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

Guilbaud, R., Butler, I.B., Ellam, R.M. , Rickard, D. and Oldroyd, A. (2011) Experimental determination of the equilibrium Fe isotope fractionation between Feaq2+ and FeSm (mackinawite) at 25 and 2°C. Geochimica et Cosmochimica Acta, 75(10), pp. 2721-2734. (doi: 10.1016/j.gca.2011.02.023)

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Publisher's URL: http://dx.doi.org/10.1016/j.gca.2011.02.023


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 FeS<sub>m</sub> and dissolved Fe(II) has direct implications the δ<sup>56</sup>Fe signatures recorded in diagenetic pyrite. The measured equilibrium Fe isotope fractionation between Fe(II)<sub>aq</sub> and FeS<sub>m</sub> is Δ<sup>56</sup>Fe<sub>Fe(II)–FeS</sub> = −0.52 ± 0.16‰ at 2 °C and Δ<sup>56</sup>Fe<sub>Fe(II)–FeS</sub> = −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<sub>m</sub> (Δ<sup>56</sup>Fe<sub>Fe(II)–FeS</sub>) equates to the fractionation factor between and FeS<sub>m</sub>(Δ<sup>56</sup>Fe<sub>Fe</sub><sup>2+</sup><sub>–FeS</sub>). 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)<sub>aq</sub>–FeS<sub>m</sub> system is slowly asymptotic to isotopic equilibrium and consequently, FeS<sub>m</sub> is likely to partially conserve kinetically derived isotopic signatures generated on precipitation. Combined with the range of published kinetic fractionations measured on FeS<sub>m</sub> precipitation, our data suggest that, subject to the degree of isotope exchange during equilibration, FeS<sub>m</sub> can display δ<sup>56</sup>Fe compositions encompassing a range of ∼1.4‰.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Ellam, Professor Rob
Authors: Guilbaud, R., Butler, I.B., Ellam, R.M., Rickard, D., and Oldroyd, A.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Journal Name:Geochimica et Cosmochimica Acta

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