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High-pressure and -temperature multianvil experiments were performed to test the effect of varying oxygen fugacity on the olivine to wadsleyite transformation. Two capsules, containing samples of [(Fe,Mg).sub.2]Si[O.sub.4], were placed in each experiment; the first buffered the oxygen fugacity with an assemblage of Re and Re[O.sub.2], whereas the second ensured the lowest possible ferric iron concentration through the presence of excess Fe metal. Measurements of coexisting olivine, wadsleyite, and ringwoodite compositions from the Fe metal saturated experiments were used to accurately determine the pressure in each experiment using established phase relations. Under the more oxidizing conditions of the Re-Re[O.sub.2] buffer, the stability field of wadsleyite was found to expand with respect to both the olivine and ringwoodite stability fields. Mossbauer spectroscopy measurements reveal [Fe.sup.3+]/ [SIGMA]Fe ratios for wadsleyite buffered by Re-Re[O.sub.2] of 0.1-0.25, while olivine appears to be [Fe.sup.3+]-free. A thermodynamic model that employs the wadsleyite end-members ([Fe.sup.3+.sub.5/3][[].sub.1/3])[Fe.sup.3+][O.sub.4]-[Fe.sub.2] Si[O.sub.4]-[Mg.sub.2]Si[O.sub.4] is used to examine the effect of varying bulk mantle [Fe.sup.3+]/[SIGMA]Fe ratio on the depth and depth interval of the 410 km seismic discontinuity. [Fe.sup.3+]/[SIGMA]Fe ratios in the range 0.02-0.12 would cause the depth interval or thickness of the 410 km discontinuity to increase from ~8 to 15 km but would have very little effect on the seismically observable absolute depth. Very large bulk mantle [Fe.sup.3+]/[SIGMA]Fe ratios (>0.2), unrepresented in recovered mantle samples, would be required to explain recent seismic observations that the depth interval of the 410 km may be >20 km beneath certain regions. Such observations are more likely to be explained by moderate local enrichments in both ferric iron and [H.sub.2]O in the mantle, most likely as a result of slab interaction. Keywords: Ferric iron, multianvil, transition zone, seismic discontinuity

Spectroscopic measurements have been made of two suites of olivine minerals synthesized under slightly different conditions in 5 10 mol% increments across the solid solution from forsterite to fayalite. Here, we present Mossbauer results for the entire Fe-Mg olivine suite, as well as the results for only the fayalite end-member as an introduction to our team's other diverse spectral-analysis techniques and data that will be presented in forthcoming papers. Experimental methods used to synthesize both suites of samples are discussed here in detail, along with specifics of the analytical techniques used to study them. Electron microprobe data and Mossbauer spectra acquired at 293 K across the solid solution are presented first to characterize and address the presence of impurities in the broad suite of samples that may affect other spectroscopic methods. We then focus specifically on the fayalite end-member to illustrate its properties using multiple techniques. Fayalite is an especially important phase for different types of spectroscopy because, by definition, it contains an equal distribution of [Fe.sup.2+] cations between the M1 and M2 octahedral sites. Thus, features associated with each of the two sites must represent equal numbers of [Fe.sup.2+] cations, removing uncertainties associated with assumptions about order/disorder of [Fe.sup.2+] and other cations. Mossbauer, Raman, thermal emission, attenuated total reflectance (ATR), specular reflectance, and visible to mid-infrared total reflectance studies are presented for fayalite. These include calculation of mid-infrared optical constants (n and k) and fundamental Mossbauer parameters: intrinsic isomer shift ([[delta].sub.1]), Mossbauer temperature ([[theta].sub.M]), and recoil-free fraction (f). Data from the different techniques are described and related, demonstrating the importance of multi-wavelength data to provide a complete characterization and understanding of the spectroscopic features in fayalite. Keywords: Fayalite, olivine, Mossbauer, Raman, thermal emission, reflectance, visible region, infrared, optical constant, attenuated total reflectance