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/T007192/1
• 负责人: Gaye Bayrakci
• 金额: $ 16.21万
• 依托单位: NATIONAL OCEANOGRAPHY CENTRE
• 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%2FT007192%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破碎形成新的海洋以及其他地方的海洋板块沉入下面的地幔而循环利用。大陆的分裂涉及到最终分裂之前的逐渐拉伸和变薄，以及从下方升起的熔岩形成新的洋壳，两侧是由变薄的大陆地壳组成的大陆边缘。大陆边缘有多种类型，从下面的地幔在融化过程的早期就开始熔化并且大量体积被添加到地壳的大陆边缘，到几乎没有证据表明这种熔化的“岩浆贫乏”边缘。直到该过程的最后。在这些全球常见的贫岩边缘，人们发现地壳可以变薄至零，地幔岩石可以暴露在海底，在那里它们与海水发生反应，发生称为蛇纹石化的过程。这种蛇纹石化作用在地球内部和海洋之间的化学物质交换中发挥着重要作用，并且在地质断层周围可能尤其强烈。虽然在过去三十年里人们对大陆地壳减薄的最后阶段进行了广泛的研究，但从地幔暴露在海底到通过熔岩喷发形成新的洋壳的转变却很少得到很好的研究。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转变的范围有多大。此外，由于在海洋中部也发现了这种地幔暴露，因此这种转变可能比通常假设的更为复杂。我们的项目将使用一种新颖的地球物理技术组合来研究在贫岩浆地区大陆破裂的最后阶段英国西南大陆边缘。那里的地壳从所有可用数据看来都是“正常”的海洋地壳，位于距钻探证实为大陆地壳约 150 公里的范围内。其间有一片蛇纹石地幔区域，现在覆盖着约 1 公里厚的泥土。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纹地幔的电阻率。电磁波会被海水强烈衰减，因此源必须强大并且必须被拖曳到靠近海底的地方。我们将使用对海底正下方的结构最敏感的拖曳式传感器和可以测量距离我们的源头数十公里处的电场和磁场的微小波动的海底探测器的组合，从而使我们能够更深入地探究。我们还将使用一些相同的海底接收器来探测从靠近船舶拖曳的声源穿过地壳的声波，并探测由自然源产生并深入地球深处的低频电磁波。我们收集的数据将使我们能够通过使用强大的计算机程序来构建地壳和地幔上部几十公里内的声速和电阻率变化的模型。这些参数提供了强大的组合，因为它们以不同的方式对岩石的性质敏感。电阻率对水以及蛇纹石化过程中形成的磁铁矿矿物的存在特别敏感。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，我们期望能够区分大陆地壳和地幔、海洋地壳和地幔以及其间物质的性质。然后，我们将把这些观察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，我们将了解新洋壳的形成实际上是如何开始的。