Tracing isotopic heterogeneities in the Early Earth’s mantle through time

地球分化为金属核心和岩石地幔，以及现代风格的地幔对流的开始，可能是地球经历过的最普遍的变化之一。这些早期的行星规模的过程对于将地球转变为宜居领域至关重要。在地球表面，地球错综复杂的内部地球动力学系统导致了裂谷系统和俯冲带的形成，这些系统在全球物质循环中发挥着重要作用，例如在地质时间尺度上控制其大气中的碳和硫含量。然而，在地球吸积后的第一个10亿年内，现代地幔对流是如何以及何时演变的仍然是个谜。研究核形成和早期地幔对流是以一种相当特殊的方式相互交织在一起的研究线索：在核形成过程中，金属熔体从硅酸盐岩浆海洋中分离出来，导致所谓的亲铁元素从地幔几乎定量地转移到核中，在地球上形成的最早的岩石中留下了诊断的化学和同位素特征。在核形成完成后不久，地球受到了有限数量的灾难性陨石撞击，将少量新鲜的铁元素注入地幔，从而部分补充了它们在硅酸盐地球中的丰度。在这里，我们将使用这个行星规模的示踪实验来阐明早期地球的内部同位素不均一性，试图确定这种陨石物质是如何在大约10亿年的时间尺度上混合到地幔中的。我们将从分析地球上最古老的岩石开始，以表征在陨石物质完全均质之前，核心形成赋予硅酸盐地幔中亲铁元素的同位素特征。然后，我们将分析地球历史上关键时期的岩石，看看在哪些时间尺度上，新到达的亲铁元素混合到了地幔中。我们的发现，加上作为“建立宜居星球”项目的一部分收集的其他证据，将使我们能够更好、更详细地描述早期地幔对流演化过程。

Differentiation of Earth into a metallic core and a rocky mantle as well as the onset of modern-style mantle convection are probably amongst the most pervasive transformations our planet has endured. These early planetary-scale processes were essential in converting the Earth into a habitable realm. On its surface, the Earth’s intricate internal geodynamic system leads to the formation of rift systems and subduction zones, which play an important role in the global material cycle, for instance in controlling the carbon- and sulfur content in its atmosphere over geological timescales. Yet, how and when modern-style mantle convection evolved within the first billion years after the accretion of the Earth remains enigmatic. Studying core formation and early mantle convection are research strands that are intertwined in a rather peculiar way: Separation of metallic melt from a silicate magma ocean during core formation caused an almost quantitative transfer of so-called siderophile elements from the mantle into the core leaving a diagnostic chemical and isotopic signature in the earliest rocks that formed on Earth. Shortly after core formation was completed, the Earth was struck by a limited number of cataclysmic meteorite impacts that injected a small amount of fresh sideropile elements into the mantle, thus partly replenishing their abundances in the silicate Earth. Here, we will be using this planetary-scale tracer experiment to shed light on the internal isotopic heterogeneity of the Early Earth by trying to identify, how this meteoritic material mixed into the Earth’s mantle over a timescale of about 1 billion years. We will start by analysing the oldest rocks on Earth in order to characterise the isotopic signature that core formation imparted on the siderophile elements in the silicate mantle before the meteoritic material was completely homogenised. We will then analyse rocks from crucial time periods throughout Earth’s history to see, over which timescales the newly arrived siderophile elements were mixed into the Earth’s mantle. Our findings, together with other evidence gathered as part of the project "Building a Habitable Planet", will enable a better and more detailed characterisation of the processes involved in the evolution of Early Earth mantle convection.