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Hybrid shallow on-axis and deep off-axis hydrothermal circulation at fast-spreading ridges


Hydrothermal flow at oceanic spreading centres accounts for about ten per cent of all heat flux in the oceans1,2 and controls the thermal structure of young oceanic plates. It also influences ocean and crustal chemistry, provides a basis for chemosynthetic ecosystems, and has formed massive sulphide ore deposits throughout Earth’s history. Despite this, how and under what conditions heat is extracted, in particular from the lower crust, remains largely unclear. Here we present high-resolution, whole-crust, two- and three-dimensional simulations of hydrothermal flow beneath fast-spreading ridges that predict the existence of two interacting flow components, controlled by different physical mechanisms, that merge above the melt lens to feed ridge-centred vent sites. Shallow on-axis flow structures develop owing to the thermodynamic properties of water, whereas deeper off-axis flow is strongly shaped by crustal permeability, particularly the brittle–ductile transition. About 60 per cent of the discharging fluid mass is replenished on-axis by warm (up to 300 degrees Celsius) recharge flow surrounding the hot thermal plumes, and the remaining 40 per cent or so occurs as colder and broader recharge up to several kilometres away from the axis that feeds hot (500–700 degrees Celsius) deep-rooted off-axis flow towards the ridge. Despite its lower contribution to the total mass flux, this deep off-axis flow carries about 70 per cent of the thermal energy released at the ridge axis. This combination of two flow components explains the seismically determined thermal structure of the crust and reconciles previously incompatible models favouring either shallower on-axis3,4,5 or deeper off-axis hydrothermal circulation6,7,8.

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Figure 1: Results of our 2D and 3D numerical experiments.
Figure 2: Mass flux analysis.
Figure 3: Comparison between observed and modelled hydrothermal fluid temperatures.

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We thank reviewers T. Driesner and P. Johnson for comments that led to more insights into hydrothermal energy transport and improved the manuscript.

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Authors and Affiliations



J.H. and J.P.M. developed the 3D numerical model. J.H. carried out the 3D simulations, did the post-processing and designed the figures. 2D simulations were done by S.T.-K. and J.H. (using the 2D model developed by S.T.-K., L.H.R., K.I. and J.P.M.). J.H. and L.H.R. wrote the initial manuscript, to which J.P.M., S.T.-K., K.I., S.P. and C.W.D. contributed geological and thermodynamic implications. Figures and text were edited and improved by all authors. All authors discussed the results and implications at all stages of the manuscript.

Corresponding author

Correspondence to Jörg Hasenclever.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Best-fit crustal permeability field.

Using the coupled crustal accretion and hydrothermal flow 2D model, we constructed a permeability field that leads to a thermal evolution of the oceanic crust that best matches the seismically inferred temperature field10 (see Fig. 1a, b). The permeability field is constructed using equations (1) and (2) in the Methods. The inset shows the permeability decrease between 600 °C and 800 °C that accounts for the vanishing pore space once the crustal rocks become ductile at higher temperatures22. Key features of this best-fit permeability field are regions of higher permeability adjacent to the hot slot, a decrease in permeability beyond a distance to the ridge axis of 5 km and a 250-m-thick highly permeable pillow lava layer at the top.

Extended Data Figure 2 3D domain split into 32 subdomains.

All 3D calculations were conducted on a computer with 32 processors. The model domain covers 16 km across-ridge (8 km on each side of the ridge axis; x-direction), 6 km along the ridge (y-direction) and up to 6.5 km depth (z-direction). The bottom boundary corresponds to the 1,000 °C isotherm of the best-fitting 2D calculation. The volume is meshed with 25 million tetrahedral elements. Mesh resolution (that is, spacing of the finite-element nodes) ranges from 25 m within the on-axis region and along the domain bottom to about 250 m in the low permeability regions beyond a distance to the ridge of 5 km. The regular triangular structures inside each larger triangle result from recursive splitting of each tetrahedron into 8 sub-elements each with one-eighth of the original volume. This recursive mesh refinement is repeated to generate all levels required for the geometric multigrid algorithm (see ref. 31 and Methods).

Extended Data Figure 3 Vertical mass flux in on- and off-axis flow component.

