Seafloor

J. C. Wiltshire, "Seafloor Cobalt Deposits A Major Untapped Resource," presented at Cohalt at the Crossroads, Intertech Conferences, Herndon, Va., June 1992. 

The most fundamental difference found between onshore and offshore drilling occurs when the wellhead is located at the seafloor. This configuration makes communication with the well more complex. A marine riser provides communication and circulation capability between the surface and the seafloor, and is used at some point during most offshore drilling. The riser consists of large-diameter (17-20) in.) steel pipe joints of approximately 50-ft lengths, with quick-connect couplings. The riser can be connected at the seafloor to a wellhead or to a subsea blowout preventer stack. A diverter system is usually attached at the... 

Observations of the ratio of oxidized plutonium to reduced plutonium may provide some insight to the observations of erratic formation and lack of equilibration in laboratory solutions at ORNL versus fairly consistent and predictable behavior in oligo-trophic lakes and marine systems. In coastal water and the relatively shallow Lake Michigan, Pu(V) is about 90 percent of the soluble plutonium, but in the upper waters of the open ocean, where it does not interact with the seafloor due to the depths,... 

Croup lb (Mg, SO4, probably K). The key property of this group is removal during seafloor hydrothermal circulation. This fits in with Broecker s original group I, tectonically controlled elements, but enlarged by two (Mg, K). 

Smith, W. H. F. and Sandwell, D. T. (1997). Global seafloor topography from satellite altimetry and ship depth soundings. Science 277,1956-1962. 

There has been considerable interest in the anaerobic metabolism of methane in the large reservoirs that lie beneath the seafloor, since little of this reaches the oxic conditions in the water column. Consortia of archaea that have so far resisted isolation and sulfate-reducing bacteria have been implicated (Orphan et al. 2002) ... 

Kuroko deposits are strata-bound and massive in form (Fig. 1.7) and syngenetically formed on the seafloor and/or sub-seafloor environment. Vein-type deposits are fissure-filling and epigenetically formed (Fig. 1.8). 

In recent years, many hydrothermal solution venting and sulfide-sulfate precipitations have been discovered on the seafloor of back-arc basins and island arcs (e.g., Ishibashi and Urabe, 1995) (section 2.3). Therefore, it is widely accepted that the most Kuroko deposits have formed at back-arc basin, related to the rapid opening of the Japan Sea (Horikoshi, 1990). 

Two hypotheses of seafloor depth at the time of mineralization have been proposed based on foraminiferal data, ca. 3500 m (Guber and Ohmoto, 1978 Guber and Merrill, 1983) and 1500 m (Kitazato, 1979). Considering seafloor depth of present-day ore formation at back-arc basins and fluid inclusion data mentioned above, shallow seafloor depth hypothesis (Kitazato, 1979) seems more likely. If the pressm e-temperature condition of Kuroko ore fluids was close to the boiling curve, the depth could be estimated to be 1,000-1,500 m, which is similar to that for present-day back-arc mineralization such as Okinawa Trough. 

The above lines of evidence suggest that the precipitation of minerals in. submarine hydrothermal ore deposits on the seafloor is taking place from the fluids with high flow rate at the orifices of the chimney (ca. 1-10 m/s) and with high degree of supersaturation under the non-equilibrium conditions. 

Some applications of the coupled fluid flow-reaction model were carried out to the ore-forming process (e.g., Lichtner and Biino, 1992). However, a few attempts to understand quantitatively the precipitations of minerals from flowing supersaturated fluids in the submarine hydrothermal systems have been done (Wells and Ghiorso, 1991). Wells and Ghiorso (1991) discussed the silica behavior in midoceanic ridge hydrothermal system below the seafloor using a coupled fluid flow-reaction model. 

The behavior of silica and barite precipitation from the hydrothermal solution which mixes with cold seawater above and below the seafloor based on the thermochemical equilibrium model and coupled fluid flow-precipitation kinetics model is described below. 

Ferruginous chert in which abundant silica occurs formed below the seafloor by the mixing of ferruginous sediments and hydrothermal components (Kalogeropourous and Scott, 1983). 

Origin of ore fluids is constrained by (1) chemical compositions of ore fluids estimated by thermochemical calculations (section 1.3.2) and by fluid inclusion analyses, (2) isotopic compositions of ore fluids estimated by the analyses of minerals and fluid inclusions (section 1.3.3), (3) seawater-rock interaction experiments, (4) computer calculations on the seawater-rock interaction, and (5) comparison of chemical features of Kuroko ore fluids with those of present-day hydrothermal solutions venting from seafloor (section 2.3). 

This kind of temporal and spatial relationship between epithermal Au vein-type mineralization and back-arc mineralization are found also in the Izu-Bonin area. Seafloor... 

