Reactive porosity waves in a dehydrating and deforming slab: a scale invariant fluid release process

Subduction of hydrated lithosphere slabs into the mantle leads to the release of hydrous fluids, which contribute to a wide range of subduction zone phenomena, such as arc volcanism and seismicity. The efficiency of slab devolatilization is crucial for maintaining the overall stability of the Earth's chemical reservoirs over geological timescales. The formation of channelized fluid flow structures during dehydration, such as olivine veins observed in the Erro Tobbio meta-serpentinites (Italy), enhances the efficiency of fluid release from the subducting slab and might be crucial for devolatilization to keep up with the rate of plate subduction.

Observed olivine-rich vein assemblages and the presence of mineral-bound H₂O in the matrix is indicative of high temperatures and partial dehydration. Porosity structures develop into vein-like structures at the onset of dehydration due to intrinsic chemical heterogeneities [1], with connectivity being already reached at low porosities [2]. As dehydration progresses, the fluid pressure in the porous network rises, and buoyancy forces lead to an upward fluid flow in the rock. Porosity waves are a potential mechanism to explain fluid flow focusing structures in rocks undergoing viscous deformation.

This work presents a 2D hydro-mechanical-chemical model for reactive porosity waves in a dehydrating and deforming serpentinite that is part of a subducting slab. Based on [3], we chose SiO₂ as the metasomatic agent in a MgO-FeO-SiO₂ system with H₂O in excess. As model input, we use chemical data of serpentinites from the Mirdita ophiolite (Albania) that has not entered a subduction zone. The 2D chemical mapping of the sample from outcrop down to µm-scale shows scale-invariant heterogeneities of SiO₂ and FeO. Similar heterogeneities occur on the km-scale where they manifest as lithological differences between serpentinized harzburgites and dunites. This dataset of chemical maps ranging continuously from the µm- to dm-scale represents a scale-independent pattern of chemical heterogeneities in a dehydrating slab.

Numerical results using this data as input show spontaneous formation of fluid-rich high-permeable channels in deforming and dehydrating serpentinite associated with further olivine formation during the reactive flow of Н₂О−SiО₂ fluid carrying low SiO₂ concentration. The numerical simulations show similar pattern to field observations from Erro Tobbio. We conclude that the formation of a fluid channeling network takes place from the µm- up to km-scale and provides the main fluid escape mechanism in subduction zones.

 

[1] Plümper, O. et al. Fluid escape from subduction zones controlled by channel-forming reactive porosity. Nat Geosci 10, 150–156 (2017).

[2] Bloch, W. et al. Watching dehydration: Seismic Indication for Transient Fluid Pathways in the Oceanic Mantle of the Subducting Nazca Slab. Geochem Geophy Geosy 19, doi:10.1029/2018gc007703 (2018)

[3] Huber, K. et al. Formation of olivine veins by reactive fluid flow in a dehydrating serpentinite. Geochem Geophy Geosy, 23, e2021GC010267 (2022).