Stable isotope studies of vent fluids and chimney minerals, southern Juan de Fuca Ridge: Sodium metasomatism and seawater sulfate reduction

Abstract

Sulfur isotope values (δ 34 S) of H 2 S in vent fluids from the southern Juan de Fuca Ridge hydrothermal sites range from 4.0 to 7.4‰ and are variably 34 S‐enriched with respect to coexisting inner wall chimney sulfides. Chimney sulfides range from 1.6 to 5.7‰. The chimneys consist of Fe‐sphalerite zoned to inner zinc sulfide and chalcopyrite (± isocubanite)‐pyrrhotite lining channels. Sulfide from inner walls of type A chimneys have the lightest δ 34 S values. Type B chimneys (porous, unzoned, low‐Fe‐sphalerite) have the isotopically heaviest chimney sulfides and occur at vent sites distal to the along‐axis shallow point of the ridge crest, hence distal to the magma chamber. These variations are largely ascribed to sulfate reduction by ferrous iron in the hydrothermal fluid in chimneys or substrate mounds, probably due to transitory entrainment of ambient sulfate‐bearing seawater. The δ 18 O values of end‐member hydrothermal fluids range from 0.6 to 0.8‰, significantly lower than the δ 18 O values at 21°N vent fluids. The δD values of the fluid samples range from −2.5 to 0.5‰. Isotopic differences from the 21°N fluids may be due to slightly higher water/rock ratios, approximately 1.0, in the southern Juan de Fuca Ridge hydrothermal system. Admixture of a small amount of residual brine from an earlier phase separation event may have contributed water with low δD values. Sulfate reduction occurs in the deep (2.3 km) hydrothermal reaction zone; a small amount of seawater sulfate passes through the zone of anhydrite precipitation during recharge of the hydrothermal system and is reduced by reaction with pyrrhotite in basalt. Sulfide from pyrrhotite is mixed with 34 S‐enriched sulfate‐derived sulfide to produce pyrite having δ 34 S values of about 3.0‰ and H 2 S having values of about 2.1‰ in the ascending fluid. Requisite acidity is provided by Na‐metasomatism at about 370°C, yielding albite‐ and epidote‐rich alteration phases.