Stable iron isotope fractionation in metal sulfidation at 600–1200 °C

Iron metal and troilite co-exist in ordinary chondrites and iron meteorites [1,2]. Here, we present results from Fe metal equilibrium sulfidation experiments in evacuated silica glass tubes at 600-1200 °C. The two distinct phases, Fe and FeS, were analyzed for their δ56Fe and δ57Fe compositions with MC-ICP-MS (Thermo Scientific Neptune Plus). The materials were ablated using a Spectra-Physics Solstice femtosecond laser ablation system at the Institute of Mineralogy, University Hannover, see [3] for analytical details.

At 600–1000 °C the sulfidation reaction occurred in the solid state and Fe metal is coated by FeS, whereas above 1000 °C we observe Fe and FeS melts. We insured isotopic equilibration by running experiments with different durations. At 600 °C and 71h we observed kinetic fractionation effects, whereas after 260h the sulfide coating was isotopically homogeneous. Overall, FeS is systematically heavier compared to the Fe metal, and the isotopic fractionation decreases with increasing temperature from 600-1200 °C. However, a comparison of the experimental results with δ56Fe data from ordinary [1] and iron [2] meteorites shows the opposite fractionation trend. In the meteorites the FeS is always isotopically lighter compared to the Fe metal. We suggest that this is due to kinetic fractionation effects that occur during cooling of Fe-FeS melts in the meteorites, whereas our quenched experiments retained the equilibrium δ56Fe fractionation.

[1] Needham A. W. et al. (2009) GCA 73:7399–7413. [2] Williams H. M. et al. (2006) EPSL 250:486–500. [3] Horn I. (2007) Spectrochim. Acta B 62:410–422.