Vol48/04/2005def 609 ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005 Key words iron – silicate melt – redox conditions 1. Introduction Fe is the most abundant 3d-transition ele- ment in geological systems and its heterovalent nature is the source of a highly variable geo- chemical behavior depending mostly on the re- dox regime during geological processes. In the case of magmatic systems, the valence-state of Fe has strong effects on the stability of Fe-bear- ing phases as well as on the physical properties of the melt phase itself. The stability of Fe- bearing phases has direct consequences on the overall chemical variation trend of a given magmatic fractionation series as is well docu- mented by the difference of tholeiitic and calc- alkaline magmatic suites. Tholeiitic series show a significant increase in Fe-content during ear- ly stages of differentiation whereas in calc-al- kaline series this enrichment is suppressed by the early crystallization of Fe-Ti oxides, which is strongly influenced by the prevailing redox regime (Wilson, 1989; Toplis and Carroll, 1995). Silicate magmas or melts containing sig- nificant amounts of Fe show considerable de- pendence in their physical properties on the prevailing redox conditions. E.g., the viscosity of andesitic melt containing 8 wt% FeOt varies by about 2 orders of magnitude in the high vis- cosity range depending on the Fe oxidation state (Fe3+/ΣFe = 0.2-0.6) (e.g., Liebske et al., 2003). Due to the significant effect of Fe oxida- tion state on the resulting products of magma genesis it is possible to deduce the redox condi- tions during magma formation, i.e. in the case of volcanic rocks, the pre-eruptive conditions. The aim of this communication is to give a brief overview on the possible methods for ex- tracting information on the redox conditions of magmas and to point out some of the possible Fe in magma – An overview Max Wilke Institut für Geowissenschaften, Universität Potsdam, Germany Abstract The strong influence of physical conditions during magma formation on Fe equilibria offers a large variety of possibilities to deduce these conditions from Fe-bearing phases and phase assemblages found in magmatic rocks. Conditions of magma genesis and their evolution are of major interest for the understanding of volcanic erup- tions. A brief overview on the most common methods used is given together with potential problems and limi- tations. Fe equilibria are not only sensitive to changes in intensive parameters (especially T and f O2) and exten- sive parameters like composition also have major effects, so that direct application of experimentally calibrated equilibria to natural systems is not always possible. Best estimates for pre-eruptive conditions are certainly achieved by studies that relate field observations directly to experimental observations for the composition of interest using as many constraints as possible (phase stability relations, Fe-Ti oxides, Fe partitioning between phases, Fe oxidation state in glass etc.). Local structural environment of Fe in silicate melts is an important pa- rameter that is needed to understand the relationship between melt transport properties and melt structure. As- signment of Fe co-ordination and its relationship to the oxidation state seems not to be straightforward. In addi- tion, there is considerable evidence that the co-ordination of Fe in glass differs from that in the melt, which has to be taken into account when linking melt structure to physical properties of silicate melts at T and P. Mailing address: Dr. Max Wilke, Institut für Geowis- senschaften, Universität Potsdam, Postfach 601553, 14415 Potsdam, Germany; e-mail: max@geo.uni-potsdam.de 610 Max Wilke sources of error for such redox estimates. In ad- dition, the structural role of Fe in silicate glass and melt at high temperature is discussed. 1.1. Redox conditions and volatiles Knowledge of the redox regime of volcanic systems is a key parameter needed to under- stand volatile budgets of magmas involved in volcanic activities. The major volatile compo- nent affected by the redox regime is sulfur. The sulfur species found in the gas phase is strong- ly related to the redox conditions. As the solu- bility of S in the melt is, of course, related to the S species present, a strong dependence on the redox conditions is observed having a mini- mum at intermediate redox conditions (e.g., Carroll and Webster, 1994). Depending on the oxygen fugacity, S is incorporated into the melt as S2– or SO42– species. Moreover, at reduced conditions Fe and S in the melt directly interact as manifested by the strong correlation of S and Fe concentrations in synthetic and natural sam- ples. This led to the hypothesis of FeS species being present in the melt (Carroll and Webster, 1994). When S saturation is reached at reducing conditions Fe-sulphide phases precipitate from the melt. At oxidizing conditions, in contrast, especially in the presence of water anhydrite may precipitate from the melt (Carroll and Rutherford, 1987). 