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

板块构造是地球科学中最重要的发现，也是地球的一个独特特征。它包括通过海底扩张形成新的构造板块，并在俯冲带将其循环回到地球深处。这一过程不断地重新铺设地球表面的三分之二。新的洋壳的形成是地球上最大的岩浆系统，涉及到沿70,000公里的全球海底扩张轴网络冷却和凝固岩浆(由地幔部分融化从下方提供)。因此，了解洋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。由于深海的岩石在很大程度上是无法接近的，科学家们试图了解岩浆是如何在扩张轴线上形成新的地壳的，他们利用地球物理(地震)实验来研究海底。然后将结果与蛇绿岩中的海洋岩石(被推入大陆并暴露在海平面以上的洋壳和上地幔的碎片)进行比较并结合观察，以建立海底扩散的科学模型。在寻找沿东太平洋隆起的岩浆室时，地球物理学家发现了沿东太平洋隆起的岩浆室，东太平洋隆起是地球上最活跃的扩张轴，在沿着东太平洋隆起延伸的下地壳顶部，地球物理学家发现了薄的(10‘S米厚)透镜状岩浆室(称为轴向熔融透镜)。它们被认为位于由少量岩浆包围的晶体组成的糊状物的顶部，这些晶体向上熔化进入上面的熔化透镜。更详细的实验表明，这些熔融透镜的物理性质沿EPR轴变化，这表明沿EPR的熔融与糊化的比例在一系列长度尺度上不同。岩浆通过岩浆片的强力侵入(形成所谓的“席状岩墙复合体”)周期性地向上从熔融透镜中排出，导致熔岩喷发到海底。然而，这张海底扩张轴的岩浆管道系统的地球物理图像(主要基于几十年前的地震实验推断)是不完整的，并且缺乏对岩浆流入和流出轴向熔融透镜系统的路径的任何约束。海底形态和喷发熔岩成分的横向变化表明，一定存在重大的沿轴(3D)熔体运移和演化，但这种情况发生的范围有多广，在地壳内处于什么水平(S)，以及通过什么机制尚不清楚。这些问题对地幔熔体产生和输送以及洋壳形成的整个过程具有广泛的影响，只有通过详细量化沿扩张轴的熔体输送轨迹并结合岩浆地球化学测定才能回答这些问题。该项目通过直接确定岩浆进入和离开轴向熔体透镜系统时遵循的迁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公里完整的扩张段绘制。这为像EPR这样快速传播的轴提供了世界上唯一的陆上模拟设备。我们将使用一种名为“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，我们将把这些观察与岩石成分的地球化学分析结合起来，以确定沿快速扩张的轴线的岩浆流制度的3D空间变化是否以及如何控制地壳构造期间岩浆的地球化学演化。这种新的方法将使我们能够为形成地球表面三分之二的岩浆系统的解剖开发一个全面的模型，测试和挑战遥感地震调查的预测。

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.