Characterising hydrothermal alteration across the Atlantis Massif: IODP Expedition 357

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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.