperatures between ~15 and 28 °C, and that the fluids involved during the formation of the diagenetic calcites and dolomites have similar δ18O values to modern seawater. Considering the estimated secular variation in seawater δ18O, the dolomite likely formed from an evaporative fluid source via a brine reflux mechanism and with a high convection rate during the late Miocene. Although isotopic compositions of the reef carbonates were overprinted by early diagenetic alteration, the Δ47values can reflect the temperature of their depositional environment if the diagenesis occurred at depths similar to the depositional depth. These results can help refine the existing dolomitization model of tropical islands and increase the utility of Δ47values of diagenetic reef carbonates in shallow burial settings as valuable records of the shallow water and climate conditions.