OLIVINE FE-METAL EQUILIBRIUM UNDER HIGH-PRESSURE - AN ATEM INVESTIGATION/

San Carlos olivine samples enclosed in soft iron capsules were anneale d in an uniaxial split-sphere apparatus, at pressures ranging from 4.6 to 9.0 GPa and temperature ranging from 1310 degrees to 1595 degrees C. We estimated the annealing fO(2), theoretically controlled by the o livine/Fe-metal equilibrium, to be 1 to 2 log units above the fO(2) of the iron/wustite buffer. Samples were investigated by analytical tran smission electron microscopy (ATEM) in order to verify that olivine an d Fe capsule did equilibrate during the annealings. TEM imaging of the olivine bulk shows a and c dislocations confined in the (010) plane, and small (0.5 mu m) spatially coupled precipitates of (1) AI-rich spi nel and (2) enstatite (volumic proportion of precipitates similar or e qual to 60 ppm). These coupled precipitates are surrounded by split c dislocation loops. Olivine composition profiles, determined by ATEM ne ar the Fe-capsule/ olivine contact, reveal a weak loss of Ni from the olivine matrix toward the capsule, as expected in such reducing condit ions. These profiles also reveal a marked incorporation of Fe from the capsule into the olivine matrix. These observations, and their interp retation in terms of olivine point defect chemistry, lead to the follo wing conclusions: (1) the starting olivine contained a high concentrat ion of Vacancies on octahedral sites (greater than or equal to 1000 pp m per site); such a high vacancy concentration is expected in San Carl os olivine which equilibrated in nature at relatively high fO(2); (2) the olivine/Fe-metal equilibrium did control fO(2) during the annealin gs, that resulted in a rapid re-equilibration of olivine at the beginn ing of the runs to the lower fO(2) imposed by the Fe capsule; this led to a strong decrease of the octahedral vacancy concentration in olivi ne. (3) Such a fO(2) decrease promoted in olivine the coupled precipit ation of both types of Al-rich spinel and enstatite precipitates. Thes e observations show that the use of Fe-capsule in high pressure experi ments is an efficient method for controlling fO(2) when studying olivi ne, and more generally Fe-bearing silicates.