We present results from three-dimensional, transient, fully coupled simulations of fluid and heat transport on a ridge flank in fast-spread ocean crust. The simulations quantify relationships between rates of fluid flow, the extent of advective heat extraction, the geometry of crustal aquifers and outcrops, and crustal hydrologic parameters, with the goal of simulating conditions similar to those seen on 18–24 M.y. old seafloor of the Cocos plate, offshore Costa Rica. Extensive surveys of this region documented a ∼14,500 km2 area of the seafloor with heat flux values that are 10–35% of those predicted from conductive cooling models, and identified basement outcrops that serve as pathways for hydrothermal circulation via recharge of bottom water and discharge of cool hydrothermal fluid. Simulations suggest that in order for rapid hydrothermal circulation to achieve observed seafloor heat flux values, upper crustal permeability is likely to be ~10-10 to 10-9m2, with more simulations matching observations at the upper end of this range. These permeabilities are at the upper end of values measured in boreholes elsewhere in the volcanic ocean crust, and higher than inferred from three-dimensional modeling of another ridge-flank field site where there is less fluid flow and lower advective power output. The simulations also show that, in a region with high crustal permeability and variable sized outcrops, hydrothermal outcrop-to-outcrop circulation is likely to constitute a small fraction of total fluid circulation, with most of fluid flow occurring locally through individual outcrops that both recharge and discharge hydrothermal fluid.
We present three‐dimensional simulations of coupled fluid and heat transport in the ocean crust, to explore patterns and controls on ridge‐flank hydrothermal circulation on the eastern flank of the Juan de Fuca Ridge. Field studies have shown that there is large‐scale fluid flow in the volcanic ocean crust in this region, including local convection and circulation between two basement outcrops separated by ~50 km. New simulations include an assessment of crustal permeability and aquifer thickness, outcrop permeability, the potential influence of multiple discharging outcrops, and a comparison between two‐dimensional (profile) and three‐dimensional representations of the natural system. Field observations that help to constrain new simulations include a modest range of flow rates between recharging and discharging outcrops, secondary convection adjacent to the recharging outcrop, crustal permeability determinations made in boreholes, and the lack of a regional seafloor heat flux anomaly as a consequence of advective heat loss from the crust. Three‐dimensional simulations are most consistent with field observations when models use a crustal permeability of 3 × 10−13 to 2 × 10−12 m2, and the crustal aquifer is ≤300 m thick, values consistent with borehole observations. We find fluid flow rates and crustal cooling efficiencies that are an order of magnitude greater in three‐dimensional simulations than in two‐dimensional simulations using equivalent properties. Simulations including discharge from an additional outcrop can also replicate field observations but tend to increase the overall rate of recharge and reduce the flow rate at the primary discharge site.
Most of the hydrothermal circulation through the ocean crust, in terms of mass, heat, and many solute fluxes, occurs on ridge-flanks. Far from the magmatic influence of mid-ocean ridges, fluid flow is driven by lithospheric heating from below and channeled through volcanic rock outcrops that serve as high-permeability conduits between the ocean and the underlying volcanic crust. Field data in this setting is sparse due to difficulties associated with accessing these remote locations, making geologically accurate modeling particularly valuable to assessing the nature of ridge-flank hydrothermal circulation. Each study in this thesis applies a combination of modeling and field observations to constrain the hydrogeologic properties and behaviors of ridge-flank hydrothermal systems, including: (1) deriving permeability estimates from flowing subsea boreholes, (2) investigating the sustainability of outcrop-to-outcrop hydrothermal flow, and (3) constraining the properties and behaviors on a well-studied outcrop-to-outcrop system. In the first study, thermal records from flowing boreholes in young oceanic crust are used to assess borehole and formation properties, including permeability, using analytic equations and a Markov chain Monte Carlo analysis to quantify uncertainty. We find the median bulk permeability at all sites to be between 0.4 to 1.5 x 10^-11 m2, with a standard deviation of 0.2 to 0.3 log-cycles at each borehole. These results are remarkably homogenous, given the much larger variability in permeability measurements in the oceanic crust. Results from the second study illuminate the controls on hydrogeologic sustainability, flow rate, and preferred flow direction in outcrop-to-outcrop hydrothermal systems. We find that sustained flow between outcrops over tens of kilometers depends on a contrast in transmittance (the product of outcrop permeability and the area of outcrop exposure) between recharging and discharging sites, and that discharge is favored through less transmissive outcrops. These systems require aquifer permeability values ranging from 10^-12 to 10^-11 m2, consistent with field measurements and values inferred from the first chapter. In the third study, a suite of three-dimensional numerical simulations are used to characterize and constrain the permeability and thickness of the upper crustal aquifer, the permeability of outcrops, and the potential for multiple discharging outcrops and azimuthal permeability anisotropy to influence hydrothermal processes at a field site on the eastern flank of the Juan de Fuca Ridge.
Most seafloor hydrothermal circulation occurs far from the magmatic influence of mid-ocean ridges, driving large flows of water, heat and solutes through volcanic rock outcrops on ridge flanks. Here we create three-dimensional simulations of ridge–flank hydrothermal circulation, flowing between and through seamounts, to determine what controls hydrogeological sustainability, flow rate and preferred flow direction in these systems. We find that sustaining flow between outcrops that penetrate less-permeable sediment depends on a contrast in transmittance (the product of outcrop permeability and the area of outcrop exposure) between recharging and discharging sites, with discharge favoured through less-transmissive outcrops. Many simulations include local discharge through outcrops at the recharge end of an outcrop-to-outcrop system. Both of these characteristics are observed in the field. In addition, smaller discharging outcrops sustain higher flow rates than larger outcrops, which may help to explain how so much lithospheric heat is extracted globally by this process.
Thermal records from boreholes in young oceanic crust, in which water is flowing up or down, are used to assess formation and borehole flow properties using three analytic equations that describe the transient thermal and barometric influence of downhole or uphole flow. We link these calculations with an iterative model and apply Markov chain Monte Carlo (MCMC) analysis to quantify ranges of possible values. The model is applied to two data sets interpreted in previous studies, from Deep Sea Drilling Project Hole 504B on the southern flank of the Costa Rica Rift and Ocean Drilling Program Hole 1026B on the eastern flank of the Juan de Fuca Ridge, and to two new records collected in Integrated Ocean Drilling Program Holes U1301A and U1301B, also on the eastern flank of the Juan de Fuca Ridge. Our calculations indicate that fluid flow rates when thermal logs were collected were ∼2 L/s in Holes 504B, 1026B, and U1301A, and >20 L/s in Hole U1301B. The median bulk permeabilities determined with MCMC analyses are 4 to 7 × 10−12 m2 around the uppermost parts of Holes 504B, 1026B, and U1301A, and 1.5 × 10−11 m2 around a deeper section of Hole U1301B, with a standard deviation of 0.2 to 0.3 log cycles at each borehole. The consistency of permeability values inferred from these four holes is surprising, given the range of values determined globally and the tendency for permeability to be highly variable in fractured crystalline rock formations such as the upper oceanic crust.