Mid-Atlantic ridge

Figure 1.34. Frequency histogram for MgO/FeO ratios (in wt%) of chlorite from the basalt studied (A) and MORE (B). Data sources are Shikazono and Kawahata (1987), Humphris and Thompson (1978) (M Mid-Atlantic Ridge) and Kawahata (1984) (C Costa Rica Rift, Galapagos Spreading Centre). The data on chlorite from MORE are taken from typical metabasalt and not from quartz-chlorite breccia and veins which formed in a hydrothermal upflow zone (Shikazono et al., 1987).

Figure 1.85. Relation between Fe20j content of epidote and that of original fresh rocks. S Seigoshi, Y Yugashima, N Noya, F Furotobe, O Ohtake, M Mid-Atlantic ridge, C Costa Rica rift. Mi Mitsuishi, Sh Shimokawa (Shikazono, 1984)...

Fig. 2.39. Na /K+ atomic ratios of well discharges plotted at measured downhole temperatures. Curve A is the least squares fit of the data points above 80°C. Curve B is another emperical curve (from Truesdell, 1976). Curves C and D show the approximate locations of the low albite-microcline and high albite-sanidine lines derived from thermodynamic data (from Fournier, 1981). Small solid subaerial geothermal water Solid square Okinawa Jade Open square South Mariana Through Solid circle East Pacific Rise 11°N Open circle Mid Atlantic Ridge, TAG.

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)... 

A similar approach led Sturm et al. (2000) to propose some °Th-" U and Pali ages for the Mark area. South of the Kane Fracture Zone of the Mid-Atlantic Ridge. 

Table 1 lists MORE studies to date and Figures 2 and 3 present data from different areas of ridge. The first study of U-series disequilibria in MORE was the pioneering work of Condomines et al. (1981) (Fig. 2A). These workers analyzed samples having a relatively wide range in composition (Mg 72 to 57) from the FAMOUS region of the Mid-Atlantic Ridge (MAR 37°N) by combined alpha spectrometry (for U and Th isotopic ratios) and mass spectrometry (isotope dilution measurements for U and Th... 

Bourdon B, Langmuir CH, Zindler A (1996a) Ridge-hotspot interaction along the Mid-Atlantic Ridge between 37°30 and 40°30TST the U-Th disequilibrium evidence. Earth Planet Sci Lett 142 175-189... 

FAMOUS zone (Mid-Atlantic Ridge, 36°50 N) Th and Sr isotopic geochemistry. Earth Planet Sci Lett 55 247-256... 

Lnndstrom CC, Gill J, Williams Q, Hanan BB (1998a) Investigating solid mantle upwelling beneath midocean ridges using U-series disequilibria. II. A local study at 33°S Mid-Atlantic Ridge. Earth Planet Sci Lett 157 167-181... 

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... 

Sinton JM, Detrick RS (1992) Mid-ocean ridge magma chambers. J Geophys Res 97 197-216 Sobolev AV, Shimizn N (1993) Ultra-depleted primary melt included in an olivine from the Mid-Atlantic Ridge. Nature 363 151-154... 

Sturm ME, Goldstein SJ, Klein EM, Karson JA, Mnrrell MT (2000) Uranium-series age constraints on lavas from the axial valley of the Mid-Atlantic Ridge, MARK area. Earth Planet Sci Lett 181 61-70 Sun S, McDonongh WF (1989) Chemical and isotopic systematics of ocean basalts implications for mantle composition and processes. In Magmatism in the Ocean Basins. Saunders AD, Norry MJ (eds) Blackwell Scientific Pnbl. Oxford, p 313-345... 

Colaijo, A., Dehairs, F. and Desbruyeres (2002). Nutritional relations of deep-sea hydrothermal fields at the mid-Atlantic ridge a stable isotope approach. Deep-Sea Res. I, 49, 395—412. 

Ellis explains that In Atlantean fantasy, the writer begins with some sort of demonstrable actuality—such as the Mid-Atlantic Ridge or the ruins of the Minoan... 

Condomines, M., Morand, P. Allegre, C. J. (1981). 230Th-238U radioactive disequilibria in tholeiites from the FAMOUS zone (Mid-Atlantic Ridge, 36°50 N) Th and Sr isotopic geochemistry. Earth Planet. Sci. Letters, 55, 247-56. 

