Sulfide-Sulfate Chimneys on the East Pacific Rise, 11° and 13° N Latitudes. Part II: Sulfur Isotopes

Gregg J. S. Bluth and Hiroshi Ohmoto

Abstract. Sulfide minerals from eleven separate hydrothermal vents, and vent-fluid H2S from five vents along the East Pacific Rise at 11° and 13° N latitudes were analyzed for sulfur isotopic compositions. On average, d34S (sulfide) values (4.1±0.60/00) are among the heaviest found for ridge-crest hot-spring systems. The d34S (sulfide) values of chimneys at 11° and 13°N are similar, although those at 13°N define a slightly wider range. Analyses of lateral samples of chimneys show a central depletion d34S of chimney walls at 13°N, and a small decrease from the interior to exterior at 11°N. Chimneys analyzed along their length show decreasing isotopic ratios upward. Measurements of coexisting sulfides suggest isotopic disequilibrium, but the d34S values of sulfides sampled from the inner walls of the chimneys are within ±0.5% of discharging vent-fluid H2S. These observations, in light of the chimney-growth model in Part I, suggest that: (i) the hydrothermal systems at vent sites from 13°N and those at 11°N are each tied to a single magma chamber at depth; (ii) the d34S (H2S) of the discharging fluids changed gradually from +1.7 in the early stage to +5.00/00 in the later stage of hydrothermal activity; (iii) the change is attributed to the increased contribution of H2S derived from reduction of seawater sulfate compared to the H2S from leaching of basalt at deeper parts of the plumbing system; and (iv) the variations in the d34S of vent-fluid H2S and of sulfides within chimney walls were probably caused by rapid chemical reactions of dissolution, reprecipitation, and replacement between hydrothermal fluids and earlier sulfide minerals in the chimney, rather than by kinetic isotope effects during precipitation of sulfides or by generation of H2S by sulfate reduction within the chimney structure.

           

           






FIG. 5. Two-stage diagram of a chimney cross-section, describing the model of sulfur isotopic evolution of fluids and chimneys at 11° and 13° N, EPR. In the early stage, vent-fluid H2S of d34S = +1.70/00 produced sulfide minerals of ~1.70/00, with no difference in the d34S(H2S) values between the main and auxiliary vent fluids. In the later stage, the d34S(H2S) of the main vent fluid was +4.70/00. Reactions between this H2S with the earlier sulfide minerals in the chimney wall produced the sulfide minerals and auxiliary vent-fluid H2S with the sulfide minerals and auxiliary vent-fluid H2S with d34S values between 1.7 and 4.70/00. The bold line indicates the isotopic trend found in some 13°N chimneys.