2. Fe-redox equilibria exploited for redox estimates 2.1. Coexisting Fe-Ti oxides Oxygen thermobarometry using Fe-Ti oxides is mainly based on the exchange equilibrium Fe2TiO4 + Fe2O3 = Fe3O4 + FeTiO3 which is strongly dependant on temperature and oxygen fugacity (e.g., Lindsley, 1991). Due to the massive amount of work performed on the stability relations in the Fe-Ti-O system Fe-Ti oxides provide pretty accurate estimates of tem- perature and oxygen fugacity (e.g., Andersen and Lindsley, 1988; Ghiorso and Sack, 1991). However, application of this method to vol- canic rocks is often restricted by the lack of Fe- Ti oxides in the eruption products or by uncer- tainties on the co-existence of oxides found in volcanic rocks. Therefore, often only a frag- mentary image of the temperature-fO2 history of the magma is achieved (e.g., Martel et al., 1999). 2.2. Partitioning of Fe between melt and plagioclase Although plagioclase contains only minor amounts of Fe, the incorporation of Fe is strong- ly dependent on oxygen fugacity. Fe3+ may easi- ly replace Al in the tetrahedral site. Fe2+ may be incorporated by replacement of Ca on the A site or by a coupled exchange of Fe2+ and Si versus 2Al on the tetrahedral site. The incorporation of Fe3+ is favored, so that the Fe-partitioning be- tween melt and plagioclase is sensitive to the prevailing redox conditions (Sato, 1989; Phin- ney, 1992; Wilke and Behrens, 1999; Sugawara, 2000, 2001). As plagioclase is usually found throughout all volcanic rocks this method can be readily applied in many cases. Plagioclase usually shows strong zoning and each zone grew from the melt at a given set of P, T and f O2 (fig. 1). If melt (glass) inclu- sions are present as well in each of the zones even the evolution of conditions may be de- duced. Since the partitioning is also influenced by temperature, melt composition and volatile content accurate estimates are only possible if the dependence of partitioning is calibrated for the system of interest. A more rigorous model for the Fe-partitioning between melt and pla- gioclase was suggested by Sugawara (2000, 2001). The model was parametrized on partitioning data from several compositions at various P, T and f O2 in the system SiO2-Al2O3-FeO-MgO- CaO-Na2O and thus provides a good basis for direct application to similar natural systems. However, Sugawara (2001) also showed that the accuracy of the P-T-f O2 estimates can be significantly improved by considering ex- change equilibria among more than two phases, Fe in magma – An overview e.g., by inclusion of pyroxene or olivine instead of using only melt and plagioclase (Sugawara, 2001). 2.3. Fe-oxidation state in volcanic glass Another possibility to estimate redox condi- tions is the direct use of the Fe oxidation state found in volcanic glass (e.g., Helgason et al., 1992a,b). Depending on the nature and size of the sample several analytical techniques pro- vide access to the Fe oxidation state of volcanic glass. Bulk methods like wet chemical analysis and Mössbauer provide quite precise data (error of Fe3+/ΣFe ± 0.03−0.1). However, samples have to be homogeneous and free of crystal in- clusions which is seldom the case. Methods with spatial resolution that also provide insight into the Fe oxidation state of volcanic glass in heterogeneous samples include Micro-Möss- bauer spectroscopy (50-100 µm spot size) (Mc- Cammon et al., 1991), Micro-XANES (X-ray Absorption Near Edge Structure, 3-30 µm spot size) (Bajt et al., 1994; Delaney et al., 1998; Bonnin-Mosbah et al., 2001; Dyar et al., 2002; Schmid et al., 2003), EELS (Electron Energy Loss Spectroscopy, e.g., van Aken et al., 1998) or high resolution X-ray emission analysis using the electron microprobe (spotsize ≥ 1 µm) (Höfer et al., 1994; Fialin et al., 2001). These methods also enable the investigation of glass inclusions within crystals and thus provide insight into the history of redox conditions during magma evolu- tion. While oxidation-state estimates from Mi- cro-Mössbauer spectroscopy may be as precise as those performed by usual Mössbauer spec- troscopy provided the Fe-content is high enough (approximately 5 wt% FeO). Estimates from Micro-XANES (Fe K-edge) usually have larger errors, i.e. an error of Fe3+/ /ΣFe: ± 0.1 − 0.2 if a general calibration as giv- en by Bajt et al. (1994) or Wilke et al. (2001) is used. Using specific calibrations for the glass 611 Fig. 1. Microscopic image of a chemically zone plagioclase phenocrysts (Plg) in a highly vesicular pumice sample from the Minoan Tuff (Santorini, Greece). Inclusions in the plagioclase host represent quenched melt (Gls) trapped at various stages during crystal growth and thus provides information on the compositional devel- opment of the erupted magma (Large black spot: Vesicle filled with resin above Plg-crystal). Bottom side of im- age 1.2 mm, crossed polarizers). 