Spatial evolution and temporal stability of hydrothermal processes at sediment-covered spreading centers: Constraints from Guaymas Basin, Gulf of California

The Guaymas Basin, Gulf of California is a hydrothermally active sediment-covered oceanic spreading center in the early stages of rifting. Hot-spring fluids were collected from its southern trough in 1998, 2003, and 2008 and analyzed for the concentration and isotopic composition of organic and inorganic aqueous species to assess subseafloor geochemical processes. Fluids discharged from the seafloor through hydrothermal edifices with measured temperatures of 91 to 317 C. In general, fluids exiting larger structures were characterized by higher temperatures than fluids venting from smaller structures. Measured pH (25 C) ranged from 5.7 to 7.1. Endmember fluids were characterized by near-zero concentrations of Mg and SO4, depletions in Na, and enrichments in K, Ca, and Cl relative to seawater, reflecting extensive interaction with sediment during circulation. Thermal maturation of sedimentary organic matter resulted in the addition of abundant sigma NH4+, CO2,sigma CH4, short-chain nalkanes, and carboxylic acids to solution. Organic compounds added to fluids during recharge pass through high temperature deep-seated reaction zones and reach high levels of thermal maturity while others may be added by entrainment of lower temperature sedimentary pore fluids in hydrothermal upflow zones. The relative abundance of many organic compounds mobilized during hydrothermal alteration is consistent with metastable states of thermodynamic equilibrium that likely record subseafloor temperature conditions. The isotope composition of low molecular weight hydrocarbons and elevated concentrations of dissolved Cl suggest that temperatures may have exceeded the two-phase boundary for seawater and subsequently cooled before discharging at the seafloor. Periods of high temperature fluid-sediment or fluid-rock interaction followed by cooling may provide a mechanism for the aqueous mobilization and concentration of sulfide-forming metals in subseafloor mineral deposits. Fluid compositions have remained constant at the southern trough of Guaymas Basin for at least 26 years, indicating decadal stability in the physical and chemical conditions in subseafloor environments as well as the km-scale regional recharge zones containing unaltered sediment.