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

在数亿年的时间里，地球表面通过大陆的碎裂形成新的海洋，而在其他地方，海洋板块下沉到地幔中，地球表面被循环利用。大陆的解体包括在最终解体之前的逐步伸展和变薄，以及从下方上升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。存在一系列大陆边缘类型，从下伏地幔在熔化过程中很早就开始融化并在地壳中增加了非常大的体积的大陆边缘类型，到直到过程最后才有证据表明这种熔融的“岩浆贫乏”边缘。在这些全球普遍存在的岩浆贫乏的边缘，人们发现地壳可以变薄到零，地幔岩石可以在海床上暴露出来，在那里它们与海水发生一种称为蛇纹岩化的过程。这种蛇纹岩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，在地质断层周围可能尤其强烈。虽然在过去的三十年里，人们对陆壳减薄的最后阶段进行了广泛的研究，但从海底暴露地幔到通过熔岩喷发形成新的洋壳的过渡研究较少。即使是设计这样一项研究也可能具有挑战性，因为人们往往不清楚这种过渡的范围有多广。此外，由于在大洋中部也发现了这种地幔暴露，这种转变可能比人们通常认为的要复杂得多。我们的项目将使用一种新的地球物理技术组合来研究英国西南部岩浆贫乏的大陆边缘的大陆破裂的最后阶段。在那里，从所有可用的数据来看，似乎是“正常”的洋壳位于大约150公里的地壳内，钻探证实它是大陆地壳。一个蛇纹状地幔区域，现在被大约1公里的泥浆覆盖，位于两者之间。在这样的地点，我们将第一次使用拖曳震源产生的电磁波来测量地壳和蛇纹状地幔的电阻率。电磁波被海水强烈衰减，因此震源必须很强大，而且必须拖到靠近海床的地方。我们将使用拖曳传感器和海底探测器的组合，拖曳传感器对海床下面的结构最敏感，海底探测器可以测量距离我们的震源数十公里处的电场和磁场的微小波动，从而使我们能够更深入地探测。我们也会使用一些相同的海底接收器，探测从靠近船只的拖曳来源通过地壳传播的声波，以及探测由自然来源产生并深入地球更深的低频电磁波。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，建立地壳和地幔下面几十公里以上的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，因为它们对岩石的性质以不同的方式敏感。电阻率对水的存在特别敏感，也对蛇纹岩作用过程中形成的一种名为磁铁矿的矿物的存在特别敏感。声速对水的存在不那么敏感，但对存在的矿物的变化更敏感。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，以及介于两者之间的物质的性质。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，我们将发现新洋壳的形成实际上是如何开始的。

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.