612 Max Wilke composition of interest the error can be sub- stantially improved to an error comparable to the one by Mössbauer analysis (Berry et al., 2003). The major advantage of this method is that spectra may also be taken at Fe-content be- low 1 wt% if the photon flux is high enough. The use of Lα,β -emission lines with the elec- tron microprobe provides information on the oxidation state with the same spatial resolution as for chemical analyses by this technique and a precision of 10-20 % rel. However, the application is usually restrict- ed by the total Fe-content (above 3.5 wt%, Fi- alin et al., 2001). The use of EELS at the Fe L- edge may provide data with a precision that may be better than 5% rel. but the volume analysed is only in the 10 nm-range and glass- es are potentially not stable under the electron beam which may be overcome by cooling the sample during measurements (Liebscher, pers. comm.). To be able to deduce the redox conditions from such data, knowledge is needed, of course, on the relationship between Fe oxidation state and temperature, pressure and melt composition. A considerable amount of effort was put into this field to establish the relationship to the above mentioned parameters using wet chemical analysis of Fe2+ in combination with electron mi- croprobe analysis for bulk Fe content or Möss- bauer spectroscopy for the determination of the Fe oxidation state (e.g., Johnston, 1964, 1965; Fudali, 1965; Sack et al., 1980; Mysen et al., 1980, 1985; Kilinc et al., 1983; Lange and Carmichael, 1989; Borisov and Shapkin, 1990; Kress and Carmichael, 1991; Moore et al., 1995; Gaillard et al., 2001; Wilke et al., 2002). Sack et al. (1980), Kilinc et al. (1983) and Kress and Carmichael (1991) used a rather simple empiri- cal equation parametrized on a large dataset of melts equilibrated at various superliquidus con- ditions to describe the dependence of Fe oxida- tion state on T, X and P. Borisov and Shapkin (1990) used a similar approach that differed only in the amount of fit- ting parameters. As these equations are purely empirical it is not clear to what extent these equations can be extrapolated to melt systems at conditions very different from those used in the parametrization of the equation, especially if they are extrapolated to significantly lower tem- peratures or to subliquidus conditions. Despite the uncertainty inherent in this approach, this equation is used in rather complex thermody- namic models predicting magmatic phase rela- tions, such as the program MELTS (Ghiorso and Sack, 1995) or the model used by Sugawara (2000, 2001) for Fe-partitioning between melt and plagioclase. Although these models can cer- tainly be considered major improvements in the modelling of magmatic systems the use of such an empirical equation in such models obviously puts some restrictions on the applicability to sys- tems that strongly differ in either composition or pressure-temperature conditions. Another approach to model the relationship of Fe oxidation state to intensive and extensive variables only recently introduced to geologi- cally relevant melts is suggested by Ottonello and Moretti (Ottonello et al., 2001; Moretti and Ottonello, 2003; Moretti, 2005). This model is based on an acid-base model for oxide melts (Fraser, 1975, 1977) which classifies a given metal oxide in terms of its acidic or basic behavior, which directly affects the polymerization reaction between single bonded, double bonded and free oxygens in sil- icate melts. Basic components produce cationic units whereas acidic components produce an- ionic units. In the case of Fe, Fe3+ shows am- photeric behavior whereas Fe2+ shows only ba- sic behavior. The disproportion of Fe3+ between anionic and cationic matrix in contrast to Fe2+ predicts that the ratio of rational activity coeffi- cients Fe3+ and Fe2+ components cannot be ex- pected to be 1 or even constant. As shown by the authors, this approach describes well Fe-ox- idation states of published datasets of glasses synthesized at known temperature and oxygen fugacity (Ottonello et al., 2001). The effect of water on the Fe-oxidation state was studied by several authors (Moore et al., 1995; Baker and Rutherford, 1996; Gaillard et al., 2001; Wilke et al., 2002). It may be con- cluded from these studies that water has no di- rect influence on the oxidation state other than that it influences the prevailing oxygen fugaci- ty. Recently the influence of water dissolved in the melt was also included in the model of Ot- tonello and Moretti (Moretti, 2005). Fe in magma – An overview The development of a correct model for pre- dicting the Fe-oxidation state of a given melt composition at P, T and f O2 of interest is of ma- jor importance, of course. However, additional problems in using the Fe-oxidation state of glass to deduce redox conditions arise from the fact that the original oxidation state may not be safely stored in the glass and might be affected by the thermal history of the volcanic glass af- ter the first quench. Burkhard (2001) has shown that the Fe-oxidation state of a basaltic glass from Kilauea, Hawaii, may already change dur- ing re-heating at relatively low temperatures just above the temperature of glass transforma- tion, well before the onset