East Pacific Rise

Hydrothermal vents have been sampled at 21 along the East Pacific Rise. The pure end member hydrothermal solutions have a temperature of 350°C and the following major ion composition (von Damm et al. (1985). Geochim. Cosmochim. Acta 49, 2197-2220). All concentrations are in mM and the pH is 3.4. Discuss the... 

Positive Eu anomaly is observed for hydrothermal solution issuing from the hydrothermal vent on the seawater at East Pacific Rise (Bence, 1983 Michard et al., 1983 Michard and AlbarMe, 1986). Guichard et al. (1979) have shown that the continental hydrothermal barites have a positive Eu anomaly, indicating a relatively reduced environment. Graf (1977) has shown that massive sulfide deposits and associated rocks from the Bathurst-Newcastle district. New Brunswick have positive Eu anomalies. These data are compatible with positive Eu anomaly of altered basaltic rocks, ferruginous chert and Kuroko ores in Kuroko mine area having positive Eu anomaly and strongly support that Eu is present as divalent state in hydrothermal solution responsible for the hydrothermal alteration and Kuroko mineralization. 

Figure 1.158. Rare earth elements in vent fluids from the East Pacific Rise (redrawn from Michard and Albarede, 1986). The Sm values for the 1984 13 N set are thought to be somewhat too high because of contamination from the sampling syringes. Data for basalt from 13°N East Pacific Rise are from Michard et al. (1983) (Scott, 1997).

Converse, D.R., Holland, H.D. and Edmond, J.M. (1984) Flow rates in the axis hot springs on the East Pacific Rise (21°N) Implications for the heat budget and the formation of massive sulfide deposits. Earth Planet. Sci. Lett., 69, 159-175. 

Michard, A. and Albarede, F., Michard, G., Minsten, J.F. and Charlou, J.L. (1983) Rare-earth elements and uranium in high-temperature solutions from East Pacific Rise hydrothermal vent field (I3°N). Nature (London), 303, 795-797. 

Ruhlin, D.E. and Owen, R.M. (1986) The rare earth element geochemistry of hydrothermal sediments from the East Pacific Rise Examination of a seawater scavenging mechanism. Geochim. Cosmochim. Acta, 50, 393-400. 

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)... 

Albarede, F., Michard, A., Minster, J.F. and Michard, G. (1981) Sr/ Sr ratios in hydrothermal waters and deposits from the East Pacific Rise at 21°N. Earth Planet. Sci. Lett, 55, 229-236. 

Bischoff, J.L., Rosenbauer, R.J., Aruscavage, P.J., Baedecker, A.L. and Crock, J.G. (1983) Seafloor massive sulfide deposits from 2I°N, East Pacific Rise Juan de Fuca Ridge and Galapagos rift Bulk chemical composition and economic implications. Econ. Geo ., 78, I711-1720. 

Graham, V.M., Bluth, G.J. and Ohmoto, H. (1988) Sulfide-sulfate chimneys on the East Pacific Rise 11° and 13°N latitudes. Part I mineralogy and paragenesis. Can. Mineral, 26, 487-504. 

Macdonald, K.C., Becker, K., Spiess, F.N. and Ballard, R.D. (1980) Hydrothermal heat flux of the black smoker vents on the East Pacific Rise. Earth Planet. Sci. Lett., 48, 1-7. 

Woodruff, L.G. and Shanks, W.C. Ill (1988) Sulfur isotope. study of chimney minerals and vent fluids from 21°N East Pacific Rise Hydrothermal sulfur sources and disequilibrium sulfate reduction. J. Geophys. Res., 93, 4562-4572,... 

Von Damm, K.L., Edmond, J.M. Measure, C.I., Walden, B. and Weiss, R.F. (1985a) Chemistry of submarine hydrothermal solutions at 21°N, East Pacific Rise. Geochim. Cosmochim. Acta, 49, 2197-2220. 

Toomey DR, Wileoek WSD, Conder JA, Forsyth DW, Blundy JD, Parmentier EM, Hanunond WC () Asynunetrie mantle dynamics in the MELT region of the East Pacific Rise. Earth Planet Sci Lett 200 287-295... 

Other early alpha spectrometry studies examined U-series disequilibria in East Pacific Rise (EPR) samples. Newman et al. (1983) measured samples from the RISE study area (21°N EPR) finding significant variations in Th/U and ( °Th)/( U) (Fig. 2B). Rubin and Macdougall (1988) measured ( °Th)/( U) and ( Ra)/( °Th) for samples from both the northern ( 10-12°isn and southern (-20-25°) EPR (Fig. 2C,D). This study was first to document large disequilibria in young MORB and interpreted the... 

The area between the Siquieros and Clipperton Transforms on the East Pacific Rise at 8-10°N (often referred to as 9°N ) has been a focus of intense study over the past 10 years. The first U-series work in this area involved combined °Th and Pa analyses of samples collected by submersible, rock coring and dredging along the southern portion of 9°N (Goldstein et al. 1993) (Fig. 3). Volpe and Goldstein (1993) analyzed three of these... 

Macdongall JD, Lugmair G (1986) Sr and Nd isotopes in basalts from the East Pacific Rise significance for mantle heterogeneity, Earth Planet Sci Lett 77 273-284 McKenzie D (1984) The generation and compaction of partially molten rock. J Petrol 25 713-765 McKenzie D (1985) °Th- U disequilibrium and the melting processes beneath ridge axes. Earth Planet Sci Lett 72 149-157... 

Sims K, Goldstein SJ, Blichert-Toft J, Perfit M, Kelemen PB, Fomari D, Michael PJ, Murrell MT, Hart SR, DePaolo DJ, Layne G, Ball L, lull M, Bender J (2002) Chemical and isotopic constraints on the genesis and transport of magmas beneath the East Pacific Rise. Geochim Cosmochim Acta 66 3481-3505... 

Toomey DR, Wilcock WSD, Solomon SC, Hammond WC, Orcntt JA (1998) Mantle seismic structure beneath the MELT region of the East Pacific Rise from P and S wave tomography. Science 280 1224-1227... 

Wood BJ, Blundy JD, Robinson JAC (1999) The role of clinopyroxene in generating U-series disequilibrium during mantle melting. Geochim Cosmochim Acta 63 1613-1620 Zou H, Zindler A, Niu Y (2002) Constraints on melt movement beneath the East Pacific Rise from °Th- U disequilibrium. Science 295 107-110... 

Asmerom et al. (2000). The depth of melting in this region is approximately 2 GPa while the prediction for equal Djh and Du is 0.8-1.3 GPa (Landwehr et al. 2001). This apparent discrepancy might reflect what Asmerom et al. (2000) suggested is a reequilibration of the melts produced at the base of the static lithosphere at shallower pressure. If there is such a reequilibration or some form of assimilation, then the initial excess °Th would decrease while the Pa excess could be maintained as advocated in the Comores (Bourdon et al. 1998) and the East Pacific Rise (Sims et al. 2002). 

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