澳大利亚西部太古代科马提岩和我国鞍山TTG岩石146Sm-142Nd和147Sm-143Nd同位素综合研究

Deciphering the primordial differentiation and evolution history of the early Earth is one of the most significant challenges in the Earth Sciences and a key to understanding the present day interactions between different Earth's layers in our dynamic Earth System, as well as providing a significant benchmark for understanding planet formation. The Archean Eon (2.5 Ga to nearly 4.0 Ga before the present) was a time that preserved the direct evidence related to the differentiation of early silicate Earth. The Archean komatiites and associated rocks are an ideal target to address issue of the Earth's early differentiation and evolution. Komatiite and komatiitic basalts were produced by high-degree melting of Earth mantle and are confined almost exclusively to the Archean. They directly record the thermal state and chemical structure of deep Earth. Early-Middle Archean TTG also preserved the direct information related to early mantle-crust differentiation, continental crust growth and differentiation, and formation of continental cratonic mantle. Thus, deciphering the primordial differentiation and evolution history of the early Earth is required integrating komatiite and TTG records. The main goal of this project is to decipher the primordial differentiation of Earth mantle its effect on shaping the Earth's chemical architecture through reconstructing the trace elemental, isotopic, and thermal secular evolution of Archean mantle by integrated study of short-lived (146Sm-142Nd) and long-lived (147Sm-143Nd) isotopes with chemical analysis. The work will develop a new conceptual understanding of the timing, processes, and products of primordial differentiation of Earth mantle, creating breakthroughs in understanding interaction between geodynamical processes and chemical variations, and thus what controls the resulting observed geochemical signatures.

研究地球早期分异和演化历史，是解读现代地球不同圈层相互作用和过程机制以及行星形成和演化的基础，是目前前沿地球科学中最具有挑战性的课题。太古代科马提岩和科马提质玄武岩是地幔大比例部分熔融的产物，直接记录了其形成时期的地幔热状态和化学结构。早太古代-中太古代(Ca.3.8-2.7 Ga)TTG岩石记录了早期壳-幔分异、大陆地壳形成和分异、古老克拉通岩石圈地幔的形成等重要地质事件。因此，太古代科马提岩和相关TTG岩石是地球早期分异和演化的关键研究对象。本次研究试图通过短半衰期146Sm-142Nd和长半衰期147Sm-143Nd同位素分析，结合常规岩石地球化学手段重建太古代地幔元素、同位素和热状态随时间的演化规律。本次工作将推动地球早期分异和演化的研究，加深对地球动力学过程和地球化学不均一性之间关系的理解，从而回答何种因素导致了目前观察到的不同储库的独特地球化学特征。

146Sm-142Nd灭绝核素系统对于早期地球演化具有重要应用价值，然而该体系对分析技术要求苛刻，特别是样品制备必须实现Ce和Nd的完全分离。本项目针对国际同行广泛采用的液-液微萃+3步离子交换分离技术，进行了系统改进。特别是采用P507树脂以固相微萃方式替代液-液微萃除Ce，单步分离Ce的去除率优于99.999%。改进后的制备技术显著传统方法，具有高回收率（92%）、高稳定性、操作简便的优势。我国鞍山地区发育有中国最老的TTG岩石，是研究早期地球演化对地球硅酸岩储库的形成和演化的绝佳对象。基于已建立的146Sm-142Nd分析技术，本研究系统分析了鞍山TTG岩石的142,143Nd同位素，首次报道了鞍山TTG岩石相对于现今硅酸岩储库具有明显的142Nd同位素正异常（µ142Nd = +9.2 ± 2.8）。研究表明：38亿年的TTG岩石基本都具有142Nd正异常特征，而33亿年的TTG岩石未发现142Nd同位素异常。142Nd结合143Nd同位素的差异，本研究为该地区存在大于38亿年的岩石样品提供了独立可靠的证据。此外，鞍山TTG岩石最显著的特征是长衰期放射成因143Nd同位素与微量元素比值存在很好的协变关系，。这种相关性表明鞍山TTG岩石同时存在高和低两种µ142Nd的源区端员。

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