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

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.