William S. D. Wilcock

Mid-ocean ridge sulfide deposits: Evidence for heat extraction from magma chambers or cracking fronts?

Numerous seafloor observations show that the sizes of high-temperature hydrothermal sulfide edifices vary dramatically with spreading rate. On fast-spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m, while on slow-spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105–106 m3. We argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast-spreading ridges, where spreading occurs predominantly by magmatism, and slow-spreading ridges, where there is a significant component of tectonic extension. Along fast-spreading ridges, a steady-state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability. In such systems, episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis. Over a time scale of the order of a decade, mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions. On slow-spreading ridges, tectonic extension maintains the fluid pathways necessary to support vigorous convection. In the waning stages of magmatism, hydrothermal circulation driven by a downward-migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits. As the depth of circulation increases, overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively. Once hydrothermal fluids penetrate the Moho, serpentinization clogs the fluid pathways and high-temperature venting ceases until it is reactivated by a fresh magmatic intrusion.

Tidal triggering of microearthquakes on the Juan de Fuca Ridge

Tidal stresses beneath the oceans can be up to an order of magnitude higher than those found in the continents because of the effects of loading by ocean tides. I have analyzed 1899 microearthquakes recorded during a 55-day experiment on the Endeavour segment of the Juan de Fuca Ridge for tidal triggering. The tidal phase of the full data set and of a declustered subset comprising 987 events appears non-random to a high level of confidence. Earthquakes occur more frequently near low tides, especially the lowest spring tides, when the extensional stresses are a maximum in all directions.

Skew of mantle upwelling beneath the East Pacific Rise governs segmentation

Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10° anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.

Cellular convection models of mid‐ocean ridge hydrothermal circulation and the temperatures of black smoker fluids

Mid-ocean ridge hydrothermal vent fields are characterized by maximum sustained venting temperatures of 320°–380°C, irrespective of the spreading rate and the inferred depth of circulation, Metamorphic assemblages, fluid inclusions, oxygen isotope data, and the salinity of vent fluids have all been used to infer maximum circulation temperatures of up to 500°–700°C. In this paper I investigate the pattern of circulation and the relationship between venting and basal temperatures using simple models of steady open-top porous convection in a rectangular medium heated from below. Solutions obtained with variable fluid properties which approximate seawater include much more recirculation than solutions obtained with uniform fluid properties, and the temperatures of upwelling fluids are greater. For a uniform permeability, models obtained with seawater properties yield a ratio of the venting temperature to the bottom temperature in the range 0.5–0.65, values which are reasonably consistent with the observations. However, a surficial layer of high permeability significantly reduces vent temperatures. Since extrusive basalts are almost certainly very permeable, this result seems incompatible with the existence of black smoker vents. It has been suggested previously that the precipitation of hydrothermal minerals where hot and cold fluids meet will lead to the formation of an impermeable shell around upflow zones. Such a shell may provide a means to tap hot fluids from depth.

Asymmetric mantle dynamics in the MELT region of the East Pacific Rise

Abstract The mantle electromagnetic and tomography (MELT) experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Scheirer et al., Science 280 (1998) 1221–1224; Evans et al., Science 286 (1999) 752–756). Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick (Canales et al., Science 280 (1998) 1218–1221). Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric (Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Evans et al., Science 286 (1999) 752–756) at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Evans et al., Science 286 (1999) 752–756). Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies. Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.

Estimates of crustal permeability on the endeavour segment of the Juan de Fuca mid-ocean ridge

Observational studies of hydrothermal venting on the Endeavour segment of the Juan de Fuca Ridge place strong constraints on the spacing and area of vent fields, the depth of circulation, and the hydrothermal heat flux. A method is described to estimate a uniform crustal permeability from these parameters under the assumptions that upflow is confined to a narrow plume underlying each vent field and downflow can be described by potential flow into a point sink at the base of each plume. For a reasonable range of parameter values, the isotropic permeability of the Endeavour lies in the range 6 × 10−13 to 6 × 10−12 m2. A significant elongation of vent fields along-axis suggests that the permeability structure is strongly anisotropic, with the across-axis permeability about an order of magnitude lower than the permeability in orthogonal directions.

