Hydrothermal activity/vents

We saw in section 2.3.2 that present-day hot spring venting and sulfide-sulfate depositions have been discovered in back-arc basins in the Western Pacific. These intense hydrothermal activities indicate that seawater-volcanic rock interactions are taking place at these environments. 

The variety of life forms to be found near hydrothermal vents does not, of course, mean that life itself originated there these geological systems are much too unstable for that. The dynamics of tectonic plates cause the vents to disappear after some decades, or at most after a few hundred years. According to Nils Holm from the Department of Geology and Geochemistry at the University of Stockholm, the discovery of the hydrothermal vents led to intense, and in some cases controversial, discussions of the question as to whether hydrothermal systems were the birthplaces of life around four billion years ago. Many geologists believe that hydrothermal activity on the primeval Earth was probably stronger than it is today, as the thick... 

Because seawater uptake dominates the REE content of metalliferous sediment, neodymium isotopic analysis of metalliferous carbonate can provide a reliable proxy for contemporaneous seawater, away from input of near-vent sulfide detritus (Mills et al., 1993). Osmium also exhibits a similar behavior and seawater dominates the isotopic composition of metalliferous sediments even close to active vent sites (Ravizza et al., 1996). Consequently, analysis of preserved metalliferous carbonate sediments has proven extremely useful in determining the past osmium isotopic composition of the oceans, both from modem marine sediments (e.g., Ravizza, 1993 Peucker-Ehrenbrink et al., 1995) and those preserved in ophiolites (e.g., Ravizza et al., 2001). Only in sediments close to an ultramafic-hosted hydrothermal system, have perturbations from a purely seawater osmium isotopic composition been observed (Cave et al., 2003, in press). 

Fig. 8.1 Diagram illustrating the major geochemical and microbial processes associated with seawater circulation in the crust and hydrothermal activity at mid-ocean ridges. Not all processes or features are found at all hydrothermal vent sites. Diagram not to scale (see text for further details).

Because few detailed studies of off-axis venting have been undertaken to date and many have been exclusively near the Juan de Fuca Ridge (JDFR see references in German etal., 1995 Becker etal., 2000 Wheat Mottl, 2000), attempts to estimate the overall chemical flux to the ocean through hydrothermal activity and its importance to ocean chemistry, relative to the contribution from rivers, have been hampered by an overall lack of knowledge (Elderfield Schultz, 1996). Discussion of the relative contributions of off- and on-axis fluxes has centred on comparing the chemical output on the axis to the greater heat (and probably water) flux on flanks (Von Damm, 2001 Mottl, 2002). 

Hydrothermal plumes are the water column manifestation of mid-ocean ridge hydrothermal activity (e.g. Baker etal., 1995). They form above areas of venting as a result of the buoyancy of the heated fluids as the fluids rise, they entrain ambient seawater, thus continuously increasing the plume volume until neutral buoyancy is achieved and the plume disperses laterally (Baker etal., 1995). These warm, buoyant plumes rise up to several hundred metres above the seafloor and extend for several kilometres away from vents [e.g. see reviews by Baker... 

German, C.R., Baker, E.T. and Klinkhammer, G. (1995) Regional setting of hydrothermal activity, in Hydrothermal Vents and Processes (eds L.M. Parson, C.L. Walker and D.R. Dixon), Geological Society Special Publication No. 87, pp. 3-15. 

Vent Field (Escanaba hole 1038), the Escanaba Trough Reference Hole (1037), which was drilled approximately 4.5 km from the nearest known hydrothermal activity, and from the Bent Hill Massive Sulfide system (holes 856 and 1035) in Middle Valley. These data show dramatic shifts in 5D and from -100 to -40 and from 15 to 5, respectively. Using the equations of Taylor (1974), relatively high w/r mass ratios of about 3-7 are indicated, as expected for the significant change in the 5D and of the muds. This w/r... 

Manus Basin is a back-arc basin located north of the New Britain island arc that consists of NE-SW trending ridge segments offset by transform faults. On the Manus Spreading Center in central Manus Basin the Vienna Woods hydrothermal field consists of a 300 m-diameter sulfide mound with an extensive foresf of active and inactive sphalerite-barite-rich, sulfide-sulfate-oxide chimneys up to 15 m-long atop it. Vent fluids from one of the active chimneys reach temperatures as high as 302°C. In the eastern Manus Basin, two important hydrothermal fields are known. The PACMANUS field includes discontinuous sulfide occurrences over a 3 by 0.8 km area and active venting of unknown nature. Volcanic rocks are andesitic to rhyodacitic. 

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