Rare Earth Elements Tracing Crustal Evolution Through Time: A Detrital Zircon Study

最近的多项证据表明，类似现代的板块构造过程在太古代（大约30亿年前）开始得相当突然，从构造上讲，地球已经经历了几个独特的动态阶段。这里首要的问题是检验地球构造岩浆演化中几个阶梯式变化的假设。这种变化应反映在大陆块体的基本组成块-中长英质火成岩的地球化学组成中。如果这确实发生了，我们是否可以更好地限制这种变化的年龄，它是突然的还是渐进的？在我们这个充满活力的星球上解开构造阶段和它们之间的过渡，对我们全面理解资源分布以及矿物和生物领域中其他进化过程的反馈具有重要意义。 在该项目中，研究人员将使用碎屑锆石U-Pb年龄和微量元素浓度（重点是稀土元素）对满足以下条件的沉积物和沉积岩的颗粒进行分析：（1）正在排出大陆块的相当大的部分，（2）包含包含大的地质时间间隔的锆石，特别是从前寒武纪开始。年龄范围目标将特别侧重于冥古宙、太古宙和元古代地区。最近的研究表明，在全球和区域范围内，年轻火成岩上测得的全岩地球化学参数与地壳厚度呈正相关。La/Yb和其他微量元素的比值是根据现代俯冲系统的中间火成岩的地壳厚度进行校准的，并已被用来显示在一些中生代造山带中地壳厚度随时间的变化。地壳厚度的隐含变化与大陆地区地壳厚度受现代板块构造影响的数千万年周期性是一致的，也表明大陆地壳的最深处（无论其厚度如何）是导致俯冲和碰撞系统中发现的中间火成岩多样性的分化工厂。

Multiple recent lines of evidence suggest that the onset of modern-like plate tectonic processes started rather abruptly in the Archean (about 3 billion years ago) and that tectonically, the Earth has evolved through several distinctive dynamic stages. The overarching question here is to test the hypothesis of several step-like changes in the tectono-magmatic evolution of the Earth. Such change should be reflected in the geochemical composition of intermediate and felsic igneous rocks - the basic building blocks of continental masses. If this did happen, can we better constrain the age of that change, was it sudden or gradual? Unraveling tectonics stages and transitions between them on our dynamic planet has important implications for our overall understanding of resource distribution as well as feedbacks with other evolutionary processes in the mineral and biologic realms. In the project, the researchers will use detrital zircon U-Pb ages and trace elemental concentrations (with a focus on the rare earth elements) on grains from sediments and sedimentary rocks that satisfy the following conditions: (1) are draining sizable segments of the continental mass, and (2) contain zircons encompassing large geologic time intervals, especially from the Precambrian. The age range target will especially focus on the Hadean, Archean and Proterozoic area. Recent work shows that whole rock geochemical parameters measured on young igneous rocks correlate positively with crustal thickness globally and regionally. La/Yb and other trace elemental ratios are calibrated against crustal thickness on intermediate igneous rocks from modern subduction systems and have been used to show variations in crustal thickness over time in a few Phanerozoic orogenic regions. The implied variation in crustal thickness is consistent with the tens of million years cyclicity of crustal thickness in continental areas being affected by modern plate tectonics and is also indicative of the fact that the deepest parts of the continental crust (regardless of its thickness) are the factories of differentiation leading to the diversity of intermediate igneous rocks found in subduction and collision systems.