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，并将其回收到俯冲带的地球深处。这个过程不断地重新铺平地球表面的三分之二。新洋壳的形成代表了地球上最大的岩浆系统，涉及沿 70,000 公里全球海底扩张轴网络的岩浆（由地幔部分熔化从下方供应）的冷却和凝固。因此，了解海洋地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交换至关重要。由于深海的岩石基本上难以接近，科学家们试图了解岩浆如何在扩张轴上形成新的地壳，因此采用地球物理（地震）实验来研究海底以下。然后将结果与蛇绿岩（被推到大陆上并暴露在海平面以上的海洋地壳和上地幔的碎片）中的海洋岩石进行比较和结合，以开发海底扩张的科学模型。沿着东太平洋隆起 (EPR)（地球上岩浆最活跃的扩张轴）沿线的岩浆室，地球物理学家发现了薄（10 米厚）透镜状岩浆室（称为“轴向熔融透镜体”）位于沿 EPR 延伸的下地壳顶部。人们认为它们位于由晶体组成的糊状物顶部，周围被少量岩浆包围，将熔体向上输送到上面的熔体透镜体中。更详细的实验表明，这些熔化透镜的物理特性沿着 EPR 轴发生变化，这表明沿着 EPR 的熔化物与糊状物的比例在一系列长度尺度上变化。通过岩浆片的强力侵入（形成所谓的“片状岩脉复合体”），岩浆从熔融透镜体中周期性地向上排出，导致熔岩喷发到海底。然而，这张海底扩张轴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论）并不完整，并且对岩浆移入和移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，一定存在显着的沿轴（3D）运输和熔体演化，但这种情况发生的范围、地壳内的水平以及通过什么机制仍然未知。这些问题对熔体产生和从地幔输送以及洋壳形成的整个过程具有广泛的影响，并且只能通过详细量化沿扩散轴的熔体传输轨迹并将其与岩浆地球化学的测定相结合来回答。该项目通过直接确定岩浆进入和退出轴向熔融透镜系统时所遵循的运移路径来解决这些问题，该系统已沿着阿曼蛇绿岩中保存的 100 公里完整扩展段绘制出来。这为 EPR 等快速伸展轴提供了世界上唯一的陆地模拟。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产生的晶体的 3D 优先排列。然后，我们将这些观测结果与岩石成分的地球化学分析相结合，以确定沿快速扩张轴的岩浆流态的 3D 空间变化是否以及如何控制地壳构造过程中岩浆的地球化学演化。这种新颖的方法将使我们能够开发一个全面的模型来分析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。