understanding of the links between the proximal plume and its 

 sources is required. 



Knowledge of the composition of parent hydrothermal fluids 

 permits estimation by modeling of the sub-seafloor sinks for 

 hydrothermal ly cycled species which may be important in some 

 elemental mass-balance determinations (Mg, for example). Venting 

 fluid/parent fluid comparisons are also important in the resolu- 

 tion of multiple sub-seafloor endmembers resulting from phase 

 separation and segregation (Von Damm, 1988) which may provide 

 dramatic enhancements in the mobilization and transport of heavy 

 metals (e.g., Fe, Mn, and Zn) within both the brine and vapor 

 phases of hydrothermal fluids (Bischoff et al., 1981; Bischoff and 

 Rosenbauer, 1987). Lastly, changes over time in the composition 

 of the primary fluid may, by inferences drawn from modeling, 

 provide insight into the geophysical underpinnings of hydrother- 

 malism. 



Venting fluids are commonly diluted several thousand-fold with 

 ambient seawater during the formation of the proximal plume 

 (7000X, Lupton et al . , 1985; 8500X, Baker and Massoth, 1986). 

 Plume anomalies for elements vented at concentrations within an 

 order of magnitude of that of seawater are, therefore, essentially 

 mixed beyond detection by all but the most precise analytical 

 techniques and, thus, are effectively "invisible." Elements within 

 this category and for which hydrothermal processes play a 

 significant role in the respective global mass balances (e.g., Mg, 

 S, Li, Rb, B, Ca, and K) are thus more efficiently studied by 

 examination of vent fluids. These elements are important relative 

 to the VENTS rationale simply by virtue of their imprint on the 

 composition of seawater. The time-scales and processes by which 

 seawater concentrations are buffered relative to these elements, 

 however, are metered relative to seawater (8-10 Ma) and 

 lithosphere (100-1000 Ma) hydrothermal cycling times whereas the 

 observable plume anomaly related effects occur over days to the 

 100' s of years typical of ocean basin mixing times. 



We know very little about the temporal variability of venting. 

 Time-series observations of vent fluid chemistry have been 

 reported for only two sites: the East Pacific Rise at 21°N and 

 Guaymas Basin where measurements were obtained at less than annual 

 frequencies extending over four to six years (Campbell et al., in 

 press). While the chemical concentrations of most major vent 

 fluid constituents remained essentially constant over this 

 interval, two-fold variations were documented for Mn and Fe which 

 are among the primary modulators of the chemical state of 

 hydrothermal plumes. The need for additional and higher frequency 

 time-series information can be addressed in part by looking 

 through the temporal window afforded by in situ chemical 

 monitoring of venting fluids. Using state-of-the-art technology, 

 such as flow-injection analysis linked to a variety of chemical 

 detectors (e.g., Johnson et al., 1985, 1986a, b), it is conceivable 



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