State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Resources, China University of Geosciences, Wuhan 430074, China.
State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China.
Molecules. 2019 Apr 10;24(7):1415. doi: 10.3390/molecules24071415.
The fundamental advances in silicon isotope geochemistry have been systematically demonstrated in this work. Firstly, the continuous modifications in analytical approaches and the silicon isotope variations in major reservoirs and geological processes have been briefly introduced. Secondly, the silicon isotope fractionation linked to silicon complexation/coordination and thermodynamic conditions have been extensively stressed, including silicate minerals with variable structures and chemical compositions, silica precipitation and diagenesis, chemical weathering of crustal surface silicate rocks, biological uptake, global oceanic Si cycle, etc. Finally, the relevant geological implications for meteorites and planetary core formation, ore deposits formation, hydrothermal fluids activities, and silicon cycling in hydrosphere have been summarized. Compared to the thermodynamic isotope fractionation of silicon associated with high-temperature processes, that in low-temperature geological processes is much more significant (e.g., chemical weathering, biogenic/non-biogenic precipitation, biological uptake, adsorption, etc.). The equilibrium silicon isotope fractionation during the mantle-core differentiation resulted in the observed heavy isotope composition of the bulk silicate Earth (BSE). The equilibrium fractionation of silicon isotopes among silicate minerals are sensitive to the Si-O bond length, Si coordination numbers (CN), the polymerization degrees of silicate unites, and the electronegativity of cations in minerals. The preferential enrichment of different speciation of dissoluble Si (DSi) (e.g., silicic acid H₄SiO₄⁰ (H4) and H₃SiO₄ (H3)) in silica precipitation and diagenesis, and chemical weathering, lead to predominately positive Si isotope signatures in continental surface waters, in which the dynamic fractionation of silicon isotope could be well described by the Rayleigh fractionation model. The role of complexation in biological fractionations of silicon isotopes is more complicated, likely involving several enzymatic processes and active transport proteins. The integrated understanding greatly strengthens the potential of δSi proxy for reconstructing the paleo terrestrial and oceanic environments, and exploring the meteorites and planetary core formation, as well as constraining ore deposits and hydrothermal fluid activity.
本文系统地展示了硅同位素地球化学的基础进展。首先，简要介绍了分析方法的不断改进以及主要储层和地质过程中的硅同位素变化。其次，重点强调了与硅络合/配位和热力学条件相关的硅同位素分馏，包括具有可变结构和化学成分的硅酸盐矿物、硅质沉淀和成岩作用、地壳表面硅酸盐岩石的化学风化、生物吸收、全球海洋硅循环等。最后，总结了硅同位素在陨石和行星核形成、矿床形成、热液活动以及水圈硅循环中的相关地质意义。与与高温过程相关的硅热力学同位素分馏相比，低温地质过程中的分馏更为显著（例如，化学风化、生物/非生物沉淀、生物吸收、吸附等）。地幔-核分异过程中硅的平衡同位素分馏导致了地球全硅（BSE）中观察到的重同位素组成。硅酸盐矿物之间硅同位素的平衡分馏对 Si-O 键长、Si 配位数 (CN)、硅酸盐单元的聚合度以及矿物中阳离子的电负性敏感。不同可溶性硅（DSi）形态（例如硅酸 H₄SiO₄⁰ (H4) 和 H₃SiO₄ (H3)) 在硅质沉淀和成岩作用以及化学风化中的优先富集，导致大陆地表水的硅同位素主要呈正同位素特征，其中硅同位素的动力分馏可以很好地用瑞利分馏模型来描述。络合在生物硅同位素分馏中的作用更为复杂，可能涉及几种酶促过程和主动转运蛋白。综合认识极大地增强了 δSi 代用指标在重建古陆地和海洋环境、探索陨石和行星核形成以及约束矿床和热液活动方面的潜力。
