anomalies within the proximal plumes overlying vents, fields of 

 vents, and entire segments of ridgecrest. The resulting in- 

 ventories have been coupled with measurements of ambient 

 advection to provide estimates of the hydrothermal fluxes 

 transported away from the respective ridgecrest regions (Baker and 

 Massoth, 1986, 1987). Thus, the emphasis within the VENTS Program 

 relative to the determination of chemical source strengths has 

 been on the assessment of plume fluxes rather than on vent fluid 

 fluxes. 



Why, then, should we pursue a costly and labor-intensive vent 

 fluid sampling program? From a geochemical prespective, a four- 

 fold rationale guides our serious efforts in this regard: 



o We need to understand better the relationship between the 

 proximal plume and its precursors: high-temperature (T > 

 100°C) vent fluids, the heretofore much-neglected low- 

 temperature vent fluids, and ambient seawater. 



o We need to know the composition of the parent hydrothermal 

 fluids. 



o We need to assess the magnitude of the "invisible plume," 

 and, 



o We need to establish a statistical basis upon which we can 

 extend our instantaneous measurements of vent fluid and 

 plume chemistry to time scales appropriate to the basin- 

 wide effects that we propose to validate. 



We expand on these in the following text. 



The answers to such questions as: "What are the relative 

 chemical, thermal, and mass contributions of each of the parent 

 reservoirs to the overlying plume?" and, even more basic, "Are the 

 chemical signatures of the respective reservoirs significantly 

 different?" remain unsatisfyingly incomplete. Yet, these answers 

 and the attendant implications are fundamental to our understand- 

 ing of submarine hydrothermal circulation processes and our 

 assessment of source strengths. Directly analyzing samples from 

 these reservoirs provides information needed to attain these 

 answers. Relating this information to the proximal plume, 

 however, is complicated by the dissolved-to-particulate phase 

 transformations that occur within the buoyant plume. These phase 

 transformations provide significant sinks for some vented species 

 such as reduced sulphur and the chalcophillic metals (e.g., Fe, 

 Cu, and Zn) which combine to form dense particulates that rapidly 

 settle from the plume. Consequently, data must be gathered by 

 submersible at closely spaced intervals throughout the steep 

 geochemical and thermal gradients typical of the buoyant plume in 

 order to track both the mixing and phase transformation processes. 

 Finally, there is an expanding vent fluid data base arising from 

 ongoing research at hydrothermal ly active sites located throughout 

 the global ridgecrest system. In order for us to extend plume- 

 based predictive capabilities to these study regions a firm 



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