Elsevier

Geochimica et Cosmochimica Acta

Volume 67, Issue 22, 15 November 2003, Pages 4389-4409
Geochimica et Cosmochimica Acta

Experimental determination of activities of FeO and Fe2O3 components in hydrous silicic melts under oxidizing conditions

https://doi.org/10.1016/S0016-7037(03)00376-4Get rights and content

Abstract

The critical role of iron on crystal-silicate liquid relationships and melt differentiation is mainly controlled by the redox conditions prevailing in magmas, but the presently available database merely constrains the thermodynamic properties of iron-bearing components in strongly reduced and anhydrous molten silicate where iron is in the ferrous form. This paper provides new standard states for pure ferrous (FeOliq) and ferric (Fe2O3liq) molten iron oxides and extends the experimental database towards oxidizing and water-bearing domains. Iron-iridium, iron-platinum alloys, magnetite or hematite were equilibrated with synthetic silicic liquids at high temperature and high pressure under controlled oxygen fugacity (fO2) to determine activity-composition relationships for FeOliq and Fe2O3liq. Between 1000 and 1300°C, the fO2 ranges from that in air to 3-log units below that of the nickel-nickel oxide buffer (NNO). Experiments were performed on both anhydrous and hydrous melts containing up to 6-wt.% water. Incorporation of water under reducing conditions increases the activity coefficient of FeOliq but has an opposite effect on Fe2O3liq. As calcium is added to system, the effect of water becomes weaker and is inverted for Fe2O3liq. Under oxidizing conditions, water has a negligible effect on both activities of FeOliq and Fe2O3liq. In contrast, changes in redox conditions dominate the activity coefficients of both FeOliq and Fe2O3liq, which increase significantly with increasing fO2. The present results combined with the previous work provide a specific database on the energetics of iron in silicate melts that cover most of the condition prevailing in natural magmas.

Introduction

The construction of thermodynamic models for multicomponent melts and magmas represents an important direction of research in petrology. Calibration of such models requires determination of both standard states and mixing properties for the melt components. Presently, the most abundant source of data constraining the mixing properties of silicate melts is phase equilibria (Berman and Brown, 1987), as illustrated by several models (e.g., Berman and Brown, 1984, Eriksson and Pelton, 1993, Ghiorso and Sack, 1995, Kirschen and Pichavant, 2001. To test and verify predictions of these models and to provide supplementary constraints for the calibration of the mixing relationships of multicomponent silicate melts, additional measurements such as determinations of activities of the melt components are needed.

Among the different components to be considered, the iron oxide melt components are critical because both ferric and ferrous iron exist in different proportions in magmas, depending on fO2, P, T and chemical composition (e.g., Thornber et al., 1980, Mysen and Virgo, 1989, Kress and Carmichael, 1991, Gaillard et al., 2001). In the literature of material science, numerous studies on the energetics of iron in molten silicate were performed on synthetic systems that are, however, very different from natural magmas compositions (Matsuzaki et al., 1998). Surprisingly, despite the fact that iron is the most abundant multivalent element present in magmas, the thermodynamic behavior of other easily reducible cations such as Ni or Co has retained more attention in the geological literature (O'Neill and Eggins, 2002). Also, the presently available database is mostly devoted to the thermodynamic properties of iron in strongly reduced and anhydrous molten silicate where iron is in the ferrous form (Doyle and Naldrett, 1986). Changes in ferric-ferrous ratio have important effects on Fe-Ti oxide saturation curves Osborn, 1959, Hamilton et al., 1964. The position of Fe-Ti oxides in the crystallization sequence controls the iron content (and ferric-ferrous ratio) of the residual melt, and consequently the stability and composition of major silicate phases Sisson and Grove, 1993, Martel et al., 1999, Pichavant et al., 2002). Fe-Ti oxides can be near-liquidus phases in basalt for fO2 above the nickel-nickel oxide buffer (NNO) under hydrous conditions Hamilton et al., 1964, Sisson and Grove, 1993, Martel et al., 1999, Pichavant et al., 2002. Conversely, under anhydrous and reducing conditions, Fe-Ti oxides crystallize near the solidus Lapin et al., 1985, Snyder et al., 1993, Toplis and Carroll, 1995. For modeling Fe-Ti oxide saturation and liquid lines of descent, an accurate calibration of the thermodynamic properties of ferrous and ferric iron in multicomponent melts is therefore needed. The phase equilibria calculated from models presently available do not reproduce accurately experimental Fe-Ti oxide saturation curves Ghiorso and Sack, 1995, Toplis and Carroll, 1996, Ariskin, 1999. This problem demonstrates the need for additional specific constraints on the ferrous and ferric melt components under relevant redox conditions. This paper provides new measurement of the activities of liquid FeOliq and Fe2O3liq in SiO2-rich melt, in the presence of significant amounts of Fe3+ and under both anhydrous and hydrous conditions.

