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 geological and geophysical characteristics of the mid-ocean ridge cannot be 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 hydrothermal plumes. In November 1991, we mapped hydrothermal temperature (DELTAtheta) and light attenuation (DELTAc) anomalies above the East Pacific Rise (EPR) continuously from 8-degrees-40' to 11-degrees-50'N, a fast spreading ridge crest portion bisected by the Clipperton Transform Fault. Plume distributions show a precise correlation with the distribution of active vents where video coverage of the axial caldera is exhaustive (90-degrees-09'-54'N). Elsewhere in the study area the sketchy knowledge of vent locations gleaned from scattered camera tows predicts only poorly the large-scale hydrothermal pattern revealed by our plume studies. Plumes were most intense between 9-degrees-42' and 9-degrees-54 N', directly over a March/April, 1991, seafloor eruption. These plumes had exceptionally high DELTAc/DELTAtheta ratios compared to the rest of the study area; we suggest that the phase-separated gas-rich vent fluids discharging here fertilize an abundant population of bacteria. Hydrothermal plume distributions define three categories: intense and continous (8-degrees-48'-8-degrees-58'N, 9-degrees-29'-10-degrees-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 category is virtually congruent with areas that are, respectively, magmatically robust, magmatically weak and magmatically starved, as inferred from previous measurements of axial bathymetric undulations, cross-axis inflation and magma chamber depth and continuity. This congruency implies a fine-scale spatial and temporal connection between magmatic fluctuations and hydrothermal venting. We thus speculate that, at least along this fast spreading section of the EPR, cyclic replenishment, eruption and freezing of the thin axial melt lens exerts greater control over hydrothermal venting than the more enduring zones of crystal mush and hot rock. We found intense, and continuous, plumes along 33% of the surveyed ridge crest, an observation implying that any point on the ridge is, on average, hydrothermally active one-third of the time. Combining this result with the 20% plume coverage found along the medium-rate Juan de Fuca Ridge suggests that superfast (approximately 150 mm/yr) spreading ridges should support vigorous venting along approximately 50% of their length, if spreading rate and along-axis plume coverage are linearly related.