From Ridge to Trench, MoHole to Bend-Faults

地幔几乎构成了地球的全部体积，它被认为是由类似于石质陨石的物质构成的。地球上几乎所有地方都有一层薄薄的地壳，这是地幔反复融化和再融化的产物。自20世纪50年代以来，在地壳上钻一个“莫孔”，以获得第一个地幔的原位样本，一直是地球科学家的目标，堪比从月球、火星或金星上获取样本。这些样本是证实或否定几十年来关于地球海洋和大陆地壳如何形成和演化、火山形成的地质过程、板块构造以及地球生命的先决条件的研究的唯一途径。大陆地壳通常厚于30千米，使地幔难以触及。海洋下的地壳覆盖了地球表面的65%，通常只有5.5公里薄，现在这项任务是可能的。实现这一目标的成本与普通太空任务类似，但需要国际社会的共同努力。日本人已经设计并建造了一艘价值10亿英镑的钻探船“地球号”（Chikyu），其中“莫霍”号是一个关键目标。中美洲近海是一个特别有希望的MoHole地点，因为水深不到4公里，板块年龄在20Myr左右，减少了技术挑战，而且海洋地壳形成于一个经过充分研究的快速（10cm/年）洋中扩张中心。此外，这是世界上唯一一个我们可以增加莫霍尔钻探来探索地壳和地幔变化的地区，当构造板块弯曲俯冲到地球内部的海沟时。中美洲近海是唯一一个板块弯曲发生在低于Chikyu钻井作业4.5公里海底深度限制的地方。我们现在认识到，海沟附近的板块弯曲很可能与海洋地壳下方的海水和地幔之间发生的重大化学反应有关，海洋地壳在上升并在洋中脊下方融化后大幅冷却。海沟外侧形成的弯曲断层似乎是这一过程的关键，它为海水穿过地壳进入浅层地幔提供了通道。为石油勘探而开发的声学（地震）工具显示，这些断层位于弯曲的地壳中，并在几个地方深入地幔10公里处。我们从降低的地震波速度得知，海水和寒冷的地幔发生了反应，形成了大量富含水的产物——蛇纹岩。如果在板块弯曲过程中形成了大量的蛇纹石，这将对地球的全球碳和水循环产生深远的影响。作为蛇纹石化反应产物形成的碳酸盐可以将一定量的二氧化碳转移到地幔中，与地表风化和造山所消耗的二氧化碳相当。深层化学合成生命也可能利用蛇纹石化反应释放的能量。这个区域可能是生命渗透到地球最深处的地方，甚至可能是生命起源的环境。为了验证这些想法，需要钻穿地壳，进入这些弯曲断层附近的浅层地幔，对岩石和流体进行取样。在本研究中，我们将测试中美洲近海板块是否适合这两个深钻项目。我们将用地震方法测量地壳厚度，以及地壳和上地幔的性质。我们将确定弯曲断层在哪里形成，它们是如何演变的，以及地壳和地幔的性质是如何变化的。我们将利用名为Autosub的先进机器人潜艇绘制海底地图，并测量海底5米以下岩石的温度，专门寻找热水经过活跃蛇纹岩化区域深处后返回海底的弯曲断层沿线的地点。找到这些地点和任何存在的生命形式本身将是我们对地球基本知识的一个重大飞跃。

The Earth's mantle forms almost all of the Earth's volume, and is thought to be made of material similar to stony meteorites. Almost everywhere on Earth it lies beneath a thin layer of crust that is the product of repeated mantle melting and remelting. Since the 1950s, drilling a "MoHole" through Earth's crust to obtain the first in-place sample of its mantle has been a goal for Earth scientists comparable to obtaining samples from the Moon, Mars, or Venus.These samples are the only way to confirm or disprove many decades of work on how Earth's oceanic and continental crust form and evolve, the geological processes that create volcanoes, Plate Tectonics, and the preconditions for Life on Earth. Continental crust is usually thicker than 30km, putting the mantle out of reach. The crust beneath the oceans, which covers 65% of the Earth's surface, is often as thin as 5.5km and the task is now possible. This goal is achievable at a cost similar to a moderate space mission but requires an international community effort, and the Japanese have designed and built a £1000M drillship, the Chikyu, with the MoHole as a key objective.Offshore Central America is an especially promising site for the MoHole because the water depths of less than 4km and the plate age around 20Myr reduce the technical challenges, and the ocean crust formed at a well-studied fast (10cm/yr) mid-ocean spreading centre. In addition, this is the world's only region where we can augment MoHole drilling to explore changes to the crust and mantle when a tectonic plate bends to subduct into Earth's interior at an oceanic trench. Offshore Central America is the only place where this plate bending occurs shallower than the 4.5km seafloor-depth limit to Chikyu drilling operations.We now realize that plate bending near a trench is likely to be associated with significant chemical reactions between seawater and mantle just below the oceanic crust that has cooled significantly since it rose and melted beneath a mid-ocean ridge. Bend-faults formed just outboard of the trench appear to be key in this process, providing pathways for seawater through the crust and into the shallow mantle. Acoustic (seismic) tools developed for oil-exploration show these faults in the bending crust and up to 10km into the mantle in several places. We know from reduced seismic wavespeeds that seawater and cold mantle have reacted to form large amounts of a water-rich product called serpentine.If large amounts of serpentine are formed during plate bending, this would have profound implications for the Earth's global carbon and water cycles. Carbonate formed as a serpentinization reaction product could transfer an amount of carbon dioxide into the mantle comparable to that consumed by surface weathering and mountain building. Deep chemosynthetic life is also likely to take advantage of the energy released by serpentinization reactions. This region may be where life penetrates most deeply into Earth and may even be the environment where life started. To test these ideas requires drilling through the crust and into the shallow mantle near one of these bend faults to sample the rocks and fluids.In this study we will test whether the plate offshore Central America is suitable for these two deep drilling projects. We will measure the crustal thickness, and the properties of the crust and upper mantle using seismic methods. We will determine where bend-faults form, how they evolve, and how the properties of the crust and mantle change as they do. We will specifically search for sites along a bend-fault where hot water returns to the seafloor after passing deep within the region of active serpentinization by mapping the seabed with an advanced robot submarine called Autosub and measuring the temperature of rocks 5m beneath the seabed. To find these sites and any lifeforms present would on its own be a major jump in our basic knowledge of the Earth.