Section snippets

Background and previous work

Activities of FeO in a wide range of silicate melt compositions (aFeOliq) have been determined by geoscientists and material scientists (ref. in Table 1). The different approaches are summarized in Table 1. In most studies, activities of FeOliq were determined by equilibrating pure metallic iron with a silicate melt at 1 atm with fO2 slightly below the iron-wustite (IW) buffer. Under these conditions, iron is present in the melt almost solely as Fe2+. The corresponding equilibrium may be

Principle

In this study, we have retained the principle of measuring aFeOliq and aFe2O3liq under relatively oxidizing fO2 conditions by equilibrating a Fe-bearing alloy with the silicate melt. Noble metal alloys (Fe-Ir) were preferred to the Fe-Ni alloys used in previous studies for several practical reasons. (1) Fe-Ir alloys are stable over a wide range of fO2, being limited only by the Ir-IrO2 equilibrium (log fO2 ∼ 9 log units above NNO). In comparison, aFeOliq and aFe2O3liq cannot be measured at fO2

Starting materials

Compositions of the 15 starting glasses are listed in Table 2. They have SiO2 contents ranging between ∼69 and ∼78 wt.% and FeOtot between ∼0.5 and ∼9 wt.%. The first series of glasses is calcium-free with several compositions (# 2, 4, 6, 8, 9) used previously to determine the effect of H2O and fO2 on the ferric-ferrous ratio of silicic melts (Gaillard et al., 2001). A second series of glasses contains different proportion of calcium. All glasses have nearly identical Si4O8 contents (between 56

Equilibrium and internal consistency

Determination of aFeOliq and aFe2O3liq requires attainment of chemical equilibrium, either between Fe-Ir alloy and melt for fO2 < NNO+2 (equilibria (1) and (5)) or between hematite and melt above NNO+2 (equilibria (8), (9)). Attainment of equilibrium was tested by performing time-series experiments. The activity determinations were cross-checked with independent equilibria, involving either noble metals others than Ir or magnetite. Strictly speaking, no reversals were performed (i.e.,

Water and fe3+/Fe2+

Here above we have put in evidence different effects of water on aFeOliq and aFe2O3liq that are both fO2 dependent. In terms of Fe2O3/FeO - fO2 relationships, our results suggest that water addition increases ferric/ferrous under reducing conditions and has a less important or no effect under oxidizing conditions. This is in very good agreement with previous experimental studies Baker and Rutherford, 1996, Gaillard et al., 2001 identifying similar effects of water. The recent results of Wilke

Acknowledgements

This study, which is part of Fabrice Gaillard's PhD thesis, was supported by the Région Centre and by the EC TMR network “Hydrous Silicate Melts”. The constructive reviews of Rebecca Lange and an anonymous scientist are acknowledged. Bjorn Mysen, who did the editorial work and his own review, is also greatly thanked.

Associate editor: B. Mysen

References (66)

  • A. Holzheid et al.

    The activities of NiO and FeO in silicate melts

    Chem. Geol.

    (1997)
  • M. Kirschen et al.

    A thermodynamic model for hydrous silicate melts in the system NaAlSi3O8-KAlSi3O8-Si4O8-H2O

    Chem. Geol.

    (2001)
  • H.S.t.C. O'Neill et al.

