The 3D anatomy of magma transport at fast-spreading ocean ridges

快速扩张的洋脊岩浆输送的 3D 解剖

• 批准号: NE/V012584/1
• 负责人: Antony Morris
• 金额: $ 83.5万
• 依托单位: Plymouth University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FV012584%2F1
• 关键词: 3D; anatomy; magma; transport; fast

Plate tectonics is the most important discovery in Earth Science and is a unique characteristic of our planet. It involves formation of new tectonic plates by seafloor spreading and their recycling back into the deep Earth at subduction zones. This process continuously repaves two-thirds of the Earth's surface. The formation of new oceanic crust represents the largest magmatic system on Earth, and involves the cooling and solidification of magma (supplied from below by partial melting of the Earth's mantle) along the 70,000 km global network of seafloor spreading axes. Understanding the details of how ocean crust forms is therefore critical to understanding the exchange of heat and mass from the solid Earth to the oceans and atmosphere. Since the rocks of the deep oceans are largely inaccessible, scientists trying to understand how magma builds new crust at spreading axes employ geophysical (seismic) experiments to investigate the sub-seafloor. Results are then compared to and combined with observations made on oceanic rocks in ophiolites (fragments of oceanic crust and upper mantle that have been pushed onto the continents and exposed above sea-level) to develop scientific models of seafloor spreading.In the search for magma chambers along the East Pacific Rise (EPR), the most magmatically active spreading axis on Earth, geophysicists have discovered thin (10's m thick) lens-shaped magma chambers (known as 'axial melt lenses') at the top of the lower crust that extend along the EPR. These are thought to sit on top of mushes made up of crystals surrounded by small amounts of magma, that feed melt upwards into the overlying melt lens. More detailed experiments have shown that the physical properties of these melt lenses change along the EPR axis, suggesting that the proportion of melt to mush along the EPR varies on a range of length-scales. Upwards expulsion of magma from the melt lens happens periodically via forceful intrusion of sheets of magma (forming so-called "sheeted dyke complexes"), leading to eruption of lava on to the seafloor. This geophysical picture of the magmatic plumbing system of seafloor spreading axes (based mostly on decades-old inferences from seismic experiments) is incomplete, however, and lacks any constraints on the pathways followed by magma migrating into and out of axial melt lens systems. Lateral variations in seafloor morphology and erupted lava compositions suggest that there must be significant along-axis (3D) transport and evolution of melt, but how extensively this occurs, at what level(s) within the crust, and by what mechanisms remain unknown. These questions have broad implications for the overall process of melt generation and delivery from the mantle and formation of ocean crust, and can only be answered by quantifying melt transport trajectories along a spreading axis in detail and by combining this with determinations of magma geochemistry.This project addresses these questions by directly determining the migration pathways followed by magma as it enters and exits from an axial melt lens system that has been mapped out along a 100 km complete spreading segment preserved in the Oman ophiolite. This provides the world's only on-land analog for fast-spreading axes like the EPR. We will use a technique called 'anisotropy of magnetic susceptibility' or 'AMS' to measure the 3D preferred alignments of crystals resulting from the flow of magma during the formation of crustal rocks. We will then combine these observations with geochemical analyses of rock compositions to establish whether and how 3D spatial variations in magma flow regimes along a fast-spreading axis control the geochemical evolution of magmas during crustal construction. This novel approach will allow us to develop a comprehensive model for the anatomy of the magma systems responsible for forming two-thirds of the Earth's surface, testing and challenging the predictions of remotely-sensed seismic investigations.

板块构造是地球科学中最重要的发现，也是我们星球的独特特征。它涉及通过海底扩张形成新的构造板块，并在俯冲带将其再循环回地球深处。这一过程持续不断地修复着地球表面的三分之二。新洋壳的形成是地球上最大的岩浆系统，涉及沿着70 000公里全球海底扩张轴网络的岩浆冷却和凝固（由地幔部分熔融从下面提供）。因此，了解海洋地壳如何形成的细节对于了解固体地球与海洋和大气之间的热量和质量交换至关重要。由于深海的岩石基本上无法接近，科学家试图了解岩浆如何在扩张轴上形成新的地壳，他们利用地球物理（地震）实验来调查海底下。然后，将结果与Ophiphitrium海洋岩石上的观察结果进行比较并结合起来（被推到大陆上并暴露在海平面以上的洋壳和上地幔碎片），以建立海底扩张的科学模型。在沿地球上最活跃的岩浆扩张轴--东太平洋隆起（EPR）沿着寻找岩浆房的过程中，地球物理学家已经在下地壳的顶部发现了薄的（10米厚）透镜状岩浆房（称为“轴向熔融透镜”），这些岩浆房沿着EPR延伸。这些被认为位于由被少量岩浆包围的晶体组成的糊状物的顶部，这些岩浆将熔体向上输送到上面的熔体透镜中。更详细的实验表明，这些熔体透镜的物理性质沿着EPR轴发生变化，这表明熔体与糊状物的比例沿着EPR在一系列长度尺度上变化。岩浆从熔体透镜中向上喷出，通过岩浆片的强力侵入（形成所谓的“席状岩墙复合体”）定期发生，导致熔岩喷发到海底。然而，这幅海底扩张轴的岩浆管道系统的地球物理图（主要基于几十年前的地震实验推断）是不完整的，而且对岩浆流入和流出轴向熔融透镜系统的路径缺乏任何限制。海底形态和喷发熔岩成分的横向变化表明，一定有显著的熔体沿轴（3D）运输和演变，但这种情况发生的范围有多大，在地壳内的什么水平，以及通过什么机制仍然是未知的。这些问题对地幔熔体生成和输送以及洋壳形成的整个过程具有广泛的影响，这个问题只能通过量化熔体沿沿着扩张轴的迁移轨迹并结合岩浆地球化学的测定来回答，本项目通过直接测定岩浆进入和离开轴向熔体透镜时的迁移路径来解决这些问题该系统已被绘制出沿着100公里的完整的扩展段保存在阿曼蛇绿岩。这为EPR等快速扩展轴提供了世界上唯一的陆上模拟。我们将使用一种称为“磁化率各向异性”或“AMS”的技术来测量地壳岩石形成过程中岩浆流动所产生的晶体的3D优选排列。然后，我们将结合联合收割机这些观察与岩石成分的地球化学分析，以确定是否和如何三维空间变化的岩浆流动制度沿着快速蔓延的轴控制地球化学演化的岩浆在地壳建设。这种新颖的方法将使我们能够开发一个全面的模型，用于解剖负责形成地球表面三分之二的岩浆系统，测试和挑战遥感地震调查的预测。