Rapid evaluation of organic and pyritic δ³⁴S values in immature sedimentary rocks using a Rock-Eval 7S® coupled to MC-ICPMS

Sulfur isotopic values (δ³⁴S) in sedimentary organic matter and pyrite provide essential information about the global S cycle. The measurement of δ³⁴S values is commonly used to reconstruct paleo-environmental conditions of sediment deposition. In petroleum geochemistry, δ³⁴S values are used to trace relationships between disparate elements of petroleum systems. However, the determination of organic and pyrite δ³⁴S values requires a tedious and time-consuming (weeks) stepwise procedure. Here we used the latest version of the Rock-Eval® 7S analyzer, which can quantify different S-fractions in sedimentary rocks. We have coupled the Rock-Eval® pyrolysis cycle with a multi-collector inductively coupled plasma mass spectrometer (RE/MC-ICPMS). The analysis requires a small sample size (5 to 90 mg), and minimum sample preparation of acid treatment to avoid the carbonate effect on the pyrolyzed pyrite-S. The RE/MC-ICPMS system allows for measuring the pyrolysable organic-S and pyrite-S and directly determining δ³⁴S values from a decarbonated rock. The new method was applied to a set of immature sedimentary rocks of various paleoenvironmental origins and geographic locations. The rapid analysis by the RE/MC-ICPMS, which takes only 20 min per sample, achieved an average precision of 0.5‰, for peaks above 5 nanomole of S. Accuracy was determined relative to a common wet chemistry method followed by an offline EA-IRMS analysis and reached an agreement of <1‰.