Back-arc spreading centers

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. 

Conder J. A., Weins D. A., and Morris J. (2002) On the decompression melting structure at volcanic arcs and back-arc spreading centers. Geophys. Res. Lett. 29(15), 4pp. 

Zhao D., Yingbiao X., Weins D. A., Dorman L., Fhldebrand J., and Webb S. (1997) Depth extent of the Lau back-arc spreading center and its relation to subduction processes. Science 278, 254-257. 

The objective of this chapter is to review available stable isotopic data on seafloor hydrothermal systems. However, this goes far beyond a simple literature review because much new, previously unpublished data, collected by the author, is included. In addition, an important goal of this chapter is to interpret the stable isotope systematics of seafloor hydrothermal systems in the context of fluid-rock reactions and geochemical reaction calculations. Boiling and supercritical phase-separation, volcanic eruption and dike-emplacement events, addition of magmatic volatiles, and bacterial fractionation processes will be discussed where applicable. In addition to the commonly measured stable isotopes of C, O, H, and S, stable isotope ratios of B, Li, N, Cl, Cu, and Fe are included where data are available. Much new data has appeared since the last comprehensive overview of stable isotopes in seafloor hydrothermal systems (Shanks et al. 1995). This includes a wealth of information on hydrothermal systems related to volcanic arcs, back-arc spreading centers, seamounts, and serpentinized ultramafics. 

Seafloor hydrothermal systems that are reasonably well studied in terms of stable isotope values of fluids, hydrothermal precipitates and hydrothermal alteration are presented as case studies in this section. Ranges of sulfur isotope values from given deposits are summarized in Figure 11. It is clear that most of the mid-ocean ridge sulfides (designated S SmbS to indicate metal sulfide minerals), ) us those from Manus and Mariana back-arc spreading centers, are quite similar with ranges of about l-5%o. 

Mariana Trough is a back-arc spreading center that occurs between a remnant arc and the currently active Mariana arc. On the flanks of axial volcanoes in the central Mariana Trough are several vent fields with measured temperatures up to 287°C and sulfide-sulfate chimneys comprised of sphalerite, galena, and barite, due to metal sources from the underlying andesitic crust. An unusual occurrence in the northeast portion of the Mariana Arc is a serpentinite mud volcano called Conical Seamount. Carbonate (calcite, aragonite) and silicate (Mg silicate) chimneys occur near the mud volcano summit. Associated fluids are cold, sulfate-sulfide-carbonate-silica-rich, and have pHs as high as... 

Seafloor hydrothermal activity at mid-ocean ridges and back-arc spreading centers has a major impact on the chemistry of the oceans (Edmond et al. 1979a, 1982) and has been responsible for... 

Since the discovery of high-temperature hydro-thermal vents at the East Pacific Rise 21°N in 1979, hydrothermal fluids have been sampled at numerous sites at mid-ocean ridges and back-arc spreading centers. As noted above, in most high-temperature vent fluids, both Mg and SO show a negative correlation with temperature, and an extrapolation to zero Mg and zero SO intersects the temperature axis at a point corresponding to the end-member temperature (Fig. 13.9). Controls on the major element compositions of these fluids... 

Fig. 13.13 Schematic diagram showing the geotectonic setting of a seafloor back-arc spreading center, showing the relationship to the active volcanic front of the arc and the subducting slab.

Collier, J. and Sinha, N., 1990. Seismic images of a magma chamber beneath the Lau Basin back - arc spreading center. Nature, 346 646-648. 

Ra, respectively Moreira et al. 1995 Sarda et al. 2000), the Southwest Indian Ridge near Bouvet Island (14.9 Ra Kurz et al. 1998) and the Manus Basin back-arc spreading center (15.1 Ra Shaw et al. 2001). In each of these areas, the high He/ tae signal can be attributed to introduction of high He/ He material from a nearby mantle hotspot. The apparent upper limit to the measured He/ He in these cases is usually presumed to stem from dilution with ambient upper mantle having He/" He between 7 and 9 Ra. 

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