Depth-dependent permeability of hydrothermal discharge zones: measurements through tidal modulation and implications for mid-ocean ridge heat budgets

Abstract:

The efficiency of hydrothermal circulation as a heat and mass exchanger is strongly modulated by the permeability structure of newly formed oceanic crust, which is known to vary spatially and temporally. Here we use the modulation of discharge temperatures by oscillatory tidal loading to estimate permeability vs. depth profiles beneath hydrothermal discharge zones at three basalt-hosted hydrothermal fields.

Phase lags between fluctuations in discharge temperature and seafloor tidal loading have previously been inverted for the permeability of the underlying discharge zone using the poroelastic modulation model of Jupp and Schulz [2004]. This has yielded a wide range of permeability estimates ranging from 10^-13­–10^-9 m². Here we extend the model to a stratified medium comprising a top layer (2A) of permeability k_2A overlying a bottom layer (2B) of permeability k_2B. We apply it to three basalt-hosted hydrothermal fields on a slow- (Lucky Strike), intermediate- (Main Endeavour Field), and fast-spreading (East Pacific Rise, EPR-9ºN) ridge, where stratification is well known from seismic studies.

We estimate a layer 2A permeability of ~10^-10 m² at Lucky Strike, in sharp contrast with EPR-9ºN, where k_2A ~ 10^-13 m². At the Main Endeavour field, two different sites located above different distinct discharge zones (as indicated by magnetic studies) yield a high and a low permeability. The permeability of layer 2B is not as well constrained as that of layer 2A, with possible values ranging from 10^-14–10^-12 m². We note, however, that the variability in measured phase lags across hydrothermal fields is compatible with a uniform layer 2B permeability of ~10^-13 m². Using theoretical scalings for high-Rayleigh porous convection, we demonstrate that the permeability of layer 2B sets the "effective permeability" of the entire convective system, and therefore the efficiency of heat extraction through young oceanic crust. A uniform layer 2B permeability would thus reconcile the observed variability of shallow permeability estimates with the effective value of 10^-13 m² that is often needed to account for the heat transferred at many hydrothermal sites.