David P. Dobson

The viscosity of liquid iron at the physical conditions of the Earth's core

It is thought that the Earths outer core consists mainly of liquid iron and that the convection of this metallic liquid gives rise to the Earths magnetic field. A full understanding of this convection is hampered, however, by uncertainty regarding the viscosity of theouter core. Viscosity estimates from various sources span no less than 12 orders of magnitude,, and it seems unlikely that thisuncertainty will be substantially reduced by experimental measurements in the near future. Here we present dynamical first-principles simulations of liquid iron which indicate that the viscosity of iron at core temperatures and pressures is at the low end of the range of previous estimates — roughly 10 times that of typical liquid metals at ambient pressure. This estimate supports the approximation commonly made in magnetohydrodynamic models that the outer core is an inviscid fluid undergoing small-scale circulation and turbulent convection, rather than large-scale global circulation.

First-principles constraints on diffusion in lower-mantle minerals and a weak D′′ layer

Post-perovskite MgSiO3 is believed to be present in the D′′ region of the Earth’s lowermost mantle. Its existence has been used to explain a number of seismic observations, such as the D′′ reflector and the high degree of seismic anisotropy within the D′′ layer. Ionic diffusion in post-perovskite controls its viscosity, which in turn controls the thermal and chemical coupling between the core and the mantle, the development of plumes and the stability of deep chemical reservoirs. Here we report the use of first-principles methods to calculate absolute diffusion rates in post-perovskite under the conditions found in the Earth’s lower mantle. We find that the diffusion of Mg2+ and Si4+ in post-perovskite is extremely anisotropic, with almost eight orders of magnitude difference between the fast and slow directions. If post-perovskite in the D′′ layer shows significant lattice-preferred orientation, the fast diffusion direction will render post-perovskite up to four orders of magnitude weaker than perovskite. The presence of weak post-perovskite strongly increases the heat flux across the core–mantle boundary and alters the geotherm. It also provides an explanation for laterally varying viscosity in the lowermost mantle, as required by long-period geoid models. Moreover, the behaviour of very weak post-perovskite can reconcile seismic observation of a D′′ reflector with recent experiments showing that the width of the perovskite-to-post-perovskite transition is too wide to cause sharp reflectors. We suggest that the observed sharp D′′ reflector is caused by a rapid change in seismic anisotropy. Once sufficient perovskite has transformed into post-perovskite, post-perovskite becomes interconnected and strain is partitioned into this weaker phase. At this point, the weaker post-perovskite will start to deform rapidly, thereby developing a strong crystallographic texture. We show that the expected seismic contrast between the deformed perovskite-plus-post-perovskite assemblage and the overlying isotropic perovskite-plus-post-perovskite assemblage is consistent with seismic observations.

In-situ measurement of viscosity and density of carbonate melts at high pressure

We present the first measurements of carbonate melt viscosity and density at mantle pressures and temperatures and provide important data for modelling carbonatite behaviour within the mantle. Synchrotron radiation was used to observe falling spheres with high atomic number in situ, allowing precise determination of high terminal velocities over short fall distances. The measured viscosities of 1.5 (5) X 10m2 to 5 (2.5) X 10e3 Pas are the lowest of any known terrestrial magma types and these measurements extend the region of measurable viscosity at high pressure by at least 2 orders of magnitude. Accurate measurements of K,Ca(CO,), melt density were performed at atmospheric pressure:

Thermal expansion and crystal structure of cementite, Fe3C, between 4 and 600 K determined by time-of-flight neutron powder diffraction

The cementite phase of Fe3C has been studied by high-resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic (Pnma) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with Tc ≃ 480 K, greatly affects the thermal expansion coefficient of the material. The average volumetric coefficient of thermal expansion above Tc was found to be 4.1 (1) × 10−5 K−1; below Tc it is considerably lower (< 1.8 × 10−5 K−1) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a third-order Gruneisen approximation to the zero-pressure equation of state, combined with a magnetostrictive correction based on mean-field theory.

Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle boundary

Ultralow-velocity zones (ULVZs) are regions of the Earths core–mantle boundary about 1–10 kilometres thick exhibiting seismic velocities that are lower than radial-Earth reference models by about 10–20 per cent for compressional waves and 10–30 per cent for shear waves. It is also thought that such regions have an increased density of about 0–20 per cent (ref. 1). A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt, core–mantle reaction zones or underside sedimentation from the core. Here we suggest that ULVZs might instead be relics of banded iron formations subducted to the core–mantle boundary between 2.8 and 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, such banded iron formations were deposited in the worlds oceans during the late Archaean and early Proterozoic eras. We argue that these layers, as part of the ocean floor, would be recycled into the Earths interior by subduction, sink to the bottom of the mantle and may explain all of the observed features of ULVZs.