of crystallization. This reaction is intrinsically controlled and does not depend on any exchange of oxygen with the surrounding environment. Such a change may also occur in natural samples if rel- atively thin lava lobes that have not completely cooled down are re-heated by successive lava flows. Burkhard (2003) shows that such a re- heating may even lead to complete crystalliza- tion of the formerly vitrified parts of the bottom lava lobe. In more silicic melt compositions such effects are certainly less probable as the difference in eruptive style provides few possi- bilities for alteration by re-heating and thus the pristine oxidation state may well be preserved. The small effect of the quench rate during the first quench after eruption is confirmed by ex- perimental observations. The Fe-oxidation state was found not to depend on the usual experi- mentally accessible quench rates (Dyar et al., 1987; Wilke et al., 2002). 3. Local structural environment of Fe in silicate melts Although the structural environment of Fe in silicate melts is only of minor interest in the light of redox estimation in magmatic systems, it plays a major role in transport properties of silicate melts. The effect of oxidation state on the viscosity described at the beginning indi- cates that Fe3+ may be considered as a network former and Fe2+ as a network modifier. Although this assignment might well de- scribe the influence of oxidation state on vis- cosity, it still leaves open the question of the ex- act local structure around the two possible Fes- pecies. Many spectroscopic investigations have been performed to gain further insight into the co-ordination environment of Fe in melt and glass and have produced a variable picture on the co-ordination found for Fe3+ and Fe2+. Fe3+ is often assigned to be tetrahedrally co-ordinated although octahedrally co-ordinated is also re- ported (e.g., Dyar, 1985; Mysen et al., 1985, Virgo and Mysen, 1985; Hannoyer et al., 1992; Galoisy et al., 2001; Partzsch et al., 2003). For Fe2+, assignments are even more variable rang- ing from octahedral, over trigonal bipyramidal to tetrahedral co-ordination (Waychunas et al., 1988; Keppler, 1992; Jackson et al., 1993; Ros- sano et al., 1999, 2000). The controversial as- signment of co-ordination polyhedra is certain- ly related to the influence of the bulk composi- tion and/or polymerization of the glasses used, but may be in part also related to the different techniques used. E.g., crystal-field spectra usu- ally show strong clear evidence for Fe3+ in tetra- hedral co-ordination, whereas evidence for higher co-ordinated Fe3+ may be superimposed by bands related to Fe2+ (Hannoyer et al., 1992). The presence of higher co-ordinated Fe3+ may then be cross-checked or more easily detected by the use of Mössbauer spectroscopy or X-ray absorption spectroscopy (Virgo and Mysen, 1985; Hannoyer et al., 1992; Partzsch et al., 2003; Wilke et al., 2004a). Spectral features are usually assigned to specific co-ordination poly- hedra by comparison of the spectra taken on the glass to spectra of model compounds with Fe in well known structural environment, implying that the lack of long-range order has only little effect on the position of spectral bands or fea- tures. Beside the lack of long-range order, as- signments are probably rendered more difficult by the fact that there is no unique co-ordination polyhedron for Fe. Instead, the co-ordination polyhedra vary from site to site leading to a distribution of the lo- cal structure around Fe in the glass or melt. This site-to-site distribution is probably best reflected in the distribution of hyperfine parameters that can be determined from Mössbauer spectral analysis (Alberto et al., 1996; Dunlap, 1997; 613 614 Max Wilke Rossano et al., 1999; Wilke et al., 2002). Thus assignment to distinct co-ordination polyhedra is problematic and would not even describe the sit- uation of Fe in glass or melt adequately, so that most spectroscopic analyses provide only infor- mation on the average co-ordination. 3.1. Difference between melt and glass Interpretations on the melt structure based on observations made on glasses assume that the structure found in the glass represents the one that would also be found in the melt. However, the structure found in glass is the one frozen in from the liquid at temperatures around the glass transition, which potentially dif- fers from the melt structure found at higher tem- peratures. First evidence for differences in the co- ordination of Fe between melt and glass may al- ready be taken from the fact that Mössbauer hy- perfine parameters vary as a function of quench rate (Dyar et al., 1987; Wilke et al., 2002). First direct experimental evidence in the case of Fe was given by Waychunas et al. (1988) and Jack- son et al. (1993). These authors have shown evi- dence by collecting XANES and EXAFS spectra at ambient conditions and high temperature that the co-ordination of Fe2+ found in glass differs from the one found in the melt. An increase in the intensity of the pre-edge