NSFGEO-NERC: Magnetotelluric imaging and geodynamical/geochemical investigations of plume-ridge interaction in the Galapagos

了解融化过程中熔体如何在我们星球的深部，即地幔中聚集，仍然是地球科学中一个关键和基本的开放问题。这对地球动力学和火山科学等不同的主题都有重要的影响。尽管熔体在地幔中的传输通常被模拟为一个扩散过程，但各种地质、地球化学、实验和理论结果表明，在长距离(100公里)的横向熔体传输过程中，这可能是通过宽度为10s到Km的通道网络发生的。然而，要通过自然观测来验证这些理论模型，并评估不同构造环境中地幔熔体沟道化的重要性，一直是一项具有挑战性的工作。能够提供理想的自然实验室来理解熔体沟道化运移的地球动力学环境之一，就是地幔羽流与附近的大洋中脊(&lt；1000公里)的相互作用。这是因为来自地幔热柱的熔体提供了一种独特的地球化学示踪剂来追踪熔体的运输过程。观察到的这种相互作用的主要特征是存在线性火山链(火山线状构造)。一个经典的例子是加拉帕戈斯的沃尔夫-达尔文线，这是一个大约200公里长的火山地貌，从上面延伸出来，目前在加拉帕戈斯扩张中心以南约250公里的地幔热柱内发生融化。在我们以前的工作中，我们发现对加拉帕戈斯线状构造的各种地球物理和地球化学观测可以在一个模型中得到自然解释，这些观测结果位于连接加拉帕戈斯羽流和加拉帕戈斯扩散中心的挥发和熔体丰富的通道网络之上。这样的火山线状构造在世界各地的其他羽流-山脊相互作用的环境中也被发现(例如留尼汪岛、复活节和发现号)。我们建议结合新收集的地球物理数据和对加拉帕戈斯线状构造和加拉帕戈斯扩张中心的新的地球化学观测。具体地说，我们建议使用一组从考察船上掉入水中的约60台最先进的宽带海洋仪器，测量沿火山线状构造和受羽流影响的海脊段约60至100公里深处的电导率。合成模型表明，与我们假设的熔体通道相关的电导信号很可能是可检测到的。我们将把地球物理测量的结果与加拉帕戈斯羽流流向海脊的地球动力学模型结合起来，以检验我们的发现，并区分不同可能的熔体沟道化机制。最后，在地球物理仪器记录数据的同时，我们建议沿加拉帕戈斯北部火山线条和线条扩展山脊交汇处挖掘火成岩样品。我们将对它们的地球化学进行分析，以限制来自地幔热柱的熔体的贡献。这项工作将使我们理解在地球核-地幔边界附近产生的地幔热柱如何与“浅”构造特征(大洋中脊)相互作用，以及总体上的地幔熔体输送过程取得重大进展。此外，我们的工作将为地球深部过程对地表系统的相互作用提供重要的启示。这是因为我们认为代表加拉帕戈斯地幔熔体运输的火山线状构造是东太平洋海洋生物(如鲸鲨)迁徙的基础。我们的研究将为这些地形特征如何在海底形成以及它们对海洋生态系统的潜在长期影响提供重要的制约因素。

Understanding how melt aggregates in our planet's deep interior, i.e. its mantle, during melting remains a critical and fundamental open question in the Earth Sciences. This has important impliactions for topics as diverse as geodynamics and volcano science. Although the transport of melt in the mantle has been typically modelled as being a diffuse process, a variety of geological, geochemical, experimental and theoretical results suggest that this might occur via a network of channels that are 10s of m to km in width during long-distance (100s of km) lateral melt transport. However, it has been challenging to validate these theoretical models via natural observations and assess the importance of melt channelisation in the mantle across different tectonic settings.One geodynamic setting that provides an ideal natural laboratory to understand this channelised transport of melt is the interaction of mantle plumes with nearby mid-ocean ridges (< 1000 km distance). This is because melts derived from a mantle plume provide a distinct geochemical tracer for tracking melt transport processes. The key observed characteristic of this type of interaction is the presence of linear chains of volcanoes (volcanic lineaments). A classic example is the Wolf-Darwin Lineament in Galápagos, a ~ 200 km long volcanic feature extending from above a region where there is currently melting taking place within a mantle plume located ~ 250 km south of the Galápagos Spreading Centre. In our previous work we find that a variety of geophysical and geochemical observations for the Galápagos lineaments are naturally explained in a model where they overlie a network of volatile- and melt-rich channels connecting the Galápagos plume to the Galápagos Spreading Centre. Such volcanic lineaments are found in other plume-ridge interaction settings worldwide (e.g. Reunion, Easter, and Discovery).We propose to use a combination of newly collected geophysical data and novel geochemical observations of the Galápagos lineaments and the Galápagos Spreading Centre. Specifically, we propose to use an array of ~ 60 state-of-the-art broadband marine instruments, dropped overboard from the research ship, to measure electrical conductivity in sections at depths of ~60 to 100 km along and across the volcanic lineaments and the plume-affected ridge segments. Synthetic modelling demonstrates that the conductivity signals associated with the melt channels that we hypothesize are very likely to be detectable. We will couple the results from the geophysical survey with geodynamical models for Galápagos plume flow towards the ridge in order to test our findings and discern among the different possible melt channelisation mechanisms. Finally, while the geophysical instruments are recording data, we propose to dredge samples of igneous rocks along both the northern Galápagos volcanic lineaments and the lineament-spreading ridge intersections. We will analyse these for their geochemistry in order to constrain the contribution of melts from the mantle plume. This work will lead to significant, if not transformative, advances in our understanding of how mantle plumes generated near Earth's core-mantle boundary interact with 'shallow' tectonic features (mid-ocean ridges), and mantle melt transport processes in general.Furthermore, our work will shed important light on the interaction of deep Earth processes on surface systems. This is because the volcanic lineaments that we believe represent the surface expressions of melt transport in the mantle in the Galapagos are fundamental to the migration of marine species in the eastern Pacific (e.g. whale sharks). Our study will provide important constraints on how these topographic features form on the ocean floor and also their potential long-term influence on marine ecosystems.