Fe isotope exchange between Fe(II)aq and nanoparticulate mackinawite (FeSm) during nanoparticle growth

We detail the results of an experimental study on the kinetics of Fe isotope exchange between aqueous Fe(II)aq and nanoparticulate mackinawite (FeSm) at 25 °C and 2 °C over a one month period. The rate of isotopic exchange decreases synchronously with the growth of FeSm nanoparticles. 100% isotopic exchange between bulk FeSm and the solution is never reached and the extent of isotope exchange asymptotes to a maximum of ~ 75%. We demonstrate that particle growth driven by Ostwald ripening would produce much faster isotopic exchange than observed and would be limited by the extent of dissolution–recrystallisation. We show that Fe isotope exchange kinetics are consistent with i) FeSm nanoparticles that have a core–shell structure, in which Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation–growth mechanism. We argue that because of the structure of FeSm nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. FeSm is a reactive component in diagenetic pyrite forming systems since FeSm dissolves and reacts to form pyrite. Isotopic mobility and potential equilibration between FeSm and Fe(II)aq thus have direct implications for the ultimate Fe isotope signature recorded in sedimentary pyrite.