EXPERIMENTAL STUDY OF SULFUR ISOTOPE FRACTIONATION BETWEEN COEXISTENT SULFIDE MINERALS

Isotopic fractionation of sulfur between synthetic sulfide mineral pairs (FeS2PbS, ZnSPbS and FeS2ZnS) was investigated as a function of temperature (150-630°C) and time (up to six months) of reaction. The coexistent mineral pairs were prepared from elements in sealed evacuated glass tubes with excess sulfur present. The results reveal that the fractionation factor among these pairs decreases with increase of temperature of synthesis, and that the enrichment of the heavy isotope (34S) is found to be in the order of FeS2 (pyrite) >ZnS (sphalerite) >Pbs (galena), similarly to what is observed in natural specimens. Isotopic equilibrium seems to be attained in a comparatively short period among these sulfide phases (less than 3 months at 150°C and only a few days of 630°C). The equilibrium constants for the isotopic exchange reactions, 1 2 Fe32S2 + Pb34S = 1 2 Fe34S2 + Pb32S, Zn32S + Pb34S = Zn34S + Pb32S, and 1 2 Fe32S2 + Zn34S = 1 2 Fe34S2 = 1 2 Fe34S2 + Zn32S obtained in the present study are found to be fairly close to the values predicted by theoretical consideration [1]. © 1969.