The system Mg 2SiO 4Fe 2SiO 4 at high pressures and temperatures

Abstract

Previous investigations on the olivine-spinel transformation are reviewed. The present investigation was carried out on synthetic (MgFe) 2SiO 4 olivine solid solutions using a Bridgman-anvil apparatus equipped with an internal heater. The experimental method and methods of calibration are described and the precision evaluated. More than 140 runs have been carried out in the pressure range 50–200 kb and approximately at 1000 °C. After quenching, the phases produced were examined by X-ray diffraction and optical methods. A continuous series of spinel solid solutions from Fe 2SiO 4 to (Mg 0.8Fe 0.2) 2SiO 4 was synthesized, and lattice parameters and refractive indices determined. The density of pure Mg 2SiO 4 was determined by extrapolation to be 3.56 g/cm 3, some 10.6% denser than forsterite. A phase diagram for the Fe 2SiO 4Mg 2SiO 4 system was constructed on the basis of the experimental results. In compositions between Fe 2SiO 4 and (Mg 0.8Fe 0.2) 2SiO 4, olivines transform to spinels with a large two-phase loop of coexisting olivine plus spinel. Between (Mg 0.8Fe 0.2) 2SiO 4 and pure Mg 2SiO 4, olivines transform to a new orthorhombic phase, β-Mg 2SiO 4 which is 8% denser than forsterite. This phase is believed to be stable in its synthesis field. The olivine to beta phase transformation at 1000 °C occurs at approximately 120 kb. Reconnaissance investigations were also carried out on the Co 2SiO 4Mg 2SiO 4 system, in which the beta phase possesses a more extensive field of occurrence. The application of the experimental phase diagram to the constitution and seismic structure of the mantle is discussed. Assuming that the olivine of the mantle has an Mg/(Mg+Fe) ratio of 0.89, olivine would partially transform with increasing depth first to spinel. At a greater depth spinel reacts to produce beta phase, and at still greater depths, the remaining olivine transforms to beta phase. The depth interval over which these transformations occur is 27 km and contains a first order density discontinuity at the spinel-beta phase reaction point. Assuming a temperature in the mantle of 1600 °C at a depth of about 400 km, and a gradient for the transformations of 30 b/°C, the olivine-spinel-beta phase transformations would occur over an interval of about 27 km, with a median depth of 397 km and with a first order discontinuity at 403 km. This agrees closely with the depth of a major seismic discontinuity in the mantle. The experimental data thus provide a satisfactory explanation of this important geophysical feature of the mantle.