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Oxygen fugacity (f[o.sub.2]) is an important parameter because it influences the crystallization sequences of magmas, as well as the composition of the resultant minerals. Traditionally f[o.sub.] has been estimated using coexisting Fe-Ti oxide compositions or [Fe.sup.3+]/[Fe.sup.2+] ratios in silicate melts, for which the relationship between f[o.sub.2] and [Fe.sup.3+] has been calibrated. Problems arise in systems where oxides and/or glasses (melts) are absent, or when oxides are suspected to have been re-equilibrated. Therefore, it is important to develop alternate ways to look at f[o.sub.2] in volcanic rocks. In this work. we describe a set of experiments to establish the relationship between the [Fe.sup.3+] content of clinopyroxene and a basaltic melt as a function of oxygen fugacity. The starting bulk compositions were chosen to be representative of martian parental melts, with pyroxene (either augite or pigeonite) on their liquidi, in order for the results to be applicable to the martian meteorite magma source regions. Experimental conditions ranged from f[o.sub.2] = IW to ~MW+8.6 at 1165 [degrees]C for pigconite/melt pairs and at 1235 [degrees]C for melts alone, and f[o.sub.2] = IW-1 to IW+4 at ~1190 [degrees]C for augite/melt pairs. Results show that [Fe.sup.3+] varies from 0-65% of the total Fe over this range of experimental conditions, and mass D (wt%, [Fe.sup.3+.sub.Py]/wt%[Fe.sup.3+.sub.melt]) ranged from 0.75-1.44 and from 0.00-0.77, for pigeonite and augite respectively. XANES and Mossbauer analytical methods give similar results within known error bars of the two techniques, with the best agreement in compositions with >10% [Fe.sup.3+]. Our data show that only at high values of f[o.sub.2] (>IW + 3.5. or [approximately equal to] QFM) can [Fe.sup.3+] distribution in pyroxene be used to estimate crystallization oxygen fugacity.

A new method has been developed to orient 50-100 [micro]m size single crystals to obtain XANES or EXAFS spectra along any crystallographic orientation relative to a polarized synchrotron X-ray beam. The method uses a modified spindle stage and an X-ray goniometer head to hold a crystal and the glass fiber on which it is mounted. The orientation of the crystallographic axes within the crystal can be determined by X-ray diffraction in all cases or with the use of the polarizing light microscope if the minerals are biaxial. Once an optical orientation direction has been located, it can be oriented parallel to the polarization direction of the synchrotron source for spectral acquisition. Mineral samples representing the three optical classes (i.e., isotropic, uniaxial, and biaxial) were oriented along their a, b, and c crystallographic axes, and Fe-XANES spectra were obtained. In all cases the interactions of polarized X-rays with the crystal structures are analogous to the interactions of visible light (e.g., as represented by refractive index). For the isotropic almandine sample the XANES spectra were similar at all orientations. For the uniaxial buergerite and scapolite samples the spectra were similar along the a and b axes and different along c. A detailed set of spectra acquired at multiple (known) optical orientations in the scapolite crystal demonstrates that spectral features vary in intensity as a function of orientation, with maxima and minima when the X-rays are polarized along optical orientations. For the biaxial fayalite sample, the XANES spectra differed along all three crystallographic axes, as expected by analogy to optical properties.

Synchrotron micro-XANES (X-ray absorption near-edge structure) spectroscopy (SmX) was used to measure [Fe.sup.3+] and [Fe.sup.2+] distribution among minerals in standard thin sections with an X-ray beam size of 10 x 15 [micro] m. Measurements were made at beamline X26a, National Synchrotron Light Source, Brookhaven National Lab. Samples studied included metapelites from garnet to upper sillimanite zone rocks that coexist with graphite, graphite/ilmenite, or varying combinations of ilmenite, magnetite, and hematite. SmX results are compared with Mossbauer spectroscopic measurements of [Fe.sup.3+] on mineral separates from the same rocks. Results show excellent agreement (within [+ or -] 5-10%) between Mossbauer and SmX for the Fe-rich phases biotite, chlorite, staurolite, and garnet. Mossbauer spectra of muscovite typically show lower values than SmX for [Fe.sup.3+]/[SIGMA]Fe (especially at low grades), suggesting contamination of the muscovite separates by fine-grained chlorite. However, heterogeneity of [Fe.sup.3+] and [Fe.sup.2+] is probably the chief source of discrepancy between the bulk and micro-scale measurements. The % [Fe.sup.3+] (relative to total Fe) in these samples ranges from a high of 90% in muscovite to a low of 0-2% in garnet, with Ms > St = Bt = Chl> Tur > Grt in graphite/ilmenite-bearing rocks. SmX measurements suggest that the averaged [Fe.sup.3+]/[SIGMA]Fe in each mineral species does not change as a function of grade, but varies as a function of buffering assemblage, especially on a very localized scale. This effect shows that the oxygen buffering capacity of mineral assemblages is very large compared to the oxidizing/reducing potential of metamorphic fluids. However, distribution of Fe atoms among phases at each grade also reflects crystal chemistry. Across all grades, the [Fe.sup.2+]/Mg ratio is such that Grt >> St > Bt > Chl> Tur > Ms. [Fe.sup.3+]/Al is highest in biotite in garnet and sillimanite-zone rocks. For [Fe.sup.3+]/Al, Bt > Chl> Tur > St > Ms, and for [Fe.sup.3+]/[Fe.sup.2+], partitioning behaves as Ms >> Bt [approximately equal to] St [approximately equal to] Chl> Tur > Grt in graphite/ilmenite-bearing rocks. Overall, partitioning of [Fe.sup.3+] in any of the minerals of interest reflects both oxide assemblage and crystal-chemical constraints on the amount of [Fe.sup.3+] that can be substituted into the structure.

