Modeling periodic megaplume emission by black smoker systems

Abstract

Two huge bodies of anomalously warm water, megaplumes, have been discovered above the normal black smoker plumes on the southern Juan de Fuca Ridge. Their chemistry indicates that they contain high‐temperature mature hydrothermal fluid mixed with entrained seawater. Their excess heat content (estimated at about 10 17 J) is equivalent to that in about 0.1 km 3 of black smoker fluid. Their buoyancy flux, estimated from the height of rise of the plume, indicates flow rates of up to about 20,000 kg s −1 . The size, buoyancy flux, and hydrothermal characteristics of megaplumes strongly suggest that they are the result of a massive, catastrophic emission from a black smoker system. Such flow rates require a high recharge permeability (10 −12 m 2 or higher). An abrupt increase in discharge permeability is essential to allow black smoker flow rates to increase by orders of magnitude. This can be achieved in two ways. The most obvious way is a tectonic stretching event, such as those observed in Iceland, where some spreading segments undergo periodic extension with episodes of activity lasting a few years. This would result in a fundamental change in the hydraulic properties of the system. Megaplume emission would be associated with periods of tectonic activity, and it is estimated that several tectonically induced megaplumes might occur each year globally. We investigate hydrofracturing as an alternative mechanism for spontaneous megaplume discharge. This relies on the presence of a clogged cap to the discharge and a clogged cylindrical shell around the discharge pipe, both the result of subseafloor precipitation of sulfides and quartz through mixing of hydrothermal fluid with cool recharge water. With this structure, decrease in fluid density can lead to pressures sufficient to fracture the clogged cap and release a megaplume. Fluid density can be decreased either by injection of magmatic CO 2 , in which case megaplumes would be related to magmatic evolution or, if heat transport in a system is less than the rate of heat supply, by increase of fluid temperature to about 400°C, where there is rapid, nonlinear expansion of seawater at subseafloor pressures. We model hydrofracturing by increasing temperature and show that it can occur under geologically realistic conditions. We calculate the change in temperature, buoyancy pressure, and flow rate before and after initial fracture and show that megaplume flow rates can be generated by hydrofracturing. Periodic emission of megaplumes from a normal black smoker system is possible if the hydrofractures are clogged during the latter stages of megaplume activity. Sites of hydrofracture‐induced megaplumes should be distinguished by mounds of fractured and recemented sulfides surrounding hydrothermal vent areas.