American Mineralogist, Volume 86, pages 456–465, 20010003-004X/01/0004–456$05.00 456INTRODUCTIONThe redox state of Earth’s mantle is a critical parameter toconstrain models on the origin and the evolution of the Earth.Oxygen fugacity plays an important role in volatile speciation,* E-mail: [email protected]+/ΣFe vs. FeLα peak energy for minerals and glasses: Recent advances with the elec-tron microprobeMICHEL FIALIN,1,* CHRISTIANE WAGNER,2 NICOLE MÉTRICH,3 ERIC HUMLER,4 LAURENCEGALOISY,5 AND ANTOINE BÉZOS41Centre de Microanalyse Camparis-CNRS, Université Paris 6, 4 place Jussieu, 75252 Paris cedex 5, France2Laboratoire de Pétrologie, Modélisation des Matériaux et Processus, Université Paris 6, 4 place Jussieu, 75252 Paris cedex 5, France3Laboratoire Pierre Süe, UMR 9956-CNRS, CE-Saclay, 91191 Gif sur Yvette, France4Laboratoire de Géosciences Marines-CNRS, Université Paris 6 et 7, Institut de Physique du Globe, 4 place Jussieu, 75252 Paris cedex 5,France5Laboratoire de Minéralogie-Cristallographie, UMR 7590-CNRS, Universités Paris 6 et 7, Institut de Physique du Globe, 4 place Jussieu,75252 Paris cedex 5, FranceABSTRACTThis paper describes a preliminary study that attempts to determine the oxidation state of Fe(Fe3+/ΣFe) with the electron microprobe (EMP) by measuring the self-absorption induced shift ofthe FeLα peak emitted from minerals and glasses. In transition metals of the first row, the L-spectraexhibit common distortions, namely peak position shifts, peak shape alterations, and changes in theLβ/Lα ratios, caused by the large difference in the self-absorption coefficients (µ/ρ) on either sidesof the L3 absorption edges that are in close proximity to the Lα peak maxima. Measurements per-formed on α-Fe2O3 and FexO oxides have shown that self-absorption effects are stronger for thelater oxide, leading to enhanced Fe2+Lα peak shift toward longer wavelengths as the beam energyincreases. First measurements performed on silicates have confirmed that enhanced self-absorptionof FeLα occurs on Fe2+ sites. The measurements consisted of plotting the FeLα peak position at afixed beam energy (15 keV) against the total Fe concentration for two series of Fe2+- and Fe3+-bearing silicates. In a first step, these data have shown that both Fe2+Lα and Fe3+Lα peaks shiftcontinuously toward longer wavelengths as the Fe concentration increases, with enhanced shifts forFe2+Lα. For silicates containing only Fe2+ or Fe3+, no effects of the site geometry were detected onthe variations of the FeLα peak position. A second set of plots has shown the variations of the peakposition relative to the previous Fe2+-Fe3+ curves of step 1, as a function of the nominal Fe3+/ΣFe, fora series of reference minerals (hydrated and non-hydrated) and basaltic glasses. Data from chainand sheet silicates (e.g., pyroxenes, amphiboles, micas) exhibited strong deviations compared toother phases (e.g., garnets, Al-rich spinels, glasses), due to reduced self-absorption of FeLα.Intervalence-charge transfer (IVCT) mechanisms between Fe2+ and Fe3+ sites may be the origin ofthese deviations. These crystal-structure effects limit the accuracy of the method for mixedFe2+-Fe3+ valence silicates. Precisions achieved for further Fe3+/ΣFe measurements strongly dependon the total Fe concentration. For basaltic glasses containing an average of 8 wt% Fe and 10%Fe3+/ΣFe, the precision is about ±2% (absolute). For low Fe concentrations (below 3.5 wt%), theuncertainty in the peak position measured by the EMP spectrometers leads to error bars that aresimilar to with the separation of the curves fitted to the Fe2+ and Fe3+ plots, which is propagated asprohibitive lack of precision for Fe3+/ΣFe (>70% relative). A major limitation of microbeam meth-ods in general deals with beam damage. This aspect has been carefully studied for basaltic glasses,and optimal beam conditions have been established (in general, electron doses higher than thosecorresponding to 130 nA and 30 µm beam diameter should be avoided to prevent large beam in-duced oxidation phenomena). Additional work, in progress, concerns: (1) other beam-sensitive phasessuch as hydrated glasses; and (2) minerals in which FeLα is affected by large matrix effect correc-tions (e.g., Cr- and Ti-rich oxides where FeLα is strongly absorbed), for which the self-absorption-induced shift of FeLα is different from that of common silicates and glasses.physical properties of mantle rocks, core-mantle interactions,and the atmospheric chemistry through time (Kadik andLukanin 1985; Kasting et al. 1993). The redox state of the up-per mantle is commonly estimated through values of oxygenfugacity calculated on the basis of Fe2+/Fe3+ equilibrium amongmineral assemblages such as olivine-orthopyroxene-spinel(Ballhaus et al. 1990; O’Neill et al. 1993) or determined byFIALIN ET AL.: MICROPROBE ESTIMATES OF Fe3+/ΣFe 457wet chemical analyses on minerals or glasses (Christie et al.1986). For example, systematic relationships have been pointedout between upper mantle oxidation states and tectonic regimes(e.g., Wood et al. 1990; Canil et al. 1994). However, the effectsof different processes on mantle redox conditions are still de-bated: are the large variations in the redox state linked to C-O-H fluids (Mattioli et al. 1989) or do they result only frommagmatic processes, partial melting, and melt infiltration(Amundsen and Neumann 1992; McGuire et al. 1991)? Manyof the controversies about this subject ultimately relate to thepaucity and irrelevance of the available data. This lack of datamay be the major justification for continuing to search for tech-niques of determining Fe3+/ΣFe.Iron commonly occurs in two different oxidation states. Theredox state of Fe is strongly dependent on the interaction withC-O-H fluids in natural magmas. These fluids play quite dif-ferent structural roles (Waychunas et al. 1988; Cooney andSharman 1990; Jackson et al. 1990) and have considerable in-fluence on properties of silicate melts such as density or vis-cosity. The effect of the oxidation state of Fe and oxygen, thechanges in Fe3+/ΣFe, and interactions of Fe and O in silicatemelts have been widely debated (e.g., Dyar 1985; Mysen 1988).The determination of Fe3+/ΣFe along with the sites occupiedby Fe3+ and Fe2+cations and their relationships with the silicateframework provide a basis from which the thermal history, andhence the melting processes by which a particular
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