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Inclusions in diamonds from São Luiz, Brazil are believed to have come from the transition zone and lower mantle (work by B. Harte and co-workers). Among the many unique opportunities presented by these inclusions, we have the possibility to determine the oxygen fugacity of their source region by measuring Fe³⁺ compositions using Mössbauer spectroscopy with our recently developed technique for investigating small samples (see Annual Report 1993).

The São Luiz inclusion suite includes a number of different mineral associations. The most abundant phase is magnesiowüstite, followed by phases with stoichiometry (Mg,Fe)SiO₃ and CaSiO₃, inferred to come from lower mantle perovskite phases, and garnets with a pyrope-almandine composition. Iron in coexisting pairs of magnesiowüstite-(Mg,Fe)SiO₃ and magnesiowüstite-garnet is partitioned preferentially into the magnesiowüstite phase, with partition coefficients similar to those observed in samples synthesised at high P,T. We also studied two eclogitic garnet inclusions believed to have come from the mantle transition zone.

We found only small amounts of Fe³⁺ in all four magnesiowüstite inclusions studied. This is consistent with recent results (see Sect. 3.3h) that show low solubility of Fe³⁺ in magnesiowüstite at high P,T regardless of oxygen fugacity, in contrast to the very high solubility at lower pressures and temperatures. The low Fe³⁺ content of the magnesiowüstite inclusions supports the argument that they originated from the lower mantle, since there are many possibilities for oxidation of the inclusions through contamination, but few possibilities for reduction. In contrast, all five silicate inclusions studied contained extremely high amounts of Fe³⁺. According to present understanding of the crystal chemistry of transition zone and lower mantle phases, this would imply a high oxygen fugacity in the region where the inclusions were formed.