Hydration of the Oceanic Lithosphere: Impact on Hydrothermal Fluid Chemistry and Seismicity.

Slow and ultraslow spread oceanic lithospheres consist of a mixture of magmatic rocks and mantle rocks with variable alteration degrees. However, the nature, extent and distribution of alteration mineral assemblages are not well constrained. Understanding this alteration pattern at ridges is key to determining the nature of hydrothermal fluids and the seismic structure of the oceanic lithosphere, with implications for seismogenesis at ridges, transform fault zones, and subduction zones. Here, we present 2D numerical models that aim to explore the nature of alteration mineral assemblages and the seismic structure of the oceanic lithosphere during ultraslow magma-poor spreading. For this, we couple thermodynamic calculations with a visco-elasto-plastic model. We simulate the formation of the oceanic lithosphere, ongoing faulting, magmatism, hydrothermal cooling and hydration reactions, starting from continental extension to oceanic spreading. We compare our results with the Gakkel Ridge and the magma-poor section of the Southwest Indian Ridge at 6430 East; as both present similarities in the magma production rate and mineral assemblages suggesting similar conditions of hydrothermal alteration 1-3. Our model reproduces the observed seismic structure of this part of the oceanic lithosphere and its alteration mineral assemblages. Importantly, we show that the interaction between faulting, hydrothermal cooling and hydration reactions results in a complex compositional nature of the oceanic lithosphere. In particular, we find a correlation between the spatial distribution of seismicity peaks and changes in mineral stability fields at mid-ocean ridges. We discuss the impact of such a compositional complexity on hydrothermal vent chemistry and the seismogenic behavior of the oceanic lithosphere.1- Patterson, S.N., et al., High temperature hydrothermal alteration and amphibole formation in Gakkel Ridge abyssal peridotites. Lithos, 2021. 392: p. 106107.2- Bickert, M., M. Cannat, and D. Brunelli, Hydrous fluids down to the semi-brittle root zone of detachment faults in nearly amagmatic ultra-slow spreading ridges. Lithos, 2023. 442: p. 107084.3- Dessimoulie, L., et al., Major and trace elements exchanges during fluid-rock interaction at ultraslow-spreading oceanic lithosphere: Example of the South West Indian Ridge (SWIR). Lithos, 2020. 352: p. 105233.