The Seismic Attenuation Structure of a Fast-Spreading Mid-Ocean Ridge

The two-dimensional P-wave attenuation structure of the axial crust of the East Pacific Rise was obtained from an inversion of waveform spectra collected during an active-source seismic tomography experiment. The structure shows that attenuation near the surface is high everywhere but decreases markedly within 1 to 3 kilometers of the rise axis. The near-axis variation is attributed to the thickening of the surface basalt layer and possibly to in situ changes in porosity related to hydrothermal circulation. High attenuation is also observed beneath the rise axis at depths ranging from about 2 kilometers (less than 1 kilometer beneath the axial magma lens) to the base of the crust. The levels of attenuation in this deeper region require at most only a small fraction of partial melt.

Thermal fluxes associated with the 1993 diking event on the CoAxial segment, Juan de Fuca Ridge: A model for the convective cooling of a dike

The 1993 diking event on the CoAxial segment, northern Juan de Fuca Ridge, generated at least three hydrothermal event plumes high in the water column and lower chronic plumes along ∼40 km of the ridge axis. A 2-year time series of temperature and light attenuation measurements within the water column shows a rapid decay of the maximum rise height of the chronic plumes over the first 3 months following the eruption. We use these measurements to estimate the hydrothermal heat fluxes into the chronic plumes at two different sites. We hypothesize that the chronic plumes resulted from the convective cooling of a dike and construct a simple two-dimensional, numerical model of this process for a vertical dike intruded into a cold, saturated porous layer. The width of the dike and the permeability and thickness of the porous layer control the transfer of heat from the dike to the seafloor. We investigate the effects of these parameters on the temporal evolution of the surficial heat fluxes. The results show that simple convective cooling of the dike can reproduce the observed temporal evolution of the heat fluxes for permeabilities in the range 10−11-10−12 m2 and a 3- to 5-m-wide dike. The high permeabilities obtained are consistent with flow through highly permeable extrusives or with the creation of a zone of high permeability near the dike walls due to fracturing. The width of the dike may well be overestimated by our model because of the limitations of the continuum approximation for the permeability in the narrow upflow zone adjacent to the dike.

Characteristics of high Rayleigh number two-dimensional convection in an open-top porous layer heated from below

Using a control-volume method and the simpler algorithm, we computed steady-state and time-dependent solutions for two-dimensional convection in an open-top porous box, up to a Rayleigh number of 1100. The evolution of the convective system from onset to high Rayleigh numbers is characterized by two types of transitions in the flow patterns. The first type is a decrease in the horizontal aspect ratio of the cells. We observe two such bifurcations. The first occurs at Ra = 107.8 when the convective pattern switches from a steady one-cell roll to a steady two-cell roll. The second occurs at Ra ≈ 510 when an unsteady two-cell roll evolves to a steady four-cell roll. The second type of transition is from a steady to an unsteady pattern and we also observe two of these bifurcations. The first occurs at Ra ≈ 425 in the two-cell convective pattern; the second at Ra ≈ 970 in the four-cell pattern. Both types of bifurcations are associated with an increase in the average vertical convective heat transport. In the bi-cellular solutions, the appearance of non-periodic unsteady convection corresponds to the onset of the expected theoretical scaling Nu ∝ Ra and also to the onset of plume formation. Although our highest quadri-cellular solutions show signs of non-periodic convection, they do not reach the onset of plume formation. An important hysterisis loop exists for Rayleigh numbers in the range 425–505. Unsteady convection appears only in the direction of increasing Rayleigh numbers. In the decreasing direction, steady quadri-cellular flow patterns prevail.

Development of a seawater battery for deep-water applications

Dissolved-oxygen seawater batteries rely on the corrosion of a reactive metal anode and the reduction of oxygen at an inert cathode to generate a potential of about 1 V when immersed in seawater. Because oxygen is not very soluble in seawater, such batteries are characterized by small cathodic current densities and are therefore most suitable for long-term low-power applications. A number of batteries have been built recently using sophisticated electrodes. The objective of this study was to evaluate the feasibility of building a seawater battery using cheap readily-available materials. The results show that magnesium anodes outperform aluminum. Although the cathodic current densities achieved in this study are markedly lower than reported previously, cathodes composed of copper (or stainless-steel) mesh perform reasonably well and facilitate the design of compact batteries with an open structure and large cathodic areas. Deep- and shallow water tests show that a battery occupying 1 m3 could produce 5 W for a year or more in oxygen-saturated waters and 1–2 W in the oxygen-starved waters of the North Pacific seafloor. The energy density of such a battery including the flotation necessary for optimal performance and recovery is estimated to be 150–400 Wh kg−1.