Résumé
Sulfur isotope ratios were measured by SIMS analysis of individual sulfides in the hydrothermally-altered sedimentary and volcanic sequence of the 3.5 Ga chert-barite deposit of the Dresser Formation, Western Australia. The rocks studied are drill cores that intercepted altered komatiites and associated network of silica- and barite-feeder veins, bedded barite and bedded carbonate. Sulfides in black chert veins yield positive delta (super 34) S values ( approximately 0 to 5.4ppm), and positive (1.1 to 3.9ppm) or negative (down to -1.1ppm approximately barite isotope composition) delta (super 33) S values. Sulfides from the host basalt show delta (super 34) S and Delta (super 33) S values of -2.3 and 0.6ppm, and -1.3 and 0.5ppm, respectively, the least altered basalts plotting near 0ppm. Microscopic sulfides along non-recrystallized barite overgrowth zones display (super 34) S-depleted values down to -22.6ppm and positive Delta (super 33) S values up to 6.1ppm. These were attributed to microbial disproportionation of elemental sulfur (S degrees ) derived from the photolysis of volcanic SO (sub 2) (Philippot et al., 2007). These results contrast with bulk rock analysis (Shen et al.; 2001; 2009; Ueno et al.; 2008) for which black chert veins and sulfides in barite define two opposite linear trends intersecting at the mean isotope value of altered basalts. These trends are best attributed to mixing arrays between a slightly modified magmatic reservoir (altered basalts) and two atmosphere/hydrothermal-derived reservoirs, a positive delta (super 34) S and Delta (super 33) S reservoir (black chert veins) and a negative delta (super 34) S and Delta (super 33) S reservoir derived from the thermal and/or microbial reduction of sulfate. Calculation of the stability limits of aqueous sulfur species (SO (sub 4) , HSO (sub 4) , H (sub 2) S, HS, S degrees ) indicate that at low T (40-60 degrees C), the stability field of S degrees is restricted to a narrow range of low pH and logf(O (sub 2) ). This implies that the use of S degrees by microorganisms to form (super 34) S-depleted sulfides with positive Delta (super 33) S values must have occurred as soon as atmospheric S degrees reached the water column. In contrast, the recognition that the sulfate stability field shrinks to a narrow range of pH and redox conditions with increasing T (40 to 200 degrees C) and that the (super 34) S-depleted sulfides with negative Delta (super 33) S values are present in both the bedded and feeder vein barite is strong support for a sulfate reduction process of thermal rather than of microbial origin.