Title

Aqueous volatiles in hydrothermal fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: temporal variability following earthquake activity

Publication Date

2003

Document Type

Article

Abstract

The Main Endeavour Field, northern Juan de Fuca Ridge, experienced intense seismic activity in June 1999. Hydrothermal vent fluids were collected from sulfide structures in September 1999 and July 2000 and analyzed for the abundance of H2, H2S, CH4, CO2, NH3, Mg and Cl to document temporal and spatial changes following the earthquakes. Dissolved concentrations of CO2, H2, and H2S increased dramatically in the September 1999 samples relative to pre-earthquake abundances, and subsequently decreased during the following year. In contrast, dissolved NH3 and CH4 concentrations in 1999 and 2000 were similar to or less than pre-earthquake values. Aqueous Cl abundances showed large decreases immediately following the earthquakes followed by increases to near pre-earthquake values. The abundances of volatile species at the Main Endeavour Field were characterized by strong inverse correlations with chlorinity. Phase separation can account for 20–50% enrichments of CO2, CH4, and NH3 in low-chlorinity fluids, while temperature- and pressure-dependent fluid–mineral equilibria at near-critical conditions are responsible for order of magnitude greater enrichments in dissolved H2S and H2. The systematic variation of dissolved gas concentrations with chlorinity likely reflects mixing of a low-chlorinity volatile-enriched vapor generated by supercritical phase separation with a cooler gas-poor hydrothermal fluid of seawater chlorinity. Decreased abundances of sediment-derived NH3 and CH4 in 1999 indicate an earthquake-induced change in subsurface hydrology. Elevated CO2 abundances in vent fluids collected in September 1999 provide evidence that supports a magmatic origin for the earthquakes. Temperature–salinity relationships are consistent with intrusion of a shallow dike and suggest that the earthquakes were associated with movement of magma beneath the ridge crest. These data demonstrate the large and rapid response of chemical fluxes at mid-ocean ridges to magmatic activity and associated changes in subsurface temperature and pressure.

Original Citation

Seewald, J.S., Cruse, A., Saccocia, P.J. (2003). Aqueous volatiles in hydrothermal fluids from the Main Endeavour Field, northern Juan de Fuca Ridge: temporal variability following earthquake activity. Earth and Planetary Science Letters,216(4), 575-590. https://doi.org/10.1016/S0012-821X(03)00543-0