Jean-Paul Poirier

The chemical composition of the Earth

Abstract The bulk composition of the Earth and the composition of the mantle and core are calculated using the ratios of major and trace elements. The ratios of elements which do not enter the core (lithophile) are the same in the bulk Earth as in the mantle. Bulk earth ratios involving an element that does enter the core (siderophile) are therefore determined from meteorite correlation diagrams of siderophile-lithophile ratios vs. lithophile-lithophile ratios and from primitive mantle composition in elements which do not enter the core (e.g., Al). The composition of the core is determined by difference, without resorting to assumptions about core formation processes. It is found that the core contains about 7.3 wt% silicon and 2.3 wt% sulphur. To account for the seismologically determined density deficit of the core, about 4 wt% oxygen must be added. The present results are compatible with the idea that the core material equilibrated at low pressure, in reducing conditions. Furthermore, we propose that the Earth is closer to CM rather than to C1 for non-volatile element ratios.

Light elements in the Earth's outer core: A critical review

Abstract There is little doubt that densities for the Earths outer core, inferred from seismology, require that it is constituted of an alloy of liquid iron and light elements. However, the nature of the light alloying elements is still uncertain as it depends in a large measure on the conditions of accretion of the Earth and formation of the core. The arguments brought forward for or against silicon, oxygen, sulphur, hydrogen and carbon are critically reviewed. There is no reason to consider that only one element is present in the outer core. Experimentally determined and/or calculated ternary and quaternary phase diagrams are needed to provide constraints on the nature of the light elements.

The age of the inner core

Abstract The energy conservation law, when applied to the Earth’s core and integrated between the onset of the crystallization of the inner core and the present time, gives an equation for the age of the inner core. In this equation, all the terms can be expressed theoretically and, given values and uncertainties of all relevant physical parameters, the age of the inner core can be obtained as a function of the heat flux at the core–mantle boundary and the concentrations in radioactive elements. It is found that in absence of radioactive elements in the core, the age of the inner core cannot exceed 2.5 Ga and is most likely around 1 Ga. In addition, to have an inner core as old as the Earth, concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds.

A logarithmic equation of state

Abstract The isothermal Eulerian Birch–Murnaghan equation of state is currently used in geophysics, despite its recognized shortcomings for very large compressive strains. We propose an equation of state constructed from the Hencky logarithmic strain, rather than from the Eulerian strain. The logarithmic equation of state has a simple expression and is valid in a greater range of pressures than the Birch–Murnaghan equation of state.

On cooling of the Earth's core

Abstract We have constructed a self-consistent model for cooling of the Earths core in which the thermal history of the core is computed as a function of the time evolution of the heat flux delivered to the mantle across the core-mantle boundary. The temperature profile in the convecting core is first assumed to be adiabatic, and its evolution in time is calculated with the only constraint that energy be globally conserved. When the temperature at the centre drops below the freezing point of the core alloy, the inner core starts growing and cools by conduction; it is found that it cannot have reached its present size in more than 1.7 billion years. If the heat flux delivered to the mantle becomes less than that conducted down the adiabat, the temperature profile becomes subadiabatic in a shell at the top of the core, through which heat is evacuated by conduction. Although it is stable against thermal convection, this shell is not necessarily stagnant and may be the seat of motions owing to compositional convection.

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.

‘Eclogitic’ minerals in a shocked basaltic meteorite

Analytical transmission electron microscopy (ATEM) was used to characterize mineral parageneses occurring in melt veins of shock origin in the Martian meteorite Zagami. We discovered a new ultra-high pressure ‘eclogite facies’ mineral assemblage consisting of omphacite, stishovite, and KAlSi3O8 hollandite. These tiny (<200–300 nm) high-pressure minerals are embedded in silicate glass, which forms the groundmass of the shock veins. The microtexture of the veins and zoning of the clinopyroxene indicate the formation of minerals by crystallization from a high-pressure melt. Using calibration data from shock and multi-anvil experiments we infer that frictional melting occurred in veins at minimum temperatures of 2400–2500°C and at a pressure of ca. 30 GPa. Subsequently, the high-pressure phases crystallized during decompression in the pressure interval from 25–5 GPa.

Does infiltration of core material into the lower mantle affect the observed geomagnetic field

Abstract A quantitative estimate of the depth of penetration of liquid iron from the core into the base of the lower mantle has been obtained, from laboratory data and theoretical models. The thickness of the infiltrated layer is found to be of the order of 1–100 m depending on the grain size of the mantle material. Even with the assumption that convection in the D″ layer entrains the electrically conducting core fluid over 100 km, the effective conductivity of the layer is hardly modified and the perturbation caused to the secular variation field is negligible. The same conclusion obtains in the improbable limiting case where all the infiltrated core fluid is gathered in a single mass.

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.

Comparison of the raman microprobe spectra of (Mg, Fe)2SiO4 and Mg2GeO4 with olivine and spinel structures

Raman microprobe (RMP) spectra were produced for each of the olivine and spinel structured phases of Mg2GeO4 and (Mg, Fe)2SiO4.The assembled data show that bands due to the tetrahedra in silicate and germanate olivines shift in a way that indicates a dominant mass effect. This correspondence is difficult to make in spinels due to differences in structural type. Differences in Fe/Mg content of olivine shift the tetrahedral vibration bands only slightly, but their linear shifts could be used to indicate the composition of the phase.