Characterising hydrothermal alteration across the Atlantis Massif: IODP Expedition 357

描述亚特兰蒂斯地块热液蚀变的特征：IODP Expedition 357

• 批准号: NE/P000061/1
• 负责人: Michelle Harris
• 金额: $ 3.52万
• 依托单位: Plymouth University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2016
• 资助国家: 英国
• 起止时间: 2016 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FP000061%2F1
• 关键词: Characterising; hydrothermal; alteration; across; Atlantis

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.

海洋约占地球表面的三分之二，但最古老的海底年龄不到2亿年，因为它是在板块构造周期中不断产生和毁灭的。洋底是由形成于大洋中脊的火山岩组成的，这是一个全球性的水下火山链，围绕海洋延伸约60，000公里，两个构造板块正在相互远离。两个构造板块相互远离的速度在大洋上各不相同。目前，全球50%的洋中脊系统正在以缓慢的扩张速度（<40毫米/年，例如大西洋中脊）扩张。通过对海洋地壳的疏浚和科学钻探以及对蛇绿岩的研究，海洋地壳的整体结构已经确定。“典型的”洋壳具有分层的地层，喷发的熔岩覆盖在侵入的补给通道和冻结的岩浆房（辉长岩）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，约50%是由构造伸展沿着剥离断层形成的，这些断层将辉长岩和地幔岩带到海底。一旦新的洋壳形成，冷海水沿着沿着裂缝向下渗透到地壳中，变热并与火山岩反应，直到热的热液流体变得有浮力并在海底离开地壳。这些反应通过形成新的热液矿物和热液流体来改变岩石的化学性质，因此是为了理解全球化学交换而量化的重要过程。形成的新矿物强烈依赖于初始岩石和反应热液流体的温度。在缓慢扩张的海脊，辉长岩和地幔岩在海底的暴露导致不同的化学反应，特别是地幔岩经历了蛇纹岩的广泛蚀变。蛇纹石化反应伴随着裂缝中碳酸钙矿物的形成。目前正在研究通过流体/岩石反应形成碳酸钙作为二氧化碳的潜在长期储存。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的一个例子。一个被称为迷城热液场的热液喷口系统存在于南极洲的南端，并受到蛇纹化反应的驱动。低温（<100 ℃）、高pH值的热液流体通过碳酸盐-水镁石结构在迷城扩散排出。在这项研究中，在国际大洋钻探项目第357次考察期间对亚特兰蒂斯地块进行科学海洋钻探所获得的新样本将用于调查热液循环在形成海洋地壳中的作用，这些海洋地壳沿着这些长期存在的剥离断层。第一次在整个ARF样本的年龄横断面已经恢复允许洞察如何脱离的变化和演变，因为它逐渐年龄。通过研究在流体/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相结合，以量化亚特兰蒂斯地块热液循环期间发生的化学变化。这一综合办法将有助于确定沿沿着拆离断层的热液循环对全球地球化学循环的更广泛热液收支的贡献。

项目成果

Magmatism, serpentinization and life: Insights through drilling the Atlantis Massif (IODP Expedition 357)

• DOI: 10.1016/j.lithos.2018.09.012
• 发表时间: 2018-12
• 期刊: Lithos
• 影响因子: 3.5
• 作者: G. Früh-Green;B. Orcutt;S. Rouméjon;M. Lilley;Y. Morono;C. Cotterill;S. Green;J. Escartín

Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

蛇纹石化和多相蚀变亚特兰蒂斯地块橄榄岩的地球化学（IODP Expedition 357）：熔岩与流体岩过程的岩石成因和区分

• DOI: 10.1016/j.chemgeo.2021.120681
• 发表时间: 2022
• 期刊: Chemical Geology
• 影响因子: 3.9
• 作者: Whattam S