Olivine/Fe-metal equilibrium under high pressure: an ATEM investigation

Abstract 

San Carlos olivine samples enclosed in soft iron capsules were annealed in an uniaxial split-sphere apparatus, at pressures ranging from 4.6 to 9.0 GPa and temperature ranging from 1310º to 1595 ºC. We estimated the annealing fO₂, theoretically controlled by the olivine/Fe-metal equilibrium, to be 1 to 2 log units above the fO₂ of the iron/wustite buffer. Samples were investigated by analytical transmission electron microscopy (ATEM) in order to verify that olivine and 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 µm) spatially coupled precipitates of (1) Al-rich spinel and (2) enstatite (volumic proportion of precipitates ≃60 ppm). These coupled precipitates are surrounded by split c dislocation loops. Olivine composition profiles, determined by ATEM near the Fe-capsule/olivine contact, reveal a weak loss of Ni from the olivine matrix toward the capsule, as expected in such reducing conditions. These profiles also reveal a marked incorporation of Fe from the capsule into the olivine matrix. These observations, and their interpretation in terms of olivine point defect chemistry, lead to the following conclusions: (1) the starting olivine contained a high concentration of vacancies on octahedral sites (≥1000 ppm per site); such a high vacancy concentration is expected in San Carlos olivine which equilibrated in nature at relatively high fO₂; (2) the olivine/Fe-metal equilibrium did control fO₂ during the annealings, that resulted in a rapid re-equilibration of olivine at the beginning of the runs to the lower fO₂ imposed by the Fe capsule; this led to a strong decrease of the octahedral vacancy concentration in olivine. (3) Such a fO₂ decrease promoted in olivine the coupled precipitation of both types of Al-rich spinel and enstatite precipitates. These observations show that the use of Fe-capsule in high pressure experiments is an efficient method for controlling fO₂ when studying olivine, and more generally Fe-bearing silicates.