Acasta ~4.0 Ga片麻岩Si-O同位素组成及其对地球早期构造环境的制约

板块构造的起源是地质学核心科学问题之一，而板块俯冲是现代板块构造的重要标志。因此检验地球上最古老的岩石是否有板块俯冲的痕迹，是制约板块构造起始时间的重要切入点。最近Si-O同位素常被用来约束太古代构造环境，但不同的分析方法获得了相互矛盾Si同位素组成特征以及不同的构造环境。本项目将以地球上最古老的Acasta片麻岩为研究对象，在野外资料、岩石学、地球化学与锆石年代分析的基础上，通过系统的全岩Si同位素，以及锆石和石英高精度原位Si-O同位素分析，拟解决以下问题。1）讨论太古代片麻岩Si-O同位素体系的封闭性，为以后Si-O同位素体系的应用提供建议和依据；2）识别地球最古老岩石的初始Si-O同位素组成。如果具有重Si特征，则表明早太古代长英质岩是由蚀变洋壳直接熔融而来，指示了板块俯冲环境；相反如果与地幔Si值相似则不能指示俯冲环境。最终该结果将对地球早期构造环境提供进一步的约束。

The operation of plate tectonics makes planet Earth unique within our solar system. The theory of plate tectonics was first proposed more than 50 years ago, however, the key question of ‘when and why did plate tectonics begin on Earth?’ still 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 test whether the oldest rocks on Earth carry any hints of subduction that might constrain the earliest operation of plate tectonics. Si-O isotopes have recently been identified as a method capable of testing whether subduction existed or not in Archean time. However, multiple initial studies, using both whole rock and zircon in situ Si-O isotopes, have so far yielded contradictory results. This project will employ Si-O isotopes on the ~4.0 Ga Acasta Gneiss Complex in northwest Canada—the oldest rocks in the world—in order to definitively constrain the tectonic setting of early Earth. We will integrate field observations, petrology, geochemistry, whole rock Si-O isotopes, and ultra-high precision in situ Si-O isotopic analyses on zircons and quartz grains to address two fundamental questions. First, by using both whole rock and ultra-high precision techniques, to test the Si-O isotopes of the oldest rocks, we will be able to reconcile the reasons for the previous contradictory results acquired with different methods. Identifying potential pitfalls when using Si-O isotopes to interpret Archean tectonic settings will establish a new protocol for the developing isotopic method. Second, we will then be able to resolve the critical question of whether the earliest Archean rocks are rich in high Si isotopes or not. An enriched signature, if identified, would indicate that earliest Archean granitoids were formed by melting of subducted basaltic crust enriched in sedimentary silica through interaction with seawater, requiring the operation of subduction on early Earth. Identifying a mantle-like signature would on the other hand challenge the notion of operational plate tectonics at this time. Either outcome will carry fundamental implications for early Earth.