Comparison of Synchrotron microXANES determination of Fe/ΣFe with Mössbauer values for clean min- eral separates of pyroxene from Martian meteorites

Introduction: The oxidation state of Fe in Martian meteorites is a parameter of great interest and the ability to determine this value in micrometer scale samples is important [1]. Intense, tunable x-ray sources at large synchrotron storage rings are being exploited to examine the Fe K-absorption edge with energy resolution of ~1-1.5eV in spots of 10x15 μm on thin sections of a wide variety of materials including several Martian meteorites [2-5]. Synchrotron microXANES (SmX) spectroscopy is the technique that provides the most flexible capability for investigating Fe/ΣFe. Variation of Fe/ΣFe is manifested as a function of the energy of the pre-edge to the Fe absorption edge produced by the sample. SmX is at present the only technique that can be used with conventional polished thin sections [6]. Data for a broad spectrum of minerals have been produced and indicate that SmX can be used with a large variety of samples types [1]. Results for the Martian meteorites have been available since 1998 [3] but the implied oxygen fugacities for the SNC’s derived early results were believed to be inconsistent with independent techniques [8,7]. Detailed examination of the results for several mineral groups demonstrated that the ‘universal’calibration [6] used for earlier results was systematically overestimating the Fe/ΣFe in minerals such as pyroxene and feldspar in which the Fe/ΣFe is low (<~10%). As a result, very detailed investigations of the relationship between SmX spectral features and large suites of independently characterized pyroxenes and other minerals were initiated. These studies led to revision of the Martian values [4] and the recognition of two major sources of error: (a) calibrations based on oxides and olivine were not suitable for pyroxene, amphibole, micas and feldspars; (b) the strong linear polarization of incident x-ray beam used to excite the samples is manifested in spectral variations caused by grain orientation in several minerals including pyroxene [4]. In addition, minerals with Fe in symmetrical sites typical of octahedral coordination, have very weak pre-edge structures. The low intensity of the peaks induces errors because the associated statistical fluctuations can be significant. For this reason, very significant improvements have been made to the beamline geometry used for the measurements. These improvements are continuing with the recent (2003) installation of a nine element Ge detector to replace the single Si(Li) detector previously used. This should provide at least a factor of 3 improvement in precision and probably better because of the significantly lower deadtime associated with modern digital detectors. Revisions to the synchrotron microXANES technique involve the use of both mineral-group-specific calibration lines based on arrays of standards for which Fe/ΣFe data are available from Mössbauer spectroscopy and wet chemical analyses. The effect of crystal orientation is still under investigation. Systematic studies of the spectral response as a function of the optic axial plane in carefully selected arrays of minerals, have demonstrated that the effect varies from very strong in some micas to very weak in many feldspars. The orientation studies are continuing with the aim of developing a suite of techniques that will allow orientation effects to be eliminated. The optical orientation of the target grain can be determined in some cases. In other cases it may be necessary to make selected area XRD measurements and, finally, in many cases the use of a modified geometry for the x-ray microprobe beamline known as Magic Angle XANES has been shown to suppress much of the orientation dependence of the spectral signature. Martian Pyroxene: The microXANES results remain at the high end of the expected range. To further test the validity of the synchrotron microXANES results, mineral separates of the constituent minerals of Martian meteorites were prepared and their Fe/ΣFe determined directly by Mössbauer spectroscopy. The average microXANES and Mössbauer results are compared in Table 1.