Spreading ridges

Fiji Transform Fault Extensional Relay Zone A (16°10 S, 177°25 E) 1869-2335 Short spreading ridge axis which displaces Fiji transform fault as interpreted by Jarvis et al. (1994). Hydrothermal sulfide impregnation in MORB-like ba.salt dredged from axial valley. M etite, pyrrhotite, chalcopyrite and opal on fracture surface. 

Northeastern Lau Basin Papatua expedition site (15°17 S, 174"45 W) 2100 Axial region of northeasterly trending active spreading ridge of the northern Lau Basin. Dredged black smoker chimney samples. Wurtzite, pyrite, chalcopyrite, barite, and amorphous silica. Thin film of Mn-oxyhydroxide. 

Ishibashi, J., Wakita, H., Nojiri, Y, Grimand, D., Jean-Baptisto, P, Game, T., Auzende, J.M. and Urabe, T. (1994b) Helium and carbon geochemistry of hydrothermal fluids from the North Fuji Basin Spreading ridge. Southern Pacific. Earth Planet. Sci. Lett., 128, 183-197. 

Basalts (typical average in oceanic spreading ridges)... 

Marine sediments cover the ocean floor to a thickness averaging 500 m. The deposition rates vary with topography. The rate may be several millimetres per year in nearshore shelf regions, but is only from 0.2 to 7.5 mm per 1000 years on the abyssal plains. Oceanic crustal material is formed along spreading ridges and moves outwards eventually to be lost in subduction zones, the major trenches in the ocean. Because of this continual movement, the sediments on the seafloor are no older than Jurassic in age, about 166 million years. 

The fact that basaltic volcanism is widespread in the deep sea is established and well proved. This volcanism is present in most areas of the deep sea and especially concentrated on spreading ridges. 

Figures 4-6 show the isotopic compositions of MORBs from spreading ridges in the three major ocean basins. Figures 4(b) and 5(a) also show isotope data for marine sediments, because these are derived from the upper continental crust and should roughly represent the isotopic composition of this crust. In general, the isotopic relationships between the continental and oceanic crust are just what is expected from the elemental parent-daughter relationships seen in Figure 3. The high Rb/Sr and low Sm/Nd and Lu/Hf ratios of continental materials relative to the residual mantle are reflected by high Sr/ Sr and low " Nd/ " Nd and Hf/ Hf ratios (not shown).

LREE/HREE ratios, a difference which is also apparent from cpx compositions (Rivalenti et al., 1996). The lanthanum concentrations are typically in the range 0.005-0.05 X chondrites. Based on their REE distribution in cpx (Figure 18), the abyssal peridotites from slow-spreading ridge systems ([Pg.831]

Girardeau J. and Mercier J. C. C. (1988) Petrology and texture of the ultramafic rocks of the Xigaze Ophiolite (Tibet) constraints for mantle structure beneath slow-spreading ridges. Tectonophysics 147, 33-58. 

Kelemen P. B., Hirth G., Shimizu N., Spiegehnan M., and Dick H. J. B. (1997) A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges. Phil. Trans. R. Soc. London Ac 355, 283—318. 

Niu Y. and Hekinian R. (1997a) Basaltic liquids and harzburgitic residues in the Garrett Transform a case study at fast-spreading ridges. Earth Planet. Sci. Lett. 146,... 

Tisseau C. and Tonnerre T. (1995) Non steady-state thermal model of spreading ridges implications for melt generation and mantle outcrops. In Mantle and Lower Crust Exposed in Oceanic Ridges and in Ophiolites (eds. R. L. M. Vissers and A. Nicolas). Kluwer Academic Press, Dordrecht, Boston, London, pp. 181-214. 

Dick H. J. B. (1989) Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism. In Magmatism in the Ocean Basins, Geological Society Special Publication No. 42 (eds. A. D. Saunders and M. J. Norry). Geological Society of London, pp. 71 -105. 

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