Micro-scale isotopic variability of low-temperature pyrite in fractured crystalline bedrock - A large Fe isotope fractionation between Fe(II)aq/pyrite and absence of Fe-S isotope co-variation

This study assessed Fe-isotope ratio (Fe-56/Fe-54, expressed as delta Fe-56 relative to the IRMM-014 standard) variability and controls in pyrite that has among the largest reported S-isotope variability (maximum delta S-34: 140 parts per thousand). The pyrite occurs as fine-grained secondary crystals in fractures throughout the upper kilometer of granitoids of the Baltic Shield, and was analyzed here for delta Fe-56 by in situ secondary ion mass spectrometry (SIMS). Part of these pyrite crystals were picked from borehole instrumentation at depths of > 400 m below sea level (m.b.s.l.), and thus are modern (known to have formed within 17 years) and can be compared with the delta Fe-56 of the source dissolved ferrous iron. The delta Fe-56 values of the modern pyrite crystals (-1.81 parts per thousand to + 2.29 parts per thousand) varied to a much greater extent than those of the groundwaters from which they formed (-0.48 parts per thousand to + 0.139 parts per thousand), providing strong field evidence for a large Fe isotope fractionation during the conversion of Fe(II)(aq) to FeS and ultimately to pyrite. Enrichment of Fe-56 in pyrite relative to the groundwater was explained by equilibrium Fe(II)(aq)-FeS isotope fractionation, whereas depletion of Fe-56 in pyrite relative to the groundwater was mainly the result of sulfidization of magnetite and kinetic isotopic fractionation during partial transformation of microsized FeS to pyrite. In many pyrite crystals, there is an increase in delta S-34 from crystal center to rim reflecting Rayleigh distillation processes (reservoir effects) caused by the development of closed-system conditions in the micro-environment near the growing crystals. A corresponding center-to-rim feature was not observed for the delta Fe-5(6) values. It is therefore unlikely that the groundwater near the growing pyrite crystals became progressively enriched in the heavy Fe isotope, in contrast to what has been found for the sulfur in sulfate. Other pyrite crystals formed following bacterial sulfate reduction in the time period of mid-Mesozoicum to Quaternary, had an almost identical Fe-isotope variability (total range: -1.50 parts per thousand to + 2.76 parts per thousand), frequency-distribution pattern, and relationship with delta S-34 as the recent pyrite formed on the borehole instrumentation. These features suggest that fundamental processes are operating and governing the Fe-isotope composition of pyrite crystals formed in fractured crystalline bedrock over large time scales.