Generation of hydrothermal megaplumes by cooling of pillow basalts at mid-ocean ridges

Abstract

Hydrothermal megaplumes are huge volumes of anomalously warm water that are located up to 1,000 metres above the sea¯oor and appear to be generated at mid-ocean ridges. Since their discovery in 1986, there has been considerable debate concerning their origin. A theoretical model is used to argue that the cooling of pillow basalts, which are erupted at ,1,200 8C into sea water and are the most common form of submarine volcanic activity, is responsible for the megaplume formation.High-temperature hydrothermal activity is a spectacular manifestation of the formation of new crust at undersea spreading centres. Hot water issues from the sea¯oor at over 350 8C and rises into the overlying water column, mixing with cold ambient sea water as it does so. Eventually, the rising plume entrains suf®cient sea water to reach a level of neutral buoyancy where it then spreads laterally and is advected away from the vent site by deep ocean currents. The height that the plume rises above the sea¯oor is largely a function of the density strati®cation of the overlying sea water and the heat ux of the hydrothermal source. Plumes overlying vents of thè black smoker' type typically rise 100±300 m above the sea¯oor and are emitted from sources that operate over periods of years; hence they are termed`chronic plumes'. Recently, transient, detached, hydrothermal plumes have been observed up to 1,000 m above the sea¯oor. These features are termed megaplumes as they are generated over a period of days by a heat¯ux several orders of magnitude greater than that which generates chronic plumes (see ref.1 for a review of chronic and transient hydrothermal plumes). Generation of megaplumes has been ascribed to rapid emptying of a high-temperature (,350 8C) hydrothermal reservoir in the crust, of the type underlying black smokers 2±5 , or the result of hydrothermal circulation within the oceanic crust initiated by dyke injection 6 . The eruption of pillow basalts onto the sea¯oor is a ubiquitous feature of undersea spreading centres, and must involve thermal perturbation of the overlying water column as they are erupted at temperatures of ,1,200 8C and cool to ambient conditions (,2 8C) within days to weeks. We have examined the physical and chemical features that might be expected from cooling of pillow basalts in the deep oceans, and conclude that this process is a viable mechanism for the formation of hydrothermal megaplumes. This simple explanation has allowed us to constrain more closely the global ux and generation of megaplumes. Previous observations and modelsThe ®rst observation of a megaplume (called MPI) was over the southern Juan de Fuca ridge (SJFR) 7 . MPI had a thickness of 700 m and a diameter of 20 km. Its centre was ,700 m above the sea¯oor (depth of sea¯oor, 2,300 m) and its top reached ,1,000 m above the sea¯oor (as delineated by the 0.12 8C temperature difference anomaly contour). The maximum temperature anomaly was only 0.28 8C, but the large volume of the megaplume indicated that it contained a heat anomaly of 67 3 10 15 J. ...

Observations of a hydrothermal plume in a karstic lake

“…In that case, the plume would be termed a ''chronic plume'' (Palmer and Ernst 1998). Furthermore, it would be interesting to check the characteristics of the plume for different environmental conditions of the water column, such as the winter mixed period.…”

Fragmentation and dispersal of komatiitic pyroclasts in the 3.5-3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa

“…8A), contains ADK fl ow rocks (Anhaeusser, 2001) and represents a possible ADK source. Fine komatiitic ash may have been elutriated from the hyaloclastite eruption site by thermal convection plumes (Palmer and Ernst, 1998) or transported by mass-fl ow and reworked by ambient currents. However, these tuffs would represent extremely thick, fi negrained hyaloclastites unknown from modern and past environments.…”

Eos, Transactions, American Geophysical Union Volume 87, Number 4, 24 January 2006

“…MOR magmatic activity can dramatically alter the intensity of hydrothermal discharge from the seafloor, epitomized by the release of 'event plumes' [Baker et al, 1987], the nearinstantaneous release of enormous volumes of deep-crustal hydrothermal fluid through new fissures [e.g., Lowell and Germanovich, 1995], and/or rapid cooling of new lava flows [e.g., Palmer and Ernst, 1998]. Chronic, continuous venting also can be enhanced greatly by magma injection events.…”