Spreading centers

Herzig, RM., Becker, K.P., Stofeers, R, Backer, H. and Bulum, N. (1988) Hydrothermal silica chimney fields in the Galapagos spreading center. Earth Planet. Sci. Lett., 89, 261-272. 

Back-arc spreading center 1 North Fiji Basin, Station 4 (16°59 S. 173°55 E) 1980 Axial graben at topographic high of north-central segment near triple junction. Sheet lava floor. Active (r = 290°C) anhydrite chimneys standing on dead sulfide mound. Forest of dead sulfide chimneys. Anhydrite, amorphous silica in dead chimneys pyrite, marcasite, chalcopyrite, sphalerite, wurtzite, goethite. 

Central Manus Basin Vienna Woods (3° ID S, 150°17 E) 2500 2-km-wide axial rift graben of the northeast spreading center. Mostly massive pillow lava floor. Sulfide chimneys up to 20 m high are venting clear, milky and black fluids. Sulfate smokers are also present. Sphalerite, wurtzite, pyrite, marcasite, chalcopyrite, galena, amorphous silica, barite. Sulfate chimney anhydrite, silica, barite. 

Eastern Manus Basin Desmos cauldron (3 42 S, 151°52 E) 2000 Caldera of basalt/basaltic andesite at an intersection of a spreading center and a transform fault Sulfide ores were not recovered. Megaplume-like methane anomalies in water column over the caldera. Ferruginous oxide deposits. Pyrite and native sulfur disseminated in basaltic andesite. 

Western Woodlaik Basin Franklin Seamount (9°55 S, 151L50 W) 2143-2366 Westernmost propagating tip of spreading center. Basaltic andesite and inferred sodic rhyolite. Spires aud mounds of Fe-Mn-Si oxide up to several meters thick and 200 m in extent. Venting 20-30 C clear solution. Inactive barite silica chimneys contain up to 21 ppm Au. vSi-bearing Fe oxyhydroxide. 

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)... 

Turekian, K.K., Cochran, J.K. and Krishnaswami, S. (1981) The flow rates of Galapagos spreading center hydrothermal waters determined with natural radionuclides. EOS, 62, 914. 

Lister, C.R.B. (1973) Hydrothermal convection at seafloor spreading centers source of power or geophysical nightmare Geol. Soc. Am. Ab.st. Programs, 5, 74. 

Michael PJ, Forsyth DW, Blackman DK, Fox PJ, Hanan BB, Harding AJ, Macdonald KC, Neumarm GA, Orentt JA, Tolstoy M, Weiland CM (1994) Mantle control of a dynamically evolving spreading center Mid-Atlantic Ridge 31-34°S. Earth Planet Sci Lett 121 451-468... 

Kelley, D. S., J. A. Baross and J. R. Delaney, 2002, Volcanoes, fluids, and life at mid-ocean ridge spreading centers. Annual Review of Earth and Planetary Sciences 30, 385—491. 

Laveme C (1993) Occurrence of siderite and ankerite in young basalts from the Galapagos Spreading Center (DSDP Holes 506G and 507B). Chem Geol 106 27-46... 

Hydrothermal vents are another source of water entering the ocean. These vents are submarine hot-water geysers that are part of seafloor spreading centers. The hydrothermal fluids contain some major ions, such as magnesium and sulfete, in significantly different ratios than foimd in seawater. The importance of hydrothermal venting in determining the chemical composition of seawater is described in Chapters 19 and 21. 

Some trace metals are transported into the ocean as a component of hydrothermal fluids. This process is discussed further in Chapter 19- To briefly summarize, hydrothermal fluids are produced when seawater penetrates into cracks in the crust near tectonic spreading centers. The seawater is heated as it comes into contact with magma. The hot seawater leaches a number of trace metals from the magma. The resulting hydrothermal fluids are acidic and do not contain O2, so most of the metals are present in reduced form. Because of their high temperatures, the hydrothermal fluids have a lower density than cold seawater. Their increased buoyancy causes them to rise until they are emitted into the deep sea. Admixture with cold, oxic, alkaline seawater causes the hydrothermal metals to undergo various redox and precipitation reactions. 

The ridge crest is a dynamic setting in which volcanic activity creates new vents while old ones die. On fast-spreading centers, hydrothermal circulation supports focused discharges through chimneys that have an average life span of a few decades. 

Cross-axis profiles of spreading centers. Topography, magma chamber depth, frequency of eruptive... 

Hydrothermal Of the hot-water systems that are present at acUve mid-ocean spreading centers. Hydroxyl A chemical group composed of an oxygen atom bound to a hydrogen atom (i.e., -OH). Hypersaline Water with a salinity in excess of that at which halite will spontaneously precipitate. Hypoxic Waters with dissolved oxygen concentraUons less than 2 to 3 ppm (2mL/L). 

Metalliferous sediments Metal-rich sediments. Most are found around active spreading centers because hydrothermal activity is the primary source of the metals. 

Pillow basalts Large mounds of basalt that form when lava is extruded onto the seafloor at active spreading centers. 

Zeolites Authigenic clay minerals whose production is associated with geochemical processes occurring at spreading centers, including volcanism and chemical weathering of ocean crust by seawater. 

Fehn, U., Green, K.E., Von Herzen, R.P., Cathles, L.M. 1983. Numerical models for the hydrothermal field at the Galapagos spreading center. Journal of Geophysical Research, 88, 1033-1048. 

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