Frédéric Béjina

Activation volume of Si diffusion in San Carlos olivine: Implications for upper mantle rheology

The effect of pressure on silicon diffusion in San Carlos olivine has been determined using a uniaxial split-sphere apparatus (USSA-2000) and the nuclear reaction analysis technique (NRA) on the 30Si isotope. Experiments were performed at high temperature, T = 1763 K, and pressures between 4 and 9 GPa. The specimens were inserted into a pure Fe capsule, which is very effective in maintaining the oxygen fugacity within the stability field of olivine, as well as providing a soft medium to mechanically protect the crystals. Diffusion profiles along the b→ crystallographic axis and of characteristic length of the order of 50 nm were obtained after annealing the olivine samples between 1 and 4 hours. We find the activation volume for silicon diffusion in San Carlos olivine to be VSi = +(0.7±2.3) × 10−6 m3/mol after a correction for oxygen fugacity which is pressure-dependent. This result demonstrates that pressure has practically no effect upon silicon diffusion under our temperature and pressure conditions. Extrapolation of our high-pressure measurements to 1 atm gives a Si diffusion coefficient, log(DSi) = −18.9±1.0 (with DSi in m2/s). Finally, our experiments show that according to the point-defect model of Jaoul [1990], the activation volume for creep of olivine at high temperature is dominated by the effect of pressure on the Mg defect concentration and has an activation volume close to 6 × 10−6 m3/mol.

Dense fine-grained aggregates prepared by spark plasma sintering (SPS), an original technique in experimental petrology

We have fabricated dense polyphase mixtures (silicate and oxide minerals, metal, and silicate melt) by Spark Plasma Sintering (SPS), a technique that has only recently been applied to silicates. We describe the SPS method and some of the characteristics of sintered specimens of forsterite, forsterite + MgO, forsterite + metal (Ni or Fe) and forsterite + metal + silicate melt. We show that SPS is a very efficient method to quickly prepare dense (>99 %) polyphase aggregates with homogeneous microstructures (well distributed phases, narrow grain size distributions, polygonal grain boundaries. . .) and small grain sizes with little or no grain growth during sintering. We observe the development of slight Shape Preferred Orientations (SPO), especially for the metal phases, due to the very simple setup we used here. Overall, SPS offers substantial advantages over traditional sintering techniques, making it perfectly suitable for experimental studies in Earth sciences, in particular those involving kinetic processes.

Use of the spark plasma sintering technique for the synthesis of dense mineral aggregates suitable for high-pressure experiments

In this paper, we present a novel sintering technique, called spark plasma sintering, that we apply for the synthesis of mineral aggregates used for experimentation. We show how dense polycrystalline aggregates of forsterite Mg2SiO4 and MgO mixed in various proportions can be obtained within minutes and with limited grain growth. Finally, we briefly mention the high-pressure experiments we performed with such samples.