Abstract. We address the question of formation of event plumes following dike emplacement in a hydrothermal upflow zone at a mid-ocean ridge. We assume a preexisting low-to moderate-temperature single-pass hydrothermal system and suggest that dike emplacement provides a damaged zone of high permeability along its margins as well as the heat required to drive the event plume. We also consider the role of thermoelastic stresses in limiting the heat output of the event plume. Our calculations show that event plumes can result from dike emplacement into a preexisting, moderate-temperature hydrothermal system, provided the local permeability generated by the dike is emission, the circulation decays rapidly to its original temperature; however, the heat output from the chronic plume is greater because of the increased permeability resulting from dike emplacement. The decay of heat output to preevent plume levels requires that the newly created permeability be sealed, perhaps as a result of chemical precipitation in the cracks.

[1] It is well known that in many cases rock permeability depends upon in situ stress conditions and on the pressure of the flowing fluid. Parallel and quasi-parallel joints represent one of the most often observed permeability structures. Frequently, joint sets are closely spaced and although joint mechanical interaction could significantly affect their aperture, the interaction is usually ignored in the evaluation of permeability. In this paper, on the basis of accurate computations of the interaction between the parallel fractures and conducted physical experiments, we suggest that the internal pressure can, in fact, close the pressurized joints. In general, there is a critical spacing between the parallel fractures below which their surfaces start contacting under the extensional load. However, the two edge fractures (end members) in the set remain widely open because they are not suppressed from one side. These effects dramatically change rock permeability and the fluid flow pattern.

Abstract. We investigate the effects of temperature-dependent permeability in a hydrothermal upflow zone on the evolution of a seafloor hydrothermal system. Our mathematical modeling of a seafloor hydrothermal system with temperaturedependent permeability suggests that the system can be in one of two stable regimes of heat transfer. In one regime, heat conduction and thermoelastic effects do not play a significant role because the rock thermal expansion coefficient is too low or the rock porosity is too high. In the other regime, thermoelastic stresses reduce the permeability by orders of magnitude, and some fraction of the heat is transferred by conduction. In both regimes, essentially the same amount of heat is transported by the upflow but with different flow parameters. When thermoelastic stresses reduce the permeability, discharge occurs at relatively high temperature and low flow velocity, whereas when thermoelastic stresses have little effect on the permeability, discharge occurs at much lower temperature and higher flow velocity. The existence of two stable states of the hydrothermal system results from the nonlinearity of the dependency of permeability upon temperature and from specifying the heat flux entering the system. Consequently, a hydrothermal system can be on one or another solution branch, depending on small variations in system parameters, its history, and boundary conditions. At the top of the hydrothermal system the temperature difference between the branches can reach several hundred degrees. Relatively small changes in basal heat flux or rock permeability in the upflow zone can rather quickly switch the system from one stable branch to another. Such permeability changes may result from magmatic events, earthquakes, or chemical dissolution or precipitation. The calculations show that it is easier to drive the system from the high-temperature branch to the low-temperature one than vice versa.

[1] It is well known that the permeability of a set of joints can significantly vary in response to in situ stress conditions and pressure of the flowing fluid. Frequently, joint sets are closely spaced, and although joint mechanical interaction could significantly affect their aperture, the interaction is usually ignored in the fluid flow models. It is rather obvious that this approach corresponds to the upper bound for flow rate and rock permeability. By taking into account the interaction between the joints, we show that modeling a joint set by an infinite array provides the lower bound. The difference between these bounds, however, can be rather large, so they may not always be used with the sufficient accuracy. From the conceptual standpoint, it is often tempting to model a set with a finite number of joints by an infinite array. The results obtained in this work clearly demonstrate that such a model may result in a significant underestimation (by orders of magnitude) of both the permeability and flow rate. Similarly, the assumption of noninteracting joints may significantly overestimate (also by orders of magnitude) the stress-dependent permeability and flow rate compared to those computed more accurately when accounting for joint interaction. Because the internal pressure can, in fact, close the pressurized joints while two edge joints (end-members) in the set remain widely open (since they are not suppressed from one side by the adjacent joints), unless the number of joints in the set is exceedingly large (typically, >10 3 ), the fluid flow through the joint set becomes highly heterogeneous, focusing in the edge joints. As a result, the permeability/flow rate dependence on the joint spacing is not monotonic but has a maximum and a minimum. The derived closed-form expression for flow rate/permeability ratio is asymptotically accurate and allows computations for rather arbitrary joint sets.

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