Christian Liebske

Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle

The oxidation state recorded by rocks from the Earths upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1u2009wt% of a metallic iron-rich alloy. The mantles oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.

Mechanisms of metal–silicate equilibration in the terrestrial magma ocean

It has been proposed that the high concentrations of moderately siderophile elements (e.g. Ni and Co) in the Earth’s mantle are the result of metal–silicate equilibration at the base of a deep magma ocean that formed during Earth’s accretion. According to this model, liquid metal ponds at the base of the magma ocean and, after equilibrating chemically with the overlying silicate liquid at high pressure (e.g. 25–30 GPa), descends further as large diapirs to form the core. Here we investigate the kinetics of metal–silicate equilibration in order to test this model and place new constraints on processes of core formation. We investigate two models: (1) Reaction between a layer of segregated liquid metal and overlying silicate liquid at the base of a convecting magma ocean, as described above. (2) Reaction between dispersed metal droplets and silicate liquid in a magma ocean. In the liquid-metal layer model, the convection velocity of the magma ocean controls both the equilibration rate and the rate at which the magma ocean cools. Results indicate that time scales of chemical equilibration are two to three orders of magnitude longer than the time scales of cooling and crystallization of the magma ocean. In the falling metal droplet model, the droplet size and settling velocity are critical parameters that we determine from fluid dynamics. For likely silicate liquid viscosities, the stable droplet diameter is estimated to be ∼1 cm and the settling velocity ∼0.5 m/s. Using such parameters, liquid metal droplets are predicted to equilibrate chemically after falling a distance of <200 m in a magma ocean. The models indicate that the concentrations of moderately siderophile elements in the mantle could be the result of chemical interaction between settling metal droplets and silicate liquid in a magma ocean but not between a segregated layer of liquid metal and overlying silicate liquid at the base of the magma ocean. Finally, due to fractionation effects, the depth of the magma ocean could have been significantly different from the value suggested by the apparent equilibration pressure.

A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts

Abstract This study presents improvements of internally heated pressure vessels to realize high-pressure experiments at controlled fO₂ in low-viscosity systems such as basaltic ones. The new design is a combination of two experimental techniques: a hydrogen sensor membrane made of platinum to measure fH₂, and therefore fO₂, and a rapid-quench system to avoid crystallization of low-viscosity melts during quench. The experimental setup has been tested successfully at temperatures up to 1250 °C and pressures up to 500 MPa. Basaltic melts containing up to 9.38 wt% water can be quenched as bubble-free and crystal-free glasses. The improvements allow synthesis of hydrated glass or partly crystallized samples with a large volume (for further studies) and to perform routine phase-equilibrium studies in basaltic systems at geologically relevant conditions. We used the new technique to determine the effect of fO₂ on water solubility in a melt with MORB composition. The results show that there is a small but significant decrease of water solubility with decreasing fO₂ from MnO-Mn3O4 to QFM buffer conditions in the pressure range 50-200 MPa. Kinetic problems in crystallization experiments in basaltic systems and the duration necessary to attain equilibrium Fe2+/Fe3+ ratio in the charge are discussed.

Mars: A New Core-Crystallization Regime

The evolution of the martian core is widely assumed to mirror the characteristics observed for Earths core. Data from experiments performed on iron-sulfur and iron-nickel-sulfur systems at pressures corresponding to the center of Mars indicate that its core is presently completely liquid and that it will not form an outwardly crystallizing iron-rich inner core, as does Earth. Instead, planetary cooling will lead to core crystallization following either a “snowing-core” model, whereby iron-rich solids nucleate in the outer portions of the core and sink toward the center, or a “sulfide inner-core” model, where an iron-sulfide phase crystallizes to form a solid inner core.

