34S 32S Variations in trace sulphide and sulphate in carbonate rocks of a Devonian reef, Alberta, Canada, and the Precambrian Siyeh formation, Montana, U.S.A.

Concentrations and δ³⁴S-values for trace sulphide and sulphate were determined in 17 dolomites from a Middle Devonian reef, Alberta, Canada, and 7 dolomites from the Precambrian Siyeh Formation, Montana, U.S.A. The isotope patterns in the two localities were quite different. The former can be isotopically divided into two groups. Group I has < 1500 ppm sulphate and relatively low δ³⁴S-values (+ 18 to + 21%o). Group II has sulphate contents > 1500 ppm and δ³⁴S-values from +23 to +25%. The fraction of sulphate, [SO₄^2-]/[SO₄^2- +S^2-], ranges from 0.7 to 0.95 and is quite similar in both groups. The lower δ³⁴S-values for sulphate in Group I are similar to those of the anhydrite which caps the reef. Dolomites from the Siyeh Formation have sulphate and sulphide contents of 54 to 1388 and 0 to 3592 ppm S, respectively. Except for one sample, the δ³⁴S-values of sulphate increase from +0.8 to + 14.3%o with decreasing sulphate content. The isotope fractionation between sulphate and sulphide is nearly constant (5 ± 2%). If this is interpreted as representing SO₄^· reduction, the kinetic isotope effect is rather small (k₃₂/k₃₄=1.005) and the initial δ³⁴S-value of the sulphate was ∼ +0.8%. Trace sulphate and sulphide in carbonate rocks appears to have great potential for studying secular variations in the sulphur isotope composition of ancient oceans particularly during periods for which evaporite deposits have not been found.