HYDROTHERMAL PLUMES ALONG THE EAST PACIFIC RISE, 8-DEGREES-40' TO 11-DEGREES-50'N - PLUME DISTRIBUTION AND RELATIONSHIP TO THE APPARENT MAGMATIC BUDGET

The interactions between hydrothermal circulation and large-scale geol ogical and geophysical characteristics of the mid-ocean ridge cannot b e ascertained without large-scale views of the pattern of hydrothermal venting. Such multi-ridge-segment surveys of venting are accomplished most efficiently by mapping the distribution and intensity of hydroth ermal plumes. In November 1991, we mapped hydrothermal temperature (DE LTAtheta) and light attenuation (DELTAc) anomalies above the East Paci fic Rise (EPR) continuously from 8-degrees-40' to 11-degrees-50'N, a f ast spreading ridge crest portion bisected by the Clipperton Transform Fault. Plume distributions show a precise correlation with the distri bution of active vents where video coverage of the axial caldera is ex haustive (90-degrees-09'-54'N). Elsewhere in the study area the sketch y knowledge of vent locations gleaned from scattered camera tows predi cts only poorly the large-scale hydrothermal pattern revealed by our p lume studies. Plumes were most intense between 9-degrees-42' and 9-deg rees-54 N', directly over a March/April, 1991, seafloor eruption. Thes e plumes had exceptionally high DELTAc/DELTAtheta ratios compared to t he rest of the study area; we suggest that the phase-separated gas-ric h vent fluids discharging here fertilize an abundant population of bac teria. Hydrothermal plume distributions define three categories: inten se and continous (8-degrees-48'-8-degrees-58'N, 9-degrees-29'-10-degre es-01'N and 11-degrees-05'-11-degrees-27'N), weak and discontinuous (8 -degrees-58'-9-degrees-29'N) and negligible. The location of each cate gory is virtually congruent with areas that are, respectively, magmati cally robust, magmatically weak and magmatically starved, as inferred from previous measurements of axial bathymetric undulations, cross-axi s inflation and magma chamber depth and continuity. This congruency im plies a fine-scale spatial and temporal connection between magmatic fl uctuations and hydrothermal venting. We thus speculate that, at least along this fast spreading section of the EPR, cyclic replenishment, er uption and freezing of the thin axial melt lens exerts greater control over hydrothermal venting than the more enduring zones of crystal mus h and hot rock. We found intense, and continuous, plumes along 33% of the surveyed ridge crest, an observation implying that any point on th e ridge is, on average, hydrothermally active one-third of the time. C ombining this result with the 20% plume coverage found along the mediu m-rate Juan de Fuca Ridge suggests that superfast (approximately 150 m m/yr) spreading ridges should support vigorous venting along approxima tely 50% of their length, if spreading rate and along-axis plume cover age are linearly related.