Decompaction-Driven Overpressurisation of Mid-Ocean Ridge Magma Lenses

Seismic imaging of magmatically robust mid-ocean ridges (MORs) reveals the presence of sill-shaped axial melt lenses (AMLs) located a few kilometres below the seafloor, overlying and embedded within mush zones. AMLs are active features: they must undergo rapid replenishment to provide the heat that fuels high-temperature hydrothermal convection. Ocean bottom pressure sensors have shown that this replenishment causes steady uplift of the seafloor over decadal time scales, which is partially or completely reversed during MOR eruptions. Previous studies of this phenomenon have typically modelled seafloor displacements by imposing overpressurisation rates in a tensile deformation source embedded in a (visco-)elastic half-space. Very few, however, have focused on the physical mechanisms that enable overpressurisation of a magma pocket in a mush zone.

To address this gap, we test the hypothesis that AMLs represent boundary layers formed by the decompaction of partially molten rocks beneath a permeability barrier (e.g., the brittle-ductile transition). Using numerical two-phase poro-viscous flow models, we calculate the buoyant load exerted by a decompacting boundary layer on its overlying permeability barrier. By systematically varying the solid and liquid shear viscosities, bulk viscosity exponent, background porosity, and grain size, we obtain a range of overpressure buildup rates that show strong agreement with a simple scaling analysis. The bulk viscosity exponent, background porosity, and grain size exert the strongest control on the rate of overpressure buildup. We then convert our computed loading rates to seafloor uplift rates using elastic dislocation models and compare them with data from the East Pacific Rise at 9°50’N. By doing so, we demonstrate that the decompaction of magmatic mush is a viable mechanism for AML overpressurisation and seafloor inflation. Future work will aim to incorporate more realistic rheologies for the magma-mush system and assess their impact on the rates of AML inflation.