Petrogenesis of basalts from the FAMOUS area Mid-Atlantic Ridge. Earth Planet. Sci. Letters, 36, 133-56. 

Figure 13. Plot of Li isotopic composition vs. inverse concentration for sea floor altered (weathered) mid-Atlantic ridge basalts (Chan et al. 1992). Solid line is the regression of the data (R = 0.927), the dashed line shows the predicted relation of a pure mixture of seawater with unaltered basalt (Teng et al. 2004), underscoring that the altered basalts incorporated Li into secondary mineral with a fractionation factor a -0.981.

Huang L, Sturchio NC, Abrajano T, Heraty LJ, Holt BD (1999) Carbon and chlorine isotope fractionation of chlorinated aliphatic hydrocarbons by evaporation. Org Geochem 30(8A) 777-785 Jambon A, Deruelle B, Dreibus G, Pineau F (1995) Chlorine and bromine abundance in MORB the contrasting behavior of the Mid-Atlantic Ridge and East Pacific Rise and implications for chlorine geodynamic cycle. Chem Geol 126 101-117... 

Vapor and brine from the Brandon vent of the East Pacific Rise have identical Fe isotope compositions, implying that phase separation does not produce an isotopic fractionation (Beard et al. 2003a). The role that sulfide precipitation plays in controlling the Fe isotope composition of the fluid remains unknown. The precision of the two sulfide analyses reported by Sharma et al. (2001) was not sufficient to resolve if sulfide precipitation would produce Fe isotope fractionation in the vent fluid. In a detailed study of sulfldes from the Lucky Strike hydrothermal field from the mid Atlantic Ridge, however, Rouxel et al. (2004) found that sulfldes span a range in 5 Fe values from -2.0 to +0.2%o, and that pyrite/marcasite has lower 5 Fe values ( l%o) as compared to chalcopyrite. The variations in mineralogy and isotope composition are inferred to represent open-system equilibrium fractionation of Fe whereby... 

The fate of dissolved Fe from MOR vents has been investigated at the Rainbow plume from the mid-Atlantic Ridge. Plume particles were sampled from the buoyant part of the Rainbow plume, proximal to the vent, as well as particles from neutrally buoyant portions of the plume that were more distal from the vent (Severmann et al. 2003). Particles from the buoyant part of the plume have positive 5 Fe values (up to +1.2%o), whereas in the neutrally buoyant sections of the plume, the particles have a near-constant 8 Fe value of -0.2%o that matches the Fe isotope composition of the vent fluid. The high 5 Fe values of plume particles that were proximal to the vent probably reflect oxidation processes. In the neutrally-buoyant plume, all aqueous Fe(ll) had been oxidized and it appears that there was no net loss in Fe because the neutrally buoyant plume particles have the same isotopic composition as the vent fluid. Moreover, metalliferous sediments sampled below the plume match the isotopic composition of the plume particles. The implication of these data is that for at least one plume, the Fe isotope composition of the vent fluid matches that of the plume particles. The Rainbow vent fluid, however, has an unusually high Fe to S ratio and hence it is uncertain if these results can be extrapolated to other plumes that originated fi om vent fluids that had lower Fe to S ratios. 

Rouxel O, Fouquet Y, Ludden JN (2004) Subsurface processes at the Lucky Strike Hydrothermal Field, Mid-Atlantic Ridge Evidence from sulfur, selenium, and iron isotopes. Geochim Cosmochim Acta, submitted... 

Rouxel O, Eouquet Y, Ludden JN (2004) Subsurface processes at the Lucky Strike hydrothermal field, Mid-Atlantic Ridge evidence from sulfur, selenium and iron isotopes. Geochim Cosmochim Acta, in press Schauble EA, Rossman GR, Taylor HP (2001) Theoretical estimates of equilibrium Ee-isotope fractionations from vibrational spectroscopy. Geochim Cosmochim Acta 65 2487-2497 Schopf JW (1992) Paleobiology of the Archaen. Im The Proterozoic Biosphere A Multidisciplinary Study. Schopf JW, Klein C (eds) p 25-39... 

Pearce, J. 1996. Sources and settings of granitic rocks. Episodes, 24, 956-983. Schilling, J.-K. et al. 1983. Petrologic variations along the Mid-Atlantic Ridge from 29°N to 73°N. American Journal of Science, 283, 510-586. 

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