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，是我们星球的独特特征。它涉及通过海底扩散形成新的构造板，并在俯冲区域回收回到深地球。这个过程不断重新铺设地球表面的三分之二。新的海洋外壳的形成代表了地球上最大的岩浆系统，涉及沿着70,000公里全球海底扩散轴的全球网络的岩浆冷却和固化（从下方通过部分融化而提供）。因此，了解海壳如何形成的细节对于了解从固体地球到海洋和大气的热量和质量的交换至关重要。由于深海的岩石在很大程度上是无法访问的，因此科学家试图了解岩浆如何建立新的地壳，以扩散轴，采用地球物理（地震）实验来研究下层状。然后将结果与蛇绿岩（海壳和上地幔的碎片）进行比较，并与对海洋岩石的海洋岩石进行的观察结果进行比较，并将其推向大陆并在海平面上方暴露在大陆上），以开发海底传播的科学模型。透镜形的岩浆腔（称为“轴向熔体透镜”）沿EPR延伸的下层地壳顶部。这些被认为坐在由晶体组成的糊状上，这些晶体被少量的岩浆包围，它们的进食融化到上覆的熔体镜头中。更详细的实验表明，这些熔体透镜的物理特性沿EPR轴发生变化，这表明熔融沿EPR搅拌的比例在一系列长度尺度上有所不同。从融化镜头中驱动岩浆的向上驱逐通过岩浆的薄片（形成所谓的“堤堤综合体”）的强烈侵入，从而导致熔岩爆发到海底。然而，这种海底扩散轴的岩浆管道系统的地球物理图片（主要是基于数十年来从地震实验的推论）是不完整的，并且缺乏对途径的任何约束，随后是magma迁移到轴向熔体透镜系统中。海底形态和熔岩组成的横向变化表明，熔融的轴（3D）运输和演变必须有明显的融合（3D），但是在外壳内的何种水平，以及哪些机制仍然未知。 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在阿曼蛇绿岩中保存的扩散段。这为世界唯一的陆上类似物提供了EPR等快速扩张轴的陆上类似物。我们将使用一种称为“磁化性的各向异性”或“ AMS”的技术来测量在地壳岩石形成过程中由岩浆流动引起的晶体的3D优选比对。然后，我们将将这些观察结果与岩石组合物的地球化学分析相结合，以确定沿快速扩张轴沿岩浆流程度中的3D空间变化控制着岩层结构期间岩浆的地球化学演化。这种新颖的方法将使我们能够为负责形成地球表面的三分之二的岩浆系统的解剖结构开发一个综合模型，从而测试和挑战远程敏感的地震研究的预测。