The influence of pressure and composition on the viscosity of andesitic melts

The effect of pressure and composition on the viscosity of both anhydrous and hydrous andesitic melts was studied in the viscosity range of 108 to 1011.5 Pa · s using parallel plate viscometry. The pressure dependence of the viscosity of three synthetic, iron-free liquids (andesite analogs) containing 0.0, 1.06, and 1.96 wt.% H2O, respectively, was measured from 100 to 300 MPa using a high-P-T viscometer. These results, combined with those from Richet et al. (1996), indicate that viscosities of anhydrous andesitic melts are independent of pressure, whereas viscosities of hydrous melts slightly increase with increasing pressure. This trend is consistent with an increased degree of depolymerization in the hydrous melts. Compositional effects on the viscosity were studied by comparing iron-free and iron-bearing compositions with similar degrees of depolymerization. During experiments at atmospheric and at elevated pressures (100 to 300 MPa), the viscosity of iron-bearing anhydrous melts preequilibrated in air continuously increased, and the samples became paramagnetic. Analysis of these samples by transmission electron microscopy showed a homogeneous distribution of crystals (probably magnetite) with sizes in the range of 10 to 50 nm. No significant difference in the volume fractions of crystals was found in samples after annealing for 170 to 830 min at temperatures ranging from 970 to 1122 K. An iron-bearing andesite containing 1.88 wt.% H2O, which was synthesized at intrinsic fO2 conditions in an internally heated pressure vessel, showed a similar viscosity behavior as the anhydrous melts. The continuous increase in viscosity at a constant temperature is attributed to changes of the melt structure due to exsolution of iron-rich phases. By extrapolating the time evolution of viscosity down to the time at which the run temperature was reached, for both the anhydrous (at 1055 K) and the hydrous (at 860 K) iron-bearing andesite, the viscosity is 0.7 log units lower than predicted by the model of Richet et al. (1996). This may be explained by differences in structural properties of Fe2+ and Fe3+ and their substitutes Mg2+, Ca2+, and Al3+, which were used in the analogue composition. n nThe effect of iron redox state on the viscosity of anhydrous, synthetic andesite melts was studied at ambient pressure using a dilatometer. Reduced iron-bearing samples were produced by annealing melts in graphite crucibles in an Ar/CO atmosphere for different run times. In contrast to the oxidized sample, no variation of viscosity with time and no exsolution of iron oxide phases was observed for the most reduced glasses. This indicates that trivalent iron promotes the exsolution of iron oxide in supercooled melts. With decreasing Fe3+/ΣFe ratio from 0.58 to 0.34, the viscosity decreases by ∼1.6 log units in the investigated temperature range between 964 and 1098 K. A more reduced glass with Fe3+/ΣFe = 0.21 showed no additional decrease in viscosity. Our conclusion from these results is that the viscosity of natural melts may be largely overestimated when using data obtained from samples synthesized in air.

The crystal structure of pyroxenes along the jadeite–hedenbergite and jadeite–aegirine joins

Abstract The crystal-structures of seven synthetic pyroxenes along the jadeite.hedenbergite (Jd57Hd43, Jd26Hd74, Jd0Hd100) and jadeite.aegirine (Jd100Ae0, Jd76Ae24, Jd35Ae65, Jd0Ae100) joins were refined using data collected by means of single-crystal X-ray diffraction (space group C2/c, R4σ between 2.2 and 3.4%). The M2 and M1 polyhedral volumes and bond lengths increase with increasing aegirine and hedenbergite content, moreover the Ca for Na substitution along the jadeite-hedenbergite join changes the M2 coordination from 6 + 2 to 4 + 4, with remarkable tilting of the tetrahedral chains. The value of the displacement parameters follows the trend UeqM2 > UeqO2 > UeqO3 > UeqO1 > UeqM1 ≈ UeqT for all samples belonging to the jadeite-aegirine join and for pure hedenbergite; in contrast,, for pyroxenes with intermediate compositions between hedenbergite and jadeite the trend is UeqO1 > UeqO2 > UeqM2 > UeqO3 > UeqM1 ≈ UeqT, with O1 and O2 having anomalously large displacement parameters, probably due to different local structural configuration around the cations with different size and charge. Cation substitution at the M1 site of Na-pyroxenes gives rise to a different structural deformation with respect of the double substitution at both the M1 and M2 sites in (Na,Ca)(M3+,M2+)Si2O6 pyroxenes as the rigid tetrahedral chains try to accommodate both the increasing size of the M1 site and the different coordination requirement of the M2 site

