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，地球的表面通过大陆的破碎而回收，以形成新的海洋，在其他地方将海洋板沉入下面的地幔中。大陆的破裂涉及在最终分裂之前进行的渐进式拉伸和变薄，以及从下方升起的熔融岩石的新海壳形成，两侧是由大陆壳薄的大陆边缘。有一系列的大陆边缘类型，与底层地幔在此过程的早期开始融化的那些不同，并且在地壳中添加了很大的体积，而这些“岩浆”的边缘几乎没有证据表明这种融化直到过程结束的证据很少。在全球常见的这些岩浆边缘，已经发现外壳可以稀薄，地幔岩石可以在海床上暴露在海床上，在那里它们在称为蛇形化的过程中与海水反应。这种蛇形化在地球内部和海洋之间交换化学物质方面起着重要作用，并且在地质断层周围可能特别强烈。在过去的三十年中，已经对大陆壳的稀疏的最后阶段进行了广泛的研究，但通过熔融岩石喷发的爆发从海床暴露到形成新的海洋壳的过渡一直没有很好地研究。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过渡的范围。同样，由于在海洋中间也发现了这种地幔暴露，因此这种过渡可能比经常假设的更为复杂。我们的项目将使用地球物理技术的新型组合来研究英国西南部Magma-Poor大陆边缘的大陆分裂的最后阶段。在那里，从所有可用数据中看来的地壳都是“正常”的海洋地壳，位于大约150 km的地壳内，通过钻孔为大陆。蛇形地幔的一个区域，现在覆盖着泥浆约1公里，介于两者之间。在这样的位置，我们将首次使用由牵引源产生的电磁波，以测量地壳和蛇纹地幔的电阻率。电磁波被海水强烈衰减，因此源必须强大，并且必须拖到海床附近。我们将使用牵引传感器的组合，这些传感器对海底下方的结构最敏感，以及可以在距离我们的源头多达数十公里的距离内测量电气和磁场中微小波动的海床探测器，从而使我们可以更深入地探测。我们还将使用一些相同的海床接收器来检测从靠近船只拖曳的源头穿过地壳的声波，并检测到由天然来源产生的低频电磁波，并更深入地球中的数据，我们将通过使用强大的计算机和电子方式来构建巨大的计算机计划，以构建巨大的能力，并构造出巨大的能力，这些模型是在声音和电子上的变化。地幔下面。这些参数提供了强大的组合，因为它们对岩石本质具有不同的方式敏感。电阻率对水的存在特别敏感，也是一种称为磁铁矿的矿物质，该矿物质可以在蛇形化过程中形成。声速对水的存在不太敏感，但对存在的矿物质的变化可能更敏感。从我们的模型中，我们希望能够区分大陆地壳和地幔，海洋壳和地幔以及介于两者之间的材料的性质。然后，随着大陆破裂，这些观察结果将这些观察结果与物理和化学过程的计算机模型联系起来。因此，我们将找出新的海洋外壳的形成实际上是如何开始的。