TY - JOUR

T1 - The oxidation state of iron in silicic melt at 500 MPa water pressure

AU - Wilke, Max

AU - Behrens, Harald

AU - Burkhard, Dorothee J.M.

AU - Rossano, Stéphanie

N1 - Funding Information: Technical assistance during the sample synthesis by W. Hurkuck and B. Aichinger and sample preparation by O. Diedrich are highly appreciated. We thank J. Klein, formerly at the Universität Marburg, for help in spectrum acquisition. We would like to thank F. Farges for enthusiastic and fruitful discussions. Helpful comments by M.D. Dyar and H.S.C O'Neill are gratefully acknowledged. This study was funded by the Deutsche Forschungsgemeinschaft, Schwerpunktprogramm Elementverteilung (Project Be/1720/4). [RR]

PY - 2002/9/16

Y1 - 2002/9/16

N2 - The dependence of the ferric-ferrous ratio in silicate melts on oxygen fugacity was studied in the system spectroscopy. Experiments were performed under water-saturated conditions at 500 MPa, and at temperatures of 850 and 950 °C, covering a range typical for magmatic processes. The oxygen fugacity was varied in the fO2 range from Cu-Cu2O buffer to slightly more reducing conditions than the wüstite-magnetite buffer. The iron redox ratio was determined by analyzing the Mössbauer parameter distribution that was modeled based on experimental spectra collected at room temperature on the quenched samples. The obtained iron redox ratios show a linear dependence on oxygen fugacity on a logarithmic scale for both temperatures. The iron redox ratio (Fe3+/Fe2+) decreases with temperature for a given oxygen fugacity. The spectroscopic data at 850 °C are in good agreement with Fe3+/Fe2+ ratios derived from element partitioning but show considerable deviations from iron redox ratios predicted by the empirical equation given by Kress and Carmichael [Contrib. Mineral. Petrol. 108 (1991) 82]. This indicates that an extrapolation of this equation to such low temperatures may have large errors. A sample quenched slowly through the temperature range near and below Tg shows considerable differences in the obtained Mössbauer spectra compared to more rapidly cooled samples, indicating ordering of the iron environment at least in the mesoscopic range. The oxidation state, however, does not differ when compared to the more rapidly quenched melts.

AB - The dependence of the ferric-ferrous ratio in silicate melts on oxygen fugacity was studied in the system spectroscopy. Experiments were performed under water-saturated conditions at 500 MPa, and at temperatures of 850 and 950 °C, covering a range typical for magmatic processes. The oxygen fugacity was varied in the fO2 range from Cu-Cu2O buffer to slightly more reducing conditions than the wüstite-magnetite buffer. The iron redox ratio was determined by analyzing the Mössbauer parameter distribution that was modeled based on experimental spectra collected at room temperature on the quenched samples. The obtained iron redox ratios show a linear dependence on oxygen fugacity on a logarithmic scale for both temperatures. The iron redox ratio (Fe3+/Fe2+) decreases with temperature for a given oxygen fugacity. The spectroscopic data at 850 °C are in good agreement with Fe3+/Fe2+ ratios derived from element partitioning but show considerable deviations from iron redox ratios predicted by the empirical equation given by Kress and Carmichael [Contrib. Mineral. Petrol. 108 (1991) 82]. This indicates that an extrapolation of this equation to such low temperatures may have large errors. A sample quenched slowly through the temperature range near and below Tg shows considerable differences in the obtained Mössbauer spectra compared to more rapidly cooled samples, indicating ordering of the iron environment at least in the mesoscopic range. The oxidation state, however, does not differ when compared to the more rapidly quenched melts.

KW - Iron oxidation state

KW - Mössbauer spectroscopy

KW - Oxygen fugacity

KW - Silicate melt

UR - http://www.scopus.com/inward/record.url?scp=0037120065&partnerID=8YFLogxK

U2 - 10.1016/S0009-2541(02)00042-6

DO - 10.1016/S0009-2541(02)00042-6

M3 - Article

AN - SCOPUS:0037120065

VL - 189

SP - 55

EP - 67

JO - Chemical geology

JF - Chemical geology

SN - 0009-2541

IS - 1-2

ER -