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The experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/j.gca.2006.09.012


Abstract A first experimental study was conducted to determine the equilibrium iron isotope fractionation between pyrrhotite and silicate melt at magmatic conditions. Experiments were performed in an internally heated gas pressure vessel at 500 MPa and temperatures between 840 and 1000 °C for 120–168 h. Three different types of experiments were conducted and after phase separation the iron isotope composition of the run products was measured by MC-ICP-MS. (i) Kinetic experiments using 57Fe-enriched glass and natural pyrrhotite revealed that a close approach to equilibrium is attained already after 48 h. (ii) Isotope exchange experiments—using mixtures of hydrous peralkaline rhyolitic glass powder (∼4 wt% H 2O) and natural pyrrhotites (Fe 1 − x S) as starting materials— and (iii) crystallisation experiments, in which pyrrhotite was formed by reaction between elemental sulphur and rhyolitic melt, consistently showed that pyrrhotite preferentially incorporates light iron. No temperature dependence of the fractionation factor was found between 840 and 1000 °C, within experimental and analytical precision. An average fractionation factor of Δ 56Fe/ 54Fe pyrrhotite-melt = −0. 35 ± 0.04‰ (2SE, n = 13) was determined for this temperature range. Predictions of Fe isotope fractionation between FeS and ferric iron-dominated silicate minerals are consistent with our experimental results, indicating that the marked contrast in both ligand and redox state of iron control the isotope fractionation between pyrrhotite and silicate melt. Consequently, the fractionation factor determined in this study is representative for the specific Fe 2+/ΣFe ratio of our peralkaline rhyolitic melt of 0.38 ± 0.02. At higher Fe 2+/ΣFe ratios a smaller fractionation factor is expected. Further investigation on Fe isotope fractionation between other mineral phases and silicate melts is needed, but the presented experimental results already suggest that even at high temperatures resolvable variations in the Fe isotope composition can be generated by equilibrium isotope fractionation in natural magmatic systems.

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