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

Phase relations for dissociation of spinel-structured Fe₂SiO₄ ahrensite to FeO wüstite + SiO₂ stishovite have been determined up to 20 GPa and 1400 °C under the iron-wüstite buffer conditions by multianvil high-pressure and high-temperature experiments. The dissociation boundary determined is expressed by P (GPa) = 19.6 (± 1.0) – 3.0 (± 0.8) × 10⁻³ T (°C). The boundary with the negative dP/dT slope is placed by ~ 1–4 GPa at lower pressure at 1000–1200 °C than the previously determined boundaries whose slopes ranged from strongly positive to weakly negative. Thermodynamic calculation based on available thermodynamic data of ahrensite, wüstite 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 ~ 13–16 GPa and above ~ 9–12 GPa in the shock process.