Hydrothermal alteration during the aging of the ocean crust: IODP Expedition 393 South Atlantic Transect

大气、水圈、生物圈和固体地圈之间的相互作用是我们地球系统如何运作的基础。这些相互作用推动了热量、质量和化学成分的交换，这对我们宜居和活跃的星球至关重要。海水的化学成分记录了这些交换之间不断变化的平衡，了解和量化进出海洋的通量对于了解地球系统至关重要。这些交流发生的时间和持续的时间对于指导我们在全球范围内推断这些结果也很重要。新海底的形成是板块构造循环的基础步骤，它沿着海底火山链（即环绕海洋盆地的洋中脊）形成。洋中脊的海底扩张所形成的新洋壳是地球内部与表面热和质量交换的主要机制。洋中脊的大部分热量传递是通过海水在新形成的岩石中的循环进行的。海水与这些热岩石发生反应，将海水转化为加热的热液，这些热液通过壮观的黑烟喷口从海底排出，但也会成为温度较低的扩散流体。在板块构造期间，随着新形成的海底远离洋中脊和年龄的增长，热液循环继续进行。由于海底体积巨大，即使是很小的化学交换也会扩大到海洋的重要地球化学通量。构成海底的岩石通过形成新的矿物记录了这种与海水的相互作用，这些矿物可以被识别和分析，以了解热液流体的温度和路径，以及流体-岩石反应引起的化学变化。海底是如何随着年龄的增长而变化的，这是物理的、化学的还是生物的（微生物活动），这很重要，但主要是未知的。这是因为尽管在过去的50年里，科学海洋钻探已经成功地对火山上地壳进行了采样，但这种采样偏向于年龄<10Ma的海底，只有少数钻孔对最古老的海洋地壳（~170Ma）进行了采样。IODP探险队390和393是专门为解决这一采样缺口而设计的，他们将在600万至6100万年前的地壳中钻一个孔的样带。该项目将以这些新钻孔的样品为目标，这些样品代表了热液蚀变（海水-岩石循环的产物）的范围，并分析它们的地球化学，以了解和量化热液循环期间元素交换的程度。为了帮助我们了解这些反应发生的时间和时间，将对热液矿脉中的碳酸钙样本进行特定同位素（U和Pb）分析，以确定它们形成的年龄。这些新的数据集将结合在一起，以建立对这一关键地球过程中发生的地球化学交换的新见解。

Interactions between the atmosphere, hydrosphere, biosphere, and the solid geosphere are fundamental to how our Earth system operates. These interactions drive exchanges in heat, mass and chemical components that are essential for our habitable and active planet. The chemistry of seawater records the changing balance between these exchanges and understanding and quantifying the fluxes to and from the oceans are essential for understanding the Earth system. When and for how long these exchanges take place are also important to guide our extrapolation of these results to a global scale.The formation of new seafloor is the foundation step of the plate tectonic cycle and takes place along the chain of submarine volcanoes know as mid-ocean ridges that circuit the ocean basins . New ocean crust formed though seafloor spreading at the mid-ocean ridges is the principal mechanism for the exchange of heat and mass from Earths interior to the surface. Much of this heat transfer at mid-ocean ridges takes place by the circulation of seawater through the newly formed rocks. Seawater reacts with these hot rocks transforming seawater into heated hydrothermal fluids that are expelled at the seafloor through spectacular black-smoker vents but also as lower temperature diffuse fluids. Hydrothermal circulation continues during plate tectonics as the newly formed seafloor moves away from mid-ocean ridge and ages, and due to the vast volumes of the seafloor, even small chemical exchanges scale up to significant geochemical fluxes to the oceans. The rocks that make up the seafloor record this interaction with seawater by forming new minerals that can be identified and analysed to understand the temperatures and pathways of hydrothermal fluids and the chemical changes that result from fluid-rock reaction. How the seafloor changes as it ages, whether this is physical, chemical or biological (microbial activity), is important but chiefly unknown. This is because although scientific ocean drilling has successfully sampled the volcanic upper crust over the last 50 years, this sampling is biased towards seafloor <10Ma in age and there are only a few boreholes that sample the very oldest ocean crust (~170Ma). IODP Expeditions 390 and 393 have been specifically designed to address this sampling gap, and will drill a transect of holes in crust between 6-61 million years old. This project will target samples from these new drill holes that represent the range of hydrothermal alteration (the product of seawater-rock circulation) and analyse them for their geochemistry to understand and quantify the extent of elemental exchange during hydrothermal circulation. To help inform our understanding on when and for how long these reactions took place, samples of calcium carbonate from hydrothermal veins will be analysed for specific isotopes (U and Pb) to determine the age that they formed at. These new datasets will be combined to build up new insights into the geochemical exchanges happening during this key Earth process.