To understand compositional variation in igneous biotite, full analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses have been completed. Major and minor elements were determined by electron microprobe analysis, water was determined by manometry and SIMS analysis, and [Fe.sup.3+]/[Fe.sup.2+] was determined by microXANES and Mossbauer spectroscopy. Our new data, together with previous biotite analyses (total of 52 analyses), reveal correlations between [O.sup.2-] (2-F-Cl-OH) and the sum of the octahedral cations Al + Ti + [Fe.sup.3+] + Cr. This correlation allows estimation of either OH-or [Fe.sup.3+]/[Fe.sup.2+] as long as one or the other has been determined. The hydroxyl site in most mantle micas contains at least 1.0 [O.sup.2-] atoms per formula unit (apfu), indicating that the oxy-component cannot be ignored. The large oxy-component in melt inclusion micas from the martian meteorite Chassigny does not necessarily indicate oxidized or hydrous magmatic conditions because dehydrogenation may have occurred and/or because the oxy-component may be stable at low oxygen fugacity. The large variation in Ti, Al, and [Fe.sup.3+] in natural igneous micas is most likely dependent upon bulk compositional differences in each specific system such as variation of [a.sub.Ti[O.sub.2]] and [a.sub.[Al.sub.2][O.sub.3]] in silicate melts.

Synchrotron micro-XANES spectroscopy (SmX) is used to examine the amount and distribution of [Fe.sup.2+] and [Fe.sup.3+] in five samples of fayalite previously studied by Mossbauer spectroscopy. Rockport fayalite is homogeneous and the Fe in it is completely reduced. Olivine from Qianan is almost completely oxidized, and probably contains finely intergrown silica, laihunite, and hematite with hematite predominating. Pantelleria olivine has an oxidized rim around a reduced core, suggesting either a post-crystallization reaction with fayalite and oxygen going to laihunite and hematite or a change in prevailing oxidation during growth. The texture of olivine from the Mourne Mountains suggests exsolution from a fayalite-laihunite solid solution, based on the substitution of three [Fe.sup.3+] atoms for two [Fe.sup.2+] and one vacancy, that was stable at high temperature. Laihunite from the type locality (Lai-He village) shows 1-3 [[micro]meter] mottling between light and dark areas in backscatter images, but these areas cannot correspond to pure fayalite and laihunite exsolution because the bulk [Fe.sup.3+]/[Sigma]Fe of such a combination would not correspond to the SmX value of 67% [Fe.sup.3+]. Rather, the texture observed in backscatter is interpreted to represent alternating areas of magnetite-rich and laihunite-rich compositions with a similar (i.e., 67% [Fe.sup.3+]/[Sigma]Fe) extent of oxidation.

Measurements of Fe3+/ΣFe in geological materials have been intractable because of lack of access to appropriate facilities, the time-consuming nature of most analyses, and the lack of precision and reproducibility in most techniques. Accurate use of bulk Mössbauer spectroscopy is limited by largely unconstrained recoilless fraction (f), which is used to convert spectral peak area ratios into valid estimates of species concentrations and is unique to different mineral groups and compositions. Use of petrographic-scale synchrotron micro-XANES has been handicapped by the lack of a consistent model to relate spectral features to Fe3+/ΣFe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mössbauer spectra of the Fe2+-bearing almandine and Fe3+-bearing andradite end-members are used to characterize f in garnets, allowing Fe3+/ΣFe to be measured accurately. Mössbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe3+ analyses. High-resolution XANES data were then acquired from these and 15 additional previously studied samples to create a calibration suite representing a broad range of Fe3+ and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe3+/ΣFe were evaluated, along with the multivariate analysis technique of partial least-squares regression (PLS). Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe3+ in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals. [ABSTRACT FROM AUTHOR]

Deals with a study which measured iron distribution among minerals in metapelites. Background to the study; Theoretical framework for sample selection; Results and discussion.

Examines the compositional variation in igneous biotite through analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses. Effect of dehydrogenation on the original composition of biotites; Examination of biotite data from mantle samples; Analysis of mantle rocks from Mars.

Provides information on a study which examined the amount and distribution of iron, Fe2+ and Fe3+, in five samples of fayalite previously studied by Mossbauer spectroscopy. Review of related literature; Experimental methods; Results and discussion.