Experimental determination of the equilibrium Fe isotope fractionation between [formula omitted] and FeS m (mackinawite) at 25 and 2 °C

Abstract

We report the first experimentally-determined metal isotope equilibrium fractionation factors for a metal sulphide at ambient temperatures and pressures. Mackinawite, referred here as FeS m (where the subscript m indicates mackinawite), can be a reactive component in diagenetic pyrite formation and the extent of equilibration between FeS m and dissolved Fe(II) has direct implications the δ 56Fe signatures recorded in diagenetic pyrite. The measured equilibrium Fe isotope fractionation between Fe(II) aq and FeS m is Δ 56Fe Fe(II)–FeS = −0.52 ± 0.16‰ at 2 °C and Δ 56Fe Fe(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 FeS m (Δ 56Fe Fe(II)–FeS) equates to the fractionation factor between Fe aq 2 + and FeS m ( Δ 56 Fe Fe 2 + – 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–FeS m system is slowly asymptotic to isotopic equilibrium and consequently, FeS m is likely to partially conserve kinetically derived isotopic signatures generated on precipitation. Combined with the range of published kinetic fractionations measured on FeS m precipitation, our data suggest that, subject to the degree of isotope exchange during equilibration, FeS m can display δ 56Fe compositions encompassing a range of ∼1.4‰.