Figure 3. Comparison of δ³⁴S and Δ³³S between dissolved sulfate and host-rock sulfides.

Error bars (if not shown, smaller than the symbols) are 1σ uncertainty. Dissolved sulfate in drilling water (open orange circle) is also plotted for comparison. The black solid line represents mixing between pyrite minerals with different sulfur isotope compositions¹⁵ and the blue solid line represents mixing between dissolved sulfate in the fracture waters, respectively. Based on the estimated 1.5‰ to 3.4‰ enrichment³⁶ in δ³⁴S in the product sulfate during the indirect radiolytic oxidation of pyrite (IROP), the initial isotopic compositions of product sulfate are shown as the area in grey (for pyrite only), or the area defined by the dashed magenta lines (for all sulfides). Significantly, δ³⁴S values for dissolved sulfate in the fracture water are instead ∼6‰ more enriched in ³⁴S than pyrite, suggesting that an additional process of isotopic enrichment, possibly microbial sulfate reduction (MSR), has affected the measured sulfate (see text).