SULFUR ISOTOPE FRACTIONATION DURING EXPERIMENTAL PRECIPITATION OF IRON(II) AND MANGANESE(II) SULFIDE AT ROOM-TEMPERATURE

Fractionation of stable sulfur isotopes (S-34/S-32) during the precipi tation of iron(II) and manganese(II) sulfides from aqueous solutions a t room temperature (approximate to 20 degrees C) was investigated usin g batch-type experiments. The sulfides were precipitated (1) by mixing of metal(II) chloride solutions with hydrogen bisulfide solutions, an d (2) by the transformation of anhydrous metal(II) carbonates in hydro gen bisulfide solutions. It was found that the partitioning of S-34/S- 32 upon metal sulfide precipitation depends on the synthesis procedure , an indication for the influence of reaction kinetics. The sulfur iso topic composition of sedimentary metal monosulfides is close to that o f dissolved hydrogen bisulfide. Kinetic fractionation factors for meth od (1) were 0.9988 +/- 0.0007 (alpha'(FeS-HS-)) and 1.0002 +/- 0.0007 (alpha'(MnS-HS-)), and for method (2) 1.0004 +/- 0.0001 (alpha'(FeS-HS -)) and 1.0012 +/- 0.0003 (alpha'(MnS-HS-)). Therefore, sulfur isotope effects related to the precipitation of metal monosulfides are small when compared to important microbial reactions. (C) 1998 Elsevier Scie nce B.V. All rights reserved.