    The effect of melt composition on trace element partitioningAn experimental investigation of the activity coefficients of FeO, NiO, CoO, MoO2 and MoO3 in silicate melts

    Chem. Geol.

    (2002)
  • H.S.t.C. O'Neill et al.

    Experimental petrochemistry of some highly siderophile elements at high temperatures, and some implications for core formation and the mantle's early history

    Chem. Geol.

    (1995)
  • P.L. Roeder

    Activity of iron and olivine solubility in basaltic liquids

    Earth Planet. Sci. Lett.

    (1974)
  • C.R. Thornber et al.

    The effect of composition on the ferric-ferrous ratio in basaltic liquids at atmospheric pressure

    Geochim. Cosmochim. Acta

    (1980)
  • A.B. Woodland et al.

    Thermodynamic data for Fe-bearing phases obtained using noble metal alloys as redox sensors

    Geochim. Cosmochim. Acta

    (1997)
  • M. Wilke et al.

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

    Chem. Geol.

    (2002)
  • Banya S., Chiba A., and Hikosaka A. (1980) Thermodynamic table for inorganic substances. Testu to Hagane,...
  • Barin I. (1998) Thermochemical data of pure substances. VCH, Weinheim, Basel,...
  • H. Behrens et al.

    The effect of anhydrous composition on water solubility in granitic melts

    Am. Mineral.

    (2001)
  • Berman R. G. and Brown T. H. (1987) Development of model for multicomponent melts: Analysis of synthetic systems. Revs....
  • C. Bodsworth

    The activity of ferrous oxide in silicate melts

    Iron Steel Inst. J.

    (1959)
  • N.L. Bowen et al.

    The system MgO-FeO-SiO2

    Am. J. Sci.

    (1934)
  • C.W. Burnham et al.

    Thermodynamic property of water to 1000°C and 10000 bar

    Geological Society of America Special Paper

    (1969)
  • I.S.E. Carmichael

    The redox state states of basic and silicic magmasA reflection of their source regions

    Contrib. Mineral. Petrol.

    (1991)
  • Coughlin J. P (1954) Contribution to the data of theoretical metallurgy, XII, heats and free energies of formation of...
  • Deines P., Nafziger R. H., Ulmer G. C., and Woermann E. (1974) Temperature oxygen fugacity tables for selected gas...
  • J.D. Devine et al.

    Comparison of microanalytical methods for estimation of H2O content of silicic volcanic glasses

    Am. Mineral.

    (1995)
  • G. Eriksson et al.

    Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the MnO-TiO2, MgO-TiO2, FeO-TiO2, Ti2O3-TiO2, Na2O-TiO2, and K2O-TiO2 systems

    Metal. Trans.

    (1993)
  • F. Gaillard et al.

    Kinetics of iron oxidation-reduction in hydrous silicic melts

    Am. Mineral.

    (2002)
  • M.S. Ghiorso et al.

    Fe-Ti oxide geothermometryThermodynamic formulation and the estimation of intensive variables in silicic magmas

    Contrib. Mineral. Petrol.

    (1991)
  • M.S. Ghiorso et al.

    Chemical mass transfer in magmatic processesIV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures

    Contrib. Mineral. Petrol.

    (1995)
  • Cited by (56)

    • Effect of pressure on Fe<sup>3+</sup>/ΣFe ratio in a mafic magma and consequences for magma ocean redox gradients

      2017, Geochimica et Cosmochimica Acta
      Citation Excerpt :

      An important question is whether the glasses quenched under experimental conditions record the Fe3+/ΣFe ratios and melt structures established at high temperature and high pressure. Many studies have documented the comparatively rapid migration of redox fronts through amorphous silicates exposed to strong gradients in oxygen fugacity (e.g., Cooper et al., 1996; Gaillard et al., 2002, 2003a,b). However, these considerations likely do not apply to the present experiments because (a) the quenching rate is too high and because, (b) unlike quenching at 100 kPa, the mechanism of quench in high pressure experiments does not involve exposure to a gradient in fO2.

    View all citing articles on Scopus

    Present address: Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany.

    View full text