Post-spinel transition of Fe2SiO4 ahrensite at high pressure and high temperature

Phase relations for dissociation of spinel-structured Fe2SiO4 ahrensite to FeO wustite + SiO2 stishovite have been determined up to 20 GPa and 1400 & DEG;C under the iron-wustite buffer conditions by multianvil high-pressure and high-temperature experiments. The dissociation boundary determined is expressed by P (GPa) = 19.6 (& PLUSMN; 1.0) - 3.0 (& PLUSMN; 0.8) x 10(-3) T (& DEG;C). The boundary with the negative dP/dT slope is placed by similar to 1-4 GPa at lower pressure at 1000-1200 & DEG;C than the previously determined boundaries whose slopes ranged from strongly positive to weakly negative. Thermodynamic calculation based on available thermodynamic data of ahrensite, wustite and stishovite has provided the dissociation boundary which is in good agreement within the errors with that experimentally determined in this study. By combining the post-spinel phase relations in Fe2SiO4 determined in this study with available phase relations in the Fe2SiO4-Mg2SiO4 system, it is suggested that natural Fe2SiO4-rich ahrensite crystals found in the shocked Umbarger L6 chondrite crystallized in shock-induced melt pockets of the FeO- and SiO2-rich composition at pressures below similar to 13-16 GPa and above similar to 9-12 GPa in the shock process.