Fe L α peak energy for minerals and glasses : Recent advances with the electron microprobe

This paper describes a preliminary study that attempts to determine the oxidation state of Fe (Fe/ΣFe) with the electron microprobe (EMP) by measuring the self-absorption induced shift of the FeLα peak emitted from minerals and glasses. In transition metals of the first row, the L-spectra exhibit common distortions, namely peak position shifts, peak shape alterations, and changes in the Lβ/Lα ratios, caused by the large difference in the self-absorption coefficients (μ/ρ) on either sides of the L3 absorption edges that are in close proximity to the Lα peak maxima. Measurements performed on α-Fe2O3 and FexO oxides have shown that self-absorption effects are stronger for the later oxide, leading to enhanced FeLα peak shift toward longer wavelengths as the beam energy increases. First measurements performed on silicates have confirmed that enhanced self-absorption of FeLα occurs on Fe sites. The measurements consisted of plotting the FeLα peak position at a fixed beam energy (15 keV) against the total Fe concentration for two series of Feand Febearing silicates. In a first step, these data have shown that both FeLα and FeLα peaks shift continuously toward longer wavelengths as the Fe concentration increases, with enhanced shifts for FeLα. For silicates containing only Fe or Fe, no effects of the site geometry were detected on the variations of the FeLα peak position. A second set of plots has shown the variations of the peak position relative to the previous Fe-Fe curves of step 1, as a function of the nominal Fe/ΣFe, for a series of reference minerals (hydrated and non-hydrated) and basaltic glasses. Data from chain and sheet silicates (e.g., pyroxenes, amphiboles, micas) exhibited strong deviations compared to other phases (e.g., garnets, Al-rich spinels, glasses), due to reduced self-absorption of FeLα. Intervalence-charge transfer (IVCT) mechanisms between Fe and Fe sites may be the origin of these deviations. These crystal-structure effects limit the accuracy of the method for mixed Fe-Fe valence silicates. Precisions achieved for further Fe/ΣFe measurements strongly depend on the total Fe concentration. For basaltic glasses containing an average of 8 wt% Fe and 10% Fe/ΣFe, the precision is about ±2% (absolute). For low Fe concentrations (below 3.5 wt%), the uncertainty in the peak position measured by the EMP spectrometers leads to error bars that are similar to with the separation of the curves fitted to the Fe and Fe plots, which is propagated as prohibitive lack of precision for Fe/ΣFe (>70% relative). A major limitation of microbeam methods 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 those corresponding to 130 nA and 30 μm beam diameter should be avoided to prevent large beam induced oxidation phenomena). Additional work, in progress, concerns: (1) other beam-sensitive phases such as hydrated glasses; and (2) minerals in which FeLα is affected by large matrix effect corrections (e.g., Crand Ti-rich oxides where FeLα is strongly absorbed), for which the self-absorptioninduced 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 and Lukanin 1985; Kasting et al. 1993). The redox state of the upper mantle is commonly estimated through values of oxygen fugacity calculated on the basis of Fe/Fe equilibrium among mineral assemblages such as olivine-orthopyroxene-spinel (Ballhaus et al. 1990; O’Neill et al. 1993) or determined by FIALIN ET AL.: MICROPROBE ESTIMATES OF Fe/ΣFe 457 wet chemical analyses on minerals or glasses (Christie et al. 1986). For example, systematic relationships have been pointed out between upper mantle oxidation states and tectonic regimes (e.g., Wood et al. 1990; Canil et al. 1994). However, the effects of different processes on mantle redox conditions are still debated: are the large variations in the redox state linked to C-OH fluids (Mattioli et al. 1989) or do they result only from magmatic processes, partial melting, and melt infiltration (Amundsen and Neumann 1992; McGuire et al. 1991)? Many of the controversies about this subject ultimately relate to the paucity and irrelevance of the available data. This lack of data may be the major justification for continuing to search for techniques of determining Fe/ΣFe. Iron commonly occurs in two different oxidation states. The redox state of Fe is strongly dependent on the interaction with C-O-H fluids in natural magmas. These fluids play quite different structural roles (Waychunas et al. 1988; Cooney and Sharman 1990; Jackson et al. 1990) and have considerable influence on properties of silicate melts such as density or viscosity. The effect of the oxidation state of Fe and oxygen, the changes in Fe/ΣFe, and interactions of Fe and O in silicate melts have been widely debated (e.g., Dyar 1985; Mysen 1988). The determination of Fe/ΣFe along with the sites occupied by Fe and Fecations and their relationships with the silicate framework provide a basis from which the thermal history, and hence the melting processes by which a particular natural glass has formed, may be deduced. Different methods are commonly used to measure Fe/ΣFe in silicates and glasses and will be briefly reviewed in the next section. Extracting quantitative Fe/ΣFe values from the FeLα,β emission spectra measured with the EMP has been a long-term subject of study in geological applications (see Höfer et al. 1994 for a history of the related works over the past 35 years). Actually three ways have been investigated to process the Fe-Lα,β spectra: (1) measurement of the Lα peak shifts; (2) measurement of the change in Lβ/Lα intensity ratios; and (3) a hybrid of 1 and 2 (Höfer et al. 1994, 2000). We present in this study a technique based on method (1) that is applicable to minerals and glasses for which the total Fe concentration (in wt%) has been previously established. The limitations of the method are discussed. EXISTING METHODS FOR THE DETERMINATION OF Fe/ΣFe