TY - JOUR

T1 - The effect of water activity on the oxidation and structural state of Fe in a ferro-basaltic melt

AU - Botcharnikov, Roman

AU - Koepke, Jürgen

AU - Holtz, Francois

AU - McCammon, C.

AU - Wilke, M.

N1 - Funding Information: We gratefully acknowledge R. Moretti for providing us the computer programs for calculations of log f O 2 and Fe redox ratios in silicate melts. We thank G. Falkenberg and K. Rickers for the help and support during beamtime at HASYLAB in Hamburg. Otto Diedrich is acknowledged for his excellent work on the preparation of experimental glasses for analysis. Grant Henderson and two anonymous reviewers provided very constructive and helpful comments that improved the earlier version of the manuscript significantly. This research was funded by the Deutsche Forschungsgemeinschaft (KO 1723/3). We would like to thank Brent Poe for the valuable editorial work. Copyright: Copyright 2011 Elsevier B.V., All rights reserved.

PY - 2005/11/1

Y1 - 2005/11/1

N2 - Experimental investigations have been performed at T = 1200°C, P = 200 MPa and fH2 corresponding to H2 O-MnO-Mn3O4 and H2O-QFM redox buffers to study the effect of H2O activity on the oxidation and structural state of Fe in an iron-rich basaltic melt. The analysis of Mössbauer and Fe K-edge X-ray absorption nearedge structure (XANES) spectra of the quenched hydrous ferrobasaltic glasses shows that the Fe3+/∑Fe ratio of the glass is directly related to aH2O in a H2-buffered system and, consequently, to the prevailing oxygen fugacity (through the reaction of water dissociation H2O ↔ H2 + 1/2 O2). However, water as a chemical component of the silicate melt has an indistinguishable effect on the redox state of iron at studied conditions. The experimentally obtained relationship between fO2 and Fe3+/Fe2+ in the hydrous ferrobasaltic melt can be adequately predicted in the investigated range by the existing empiric and thermodynamic models. The ratio of ferric and ferrous Fe is proportional to the oxygen fugacity to the power of ∼0.25 which agrees with the theoretical value from the stoichiometry of the Fe redox reaction (FeO + 1/4 O2 = FeO1.5). The mean centre shifts for Fe2+ and Fe3+ absorption doublets in Mössbauer spectra show little change with increasing Fe3+/∑Fe, suggesting no significant change in the type of iron coordination. Similarly, XANES preedge spectra indicate a mixed (C3h, Td, and Oh, i.e., 5-, 4-, and sixfold) coordination of Fe in hydrous basaltic glasses.

AB - Experimental investigations have been performed at T = 1200°C, P = 200 MPa and fH2 corresponding to H2 O-MnO-Mn3O4 and H2O-QFM redox buffers to study the effect of H2O activity on the oxidation and structural state of Fe in an iron-rich basaltic melt. The analysis of Mössbauer and Fe K-edge X-ray absorption nearedge structure (XANES) spectra of the quenched hydrous ferrobasaltic glasses shows that the Fe3+/∑Fe ratio of the glass is directly related to aH2O in a H2-buffered system and, consequently, to the prevailing oxygen fugacity (through the reaction of water dissociation H2O ↔ H2 + 1/2 O2). However, water as a chemical component of the silicate melt has an indistinguishable effect on the redox state of iron at studied conditions. The experimentally obtained relationship between fO2 and Fe3+/Fe2+ in the hydrous ferrobasaltic melt can be adequately predicted in the investigated range by the existing empiric and thermodynamic models. The ratio of ferric and ferrous Fe is proportional to the oxygen fugacity to the power of ∼0.25 which agrees with the theoretical value from the stoichiometry of the Fe redox reaction (FeO + 1/4 O2 = FeO1.5). The mean centre shifts for Fe2+ and Fe3+ absorption doublets in Mössbauer spectra show little change with increasing Fe3+/∑Fe, suggesting no significant change in the type of iron coordination. Similarly, XANES preedge spectra indicate a mixed (C3h, Td, and Oh, i.e., 5-, 4-, and sixfold) coordination of Fe in hydrous basaltic glasses.

UR - http://www.scopus.com/inward/record.url?scp=28344438772&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2005.04.023

DO - 10.1016/j.gca.2005.04.023

M3 - Article

AN - SCOPUS:28344438772

VL - 69

SP - 5071

EP - 5085

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

IS - 21

ER -