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The effect of iron on the post-perovskite phase transition has been controversial. We have performed direct chemical analyses of co-existing perovskite and post-perovskite that were synthesized from an ([Mg.sub.0.91][Fe.sub.0.09])Si[O.sub.3] bulk composition using a laser-heated diamond anvil cell at pressures above 100 GPa and temperatures of 1700-1800 K. Analysis on quenched samples was carried out using the transmission electron microscope (TEM). The results demonstrate that crystalline perovskite grains are enriched in iron compared to adjacent amorphous parts presumably converted from post-perovskite. This indicates that ferrous iron stabilizes perovskite to higher pressures. The ferrous and ferric irons are likely to have competing effects on the post-perovskite phase transition, and therefore the effect of iron may be controlled by aluminum. Keywords: Perovskite, post-perovskite, iron partitioning, phase transition, D' layer

The oxidation state of Fe in hydrous ultrapotassic (phono-tephritic) melts coexisting with mixed [H.sub.2]O-C[O.sub.2] fluids was studied experimentally at 1200 and 1250[degrees]C and pressures from 50 to 500 MPa. The oxygen fugacity ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) varied from NNO-2.9 to NNO+2.6 in log [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], relative to the Ni-NiO oxygen buffer (NNO), as imposed by external redox conditions in experimental vessels and internal variations in water activity from 0.05 to 1 inside the capsules. The Fe redox state of the quenched melts was determined by colorimetric wet-chemical analysis. This analytical method was optimized to measure the [Fe.sup.2+]/[SIGMA]Fe ratio of milligram-sized samples within [+ or -] 0.03 (2[sigma]). The accuracy and precision was tested with international reference materials and with standards analyzed by other methods. The [Fe.sup.2+][SIGMA]Fe ratio of the experimental glasses covered a range of 0.41 to 0.85. A small negative effect of dissolved water on [Fe.sup.2+]/[SIGMA]Fe at given [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] was found, consistent with the thermodynamic model of Moretti (2005). No effect of pressure and temperature on the redox state of Fe was resolvable in the investigated P-T range. Compared to hydrous ferrobasaltic melts that were studied previously under similar conditions, systematically lower [Fe.sup.2+]/[SIGMA]Fe ratios were found for the phono-tephritic melts, in particular at low oxygen fugacities. This effect is attributed to the much higher [K.sub.2]O contents of the phono-tephrite (7.5 compared to 0.3 wt%), but the difference in Fe[O.sub.T] (7.8 wt% in the phono-tephrite and 12.9 wt% in the ferrobasalt) may have an influence as well. Comparison of the experimentally obtained relationship between log [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [Fe.sup.3+]/[Fe.sup.2+] for the studied hydrous ultrapotassic melts with commonly used empirical and thermodynamic models suggest that these models can be successfully applied to phono-tephritc melts, although such compositions were not implemented in the model calibrations. Furthermore, the new data can be used to improve the models with respect to the effects of compositional variables, such as [H.sup.2]O or [K.sub.2]O, on the redox state of Fe in silicate melts. Keywords: Fe oxidation state, FeO determination, oxygen fugacity, water activity, phono-tephrite, ultrapotassic hydrous silicate melt, Alban Hills