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

大气层、水圈、生物圈和固体地质圈之间的相互作用是我们地球系统运行的基础。这些相互作用推动了热量、质量和化学成分的交换，这些成分对我们的宜居和活跃的星球至关重要。海水的化学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。这些交换发生的时间和持续时间对于指导我们将这些结果外推到全球范围也很重要。新海底的形成是板块构造循环的基础步骤，并沿着环绕大洋盆地的海底火山链发生，被称为大洋中脊。通过在大洋中脊扩张的海底形成新的洋壳，是地球内部向表面交换热量和质量的主要机制。大洋中脊的大部分热量传递是通过海水在新形成的岩石中循环进行的。海水与这些炽热的岩石发生反应，将海水转化为加热的热液，这些热液通过壮观的黑烟喷口排出海底，但也作为较低温度的扩散液排出。在板块构造期间，随着新形成的海底远离大洋中脊和年龄，热液循环仍在继续，而且由于海底的巨大体积，即使是很小的化学交换也会扩大到向海洋的重大地球化学通量。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-岩石反应产生的化学变化。海底随着年龄的变化是如何变化的，无论是物理、化学还是生物(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，科学的大洋钻探已经成功地对火山上地壳进行了采样，但这种采样在年龄上偏向海底&10 Ma，而且只有几个钻孔采样了最古老的洋壳(~170 Ma)。IODP探险390和393是专门为解决这一采样差距而设计的，将在600-6100万年前的地壳中钻出一段横断面的洞。该项目将针对代表热液蚀变(海水-岩石循环的产物)范围的这些新钻孔的样品，对其进行地球化学分析，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品进行特定同位素(U和Pb)的分析，以确定它们形成的年龄。这些新的数据集将被结合在一起，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.