Experimental Determination of Phase Relations in SiO2-Al2O3-Fe2O3-MgO at Lower Mantle Conditions: Synthesis Results

We made synthesis experiments in the system SiO2-Al2O3-Fe2O3-MgO (SAFM) using the laser-heated diamond anvil cell at conditions approximating the upper portion of Earth`s lower mantle. Eleven starting compositions spanning five compositional joins were investigated. Starting mixtures are composed of glass, glass+oxide(s), or glass+crystalline materials. Fine Pt black was added to iron-free mixtures for a laser absorber. Seventeen experiments on finely powdered samples were made at pressures of 25-60 GPa and at temperatures of 2000-3000 K. Samples were heated in a double-sided geometry with a diode-pumped YLF laser operating in TEM01 mode. The laser spot was ~ 30 μm in diameter, and sample areas ~ 70 μm in diameter were heated by slow and repeated scanning for 15 to 20 minutes. Pressure and temperature quenched samples were analyzed for phase identification using wavelength-dispersive X-ray diffraction at station 12.2.2 of the Advanced Light Source. Multiple spectra spanning the heated regions of the samples were collected using a monochromatic (λ=0.62) X-ray beam focused to a spot ~ 30 μm in diameter. The salient findings are as follows: 1. SAM system: Al2O3 is highly soluble in Mg-perovskite (MPv=MgSiO3), possibly exceeding 50 wt% Al2O3. This shows that coupled substitution of Al3+ on both the six- and eight-coordinated sites is energetically favorable, as predicted in ab initio calculations. 2. Results along MPv-MgAlO2.5 indicate a considerable solution of Al3+ into MgPv via a vacancy forming reaction. 3. SFM system: Fe2O3 has low solubility in MPv, indicating that coupled substitution of 2Fe3+ onto six- and eight-fold sites is generally not energetically favorable. Experiments indicate that Fe3+ enters the six-fold site via an oxygen-vacancy forming reaction. 4. Results within SAFM along the join MPv-FeAlO3 show an extensive region within which MPv, a ferrite-structered Mg(Fex,Al1-x)2O4, and stishovite coexist. Phase relations indicate that as Al3+ is added to the system SFM, coupled substitution of Fe3+ (8-fold) and Al3+ (6-fold) increases over vacancy-forming substitution of Fe3+. Based on a self-consistent set of quaternary phase relations in SAFM, we predict that at conditions of the upper part of the lower mantle (e.g. 25-50 GPa), model mantle peridotite compositions should yield an oxide phase (Mg,Fe)O coexisting with Mg-perovskite dominated by defect substitution mechanisms. This is because in the quaternary system, peridotite composition is confined between the MgO apex and a plane of defect perovskite.