Depth‐Dependent Permeability and Heat Output at Basalt‐Hosted Hydrothermal Systems Across Mid‐Ocean Ridge Spreading Rates

Abstract

The permeability of the oceanic crust exerts a primary influence on the vigor of hydrothermal circulation at mid‐ocean ridges, but it is a difficult to measure parameter that varies with time, space, and geological setting. Here we develop an analytical model for the poroelastic response of hydrothermal exit‐fluid velocities and temperatures to ocean tidal loading in a two‐layered medium to constrain the discharge zone permeability of each layer. The top layer, corresponding to extrusive lithologies (e.g., seismic layer 2A) overlies a lower permeability layer, corresponding to intrusive lithologies (e.g., layer 2B). We apply the model to three basalt‐hosted hydrothermal fields (i.e., Lucky Strike, Main Endeavour and 9°46′N L‐vent) for which the seismic stratigraphy is well‐established, and for which robust exit‐fluid temperature data are available. We find that the poroelastic response to tidal loading is primarily controlled by layer 2A permeability, which is about 3 orders of magnitude higher for the Lucky Strike site (∼10−10 m2) than the 9°46′N L‐vent site (∼10−13 m2). By contrast, layer 2B permeability does not exert a strong control on the poroelastic response to tidal loading, yet strongly modulates the heat output of hydrothermal discharge zones. Taking these constraints into account, we estimate a plausible range of layer 2B permeability between ∼10−15 m2 and an upper‐bound value of ∼10−14 (9°46′N L‐vent) to ∼10−12 m2 (Lucky Strike). These permeability structures reconcile the short‐term response and long‐term thermal output of hydrothermal sites, and provide new insights into the links between permeability and tectono‐magmatic processes along the global mid‐ocean ridge.

Investigating Fine‐Scale Permeability Structure and Its Control on Hydrothermal Activity Along a Fast‐Spreading Ridge (the East Pacific Rise, 9°43′–53′N) Using Seismic Velocity, Poroelastic Response, and Numerical Modeling

“…Although we observe a similar pattern of permeability highs and lows, our study suggests permeabilities that are 1-2 orders of magnitude lower. As noted by Barreyre et al (2018), the high-permeability values derived from poroelastically triggered microearthquakes are not consistent with hydrothermal flow observations, as they would result in unrealistically high heat flow around the vents and in temperatures of fluid discharge significantly lower than those observed.…”

“…Regarding the former two models, although the results appear similar, they yield to permeability difference of about an order of magnitude higher to those derived from poroelastic approach. The observed difference may be due to (i) high spatial heterogeneity of layer 2A (note the small permeability range obtained from poroelastic approach; Table ), (ii) the assumptions present in velocity (Marjanović, Fuji, et al, ) and poroelastic (Barreyre et al, ) modeling, and (iii) different sensitivity of the methodologies (see section B in SI). To keep the model consistent with the assumption we considered for our preferred layer 2A porosity model (section ), we use the results employing Carlson's relationship (A8) as our preferred permeability model (Figure d and Table ).…”

“…For layer 2B, we adopt porosity of 3.5%, corresponding to the harmonic mean (Table A2 in SI) obtained using the porosity‐velocity relationship described by equation A5 (section A and Figure A2a in SI). Other parameters such as densities of both seawater and hydrothermal fluids, bulk modulus, and storage compressibility are from Barreyre et al (). These constraints on crustal and fluid parameters, in concert with our phase lag estimates at both M2 and K1 tidal frequencies, reduce the permissible model solution space to narrow slivers shown as black contours in Figure c.…”

“…Kilometer-scale hydrothermal flow models at MOR proposed a subrange of average permeabilities for the upper crust, 10 −15 and 10 −12 m 2 (Coumou et al, 2008;Driesner, 2010;Hasenclever et al, 2014;Lowell & Germanovich, 2004;Lowell et al, 2013;Theissen-Krah et al, 2011). Finally, a number of recently developed, indirect approaches based on the analysis of the effects of tides on vent temperatures suggested values ranging from 10 −9 to 10 −15 m 2 for a narrow zone around vent sites (Barreyre et al, 2014(Barreyre et al, , 2018Barreyre & Sohn, 2016;Crone et al, 2011). As a result, both the actual permeability and its spatial variation within the upper oceanic crust, dominated by hydrothermal circulation, remain poorly constrained.…”

<i>HydrothermalFoam</i> v1.0: a 3-D hydro-thermo-transport model for natural submarine hydrothermal systems

“…porosity/permeability staying open up to 600 -800 • C (Lister, 1974). Finally, hydrothermal systems can evolve over long times scales of up to 50-100 kyrs (Jamieson et al, 2014) but also respond to shorter events like glacial sealevel changes (Middleton et al, 2016) or magmatic as well as seismic events (Germanovich et al, 2000;Wilcock, 2004;Singh and Lowell, 2015) and even tidal pressure changes (Crone et al, 2011;Barreyre et al, 2018). These spatial and temporal scales in combination with the extreme pressure (P) and temperature (T) conditions (up to 300 MPa and 1000 • C) of submarine hydrothermal systems make direct and long-term observations challenging and pose a problem for laboratory work (Ingebritsen et al, 2010).…”