Evolution of the hydrothermal system at 9-10N, 
East Pacific Rise

Karen L. Von Damm, Ph.D.
University of New Hampshire

Monday, August 31, 1998
3:00 p.m.—Pacific Forum

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In 1991/1992 volcanic eruptions on the East Pacific Rise from ~945-51'N caused dramatic changes in the hydrothermal systems and biological communities, in comparison to what had been observed in an 1989 survey. The eruptions were followed by the establishment of new hydrothermal systems (and perturbations of hydrothermal systems that likely pre-dated the eruption). In the initial 1-2 years following the magmatic events, the hydrothermal fluids had unusually high temperatures, high gas contents, low chlorinities, low pHs, and generally low metal contents, although some of the transition metal concentrations were surprisingly high. In the ~7 years since that time, the compositions of the vent fluids have continued to evolve, and most vents have not yet stabilized. We attribute some of the changes in fluid compositions to: 1) initially very shallowly driven circulation resulting in sub-critical phase separation (boiling) and minimal water-rock interaction; 2) a gradual deepening of the reaction zone with attendant changes in the pressure-temperature conditions of water-rock reaction and phase separation, and 3) eventual change to super-critical phase separation and/or equilibration with the substrate. Each of the vents is evolving uniquely. This is in contrast to many of the vents south of the eruption area (916.8- 41'N), most of which were chemically stable between samplings in 1991 and 1994.

The diffuse, low-temperature (mostly <35C) fluids venting at this site have also undergone profound changes in composition, and diverse biological communities have now been established around them. Comparison of diffuse, low-temperature vent fluids with the chemistry of adjacent black smokers indicates that simple admixture of seawater does not explain the chemical differences between them. The changes are consistent with biological activity affecting the diffuse fluid chemistries.

The distinct compositions following the eruptions, and potential biologically induced changes in diffuse fluids, may have profound effects on the net flux of materials from hydrothermal systems to the ocean. These considerations have not been incorporated into previous flux estimates.

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Last updated: December 19, 2000