HR: 1340h
AN: MR23A-0042    [Abstracts]
TI: Experimental Determination of Phase Relations in SiO2-Al2O3-Fe2O3-MgO at Lower Mantle Conditions: Synthesis Results
AU: Walter, M
EM: M.J.Walter@bristol.ac.uk
AF: University of Bristol, Department of Earth Sciences, Bristol, BS8 1RJ United Kingdom
AU: * Tronnes, R
EM: r.g.tronnes@nhm.uio.no
AF: University of Oslo, Natural History Museum Geology Department, Oslo, N-0318 Norway
AU: Caldwell, S
EM: WACaldwell@lbl.gov
AF: Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, CA 94720 United States
AB: 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-MgAlO_2.5 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(Fe_x,Al_1-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.
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3630 Experimental mineralogy and petrology
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005