FY 1995

Characteristics of hydrothermal discharge following a magmatic intrusion

Baker, E.T.

In Hydrothermal Vents and Processes, L.M. Parson, C.L. Walker, and D.R. Dixon (eds.), Geol. Soc. Spec. Pub. No. 87, 65–76 (1995)

Seafloor hydrothermal systems are profoundly altered by magmatic fluctuations, which are inherently episodic and generally unpredictable. At present, three examples of hydrothermal systems perturbed by magmatic intrusion have been identified and sampled: 1986 on the Cleft segment of the Juan de Fuca Ridge; 1991 at 10°N on the East Pacific Rise; and 1993 on the CoAxial segment of the Juan de Fuca Ridge. From the fragmentary observations at each site three trends can be identified that may be common to magmatically perturbed hydrothermal systems. The flux of heat and mass can increase by orders of magnitude virtually simultaneously with a magmatic intrusion by the sudden and short-lived release of event plumes, while chronic discharge remains elevated for months or years afterwards. Vent fluid composition is altered by at least two processes. Phase separation is initiated or enhanced, producing fluids highly enriched in the vapour phase. The conjugate brine-enriched fluid may be stored in the crust to be flushed months or years later by convecting seawater. Magmatic degassing increases the flux of volatiles, temporarily elevating He/temperature ratios. Time series observations of magmatically altered vent fields are vital, because chemical budget extrapolations and hypotheses derived only from observations of stable hydrothermal discharge may be incomplete or unreliable.