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

地球分化为金属核和岩石地幔，以及现代地幔对流的开始，可能是我们星球经历的最普遍的转变之一。这些早期行星尺度的过程对于将地球转变为可居住的领域至关重要。在其表面，地球复杂的内部地球动力系统导致裂谷系统和俯冲带的形成，这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大气中的碳和硫含量。然而，在地球吸积后的第一个十亿年内，现代地幔对流是如何以及何时演变的仍然是个谜。研究地核形成和早期地幔对流是以一种相当奇特的方式交织在一起的研究链：在地核形成期间，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地核中，在地球上形成的最早的岩石中留下了诊断化学和同位素特征。在地核形成后不久，地球受到了数量有限的灾难性陨石撞击，将少量新鲜的铁氧石元素注入地幔，从而部分补充了硅酸盐地球中的丰度。在这里，我们将使用这个行星尺度的示踪实验来阐明早期地球的内部同位素异质性，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入地球地幔的。我们将从分析地球上最古老的岩石开始，以确定在陨石物质完全均匀化之前，核心形成对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，看看新到达的亲铁元素在哪个时间尺度上混合到地幔中。我们的发现，以及作为“建设一个宜居星球”项目的一部分收集的其他证据，将使早期地球地幔对流演化过程的特征更好，更详细。

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.