太古宙花岗质岩石Si-O同位素对岩石成因和板块构造的约束

板块构造的起源是地球科学核心问题之一, 而表壳物质循环是现代板块构造的重要表现之一, 因此检验地球表壳物质循环起始时间是约束板块构造启动时间的重要切入点。近年来, 岩浆岩的Si-O同位素联合示踪开始被用来约束太古宙构造环境, 但由于大多太古宙样品经历了强烈的变质作用, Si同位素数据是否代表原岩信息需要进一步的评估; 此外, Si同位素在高温岩浆分异过程中变化非常小, 在分析精度不够高的情况下, 其分析结果则很可能无法揭示其潜在的变化规律。本文结合当前地球早期构造环境研究进展以及Si-O同位素的应用情况: (1)重点总结介绍了太古宙花岗质岩石(主要包括英云闪长岩-奥长花岗岩-花岗闪长岩; 简称TTG)的Si同位素应用原理, 及应用Si-O同位素约束地球早期构造环境的优势; (2)分析了目前研究中存在的问题, 并给出了具体的改进建议和方法; (3)进一步总结了太古宙TTG Si同位素和全球规模的O同位素, Ge/Si比值, 及其他地质学和地球化学指标, 确认了在大约3.8 Ga开始有表壳物质再循环特征的出现; (4)最后依据当前的研究进展, 提出了未来具体研究方向。

Si-O isotope compositions of Archean granitoids and their implications for petrogenesis and plate tectonics

The operation of plate tectonics makes planet Earth unique within our solar system. However, 'When did plate tectonics begin on Earth?' remains hotly debated in the field of geology. Subduction is one of the most distinct signatures of modern plate tectonics. Thus, a critical aspect in addressing this question is to investigate when supracrustal material recycling appeared. Paired Si-O isotopes have recently been identified as powerful paired tracers of surficial materials in the Archean. However, due to the highly metamorphosed rock affinity in most Archean rocks, Si isotopes of these rocks need further evaluations on whether they represent the original Si signature. Besides, Si isotopes display very limited variation during high-temperature magmatic differentiation, which requires ultra-high analytical precision. Otherwise, the data with large analytical errors may hide any potential shifts. Here, we review the progress of the Si-O isotope studies on Archean granitoids (mostly tonalite-trondhjemite-granodiorite, TTGs), and discuss the implications and limitations of the current studies. First, we review the principles of Si isotope application on TTGs and the advantages of paired Si-O isotopes. Second, we discuss the potential problems of the pioneering studies and propose a specific evaluation process. Third, we compile the available global Si-O isotope data, Ge/Si ratios, and other geochemical and geological evidences, and confirm that supracrustal material recycling first appeared at ca. 3.8Ga. Finally, we discuss the remaining issues and provide specific topics for future research.