NSFGEO-NERC: Quantifying evolution of magmatism and serpentinisation during the onset of seafloor spreading

NSFGEO-NERC：量化海底扩张开始期间岩浆作用和蛇纹石化的演化

• 批准号: NE/T007419/1
• 负责人: Timothy Minshull
• 金额: $ 56.9万
• 依托单位: University of Southampton
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FT007419%2F1
• 关键词: NSFGEO; NERC; Quantifying; evolution; magmatism

Over periods of hundreds of millions of years, Earth's surface is recycled via the fragmentation of continents to form new oceans and elsewhere the sinking of oceanic plates into the mantle beneath. The breakup of continents involves progressive stretching and thinning prior to final breakup and the formation of new oceanic crust from molten rock that rises from below, flanked by continental margins comprised of thinned continental crust. There is a range of continental margin types, varying from those where the underlying mantle starts to melt very early in the process and very large volumes are added to the crust, to those "magma-poor" margins where there is little evidence for such melting until the very end of the process. At these magma-poor margins, which are common globally, it has been found that the crust can thin to nothing and mantle rocks can be exposed at the seabed, where they react with seawater in a process called serpentinisation. This serpentinisation plays an important role in exchange of chemicals between the Earth's interior and the ocean, and may be particularly intense around geological faults. While the final stages of thinning of the continental crust have been studied extensively over the past three decades, the transition from exposing mantle at the seabed through to forming new oceanic crust by the eruption of molten rock has been less well studied. Even designing such a study can be challenging because it is often unclear how wide this transition is. Also, because such mantle exposure has also been found in the middle of the oceans, this transition may be more complicated than often assumed.Our project will use a novel combination of geophysical techniques to study this final stage of continental breakup at a magma-poor continental margin southwest of the UK. There, crust that seems from all available data to be "normal" oceanic crust lies within about 150 km of crust confirmed by drilling to be continental. A region of serpentinised mantle, now overlain by up to around 1 km of mud, lies in between. For the first time in such a location, we will use electromagnetic waves, generated from a towed source, to measure the electrical resistivity of the crust and serpentinised mantle. Electromagnetic waves are strongly attenuated by seawater, so the source must be powerful and must be towed close to the seabed. We will use a combination of towed sensors, that are most sensitive to structures just below the seabed, and seabed detectors that can measure tiny fluctuations in electrical and magnetic fields at distances of up to tens of kilometres from our source, and thus allow us to probe deeper. We will also use some of the same seabed receivers to detect sound waves travelling through the crust from a source towed close to the ship, and to detect lower-frequency electromagnetic waves that are generated by natural sources and penetrate deeper into the Earth.The data that we collect will allow us, via the use of powerful computer programmes, to construct models of the variation of both sound speed and electrical resistivity in the crust and in the upper few tens of kilometres of the mantle beneath. These parameters provide a powerful combination because they are sensitive in different ways to the nature of the rocks. The electrical resistivity is particularly sensitive to the presence of water, and also of a mineral called magnetite that can be formed during the process of serpentinisation. The sound velocity is less sensitive to the presence of water but can be more sensitive to variations in the minerals present. From our models, we expect to be able to distinguish the continental crust and mantle, the oceanic crust and mantle, and the nature of the materials in between. We will then link these observations to computer models of the physical and chemical processes occurring as continents break apart. Thus we will find out how the formation of new oceanic crust actually starts.

在数亿年的时间里，地球表面通过大陆的分裂形成新的海洋，以及海洋板块下沉到地幔中而循环。大陆的分裂包括在最后分裂之前逐渐拉伸和变薄，以及从下面升起的熔融岩石形成新的海洋地壳，两侧是由变薄的大陆地壳组成的大陆边缘。大陆边有一系列的类型，有的是底层地幔在熔化过程中很早就开始熔化，地壳中增加了很大的体积，有的是“岩浆贫乏”的大陆边，在熔化过程结束之前几乎没有这种熔化的证据。在这些全球常见的岩浆贫乏的边缘，人们发现地壳可以薄到什么都没有，地幔岩可以暴露在海底，在那里它们与海水发生反应，形成一个称为蛇纹石化的过程。这种蛇纹化作用在地球内部和海洋之间的化学物质交换中起着重要作用，在地质断层周围可能特别强烈。虽然在过去三十年中对大陆地壳变薄的最后阶段进行了广泛的研究，但对从海底地幔暴露到熔岩喷发形成新的洋壳的过渡研究较少。即使设计这样的研究也可能具有挑战性，因为通常不清楚这种过渡的范围有多大。此外，因为这样的地幔暴露也被发现在海洋的中间，这个过渡可能比通常assumption.Our项目将使用一种新的地球物理技术的组合来研究这个大陆分裂的最后阶段在岩浆贫乏的大陆边缘西南英国。在那里，从所有现有数据来看似乎是“正常”海洋地壳的地壳位于经钻探确认为大陆地壳的大约150公里范围内。一个蛇形地幔区域，现在被大约1公里厚的泥浆覆盖，位于两者之间。我们将首次在这样的地点使用拖曳源产生的电磁波来测量地壳和蛇形地幔的电阻率。电磁波会被海水强烈衰减，因此源必须是强大的，并且必须被拖到靠近海底的地方。我们将结合使用拖曳式传感器和海底探测器，拖曳式传感器对海底下方的结构最敏感，海底探测器可以测量距离我们的源几十公里的电场和磁场的微小波动，从而使我们能够进行更深的探测。我们亦会利用一些相同的海底接收器，探测由拖曳在船只附近的声源所发出的穿透地壳的声波，以及探测由天然声源所产生并深入地球的低频电磁波。我们所收集的数据，可透过强大的电脑程式，建立地壳和下面几十公里地幔上部声速和电阻率变化的模型。这些参数提供了一个强大的组合，因为它们以不同的方式对岩石的性质敏感。电阻率对水的存在特别敏感，也对在蛇纹石化过程中形成的称为磁铁矿的矿物特别敏感。声速对水的存在不太敏感，但对存在的矿物质的变化更敏感。从我们的模型中，我们希望能够区分大陆地壳和地幔，海洋地壳和地幔，以及它们之间的物质性质。然后，我们将把这些观测结果与大陆分裂时发生的物理和化学过程的计算机模型联系起来。因此，我们将发现新的海洋地壳的形成实际上是如何开始的。