In-situ Chemistry of Hydrothermal Fluids from Black Smokers in Main Endeavour Field, Juan de Fuca Ridge

After an off-axis earthquake swarm in 1999, dramatic changes were observed in vent fluids of Main Endeavour Field, Juan de Fuca Ridge. Three month latter, we also recorded this sudden variation using a high temperature in-situ chemical sensor. The results at that time indicated some of the vent temperatures as high as 374°C. This change was also characterized by relatively high in-situ pH, high dissolved H2, and H2S concentrations in the fluids that were in excess of 5, 0.7 mmol/kg and 20 mmol/kg respectively. In order to further track time dependent changes over the past 6 years, we revisited Main Endeavour Field during the recent AT 11-31 cruise in Aug.~Sept. 2005. The high temperature chemical sensor was again used on selected dives with DSV Alvin to conduct in-situ measurements of pH, dissolved H2 and H2S concentrations along with temperatures. The data were obtained in a real time mode of 3 seconds per-reading from a series of measurements at high temperature conditions in the depth of 2200 m. Conventional gas-tight samples were also collected for verification and further study. In this study, Puffer, Sully and Bastille black smoker vent sites were specifically investigated owing to the high fluid temperatures that characterize these vents in comparison with other vents in the area. The measured temperatures for these vents were 362°C, 358°C, and 361°C respectively, which were generally about 20~30°C higher than the others currently in the area, but approximately 10°C lower than the highest temperatures measured in the aftermath of the 1999 seismic-magmatic event. Although the drops in vent temperatures were not substantial, the measured in-situ chemistry showed large departures from previous reported data. The in-situ pH values in these vents ranged from 4.43 to 4.89, in comparison with values above 5 in 1999. This difference may be linked directly to the decrease in temperature. The measured in-situ dissolved H2 and H2S concentrations were 0.12~0.18 mmol/kg and 7.96~8.90 mmol/kg, respectively. These concentrations clearly demonstrate substantially lower values than reported in the past, which cannot be simply the result of the observed decrease in temperature of the MEF vent fluids. These data reveal a strong departure from the pyrite-pyrrhotite-magnetite equilibrium state as observed in 1999 to more oxidizing conditions, consistent with the assemblage of anhydrite-anorthite-clinozoisite. Compared to dissolved H2, the decrease in H2S concentration is less obvious, but still significant in comparison with previous data. From these in-situ measurements, we suggest that the MEF vent systems have not yet fully returned to a pre-seismic condition. The comparatively high temperature and high dissolved H2S may be the cause for the current dense faunal communities observed most recently at the vent sites.