Fe Redox Determinations in Minerals, Glasses and Melts Using XANES Spectroscopy

Francois Farges (farges@uuniv-mlv.fr)¹, Max Wilke (max@geo.uni.potsdam.de), Pierre-Emmanuel Petit (ppetit@esrf.fr)² & Gordon E. Brown, Jr. (gordon@pangea.stanford.edu)³

¹ Lab. des Géomatériaux, Unviersité de Marne la Vallée, 77454 Marne la Vallée cedex 2, France

² ESRF, BP 220, 38042 Grenoble cedex, France

³ Dept. of Geology, Stanford University, Stanford, CA 94305-2115, USA

Fe K-edge pre-edge and XANES spectra for 35 crystalline Fe(II) and Fe(III)-minerals were collected under high resolutions at the ESRF Facility (Grenoble, France) on beamline ID26 using Si(220) double-crystal monochromator. In these compounds, Fe(II) sites are tetrahedral, square planar, trigonal bipyramidal, octahedral and hexahedral. For Fe(III), tetrahedral, trigonal bipyramidal and octahedral sites were investigated The pre-edge spectra show one feature (_[4]Fe(III), _[5]Fe(III)), 2 feature (_[4]Fe(II), _[5]Fe(II), _[6]Fe(III)) and 3 feature (_[6]Fe(II)). Combination of multiplets and l-DOS calculations are in agreement wit these observations. Consideration of spin-orbit interactions and crystal-field splitting (10q) of ~1 eV are able to reproduce accurately the measured pre-edges. The most robust pre-edge information in terms of Fe(III)/ Fe ratio is given by their centroid and total intensity (i.e., the sum of the integrated normalized height for the 1s -> 3d/4 p transitions). The pre-edge centroids for ferrous and ferric iron were measured absolutely in energy, at 7113.0(1) and 7114.4(1) eV, respectively, suggesting a separation between Fe(II) and Fe(III) ~1.4(1) eV. The pre-edge features increase in intensity with decreasing site centrosymmetry. Therefore, derivation of coordination information from these pre-edge intensity can be difficult. The splitting of the pre-edge feature decreases with lower Fe(II)-coordination, and from Fe(II) to Fe(III), in agreement with crystal-field theory. In order to measure the variation in pre-edge information with variable Fe(III)/ Fe, the pre-edge feature for 60 mixtures between Fe(II) and Fe(III) were investigated. The pre-edge information can vary quite linearly with Fe(III)/ Fe at a constant Fe-redox or Fe-coordination. However, non-linear variations are observed when both coordination and redox vary with Fe(III)/ Fe. Redox determinations can then be done at ±10 mol.%, provided that the site geometry for each redox state is constrained. Application of these methods to 12 minerals containing variable/unknown amounts of Fe(II)/Fe(III) (magnetite, vesuvianite, franklinite, rhodonite, etc.) supports these models.

The Experimental Determination of the Chemical Activities of MgO and SiO₂ in Primary Mantle Melts

Joerg Fechner (j.fechner@uni-koeln.de)¹, Herbert Palme (palme@min.uni-koeln.de)¹ & Sumit Chakraborty (sumit.chakraborty@rz.ruhr-uni-bochum.de)²

¹ Institut fuer Mineralogie und Geochemie, Zuelpicher Str. 49b, 50674 Koeln, Germany

² Institut fuer Mineralogie, Universitaetsstr. 150, 44780 Bochum, Germany

The two oxides SiO₂ and MgO make up more than 80% of the mass of the Earth's mantle. The chemical activites of these two components in melts from the mantle of the Earth can be determined experimentally relative to a standard state following a relatively simple procedure.

Olivine (ol) and orthopyroxene (opx) are stable mantle minerals down to a depth of ~400 km and during melting, they coexist with a melt up to ~70% partial melting. The coexistence of these two minerals uniquely defines the activities of SiO₂ and MgO relative to a given standard state. The activities of the two oxides can be determined by equilibrating ol and opx with Pd-metal (Chamberlin et al., 1994) and comparing the results with those from equilibration with pure MgO and SiO₂ standards. The ratio (Si in Pd) ol-opx mix /(Si in Pd) pure SiO₂ gives the activity of SiO₂ in the ol-opx mix (i.e. mantle) at the conditions of the experiment (same procedure for MgO).

We have performed reversal experiments at temperatures ranging from 1000°C to 1400°C and at atmospheric pressure. At 1200°C and 1400°C we found SiO₂ activities of 0.81 to 0.89 and MgO activities of 0.17 to 0.18. At lower temperatures equilibrium was not achieved. Calculated activities from these data, at 1 GPa (assuming molar volume independent of T and p) and for olivine compositions of fo90 (using interaction parameter of Kawasaki & Matsui 1983) are: 0.67 (1250°C) to 0.75 (1400°C) for SiO₂ and 0.19 (1250°C) to 0.18 (1400°C) for MgO, respectively. Using melt composition data of Baker et al., 1994, activity coefficients for SiO₂ and MgO in melts in equilibrium with fertile upper mantle peridotite MM3 were calculated to be 1.10 and 1.48 for SiO₂ for melt fractions of 2% and 27% and 0.98 (2%) to 1.15 (27%) for MgO, respectively.

Further experiments at high pressues are planned to measure these activities directly.

Chamberlin L, Beckett JR & Stolper E, Contrib. Mineral. Petrol, 116, 169-181, (1994).

Kawasaki T & Matsui Y, Geochim. Cosmochim. Acta, 47, 1661-1679, (1983).

Baker MB & Stolper EM, Geochim. Cosmochim. Acta, 58, 2811-2827, (1994).