Finding the missing evidence for Earth's magma ocean: a novel stable isotope approach

地球的现在掩盖了它暴力的过去。在地球最初的5亿年里，灾难性的撞击产生了足够的能量来融化地球的内部，创造了行星规模的融化，或称“岩浆海洋”。它们随后的冷却和结晶将决定地球的化学成分及其未来的长期宜居性。然而，我们不知道地球岩浆海洋是在哪里结晶的，是如何结晶的，它们的组成是什么，以及早期岩浆海洋物质的残余物是否仍然存在于地球深处，可能是挥发物和贵金属的重要储存库。地球深处可能存在一条关键信息：随着岩浆海洋冷却，它将开始结晶，新形成的致密晶体下沉到地幔底部。这将在地幔中产生强烈的化学分层，这种分层可能会持续到今天。这个项目的重点是为这些密集的岩浆海洋晶体寻找化学证据，从而确定地球最早历史上缺失的关键证据。由于最深的地幔无法直接采样，我们必须依靠大自然来为我们做这件事。当地幔区域升温、浮升和融化，最终产生火山活动时，就会发生这种现象；冰岛、夏威夷和其他“地幔热柱”都出现了这种现象。我们可以利用这些熔岩的化学成分来探测熔化形成它们的物质的组成，从而获得了解地球深处的窗口。现代和古代熔岩中的化学信号导致了地球内部孤立的“原始”区域的范例，通常被认为位于地球地幔的最底部，在与地球中央金属核心的边界上。有人认为，这些深部地幔区域的矿物学和组成使它们能够抵抗数十亿年来被带入对流地幔中，在那里它们可能储存挥发性和产热元素。地幔的这些区域是否起源于岩浆海洋结晶作用？岩浆海洋物质总是与对流地幔隔离吗？岩浆海洋结晶后残留的冰冻熔体或结晶物质能否被输送到上地幔，如果是的话，它能否熔化并对现代和古代原始熔岩的化学成分做出贡献？为了回答这些问题，我们需要化学示踪剂，1)直接对深岩浆海洋结晶过程中形成的矿物类型做出反应，2)当火山岩在地球表面风化时抵抗改变，以便应用于古老的熔岩，以及3)反映这些熔岩来自的地幔的主体属性。我们建议使用铁(Fe)和钙(Ca)稳定同位素作为示踪剂。对37亿年前岩石的勘测测量表明，这些示踪剂对岩石的风化历史具有很强的适应性。这些数据还包含一种诱人的暗示，即这些火山岩是来自前岩浆海洋的熔融物质残留物。我们将使用这些示踪剂来探索地球岩浆海洋的历史及其在定义当今地球化学和物理状态方面的作用。重要的步骤是：1)在岩浆海洋结晶过程中限制Fe和Ca同位素的分配。我们将通过高压实验室实验来实现这一点，在那里我们将模拟岩浆海洋结晶的条件，并分析我们产生的晶体残留物。2)对距今37亿年的火山岩进行了新的铁、钙同位素分析。3)发展了一系列热力学模型来追踪岩浆海洋结晶的Fe和Ca同位素效应，并预测这些岩浆海洋结晶堆积在上地幔中的夹带和熔融所产生的火山岩的成分。

Earth's present belies its violent past. Catastrophic impacts during the Earth's first 500 million years generated enough energy to melt the planet's interior, creating planetary-scale volumes of melt, or "magma oceans". Their subsequent cooling and crystallisation would have set the chemistry of the Earth and its future long-term habitability. However, we do not know exactly where and how the Earth's magma oceans crystallised, what their composition was and whether remnants of early magma ocean material remain present in the Earth's deep interior, potentially acting as important reservoirs for volatiles and precious metals.A key piece of information may reside in the deep Earth: as the magma ocean cooled it would have started to crystallise, with the dense newly formed crystals sinking to the base of Earth's mantle. This would have generated strong chemical layering in the mantle, which could persist to today. This project focuses on finding the chemical evidence for these piles of dense magma ocean crystals, and thus identifying a key missing piece of evidence for Earth's earliest history.As the deepest mantle is inaccessible to direct sampling, we must rely on nature to do this for us. This occurs when regions of the mantle heat up, buoyantly rise and melt, ultimately producing volcanism; a phenomenon exhibited at Iceland, Hawaii and other "mantle plumes". We can use the chemistry of these lavas to probe the composition of the material that melted to form them, thereby gaining a window into the deep Earth. The chemical signals in both modern and ancient lavas have resulted in the paradigm of isolated and "primordial" regions of the Earth's interior, often presumed to be located at the very base of the Earth's mantle, at the boundary with the planet's central metallic core. It has been suggested that the mineralogy and composition of these deep mantle domains has allowed them to resist being entrained into the convecting mantle for billions of years, where they may store volatile- and heat-producing elements. Do these regions of the Earth's mantle have their origin in magma ocean crystallisation? Has magma ocean material always remained isolated from the convecting mantle? Can residual frozen melts or crystalline material left over from magma ocean crystallisation be transported into the upper mantle, and if so, can it melt and contribute to the chemistry of modern and ancient primitive lavas?To answer these questions, we need chemical tracers that, 1) respond directly to the type of minerals that would have formed during the crystallisation of a deep magma ocean, 2) are resistant to alteration when volcanic rocks are weathered at Earth's surface so that they can be applied to ancient lavas, and 3) reflect the bulk properties of the mantle that these lavas were derived from. We propose to use iron (Fe) and calcium (Ca) stable isotopes as tracers. Reconnaissance measurements of 3.7 billion year old rocks shows that these tracers are robust to the rocks' weathering history. The data also contain the tantalising suggestion that these volcanics were derived from melting material residual from a former magma ocean. We will use these tracers to explore the Earth's magma ocean history and its role in defining the chemical and physical state of the planet today. Important steps are:1) Constraining the partitioning of Fe and Ca isotopes during magma ocean crystallisation. We will do this by high-pressure laboratory experiments, where we will simulate the conditions of magma ocean crystallisation and analyse the crystal residues that we produce. 2) Undertaking new Fe and Ca isotope analysis of volcanics ranging from 3.7 billion years old to the present. 3) Develop a series of thermodynamic models to track the Fe and Ca isotope effects of magma ocean crystallisation and to predict the composition of volcanics derived from the entrainment and melting of these magma ocean crystal piles in the upper mantle.