Geochemistry of hydrothermal vent fluids from the northern Juan de Fuca Ridge

Abstract

The presence of aqueous organic compounds derived from sedimentary organic matter has the potential to influence a range of chemical processes in hydrothermal vent environments. For example, hydrothermal alteration experiments indicate that alteration of organic-rich sediments leads to up to an order of magnitude more metals in solution than alteration of organic-poor basalt. This result is in contrast to traditional models for the evolution of vent fluids at sediment-covered mid-ocean ridge axis environments, and indicates the fundamental importance of including the effects of organic compounds in models of crustal alteration processes. However, in order to rigorously constrain their role in crustal alteration processes, quantitative information on the abundances and distributions of organic compounds in hydrothermal vent fluids is required. This thesis was undertaken to provide quantitative information on the distributions and stable carbon isotopic compositions of several low-molecular weight organic compounds (C1-C4 alkanes, C2-C3 alkenes, benzene and toluene) in fluids collected in July, 2000, at three sites on the northern Juan de Fuca Ridge: the Dead Dog and ODP Mound fields, which are located at Middle Valley, and the Main Endeavour Field, located on the Endeavour segment. At Middle Valley, the ridge axis is covered by up to 1.5 km of hemipelagic sediment containing up to 0.5 wt.% organic carbon. The Main Endeavour Field (MEF) is located approximately 70 km south of Middle Valley in a sediment-free ridge-crest environment, but previously measured high concentrations of NH3 and isotopically light CH4 relative to other bare-rock sites suggest that the chemical composition of these fluids is affected by sub-seafloor alteration of sedimentary material (LILLEY et al., 1993). Differences in the absolute and relative concentrations of NH3 and organic compounds and the stable carbon isotopic compositions of the C1-C3 organic compounds suggest that the three fields represent a continuum in terms of the extent of secondary alteration of the aqueous organic compounds, with the Dead Dog fluids the least altered, the MEF fluids the most altered and ODP Mound fluids in an intermediate state. At the two Middle Valley sites, the greater extent of alteration in the ODP Mound fluids as compared to the Dead Dog fluids is due either to higher temperatures in the subsurface reaction zone, or a greater residence time of the fluids at high temperatures. Higher reaction zone temperatures at the ODP Mound field than at the Dead Dog field are consistent with differences in endmember C1 concentrations between the two fields. The greater extent of alteration in the MEF fluids is caused by relatively oxidizing conditions in the subsurface reaction zone that promote faster reaction kinetics. Temperatures in the subsurface reaction zones calculated by assuming equilibrium among aqueous alkanes, alkenes and hydrogen are consistent with other inorganic indicators (C1 and Si concentrations) of temperature, indicating that metastable equilibrium among these compounds may be attained in natural systems. Isotopic equilibration among CH4 and CO2 appears to have been attained in ODP Mound fluids due to the high temperatures in the subsurface reaction zone and the approach to chemical equilibrium from excess methane. However, isotopic equilibrium between CH4 and CO2 was not attained in the MEF fluids, due to a short residence time of the fluids in the crust following late-stage addition of magmatic-derived CO2 to the fluids. Time series analysis indicate that Middle Valley fluid compositions are generally characterized by stable concentrations over the last decade. However, decreases in Br concentrations in Dead Dog fluids from 1990 to 2000 suggest that either a greater proportion of the fluids interact with basalt rather than sediments or that the sediment with which hydrothermal fluids interact is becoming exhausted. In contrast, the concentrations of H2 and H2S and the δ34S of H2S are quite different in fluids sampled from vents of differing ages at the ODP Mound field, despite their close spatial proximity. The observed variations are caused by the reaction of hydrogen-rich fluids within the ODP Mound massive sulfide to reduce pyrite to pyrrhotite during upflow. The replacement of pyrite by pyrrhotite is opposite to the reaction predicted during the weathering of sulfide minerals weather on the seafloor and reflects the real-time equilibration of the reduced fluids with mound mineralogy due to the very young age (<2 years) venting from Spire vent. The presence of aqueous organic compounds therefore affects not only the inorganic chemical speciation in vent fluids, but can also control the mineralogy of associated sulfide deposits. These results also indicate that vent fluid compositions do not necessarily reflect conditions in the deep subsurface, but can be altered by reactions occurring in the shallow subsurface.