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

Abstract

Hydrothermal flow at oceanic spreading centres accounts for about ten per cent of all heat flux in the oceans 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-axis or deeper off-axis hydrothermal circulation.

The geochemistry of methane isotopologues

“…3.4 showing temperatures from D/H geothermometry of H 2 -H 2 O in endmember fluids at Lost City (Proskurowski et al, 2006) shows that these timescales for fluid transit are also consistent with estimated kinetics of D/H exchange between H 2 and H 2 O. Timescales inferred here may also be compared with constraints on upflow velocities from 1D reactive transport models of fluids ascending from~750 mbsf and cooling via conduction (Seyfried et al, 2015). Actual timescales of circulating fluid may vary widely due to significant contributions of both on-and off-axis recharge and circulation (Hasenclever et al, 2014).…”

“…3.4 showing temperatures from D/H geothermometry of H 2 -H 2 O in endmember fluids at Lost City (Proskurowski et al, 2006) shows that these timescales for fluid transit are also consistent with estimated kinetics of D/H exchange between H 2 and H 2 O. Timescales inferred here may also be compared with constraints on upflow velocities from 1D reactive transport models of fluids ascending from~750 mbsf and cooling via conduction (Seyfried et al, 2015). Actual timescales of circulating fluid may vary widely due to significant contributions of both on-and off-axis recharge and circulation (Hasenclever et al, 2014).This study emphasizes that the use of bulk and position-specific D/H ratios and clumped isotopologues abundances of small organic molecules as geothermometers or geospeedometers requires an understanding of the factors controlling hydrogen exchange rates (Eiler, 2013). Rigorous exchange experiments under simulated natural conditions may yield broadly-applicable insights into interactions of CH 4 or other hydrocarbons with minerals or water.…”

“…Low-angle, large-offset, and long-lived (>1 Myr) normal faults near vent fields at slow-spreading ridges allow for fluid penetration deep into plutonic rocks of layer 3, enabling access to fresh gabbroic material and/or inclusions to be leached (Kelley, 1996;Schroeder et al, 2002;Schlindwein and Schmid, 2016). In contrast, in fast spreading environments such as the East Pacific Rise, shallow melt lenses at 1 to 2 km below seafloor may limit the depth of circulation (e.g., Hasenclever et al, 2014;and references in Alt, 1995).…”

Subduction zone forearc serpentinites as incubators for deep microbial life

“…In contrast, habitable zones in the vicinity of oceanic spreading centers are limited to the first hundred meters to few kilometers below the seafloor. The exact location of the 122°C isotherm will likely vary in depth at and around the spreading center as a result of the ridge architecture, heat flux, and hydrothermal circulation (42,43). The downhole temperature within the International Ocean Drilling Program (IODP) Hole U1309D at Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) places the 122°C upper temperature limit for microbial life at ∼1,000 mbsf (44).…”

Transport efficiency and dynamics of hydraulic fracture networks

“…However, these models are focused on the micro-to meso-scale, where single cracks and fractures can be numerically resolved. Existing numerical models for crustal scale flow, on the other hand, do not consider the interplay between fluid flow and hydraulic fracturing, but assume that the intrinsic matrix permeability of rocks is the only relevant parameter [e.g., [37][38][39]. Miller and Nur [21] developed a crustal scale cellular automaton model which was able to capture the general dynamics of large scale hydraulic fracture networks, but wasn't based on realistic concepts for fluid propagation and closing of fractures.…”