Sulfur Isotope Partitioning in Metallic Sulfide Systems

Sulfur isotopic fractionation factors involving pairs of pyrite, pyrrhotite, sphalerite, chalcopyrite, and galena have been determined experimentally over the temperature range 250 °C to 600 °C.Since chalcopyrite and pyrrhotite are not stable at higher PS 2 conditions, buffer assemblages were necessary to control PS 2 in experiments with these minerals. Since low PS 2 values and low temperatures are unfavorable to rapid isotope exchange, techniques were devised whereby equilibrium constants could be estimated indirectly in systems where direct measurements are not possible because of the time factor.Current data place the sulfide minerals in the following order of 34 S enrichment under equilibrium exchange conditions: pyrite > (pyrrhotite Formula: see textsphalerite) > chalcopyrite > galena in agreement with theoretical predictions. In agreement with theory the equilibrium exchange constant K for a given mineral pair depends upon temperature as follows: 1000 ln Formula: see text, where A denotes a constant. The A values for various mineral pairs have been determined with ± 10% uncertainties as follows: 11.0 × 10 5 (py–gn), 8.0 × 10 5 (sp–gn), 6.5 × 10 5 (cp–gn), 4.5 × 10 5 (py–cp), 3.0 × 10 5 (py–sp, py–po), 1.5 × 10 5 (sp–cp, po–cp), and Formula: see text 0 (sp–po).