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

Abstract

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‰.