feature located 10-20 eV before the main edge and a shortening of the measured Fe-O distance indicates that the co-or- dination of Fe2+ at melt conditions is lower than that found in glass. Similar observations have been made for other elements like Ti and Ni (Farges and Brown, 1996; Farges et al., 1996). An in situ Raman study on Fe-rich sodium- silicate glass and melt by Wang et al. (1993) looking to bands that are related to vibrational modes of the polymeric network has evidenced no changes above the temperature of glass transformation for oxidized samples. The high temperature spectrum of the reduced sample, however, indicates a further anionic species present in the melt and a depolymerization of the melt network, which was related by the au- thors to the ferrous iron in the melt. New results from further in situ measure- ments using high resolution X-ray absorption spectroscopy at the Fe K-edge yielded similar results to those former studies using this tech- nique (Wilke et al., 2004b; Wilke et al., un- publ.). XANES and EXAFS spectra were taken on Na2Si3O7 glass and melt that was doped with 5 wt% Fe2O3 at both reducing and oxidizing conditions. At reducing conditions only a very small increase in the pre-edge intensity is ob- served in contrast to the results by Jackson et al. (1993). The change in the pre-edge intensity is much stronger at oxidizing conditions indicat- ing that Fe3+ is affected much more strongly. An increase in the pre-edge intensity may be inter- preted by a lowering of the coordination num- ber or a change in site geometry, as the pre-edge intensity of tetrahedrally co-ordinated Fe is much higher than the one of octahedrally co-or- dinated Fe (Wilke et al., 2001). So far, this change seems not to be quenchable and thus in- dicates an immediate re-arrangement of the lo- cal structure at Tg. Although the structural differences between melt and glass are probably unimportant for defining redox equilibria in silicate melts these differences are certainly important for the un- derstanding of transport properties and their re- lation to the melt structure. 4. Conclusions Fe equilibria in magmatic systems provide valuable constraints for the understanding of the development of magmas prior to volcanic erup- tion or sub-volcanic emplacement. Each of the methods proposed here, however, has its limita- tions when applied to natural systems, although they always yield at least some rough estimate on the redox regime, of course. For more precise estimates a combination of techniques or Fe equilibria has to be used. As discussed by Sug- awara (2001) the use of Fe-partitioning between three or more phases (e.g., plagioclase, olivine and melt) is probably much more reliable than just partitioning between plagioclase and melt. A very important tool to constrain pre-eruptive conditions for a given volcanic site precisely is probably the experimental determination of phase relations as a function of T, P, fluid and melt composition (e.g., Rutherford and Devine, Fe in magma – An overview 1996; Martel et al., 1998; Cotrell et al., 1999). Beside the determination of phase stability rela- tions such experiments can also be used to cali- brate the Fe (exchange) equilibria for the bulk composition of interest, i.e. Fe partitioning among phases, Fe oxidation state in glass, etc. The modelling approaches introduced by Ot- tonello and Moretti to the volcanologist/ petrolo- gist community provide an important step for- ward to profound modelling of redox equilibria in melt systems. Such models are certainly need- ed to model melt-crystal equilibria and to predict the evolution of the composition of magmas dur- ing volcanic cycles. An extension of this model- ling approach is certainly desirable. For an understanding of the relationship be- tween melt-transport properties and melt struc- ture a better link between composition and melt structure should be established. Fe is one further example that the local environment in glass may be significantly different from that in the melt. Any model of the melt structure that may be used as an input for the compositional dependence of transport properties should be based on experi- mental evidence of the molten state. The simple parameter NBO/T (Non-Bridg- ing Oxygen per Tetrahedron), often used to ap- proximate changes in the melt polymerization with composition, differentiates the given ele- ments only by the network forming or network modifying role, neglecting completely the po- tentially large difference in the field strength of the ions. A structural model that also accounts for these differences should provide a better ba- sis for modelling compositional dependence of e.g., viscosity data. Acknowledgements The basis for this communication was and still is made possible by collaborations with F. Farges, S. Rossano, G.M. Partzsch, C. Schmidt, W. Heinrich, H.Behrens, F. Holtz, D. Burkhard, E. Welter, K. Klementiev and A. Erko whose help, commitment and support is highly appreci- ated. 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