Microscale δ34S and δ18O variations of barite as an archive for fluid mixing and microbial sulphur metabolisms in igneous rock aquifers.

The stable isotope compositions of sulphur (δ³⁴S) and oxygen (δ¹⁸O) in barite are frequently used as proxies for microbial sulphate reduction (MSR) in diverse environments, such as in relation to anaerobic oxidation of methane in marine cold seeps. There, isotopically heavy barite is used as a marker for MSR from a sulphate pool that has undergone semi-closed system conditions. Closed-system MSR is also a commonly observed feature in igneous rock hosted fracture aquifers, as shown by extremely ³⁴S-enriched pyrite. What is less well-constrained is whether δ³⁴S in barite can be used as a proxy for MSR in such systems. Here we explore the microscale heterogeneity of δ³⁴S and δ¹⁸O via secondary ion mass spectrometry and the trace element Sr via LA-ICP-MS maps in barite precipitated in granite-hosted boreholes during a 17-year experiment, at Äspö, Sweden. We compare it with δ¹⁸O_sulfate, δ³⁴S_sulfate, and δ³⁴S_sulfide of the fracture fluids and with paragenetic pyrite with δ³⁴S values reflecting closed system MSR. The δ¹⁸O values in barite (+9.4 to +16.9 ‰) represent two generations of barite, one with low values and one with high values. The latter are likely impacted by sulphur disproportionating or -oxidizing bacteria. The barite reflects a much smaller span in δ³⁴S (+14.5 to +28.6 ‰) than the pyrite (-47.2 to +53.3 ‰). This lack of extremely high δ³⁴S_barite values is proposed to be due to that barite saturation only occurred in the early parts of the Rayleigh cycle. Additionally, fluid migration has affected the δ³⁴S values to lower values, accompanied by higher Sr concentrations. Taken together, barite δ³⁴S values cannot be regarded as a reliable independent proxy for MSR in deep sulphate-poor igneous rock hosted aquifers. However, the relation between the δ³⁴S values of coeval barite and pyrite is regarded as a useful proxy for MSR-related fractionation during early stages of MSR.