Ocean-ridge

As a specific example, consider oceanic sulfate as the reservoir. Its main source is river runoff (pre-industrial value 100 Tg S/yr) and the sink is probably incorporation into the lithosphere by hydrogeothermal circulation in mid-ocean ridges (100 Tg S/yr, McDuff and Morel, 1980). This is discussed more fully in Chapter 13. The content of sulfate in the oceans is about 1.3 X lO TgS. If we make the (im-realistic) assumption that the present runoff, which due to man-made activities has increased to 200 Tg S/yr, would continue indefinitely, how fast would the sulfate concentration in the ocean adjust to a new equilibrium value The time scale characterizing the adjustment would be To 1.3 X 10 Tg/(10 Tg/yr) 10 years and the new equilibrium concentration eventually approached would be twice the original value. A more detailed treatment of a similar problem can be found in Southam and Hay (1976). 

Low-temperature circulation of seawater through mid-ocean ridge systems creates a deficiency in Mg and an excess in Ca at middepths (de Villiers and Nelson, 1999). 

A solution, still controversial, has been recently proposed. This is the loss of sulfate from seawater during hydrothermal circulation through mid-ocean ridges (Edmond et al., 1979). The flow of water through these systems is estimated to be about 1.4 x 10 L/yr, about 0.4% of the flow of rivers. However, sulfate is quantitatively removed, yielding a flux of 125 Tg S/yr, capable of balancing the river flux. The controversy is whether the chemistry involved in removing sulfate is the formation of... 

In the Chichibu Zone, the intimate association of abundant strata-bound Mn-Fe deposits, limestone-dolomite and silica (chert) with basic volcanic rocks suggests an ocean-ridge hydrothermal origin. 

In Fig. 1.85 iron contents of epidote from two different geologic environments, island arc and oceanic ridge or ophiolite, are summarized. It can be seen in Fig. 1.85 that the iron content of epidote from ridge basalt and ophiolite is generally lower than... 

Janecky, D.R. and Seyfried, W.E. Jr. (1984) Formation of massive sulfide deposits on oceanic ridge crests incremental reaction models for mixing between hydrothermal solutions and seawater. Geochint. Cosmochim. Acta, 48, 2723-2738. 

Shikazono, N. (1992) Formation models for Kuroko and hydrothermal ore deposits on oceanic ridges. Mining Geology, 42, 331-338 (in Japanese with English abst.). 

Fame G (1986) Principles of Isotope Geology, Second Edition. John Wiley and Sons, New York Fleischer RL, Raabe OG (1975) Recoiling alpha-emitting nuclei. Mechanisms for uranium-series disequilibrium. Geochim Cosmochim Acta 42 973-978 Goldstein SJ, Murrell MT, Williams RW (1993) Pa and h chronology of mid-ocean ridge basalts. Earth Planet Sci Lett 115 151-159... 

Goldstein SJ, Murrell, MT, Wilhams RW (1993) Pa and h chronology of mid-ocean ridge basalts. Earth Planet Sci Lett 115 151-159... 

Paul UH (2001) Melt retention and segregation beneath mid-ocean ridges. Nature 410 920-923 Feineman MD, DePaolo DJ, Ryerson FJ (2002) Steady-state Ra/ °Th disequilibrium in hydrous mantle minerals. Geochim Cosmochim Acta 66 A345 (abstr)... 

Salters VJM, Hart SR (1989) The hafnium paradox and the role of garnet in the source of mid-ocean-ridge basalts. Nature 342 420-422... 

Lundstrom CC (2003) Uranimn-series disequilibria in mid-ocean ridge basalts observations and models of basalt genesis. Rev Mineral Geochem 52 175-214... 

Rubin KH, Macdougall JD (1988) Ra excesses in mid-ocean-ridge basalts and mantle melting. Nature 335 158-161... 

Volpe AM, Hammond PE (1991) U- °Th- Ra disequilibria in young Mount St. Helens rocks Time constraint for magma formation and crystallization. Earth Planet Sci Lett 107 475-486 Volpe AM, Goldstein SJ (1993) Ra- °Th disequilibrium in axial and off-axis mid-ocean ridge basalts. Geochim Cosmochim Acta 57 1233-1241... 

Table 1. Summary of published U-Th-Ra-Pa data for mid-ocean ridge basalts.

U-series disequilibria have become a powerful technique for examining the melting process beneath mid-ocean ridges. All available evidence points to the disequilibria... 

Bourdon B, Zindler A, Elliott T, Langmuir CH (1996b) Constraints on mantle melting at mid-ocean ridges from global U- °Th disequilibrium data. Nature 384 231-235... 

Klein EM, Langmuir CH (1987) Global correlations of ocean ridge basalt chemistry with axial depth and cmstal thickness. J Geophys Res 92 8089-8115... 

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... 

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