Jean Peyronneau

Experimental evidence for carbonate stability in the Earth's lower mantle

We present experimental results on the stability of carbonates up to 50 GPa and at high temperatures (1500-2500 K). The experiments were conducted in a laser-heated diamond anvil cell and the run products were characterized by analytical transmission electron microscopy. Dolomite is shown to break down to a CaCO3 + MgCO3 assemblage at pressures between 20 and 50 GPa. No decarbonatation was evident, suggesting that carbonates remain stable under these conditions with respect to rocksalt oxide + CO2 assemblages. Equimolar mixed powders of dolomite + enstatite and dolomite + olivine were transformed into magnesite + calcic perovskite and into magnesite + calcic and magnesian perovskites + magnesiowustite, respectively. The very strong partitioning of Ca in silicates suggests that magnesite is the stable carbonate in the presence of silicates in the Earths lower mantle down to at least 1500 km. Finally, eutectoid or eutectic intergrowth of magnesiowustite and magnesite is observed, suggesting a possible mutual solubility between these two phases at high pressures and high temperatures. Lower mantle magnesiowustite may provide an alternative host for carbon in the Earths lower mantle.

High‐pressure and high‐temperature reactions between silicates and liquid iron alloys, in the diamond anvil cell, studied by analytical electron microscopy

We report new experimental results on mixtures of iron alloys and silicates (Fe + forsterite, Fe + (Mg0.89,Fe0.11)SiO3 enstatite, Fe + (Mg0.89,Fe0.11)2SiO4 olivine, Fe + FeS + (Mg0.83,Fe0.17)2SiO4 olivine, and FeS + (Mg0.83,Fe0.17)2SiO4 olivine) reacted at about 70 GPa and 130 GPa, at high temperature in a laser-heated diamond anvil cell. The recovered samples were studied by analytical transmission electron microscopy. We found that chemical reactions occurred between molten iron alloys and solid oxides, leading to a dissolution of oxygen in the molten metallic phase, and a drastic depletion of iron in the oxides in contact with the metallic phase. Chromium and manganese partition into the liquid iron. Oxygen is therefore a serious candidate for being a light element in the Earths core, and oxides which have experienced a high-pressure contact with molten iron should have a rather low Fe/Fe+Mg ratio.

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

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 magnesiowustite (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 magnesiowustite. 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 magnesiowustite 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.

Electron microscopy of high-pressure phases synthesized from natural olivine in diamond anvil cell

The products of the transformation of natural (Mg0.83Fe0.17)2SiO4 olivine have been prepared at various high pressures (between 25 GPa and 90 GPa), and high temperature in a laser-heated diamond-anvil cell (DAC). Studies of the high-pressure phases have been made by transmission electron microscopy (TEM), and X-ray microanalysis.The olivine/spinel boundaries exhibit all the characteristics of a diffusionless shear transition, having a finely sheared structure and a constant orientation relationship between the close-packed planes of the two structures ((100)ol∥(111)sp).The TEM observations of zones where olivine (or spinel) transforms into post-spinel phases show that the transformation possesses the features of an eutectoïdal decomposition, leading to a lamellar intergrowth of magnesiowüstite (Mg,Fe)O and perovskite (Mg,Fe)SiO3. With increasing temperature and/or decreasing pressure, the grain size of the high-pressure phases increases and obeys an Arrhenius law with an activation volume equal to zero. (Mg,Fe)O grains exhibit a very high density of dislocations (higher than 1011cm−2), whereas (Mg,Fe)SiO3 grains exhibit no dislocations but systematic twinning. The composition plane of the twins is (112) of the GdFeO3-type perovskite, corresponding to the {110} plane of the cubic lattice of ideal perovskite.

Low ferric iron content of (Mg,Fe)O at high pressures and temperatures

New results from high-pressure multi-anvil and diamond anvil cell experiments show that the Fe3+ content of (Mg,Fe)O is low at high pressures and temperatures, and relatively independent of oxygen fugacity. At 18 GPa and 1000°C, the maximum solubility of Fe3+ in Mg0.8Fe0.2O ranges from approximately 2% Fe3+/ΣFe at the Fe-FeO buffer to 3–7% Fe3+/ΣFe at the Re-ReO2 buffer. These low values are likely due to a high-pressure phase transition in the system Fe3O4-MgFe2O4, resulting in preferential partitioning of Fe3+ into the high-pressure phase. (Fe,Mg)Fe2O4 would be exsolved in (Mg,Fe)O depending on the oxygen fugacity, which could have significant effects on transport properties. Exsolved (Fe,Mg)Fe2O4 could also be useful as an oxygen barometer.

Electrical conductivity of magnesiowüstite/perovskite produced by laser heating of synthetic olivine in the diamond anvil cell

Samples prepared from synthetic single crystals of olivine have electrical conductivity that differs significantly from samples prepared, under similar conditions, from San Carlos olivine. In particular, the oxygen fugacity with which the sample was treated before being loaded into the laser-heated diamond anvil cell (LHDAC) has a large effect on the electrical conductivity of magnesiowustite/perovskite assemblages synthesized from San Carlos olivine and no significant effect on those synthesized from the synthetic olivines. This effect is likely due to the presence of nickel in San Carlos olivine. However, because the main effect of nickel in olivine is to diminish the extent of the redox stability field of olivine, this points to a possible effect of the internal oxygen fugacity of the LHDAC on the assemblages produced during transformation. Electron microscopy examination of samples transformed to magnesiowustite/perovskite assemblages in the LHDAC under identical conditions detected substantially more metallic iron in assemblages produced from San Carlos olivine. Analysis of the experimental procedure employed for transformation of silicates in the LHDAC indicates that oxygen from air trapped in the porous powder during pressurization can provide a condition of constant oxygen fugacity in the material. In the temperature gradient that exists during laser heating, a condition of constant oxygen fugacity can lead to very complicated phase assemblages, varying from highly oxidized to highly reduced, at least for materials containing transition metals.

High pressure equilibrium of nickel and cobalt between metal and mantle minerals

Abstract In order to test the effect of very high pressures on the siderophile behaviour of two elements, Ni and Co, we have carried out diamond anvil cell experiments on Ni- and Co-bearing systems, up to 70 GPa. Observation of recovered samples by analytical transmission electron microscopy shows that Ni and Co remain siderophile at least up to 70 GPa, but that their siderophile character decreases with pressure, as already observed in previous studies at lower pressures. Our results also suggest that the abundances of Ni and Co observed in the Earth’s upper mantle cannot be explained by very high pressure equilibrium between silicate perovskite, magnesiowustite, and metal.