TY - JOUR

T1 - The experimental calibration of the iron isotope fractionation factor between pyrrhotite and peralkaline rhyolitic melt

AU - Schuessler, Jan A.

AU - Schoenberg, Ronny

AU - Behrens, Harald

AU - Blanckenburg, Friedhelm von

N1 - Funding Information: We express our thanks to F. Melcher and U. Vetter (BGR: German Geological Survey) for providing natural pyrrhotite samples and for fruitful discussions regarding sulphide ore formation processes. We are also grateful to the Mineralogical Museum, University of Köln (R. Hollerbach, I. Kerkamm) for kindly providing the pyrrhotite sample K. O. Diedrich is acknowledged for preparation of the thin sections. We are greatly indebted to V. Polyakov for generously providing us with unpublished β-factor data of troilite and for fruitful discussion of sulphide β-factors in general. We also thank T. Gesing for his assistance with XRD measurements. A. Tangen, C. Thürnagel and A. Wegorzewski are thanked for their assistance in the laboratory. Thanks to K. Norton for linguistic improvement of the manuscript. We gratefully acknowledge financial support by the German Research Foundation (DFG) Grant SCHO-1071. We thank E. Schauble, F. Poitrasson, two anonymous reviewers and Associate Editor C. M. Johnson for their thoughtful and constructive reviews that significantly halped to improve the quality of the manuscript.

PY - 2006/10/25

Y1 - 2006/10/25

N2 - 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% H2O) and natural pyrrhotites (Fe1 - xS) 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/54Fepyrrhotite-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 Fe2+/ΣFe ratio of our peralkaline rhyolitic melt of 0.38 ± 0.02. At higher Fe2+/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.

AB - 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% H2O) and natural pyrrhotites (Fe1 - xS) 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/54Fepyrrhotite-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 Fe2+/ΣFe ratio of our peralkaline rhyolitic melt of 0.38 ± 0.02. At higher Fe2+/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.

UR - http://www.scopus.com/inward/record.url?scp=33845666196&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2006.09.012

DO - 10.1016/j.gca.2006.09.012

M3 - Article

AN - SCOPUS:33845666196

VL - 71

SP - 417

EP - 433

JO - Geochimica et cosmochimica acta

JF - Geochimica et cosmochimica acta

SN - 0016-7037

IS - 2

ER -