Analysis of vertical mass fluxes as a function of fluid temperature in different regions of the 3D domain: a, On-axis, within pillow basalt layer; b, on-axis between pillow basalts and mixing region above melt lens; c, on-axis within mixing region above melt lens; d, off-axis within pillow basalt layer; and e, off-axis below pillow basalts. f, This panel shows these different regions for mass flux evaluation, and the colours of each histogram (ae) correspond to the colours in f. Figure 2b in the main text shows vertical mass fluxes below the 250-m-thick pillow basalt layer and outside the mixing region overlying the melt lens: that is, Fig. 2b is the compilation of panels b and e in this figure. For completeness, we also show the vertical mass fluxes in the remaining parts of the domain. Note that all histograms are scaled by the total upward and downward mass flux in the entire domain. On-axis upflow within the highly permeable pillow basalt layer (upper bars in a) reveals shallow entrainment of cold sea water into the hot upwelling plumes, because the distribution is shifted towards colder temperatures compared to b. Most on-axis downflow in the pillow basalts (lower bars in a) occurs at low temperatures because the fluid has not yet had time to warm up to its optimum temperature range of 100 °C–200 °C for efficient downward flow (compare with b). At the top of the melt lens, hot off-axis fluids mix with the on-axis component. The upward mass flux distribution in this region (upper bars in c) shows, therefore, characteristics of both flow components. d, The off-axis vertical mass flux in the pillow basalt layer, highlighting the absence of any off-axis venting.

Extended Data Figure 4 Additional 3D experiments.

The key finding of our study is the coexistence of two different flow components—a supra-melt-lens on-axis circulation and a deep off-axis flow in the lower crust—that both contribute to on-axis venting. A mass flux analysis (Fig. 2a) shows that the on-axis component contributes 60% and the off-axis component 40% to the total discharge at the ridge. To test how robust this relative importance of the two components is, we performed additional 3D calculations, in which we deformed the bottom boundary of the 3D domain. These perturbations mimic along-axis variations of depth and lateral position of the melt lens. a, The simulation without along-axis variations of the bottom boundary; this simulation is presented in the main text and shown here for completeness. b, A simulation in which we added a sinusoidal vertical variation (amplitude 200 m, wavelength 4,000 m) to the bottom boundary. The vent field distribution is affected by this modification in that more vent fields form above the ‘highs’ and fewer above the ‘lows’. The strongest discharge still occurs exactly on-axis, where the energy input of the 400-m-wide dyking region supports the thermal plumes. c, The same experiment but without the energy input from the dyking region. We find stronger venting above the edges of the melt lens, but again more vent fields form above regions where the bottom boundary is shallower. Finally, in d we added a lateral perturbation (amplitude 400 m, wavelength 4,000 m) in addition to the vertical variation. This lateral displacement can also be recognized in the vent field distribution that crudely follows this perturbation. Nevertheless, more vent fields still form above the ‘highs’ and heat input from the dyking regions leads to the strongest venting above this region. Although the location and shape of the vent fields depend on the vertical and lateral along-axis variations of the bottom boundary, the overall flow structure of the hydrothermal fluid flow through upper and lower crust persists. We find that the relative contributions of the on- and off-axis components to the total hydrothermal mass flux are still about 60:40% (see Extended Data Fig. 5).

Extended Data Figure 5 Mass flux analysis for additional 3D experiments.

We analysed the mass fluxes in the three additional calculations (see Extended Data Fig. 4) and compared them to the calculation with uniform along-axis domain bottom (a and b; also shown in Fig. 2 of the main text). c and d, Mass fluxes in the experiment with sinusoidal vertical variation (amplitude 200 m, wavelength 4,000 m) of the bottom boundary. e and f, Mass fluxes in the experiment with the same sinusoidal depth variation but without heat input from the dyking region. We note that vent field locations shift laterally by a few hundred metres and are now located above the edges of the melt lens, which has a half-width of 600 m. g and h, Mass fluxes in the experiment with lateral perturbation (amplitude 400 m, wavelength 4,000 m) in addition to the above vertical variation. The relative contribution of on- and off-axis flow to the total discharging mass flux varies by only by few per cent in the four scenarios. Neglecting the heat input in the region of the dyke intrusions leads to a 5%–6% lower total discharging mass flux compared to the other experiments.

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Hasenclever, J., Theissen-Krah, S., Rüpke, L. et al. Hybrid shallow on-axis and deep off-axis hydrothermal circulation at fast-spreading ridges. Nature 508, 508–512 (2014).

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