Sensitivity of hydrothermal vent temperatures to changes in crustal permeability profiles

Hydrothermal circulation at mid-ocean ridges is permitted by the highly permeable young oceanic lithosphere and the presence of a shallow heat source, both of which can fluctuate on different time scales in response to tectonic and magmatic activity. Seafloor observatories increasingly allow us to quantify how hydrothermal discharge responds to these changes, by continuously measuring key properties of vent fluids such as temperature, chemical composition, or flow rate. Barreyre et al. (2025, PNAS) for example showed that hydrothermal vent temperatures at the East Pacific Rise (EPR) 9º50’N steadily increase between eruptions, as the axial melt lens inflates. The models used to interpret these measurements, however, have thus far assumed a uniform permeability along the fluid upflow path, when magmatic inflation likely imparts depth-dependent changes to the permeability field.

To remedy this, we developed SAPHYR, a semi-analytical workflow to study the behavior of an axisymmetric (1-D) hydrothermal upflow zone with a depth-dependent permeability profile, subjected to lateral heat loss. SAPHYR specifically predicts the steady-state temperature and velocity of upwelling fluids, from heat source to seafloor, given a basal heat input and background permeability profile. It is benchmarked against standard models that assume both uniform and exponentially-decaying permeability profiles.

We use SAPHYR to assess how exit fluid temperatures may evolve in response to depth-dependent perturbations of the upflow zone permeability profile. At the EPR, such perturbations could stem from changes in the mean stress of the upper oceanic crust caused by an inflating axial melt lens. To test this idea, we run a large parametric study where we compare the state of the hydrothermal discharge zone before and after imposing a perturbation, and do so for a wide range of basal heat inputs, background permeability profiles, and degree of lateral heat loss. We find that an inflating melt lens can either drive an increase or a decrease in hydrothermal vent temperatures depending on the basal heat-flow and the vent location with respect to the inflating body. Our findings explain why neighboring hydrothermal vents may respond differently to the same sub-seafloor deformation process, as was documented at the EPR. They further open a path to inverting changes in sub-seafloor permeability and stress from time series of black smoker temperatures.