X-ray microanalysis of high-pressure/high-temperature phases synthesized from natural olivine in a diamond-anvil cell

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Scientific paper

The transformation products of natural (Mg0.83Fe0.17)2SiO4 olivine have been prepared at high pressures and high temperature in a laser-heated diamond-anvil cell (DAC). The study of the quenched high-pressure phases has been performed using analytical transmission electron microscopy.
The iron/magnesium partition coefficient between perovskite (pv) and magnesiowüstite (mw), defined as Kmw - pv = χFe/χMg)mw/χFe/χMgpv, was measured in samples synthesized at pressures from 25 to 75 GPa under various heating conditions. In all cases, iron goes preferentially into magnesiowüstite. It was observed, however, that the partition coefficient decreases strongly between 25 and 40 Gpa. At pressures higher than 40 GPa, it is constant (3.5) within experimental error. Although there is a systematic increase in K as T decreases, the effect of temperature is weak at high pressures. From the values of the iron partition coefficient and previously existing thermodynamic values, we obtain a consistent thermodynamic data set for the major constituents of the lower mantle.
The equilibrium compositions of (Mg,Fe)2SiO4 spinel, (Mg,Fe)SiO3 perovskite and (Mg,Fe)O magnesiowüstite phases have been measured and a new high-pressure phase of composition (Mg,Fe)Si2O5 is shown to occur in very high-temperature regions, probably due to high-temperature breakdown or non-stoichiometry of (Mg,Fe)SiO3 perovskite.

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