Laser microprobe study of sulfur isotope variation in a sea-floor hydrothermal spire, Axial Seamount, Juan de Fuca Ridge, eastern Pacific

Abstract

The Axial Seamount site is located on the Juan de Fuca Ridge near the intersection of the Cobb-Eikenberg-Brown BEar seamount chain and the axial rift of the ridge. Several large (to 12 m) silica+sulfide+sulfate hydrothermal spires reflect high-temperature (to 300°C) hydrothermal activity. Laser microprobe analyses along traverses of five temporally distinct fluid conduits within an inactive high-temperature spire reveal significant temporal variation inδ³⁴S, both within individual conduits and between conduits. The maximum intra-conduitδ³⁴S variation (on wurtzite) is 5‰ and the maximum inter-conduit variation (on sphalerite) is 7.4‰ Inter-conduitδ³⁴S variation occurs primarily due to variable amounts of seawater mixing with hydrothermal fluids within the spire; generallyδ³⁴S_mineral decreases through time due to lesser amounts of seawater that invade more mature, heavily mineralized spires. Intra-conduitδ³⁴S variation has not been documented at this scale (5‰ within 1 mm), and the potential mechanisms responsible for this variation include: (1) within-spire seawater-hydrothermal fluid mixing, which also produces the inter-conduit variations, or (2) more deep-seated convection and redox processes in the sea-floor subsurface that alter theδ³⁴S_fluid.

Mixing of hydrothermal fluid with seawater cannot produce a range ofδ³⁴S_mineral-values of this magnitude (5‰), and deeper subsurface processes are required. Such deep-seated processes may involve the opening of new fluid conduits in the subsurface, exposing fresh basalt which will increase the reduction potential of the rock system. This in turn will promote increased reduction of seawater sulfate in the hydrothermal fluid and attendant increases ofδ³⁴S_fluid. The fluids will ulti precipitate³⁴S-enriched sulfide phases, although this excursion inδ³⁴S-values is ephemeral and will last only until the fluid has exhausted the reducing potential of the new conduit. At this point, sulfate reduction will be sharply reduced andδ³⁴S-values will correspondingly decrease. This process may explain the major (to +5‰) isotope excursions that occur within individual conduits on a very small (<1m) scale.