Hydrothermal plumes

Farrell and Holland (1983) cited ba,sed on Sr isotope study on anhydrite and barite in Kuroko deposits that the most appealing model for the formation of Kuroko strata-bound ores would seem to entail precipitation of the minerals from a hydrothermal solution within the discharge vent or in the interior of a hydrothermal plume formed immediately below above the vent exit in the overlying seawater (Eldridge et al., 1983). The study on the chimney ores from Kuroko deposits support this model which is discussed below. 

It is worth elucidating mineral particle behavior in hydrothermal plumes in order to consider the formation mechanism of chimney and massive ores on the seafloor. Using the grain size data on sulfides and sulfates, the density of the fluids and of the minerals, the relationship between vertical settling rate and grain size of sulfides and sulfates can be derived based on the following Stokes equation ... 

Mottl, M. J. and T. F. McConachy, 1990, Chemical processes in buoyant hydrothermal plumes on the East Pacific Rise near 21 ° N. Geochimica et Cosmochimica Acta 54, 1911-1927. 

Important exceptions to this are the strong horizontal gradients in chemical composition found in hydrothermal plumes (Chapter 19.4). 

Plume incidence is the fraction of ridge crest length overlain by a significant hydrothermal plume. 

Hydrothermal venting injects fluids into seawater as buoyant, jetlike pliunes. These turbulent flows mix rapidly with seawater becoming diluted by factors of lO" to 10. This mixing eventually makes the plumes neutrally buoyant, after which they are transported laterally through the ocean basins as part of the intermediate and deepwater currents. Hydrothermal plumes have the potential to greatly affect seawater chemistry. From global estimates of hydrothermal fluid emissions and dilution ratios, a volume of seawater equivalent to the entire ocean can be entrained in the hydrothermal plumes every few thousand years. 

Rayleigh convection forms above the centre of the sill, it is overprinted by the later and larger (from double to an order of magnitude more Zn metal) hydrothermal plumes driven by the horizontal gradient at the verticai boundary created as the sill cools toward the centre. Since we maximize permeabiiity at 325°C, these piumes approximateiy foiiow this isotherm. 

Lupton, J. E., Weiss, R. F., Craig, H. (1977b). Mantle helium in hydrothermal plumes in the Galapagos Rift. Nature, 267, 603 1. 

Lam, P., Cowen, J. P., and Jones, R. D. (2004). Autotrophic ammonia oxidation in a deep-sea hydrothermal plume. FEMS Microbiology Ecology 47, 191—206. 

Boyle, E. A., Bergquist, B. A., Kayser, R. A., andMahowald, N. (2005). Iron, manganese, and lead at Hawaii Ocean Time-series station ALOHA Temporal variabihty and an intermediate water hydrothermal plume. Geochim. Cosmochim. Acta 69, 933—952. 

Eupton J. E. (1996) A far-field hydrothermal plume from Loihi Seamount. Science 272, 976-979. 

Impact of Hydrothermal Plumes Upon Ocean Geochemical Cycles... 

The rest of the chapter is organized as follows. In Section 6.07.2 we discuss the chemical composition of hydrothermal fluids, why they are important, what factors control their compositions, and how these compositions vary, both in space, from one location to another, and in time. Next (Section 6.07.3) we identify that the fluxes established thus far represent gross fluxes into and out of the ocean crust associated with high-temperature venting. We then examine the other source and sink terms associated with hydrothermal circulation, including alteration of the oceanic crust, formation of hydrothermal mineral deposits, interactions/uptake within hydrothermal plumes and settling into deep-sea sediments. Each of these fates for hydrothermal material is then considered in more detail. Section 6.07.4 provides a detailed discussion of near-vent deposits, including the formation of polymetallic sulfides and... 

Hydrothermal plumes form wherever buoyant hydrothermal fluids enter the ocean. They represent an important dispersal mechanism for the thermal and chemical fluxes delivered to the oceans while the processes active within these plumes serve to modify the gross fluxes from venting, significantly. Plumes are of further interest to geochemists because they can be exploited in the detection and location of new hydrothermal fields and for the calculation of total integrated fluxes from any particular vent field. To biologists, hydrothermal plumes represent an... 

The exact height reached by any hydrothermal plume is a complex function involving key... 

Figure 12 Sketch of the hydrothermal plume rising above an active hydrothermal vent, illustrating entrainment of ambient seawater into the buoyant hydrothermal plume, establishment of a nonbuoyant plume at height z ax (deeper than the maximum height of rise actually attained due to momentum overshoot) and particle settling from beneath the dispersing nonbuoyant plume (after Helfrich and Speer, 1995).

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