The effect of the hedenbergitic substitution on the compressibility of jadeite

Abstract Four synthetic crystals belonging to the jadeite (Jd, NaAlSi2O6)-hedenbergite (Hd, CaFeSi2O6) solid solution were investigated by X-ray diffraction in situ at high pressure using a diamond anvil cell to Pmax = 10.6 GPa. The samples exhibited space group symmetry C2/c throughout the investigated pressure range and did not show any phase transformations. V0, KT0, and K were simultaneously refined by fitting a third-order Birch-Murnaghan equation of state to pressure-volume data for all samples. The following relationship between bulk modulus and molar fraction of jadeite is observed: KT0 = 108.7(2) (GPa) + 0.191(9) × [% molar Jd] + 0.0006(1) × [% molar Jd]2 The bulk modulus of hedenbergite is 19% lower than jadeite with a strong axial anisotropy that increases with increasing the Hd content. In particular, the compressibility along the b axis (the most compressible in pyroxenes) increases by about 35% going from Jd to Hd while along the c axis the increase in compressibility is about 24%. The a axis does not show any variation in the deformation rate along the join. The analysis of the crystal structure behavior with pressure for all samples clearly indicates that the main cause of the strong anisotropy on the b-c plane is related to the narrowing of the M1 octahedral chain and to anion-anion interactions increasing the packing efficiency of the anion skeletons of the crystals going from Jd to Hd.

A Critical Evaluation and Thermodynamic Optimization of the CaO-CaF2 System

A critical evaluation and thermodynamic optimization of all the available literature experimental data of the CaO-CaF2 system was conducted to obtain a set of thermodynamic functions which can reproduce all available and reliable experimental phase diagrams and thermodynamic data. The liquid solution was described using the Modified Quasichemical Model which assumes the mixing of O2 and F− in the pseudo anionic sublattice and CaF2 solid solution was described using the compound energy formalism. The discrepancies in the CaO and CaF2 liquidii among the available experimental data were resolved. The recent experimental data on the solubility of CaO in solid CaF2 phase were well reproduced, which is critical to explain the eutectic temperature of the system.

iSpectra: An Open Source Toolbox For The Analysis of Spectral Images Recorded on Scanning Electron Microscopes

iSpectra is an open source and system-independent toolbox for the analysis of spectral images (SIs) recorded on energy-dispersive spectroscopy (EDS) systems attached to scanning electron microscopes (SEMs). The aim of iSpectra is to assign pixels with similar spectral content to phases, accompanied by cumulative phase spectra with superior counting statistics for quantification. Pixel-to-phase assignment starts with a threshold-based pre-sorting of spectra to create groups of pixels with identical elemental budgets, similar to a method described by van Hoek (2014). Subsequent merging of groups and re-assignments of pixels using elemental or principle component histogram plots enables the user to generate chemically and texturally plausible phase maps. A variety of standard image processing algorithms can be applied to groups of pixels to optimize pixel-to-phase assignments, such as morphology operations to account for overlapping excitation volumes over pixels located at phase boundaries. iSpectra supports batch processing and allows pixel-to-phase assignments to be applied to an unlimited amount of SIs, thus enabling phase mapping of large area samples like petrographic thin sections.

Mould powder investigations for high speed casting

Abstract Mould powders significantly determine the stability of the continuous casting process of steel. The processes leading to melting of mould powder and solidification of mould slag were studied in situ using high temperature X-ray diffraction with additional powder X-ray diffraction and microscopic techniques. It was shown that during heating, a powder shows a specific sequence of phase relations before melting takes place. During cooling and solidification, one or more crystalline phases can be formed. The findings on high temperature properties were confirmed by the analyses of slag rims and slag films, obtained from the Corus thin slab caster. Finally, the in depth characterisation, together with some calculations on mould slag was compared with plant data of the thin slab caster. Results from this work give a better understanding of the mould powder functions at the thin slab caster and are being used to guide mould powder design for the current and even higher casting speeds.