Experimental determination of the equilibrium Fe isotope fractionation between Feaq2+ and FeSm (mackinawite) at 25 and 2 °C

We report the first experimentally-determined metal isotope equilibrium fractionation factors for a metal sulphide at ambient temperatures and pressures. Mackinawite, referred here as FeSm (where the subscript m indicates mackinawite), can be a reactive component in diagenetic pyrite formation and the extent of equilibration between FeSm and dissolved Fe(II) has direct implications the δ56Fe signatures recorded in diagenetic pyrite. The measured equilibrium Fe isotope fractionation between Fe(II)aq and FeSm is Δ56FeFe(II)–FeS = −0.52 ± 0.16‰ at 2 °C and Δ56FeFe(II)–FeS = −0.33 ± 0.12‰ at 25 °C and pH 4. At the experimental pH the equilibrium fractionation factor between all dissolved Fe(II) species and FeSm (Δ56FeFe(II)–FeS) equates to the fractionation factor between Feaq2+ and FeSm(Δ56FeFe2+–FeS)(Δ56FeFe2+–FeS). The measured fractionations are of the same order as other non-redox fractionations measured in low-temperature Fe–C–O systems. We show that at low temperature, the Fe(II)aq–FeSm system is slowly asymptotic to isotopic equilibrium and consequently, FeSm is likely to partially conserve kinetically derived isotopic signatures generated on precipitation. Combined with the range of published kinetic fractionations measured on FeSm precipitation, our data suggest that, subject to the degree of isotope exchange during equilibration, FeSm can display δ56Fe compositions encompassing a range of ∼1.4‰.