From Ridge to Trench, MoHole to Bend-Faults

从山脊到海沟，从莫孔到弯曲断层

• 批准号: NE/M021246/1
• 负责人: Timothy Henstock
• 金额: $ 46.98万
• 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: 2022
• 资助国家: 英国
• 起止时间: 2022 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FM021246%2F1
• 关键词: Ridge; Trench; MoHole; Bend; Faults

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.

地幔几乎构成了地球的全部体积，被认为是由类似于石质陨石的物质构成的。几乎在地球上的任何地方，它都位于一层薄薄的地壳之下，这是地幔反复融化和重新融化的产物。自20世纪50年代以来，在地壳中钻一个“Mohole”以获取第一个地幔样本一直是地球科学家的目标，可以与从月球、火星或金星上获取样本相媲美。这些样本是确认或反驳数十年来关于地球海洋和大陆地壳如何形成和演化、形成火山的地质过程、板块构造以及地球上生命存在的先决条件的唯一途径。大陆地壳通常厚于30公里，使地幔遥不可及。海洋下面的地壳覆盖了地球表面的65%，厚度往往只有5.5公里，现在这项任务是可能的。这一目标是可以实现的，成本与中等规模的太空任务类似，但需要国际社会的努力，日本已经设计和建造了一艘GB 1000米的钻井船--千岛号，莫霍尔是一个关键目标。中美洲近海是莫霍尔的一个特别有希望的地点，因为那里的水深不到4公里，板块年龄在20英里左右，减少了技术挑战，而且洋壳形成于一个经过充分研究的快速(10厘米/年)的中洋扩张中心。此外，这是世界上唯一一个我们可以加强莫霍尔钻探的地区，当构造板块在大洋海沟处弯曲俯冲到地球内部时，我们可以探索地壳和地幔的变化。中美洲近海是唯一一个发生这种板块弯曲的地方，其深度低于Chikyu钻井作业的4.5公里海底深度限制。我们现在意识到，海沟附近的板块弯曲可能与洋壳下方的海水和地幔之间的重大化学反应有关，自洋壳上升并融化到大洋中脊之下以来，洋壳已经显著冷却。海沟外侧形成的弯曲断层似乎是这一过程的关键，为海水通过地壳进入浅地幔提供了通道。为石油勘探开发的声波(地震)工具显示，这些断层位于弯曲的地壳中，并在几个地方进入地幔长达10公里。从降低的地震波速度我们知道，海水和冷地幔发生了反应，形成了大量富含水的产品，称为蛇纹石。如果在板块弯曲过程中形成大量蛇纹石，这将对地球的全球碳循环和水循环产生深远的影响。作为蛇纹岩化反应产物形成的碳酸盐可以将二氧化碳转移到地幔中，其数量相当于地表风化和造山所消耗的二氧化碳。深层的化学合成生命也可能利用蛇纹岩反应释放的能量。这一地区可能是生命深入地球最深的地方，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，钻入其中一个弯曲断层附近的浅地幔，以采样岩石和流体。在这项研究中，我们将测试中美洲近海板块是否适合这两个深部钻探项目。我们将使用地震方法测量地壳厚度和地壳和上地幔的性质。我们将确定弯曲断层在哪里形成，它们是如何演化的，以及地壳和地幔的性质如何随着它们的变化而变化。我们将专门搜索弯曲断层沿线的地点，通过使用名为Autosub的先进机器人潜艇绘制海床地图，并测量海床下5米处岩石的温度，在这些地点热水经过活跃的蛇纹岩化区域深处返回海底。找到这些地点和存在的任何生命形式本身就是我们对地球基本知识的一次重大飞跃。