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 1017 J) is equivalent to that in about 0.1 km3 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 m2 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 CO2, 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.
A model of hydrothermal circulation close to mid-ocean ridge crests has been developed. The flow is modelled as a transient open-loop thermosyphon, restricted to a single fracture parallel to the spreading centre. Circulation is to a depth of 500-1000m, and heat exchange occurs at the fault walls. Conduction is the sole heat transfer mechanism within the rock.The aim is to model the conditions under which deposition of ore deposits, such as those in the Cyprus ophiolites, could occur.Hot springs, with the characteristics of the black smokers at 21"N on the East Pacific Rise, are obtained from the model, but only when the vents are at the unrealistically large spacing of lOkm apart, and then water at over 350°C is vented for a maximum of only 175 yr. The model demonstrates that heat transfer through the rock by conduction alone is too slow to maintain the power output of a group of black smokers. The skin depth of the temperature field limits the total heat available to the system, but even without this constraint insufficient high-grade heat would be available from the volume of rock within the likely field of influence of a vent.It would take about 2500yr to accumulate a 3 million ton ore deposit from 1 vent or 250yr from a group of 10 vents. The rate of heat extraction and the duration of the vents must both be at least an order of magnitude greater than those obtained from this model, thus requiring a much greater source of heat than in the rocks alone. The only source likely to be great enough is the magma chamber itself.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.