Characterising hydrothermal alteration across the Atlantis Massif: IODP Expedition 357

海洋覆盖了大约三分之二的地球表面，但最古老的海底还不到2亿年，因为它是在板块构造循环中不断产生和破坏的。海底由形成于大洋中脊的火山岩组成，大洋中脊是一个围绕海洋延伸约6万公里的全球水下火山链，在那里两个构造板块正在远离彼此。这两个构造板块在大洋彼岸的移动速度各不相同。目前，全球50%的大洋中脊系统正在以缓慢的扩散速度(&lt；40毫米/年，例如大西洋中脊)扩散。通过对洋壳的挖掘和科学钻探，以及对蛇绿岩的研究，洋壳的整体结构已经确定。蛇绿岩是侵位在大陆上的洋壳碎片。典型的洋壳具有分层的地层，喷发的熔岩覆盖在侵入通道和冻结的岩浆室(辉长岩)之上。然而，沿着缓慢扩张的海脊，这种典型的地层并不总是存在的，大约50%是由沿拆离断层的构造伸展形成的，这些断层将辉长岩和地幔岩石带到海底。一旦形成新的大洋地壳，冷海水就沿着裂缝向下渗透到地壳中，变得受热，并与火山岩反应，直到热的热液变得浮力，在海底离开地壳。这些反应通过形成新的热液矿物和热液流体改变了岩石和热液的化学，因此是一个重要的量化过程，以了解全球化学交换。形成的新矿物强烈依赖于初始岩石和反应热液的温度。在缓慢扩张的海脊，辉长岩和地幔岩石在海底的暴露导致不同的化学反应，特别是地幔岩石经历了广泛的蛇纹岩蚀变。蛇纹岩反应伴随着裂缝中碳酸钙矿物的形成。通过流体/岩石反应形成的碳酸钙目前正在作为二氧化碳的潜在长期储存而被研究。了解这些环境中的热液循环对于了解这一过程并最终将其用于二氧化碳的工业储存至关重要。亚特兰蒂斯地块位于大西洋中脊，是构造伸展暴露海底辉长岩和地幔岩石的一个例子。在地块的南端存在一个被称为失落城市热液场的热液喷发系统，它是由蛇纹岩反应驱动的。低温(&lt；100℃)、高pH的热液通过碳酸盐-水镁石结构在失城扩散喷发。它是已知的仅有的五个地幔岩石上的热液喷口之一。在这项研究中，IODP 357远征期间通过对亚特兰蒂斯地块进行科学海洋钻探获得的新样本将用于研究热液循环在这些长期存在的拆离断层上形成洋壳中的作用。首次恢复了整个地块的年龄横断面，这使得人们能够深入了解滑脱是如何随着其逐渐老化而变化和演化的。通过研究流体-岩石反应过程中形成的新的热液矿物，并记录它们在不同岩石类型中的分布，可以破译热液流体的路径。这些信息将与岩石和热液矿物的地球化学分析结合起来，以量化在整个亚特兰蒂斯地块的热液循环期间发生的化学变化。这一综合办法将能够确定沿拆离断层的热液环流对全球地球化学循环更广泛的水热收支的贡献。

The oceans covers approximately two thirds of the Earth's surface yet the oldest ocean floor is less than 200 million years old because it is continuously created and destroyed through the plate tectonic cycle. The ocean floor is made of volcanic rocks that form at mid ocean ridges, a global chain of under-water volcanoes that stretch for ~60,000km around the oceans, where two tectonic plates are moving away from each other. The rate at which the two tectonic plates move away from each other varies across the oceans. Currently 50% of the global mid ocean ridge system is spreading at slow spreading rates (<40 mm/yr, e.g Mid Atlantic Ridge). From dredging and scientific drilling of the ocean crust and studying ophiolites, pieces of ocean crust that have been emplaced onto the continents, the overall structure of the ocean crust has determined. 'Typical' ocean crust has a layered stratigraphy with erupted lavas overlying intrusive feeder channels and frozen magma chambers (gabbros). However along slow spreading ridges this typical stratigraphy is not always present, and ~ 50% is formed by tectonic extension along detachment faults that bring gabbros and mantle rocks to the seafloor.Once new ocean crust is formed cold seawater penetrates downwards into the crust along fractures, becomes heated and reacts with the volcanic rocks until the hot hydrothermal fluids becomes buoyant and exit the crust at the seafloor . These reactions modify the chemistry of both the rocks by the formation of new hydrothermal minerals and the hydrothermal fluids, and are therefore an important process to quantify in order to understand global chemical exchange. The new minerals that form are strongly dependent on the initial rock and the temperature of the reacting hydrothermal fluids. At slow spreading ridges, the exposure of gabbroic and mantle rocks at the seafloor results in different chemical reactions, and mantle rocks in particular undergo extensive alteration to serpentinites. Serpentinisation reactions are accompanied by the formation of calcium carbonate minerals in fractures. The formation of calcium carbonate by fluid/rock reactions is currently being investigated as a potential long-term store of carbon dioxide. Understanding hydrothermal circulation in these environments is critical for understanding this process and ultimately exploiting it for the industrial storage of carbon dioxide.The Atlantis Massif is located on the Mid Atlantic Ridge and is an example of where tectonic extension has exposed gabbroic and mantle rocks at the seafloor. A hydrothermal vent system called the Lost City Hydrothermal Field is present on the southern end of the massif and is driven by serpentinisation reactions. Low temperature (<100degC), high pH hydrothermal fluids vent diffusively at Lost City through carbonate-brucite structures. It is one of only five hydrothermal vents that are known to be hosted on mantle rocks.In this study, new samples recovered by scientific ocean drilling of the Atlantis Massif during IODP Expedition 357 will be used to investigate the role of hydrothermal circulation in the formation of ocean crust along these long-lived detachment faults. For the first time an age transect of samples across the massif has been recovered allowing insight into how the detachment changes and evolves as it progressively ages. By studying the new hydrothermal minerals that have formed during fluid/rock reaction, and documenting their distribution within the different rock types, the pathways for the hydrothermal fluids can be deciphered. This information will be combined with geochemical analyses of the rocks and hydrothermal minerals to quantify the chemical changes that have occurred during hydrothermal circulation across the Atlantis Massif. This combined approach will allow the contribution of hydrothermal circulation along detachment faults to the broader hydrothermal budget of global geochemical cycles to be determined.