In situ geochemistry of Lower Paleozoic dolomites in the northwestern Tarim basin: Implications for the nature, origin, and evolution of diagenetic fluids
In situ geochemistry of Lower Paleozoic dolomites in the northwestern Tarim basin: Implications for the nature, origin, and evolution of diagenetic fluids
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DOI：
10.1002/2013gc005194
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发表时间：
2014-07
期刊：
影响因子：
3.7
通讯作者：
Wei Zhang;Ping Guan;X. Jian;Fan Feng;C. Zou
中科院分区：
文献类型：
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作者：
Wei Zhang;Ping Guan;X. Jian;Fan Feng;C. Zou
塔里木盆地西北部下古生界沉积岩受复杂的地球流体强烈蚀变，形成各种成岩矿物(如白云岩)。本文通过对下奥陶统成岩白云岩颗粒进行原位主量、微量、稀土元素地球化学和岩相学研究，揭示了成岩流体的地球化学特征、性质和成因。结果表明，在单个样品中，甚至在单个白云石颗粒中，可以检测到不同的地球化学信息。根据4种不同类型成岩流体的地球化学特征，已识别出5代成岩白云岩：(1)HREE富集型(PaAS标准化)、低Σ稀土元素、无Eu异常、低Mn、Ba、中等Fe、高锶含量可能与早期埋藏的白云石化流体有关，(2)Σ富集型、高REE、高Mn、Fe、低Sr可能与泥盆纪深循环地壳热液有关，(3)Eu异常明显的平坦或轻稀土富集型与二叠纪岩浆热液有关；(4)平坦的稀土配分模式，中等的Σ稀土元素，无Eu异常，低的Mn、Ba、中等的Fe和高的锶含量，可能与晚埋藏的白云石化流体有关。与全岩分析相比，原位分析的意义不仅包括无污染分析，还包括揭示单个样品或矿物颗粒内部的地球化学变化。因此，对于复杂成岩过程的地球化学研究，应优先考虑原位方法。
Lower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situ major, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS‐normalized), low ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high ΣREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep‐circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situ method demonstrated in this study, compared with the whole rock analysis, include not only contamination‐free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situ method should be preferentially considered.