In this tectonic situation, intense bimodal volcanism and associated seawater circulation occur, resulting to the formation of Kuroko deposits on the seafloor and formation of vein-type mineralization under subaerial condition and intense hydrothermal and volcanic CO2 fluxes to ocean and atmosphere. Such fluxes affect the long-term environmental changes (see Chapter 4). 

Bischoff, J.L. and Dickson, F.W. (1975) Seawater-basalt interaction at 200°C and 500 bars Implications for origin of seafloor heavy-metal deposits and regulation of seawater chemistry. Earth Planet. Sci. Lett., 25, 385-397. 

Ludden, J. and Thompson, G. (1979) An evaluation of the behavior of rare earth elements during the weathering of seafloor basalts. Earth Planet. Sci. Lett., 43, 85-92. 

Recently, several submarine hydrothermal sites have been discovered from the seafloor of back-arc depression zones and volcanic fronts near the Japanese Islands (Okinawa Trough and Izu-Bonin) (Fig. 2.29). The studies on these areas are described below. 

Several hydrothermal sites have been discovered on the seafloor of Izu-Bonin arc that is located at the eastern margin of the Philippine Sea plate (Fig. 2.32). This arc has been formed, related to the westward subduction of the Pacific plate (Fig. 2.32). Hydrothermal mineralization occurs both in back-arc depression and volcanic chain (Shichito-Iwojima Ridge). Hydrothermal venting and mineralizations are found... 

Average content (in parts per million or wt%) of selected metals in samples from the Sunrise deposit compared with other modem seafloor deposits and an average Kuroko massive sulfide deposit (lizasa et ah, 1999)... 

Average bulk compositions of samples from seafloor sulfide deposits at seamounts and back-arcs (Scott, 1997)... 

For example, assuming anhydrite-magnetite-calcite-pyrite-pyrrhotite buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations of 21 °N EPR fluid indicate a temperature of 370-385°C. However, the estimated temperatures are higher than those of the measurement. This difference could be explained by adiabatic ascension and probably conductive heat loss during ascension of hydrothermal solution from deeper parts where chemical compositions of hydrothermal solutions are buffered by these assemblages. 

Fig. 2.33. H2Saq concentration.s as a function of temperature for hot spring fluids at midocean ridges as a function of redox. Assuming AMPC (anhydrite-magnetite-pyrite-calcite) and PPM (pyrite-pyrrhotite) buffers redox in sub-seafloor reaction zones and a pressure of 500 bars, dissolved H2Saq concentrations indicate temperatures of approximately 370-385°C. Solid star Okinawa. (Modified after Seyfried and Ding, 1995.)...

Low chloride concentrations of hydrothermal solutions were obtained from North Fiji Basin (Grimaud et al., 1991) and the Endeavour Segment of the Juan de Fuca Ridge (Butterfield et al, 1994). This wide variation in chloride concentration could be explained in terms of gas-liquid separation at the shallow part from the seafloor (Von Damm and Bischoff, 1987 Von Damm, 1988 Cowan and Cann, 1988). 

Base and precious metals in modern seafloor hydrothermal systems (Hannington et al., 1995). N number of analyses... 

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 ... 

Model for the formation of sulfate-sulfide chimneys and massive deposits on the seafloor... 

The above summarized mineralogical and geochemical studies on Kuroko and Mariana chimneys (Shikazono and Kusakabe, 1999), and previous studies on midoceanic ridge chimneys, combined with the studies of mineral particle behaviors in the plumbing system, are used to develop the following plausible model for the growth history of sulfate-sulfide chimneys on the seafloor (Shikazono and Kusakabe, 1999). 

Sulfates (barite and anhydrite) precipitate due to the mixing of discharging hydrothermal solution with cold seawater above the seafloor at an early stage of hydrothermal activity. Ca and Ba in hydrothermal solution react with SO in cold seawater, leading to the precipitations of anhydrite and barite. It is observed that anhydrite precipitated earlier than barite. This may depend on the initial Ca and Ba concentrations of end member hydrothermal solutions, temperature and degree of mixing of hydrothermal solutions and... 

Bulk chemical composition data of the Besshi-type deposits (Besshi), the seafloor sulfide deposits from the Mid-Atlantic Ridge at 23°N (MAR), the Galapagos Spreading Center at 86 W (GSC) and the East Pacific Rise at 21 N (EPR) (Kase and Yamamoto, 1988)... 

Barite and sphalerite tend to precipitate at lower temperature from the hydrothermal solution mixed with a large amount of cold seawater (but mixing ratio (seawater/hydrothermal solution) may be less than 0.2). These minerals precipitate on the seafloor and/or at very shallow subsurface environment. However, chalcopyrite tends to precipitate from high temperature solutions in ore bodies and/or at the sub-seafloor sediments. Usually shale which is relatively impermeable overlies the Besshi-type ore bodies. This suggests that hydrothermal solution could not issue from the seafloor and... 

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