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Chemical analysis of 18 diffuse hydrothermal fluids collected from the Lucky Strike hydrothermal vent site reveals the presence of less than 7% high-temperature hydrothermal end-member. Temperature-Li and temperature-Mg relationships for the diffuse fluids indicated two sample groups. One group of three samples had temperatures which fell on or below the temperatures expected for conservative mixing between seawater and high-temperature vent fluids, consistent with mixing and cooling. The second grouping had diffuse fluid temperatures greater than expected from conservative mixing of high-temperature end-member fluids and seawater. This indicates that the diffuse hydrothermal effluent was heated conductively during its formation. The Lucky Strike site is characterized by a hydrothermal slab of basaltic fragments indurated by silica and barite. It is proposed that the diffuse fluids circulate within the cracks of the slab, forming a shallow short-pass hydrothermal convection system. The source of heat is likely to be high-temperature hydrothermal fluids circulating beneath the hydrothermal slab. The chemistry of the diffuse fluids shows no evidence for silica precipitation, suggesting that the slab is not actively forming from the diffuse fluids at the present.

Variations in the electrical conductivity of the mantle beneath Carty Lake in the Canadian Shield, Tucson in the southwest United States, and Honolulu and Midway in the north central Pacific were determined through the inversion of long-period magnetotelluric and geomagnetic depth sounding data. Inversion of computed response functions is carried out using a minimum structure, regularized approach. The upper mantle beneath Carty Lake is approximately an order of magnitude more resistive than the upper mantle beneath Tucson and nearly 1.5 orders of magnitude more resistive than Honolulu and Midway Island. Inversions were also carried out where the minimum structure constraint was removed at known upper mantle discontinuities. These models show a jump in conductivity of ∼1.5 orders of magnitude across the 660 km discontinuity, a result that is consistent with laboratory experiments on realistic mantle assemblages. Mantle conductivity profiles at Carty Lake are significantly more resistive than those at Tucson, Honolulu, and Midway to depths of ∼300–400 km. These observations likely reflect differing thermal states, the presence (or absence) of partial melt and volatiles, and may also be related to chemical differences between depleted and undepleted upper mantle. The observed conductivity variations may be interpreted as lateral variations in temperature, partial melt, and/or dissolved hydrogen in olivine.

A finite element numerical method is presented for computing electromagnetic induction in a heterogeneous conducting sphere by external source current excitation. The numerical model has applicability in the problem of determining the three-dimensional electrical conductivity structure of Earth's mantle. The formulation is in terms of vector and scalar potentials. Boundary value problems are derived for fully three-dimensional (3-D) and azimuthally symmetric geometries. To validate the code, we check against a quasi-analytic solution for an azimuthally symmetric configuration of eccentrically nested spheres and against an integral equation solution for an azimuthally symmetric, buried thin spherical shell model. To illustrate the capability of the code for modeling the electromagnetic response of realistic mantle structural variations, we compute solutions for an azimuthally symmetric electrical model whose lateral heterogeneity corresponds to zonal averages of the topographic relief on the 660-km seismic discontinuity. The latter is derived from differential shear wave travel times. The anomalous responses are small but either spatially correlated or anticorrelated with the relief, depending on the period. The behavior can be reproduced by simple one-dimensional (1-D) modeling. The volumetric heterogeneity associated with recent seismic tomographic models should yield a greater surface anomaly, since electromagnetic data are inherently more sensitive to bulk electrical properties than sharp internal interfaces. The finite element method presented here is general and can account for galvanic, oceanic, and non-P10 source effects. The present implementation does not include these complications, but the code is designed so that they can be added easily. The major obstacle to their inclusion at present is the scarcity of other 3-D solutions to validate our calculated results.

[1] We use a simplified model of convection in a porous medium to investigate the balances of mass and energy within a subseafloor hydrothermal convection cell. These balances control the steady state structure of the system and allow scalings for the height, permeability, and residence time of the “reaction zone” at the base of the cell to be calculated. The scalings are presented as functions of (1) the temperature TD of the heat source driving the convection and (2) the total power output ΦU. The model is then used to illustrate how the nonlinear thermodynamic properties of water may impose the observed upper limit of ∼400°C on vent temperatures. The properties of water at hydrothermal conditions are contrasted with those of a hypothetical “Boussinesq fluid” for which temperature variations in fluid properties are either linearized or ignored. At hydrothermal pressures, water transports a maximum amount of energy by buoyancy-driven advection at ∼400°C. This maximum is a consequence of the nonlinear thermodynamic properties of water and does not arise for a simple Boussinesq fluid. Inspired by the “Malkus hypothesis” and by recent work on dissipative systems, we speculate that convection cells in porous media attain a steady state in which the upwelling temperature TU maximizes the total power output of the cell. If true, this principle would explain our observation (in previous numerical simulations) that water in hydrothermal convection cells upwells at TU ∼ 400°C when driven by a heat source above ∼500°C.

[1] From August 1994 to March 1995, three 50-m-high vertical thermistor arrays designated “Giant Kelps” (GKs) were deployed around the central black smoker complex (CBC) at the TAG hydrothermal mound, Mid-Atlantic Ridge (26°08′N, 44°49′W). These were designed to monitor the temporal variability of the vertical temperature distribution in the hydrothermal plume. One small high-temperature probe “Hobo” was also deployed in one of the black smoker vents of CBC. Over the observation period, two typical characteristics are recognized in plume temperatures measured with GKs: (1) the amplitudes of temperature anomalies decrease with increasing height above the top of CBC; (2) maximum temperature anomalies on the upper thermistors occurred periodically and nearly simultaneously across the array about every 6 hours. Conversely, maximum temperature anomalies on the lower thermistors occurred periodically every 12 hours, indicating that the location of the plume discharged from CBC was forcibly moved by the change in direction of tidally modulated current flow. The heat flux from CBC was estimated from temperatures measured by GKs based on a model of buoyant hydrothermal fluid rising in a stable, stratified density environment. The estimated heat flux from CBC gradually decreases from about 86 to 55 MW over the ∼7 months of measurement, with a mean rate of decrease of 0.17 MW d−1. Since the black smoker effluent temperature measured with Hobo was almost stable over the measurement period, a plausible cause of the decrease is a reduction in the volume of hydrothermal fluid provided to the CBC (in which case the estimated mean rate of decrease in volume flux of CBC is 8.9 m3 d−1). Estimated heat flux, temperature anomalies observed by Hobo, and diffuse flow and subbottom temperature anomalies recorded by other long-term monitoring instruments before, during, and after ODP Leg 158 indicate that the drilling probably affected the fluid flow pattern within the mound but had little effect on the total heat flux from CBC.