Deciphering the fluid-rock interaction during penetration in the deep sections of the oceanic crust, Atlantis Massif (IODP Exp. 399)

International Ocean Discovery Program (IODP) Expedition 399 will recover new cores from the roots of the Atlantis Massif (30N; Mid-Atlantic Ridge), an oceanic core complex that has transformed our understanding of tectonic, magmatic and chemical processes at slow-spreading ridges. The exhumation of these deep (>1.4km) and originally dry crustal assemblages at shallow depths is achieved via detachment faulting and leads to their often profound modifications via seawater alteration (serpentinization) processes. The latter have major consequences for the properties of the oceanic lithosphere (incl. volatile contents), heat exchange between the ocean and crust and the overall bio-geochemical cycles.

In particular, the proposed investigation is to use the down core variations of trace elements and boron and strontium isotopes as dependable proxies for hydration reactions within the deep (and hot) parts of the slow spreading crust. The proposed chemical profiles across the altered oceanic crust at the Atlantis Massif will allow the much-needed quantification of the extent, rates and chemical gradients resulting from the currently unknown volumes and rates of seawater propagation with depth.

This knowledge will be critical for the understanding of the importance of alteration minerals for accommodating volatiles (H2O), as well as many other elements considered important for Life on Earth. After plate convergence, similar altered lithosphere must participate and ultimately dehydrate and release its volatiles and thus impacting arc volcanism. Accordingly, quantifying the extend of alteration (hydration) with the fluid-sensitive elemental and isotope tracers will lead to more trustworthy constraints for the important mineralogical and thermal state of the altered oceanic crust as participant in the elemental and isotope cycling across subduction zones.