The chemistry of diffuse-flow vent fluids on the galapagos rift (86°W): Temporal variability and subseafloor phase equilibria controls

Recent (2002, 2005) expeditions to the Galapagos Rift, 86°W, have conducted integrated experiments to assess temporal and spatial changes in both the chemistry and associated biology of diffuse-flow hydrothermal vents. The chemical data add significantly to time series observations of hydrothermal processes at the Galapagos Spreading Center (GSC), where seafloor venting was first discovered in 1977. Assuming quartz-fluid equilibrium (0.55 NaCl solution), end-member values for dissolved silica in 2002 and 2005 indicate temperatures ~50°–100°C lower than in 1977 (~400°C). The higher predicted temperature in 1977 is consistent with observed chloride depletion in fluids issuing from Oyster Beds vent, indicating fluid phase separation, whereas all fluid samples from 2002 and 2005 are at or above seawater concentration. Recent data for dissolved Mn further suggest a lower temperature hydrothermal reaction zone relative to three decades ago. The Li/Si ratios in 1977, however, do not differ significantly from analogous data in 2002 and 2005. This is in contrast with ratios for species known to be temperature-dependent, suggesting that other factors (e.g., fluid/rock ratio, protolith composition) may exert a greater influence on dissolved Li in the hydrothermal end-member. Geologic evidence indicates that between 1990 and 2002 a volcanic eruption covered the historical Rose Garden vent field, ending years of biological observations. Although the precise age of the lava flow is uncertain, the lack of chloride depletion and low H2S/heat ratios in the 2002 and 2005 vent fluids suggest that the 2002 expedition arrived well after the eruption based on observations from other diffuse-flow systems perturbed by volcanic activity. Lower dissolved silica, Mn, and H2S in the 2005 vent fluids relative to 2002, however, may indicate continued cooling at depth in the aftermath of the recent eruption. Geochemical modeling results of evolved seawater in equilibrium with a moderately oxidizing mineral assemblage are consistent with the chemistry of recent GSC fluid samples, predicting H2S/Fe > 1, with little Fe in fluids diluted to the extent typical of 86°W vents. In situ chemical sensor measurements indicate that at temperatures below ~10°C, dissolved sulfide species change in a complex way with temperature, suggesting the involvement of processes other than simple dilution, including, perhaps, the effects of microbial metabolism, recognized in other diffuse-flow vent systems.