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3.2 e. Reduction reactions in iron-bearing silicates (A.G. Duba, B.T. Poe, D.C. Rubie and F. Langenhorst, in collaboration with S. Chakraborty/Köln)

Recent reports of Fe-metal and San Carlos olivine being used to control oxygen fugacity in the multianvil device were investigated using a cylindrical San Carlos olivine single-crystal sealed in an Fe capsule. The capsule was then enclosed in MgO, placed in an 18M assembly, pressurized to 6 GPa, and held at 1500°C for 20 hours. The run material is now being studied using the electron microprobe and will eventually be studied by ATEM techniques. Preliminary electron-microprobe results show numerous Fe-Ni-bearing precipitates along cracks throughout the olivine single-crystal. In addition, there appears to be Ni loss to the Fe capsule and depletion of both Fe and Ni in the olivine surrounding the precipitates. These preliminary results imply that Fe and San Carlos olivine do not form an oxygen buffer. Rather, our results at high pressure support earlier work at atmospheric pressure that shows that olivine will precipitate Fe-Ni from San Carlos at the Fe-wüstite buffer and that the olivine stability field (Fig. 3.2-10) is strongly influenced by the presence of small amounts of Ni in olivine. Although one may conclude that the olivine is being reduced by the presence of the Fe capsule, things are not that simple. For example, the preliminary electron microprobe results show Mg enrichment in the olivine surrounding the precipitates. Further experiments to investigate the source of this Mg enrichment are planned, and more intensive electron-microprobe work is in progress, to be followed by ATEM studies.

Fig. 3.2-10: Oxygen fugacity/temperature plot: Solid lines indicate the stability field for olivine with 10% fayalite and no Ni. Experimental studies at atmospheric pressure, later confirmed by thermodynamic calculations, show that San Carlos olivine with about 0.2 wt% NiO will precipitate Fe-Ni at the Fe/FeO buffer (dashed line).

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