Iron–silica interaction at extreme conditions and the electrically conducting layer at the base of Earth's mantle

The boundary between the Earths metallic core and its silicate mantle is characterized by strong lateral heterogeneity and sharp changes in density, seismic wave velocities, electrical conductivity and chemical composition. To investigate the composition and properties of the lowermost mantle, an understanding of the chemical reactions that take place between liquid iron and the complex Mg-Fe-Si-Al-oxides of the Earths lower mantle is first required. Here we present a study of the interaction between iron and silica (SiO2) in electrically and laser-heated diamond anvil cells. In a multianvil apparatus at pressures up to 140 GPa and temperatures over 3,800 K we simulate conditions down to the core–mantle boundary. At high temperature and pressures below 40 GPa, iron and silica react to form iron oxide and an iron–silicon alloy, with up to 5 wt% silicon. At pressures of 85–140 GPa, however, iron and SiO2 do not react and iron–silicon alloys dissociate into almost pure iron and a CsCl-structured (B2) FeSi compound. Our experiments suggest that a metallic silicon-rich B2 phase, produced at the core–mantle boundary (owing to reactions between iron and silicate), could accumulate at the boundary between the mantle and core and explain the anomalously high electrical conductivity of this region.

In situ measurement of viscosity of liquids in the Fe-FeS system at high pressures and temperatures

Abstract The viscosity of liquid FeS and Fe-FeS eutectic was measured at pressures between 0.5 and 5.0 GPa using a synchrotron-based falling sphere technique. We obtain viscosities of 2 × 10-2 to 4 × 10-3 Pa-s in FeS at 1450 to 1700 °C and 2 × 10-2 to 8 × 10-3 Pa-s in Fe-Seut at 1150 to 1380 °C. These results are consistent with recent viscosity measurements in Fe-Seut at 5 to 7 GPa (Urakawa, in preparation), measured diffusivities (Dobson 2000) and ab initio simulated viscosity (Vočadlo et al. 2000). The results are also similar to the values for pure iron at low pressure (Shimoji and Itami 1986). A systematic increase in viscosity and activation energy is seen with increasing sulfur content. Interpolation between the data presented yields a viscosity of 1.4 × 10-2 Pa-s for an outer core composition with ~10 wt% S at the melting temperature. There is good evidence of homologous behavior for Fe-S liquids which implies that the liquid alloy at the inner core boundary may have a similar viscosity

The electrical conductivity of the lower mantle phase magnesiowüstite at high temperatures and pressures

Experimental measurements of magnesiowustite electrical conductivity at Fe/Fe + Mg 0.05 to 0.2 and Fe3+/Fe3+ + Fe2+ 0.01 to 0.7 at high pressure and high-temperature are presented. Below 1000 K, conduction occurs by a small-polaron process of electron hopping between ferric and ferrous sites, but above 1000 K there is a change in mechanism. This high-temperature mechanism is postulated to be a large-polaron process in which holes are promoted in the oxygen valence band via the reactions: 1/2 O2 = OOx + VMg″ + 2h• and FeFe• = FeFex + h•. The hole and its associated polarization field are free to move in the valence band until trapped by a ferrous ion. Activation energies for the low-temperature, small-polaron regime are ∼0.3 eV across the range of Fe/Fe + Mg and Fe3+/Fe3+ + Fe2+ studied, in agreement with previous studies. The high-temperature, large-polaron activation energy decreases with increasing Fe/Fe+Mg and decreasing Fe3+/Fe3+ + Fe2+, ranging, from 0.4 to 1.1 eV. Both regions show a small, negative activation volume (ΔVlt = −0.33 (19) cm3 mol−1; ΔVht = −0.26(69) cm3 mol−1), consistent with previous high pressure studies of electronic conduction mechanisms. A compilation of the available data shows a discrepancy between measurements at low and high-temperatures, consistent with the new results presented here. At the temperature of the lower mantle, the dominant conduction mechanism in magnesiowustite will be the more mobile large-polaron process. This is less sensitive to iron content than small-polaron conduction at Fe/Fe+Mg < 0.17 (the likely compositional range of lower mantle magnesiowustite) and has a different temperature dependence from the low-temperature process.

SYNTHESIS OF CUBIC DIAMOND IN THE GRAPHITE-MAGNESIUM CARBONATE AND GRAPHITE-K2MG(CO3)2 SYSTEMS AT HIGH PRESSURE OF 9-10 GPA REGION

Cubic diamond was synthesized with two systems, (1) graphite with pure magnesium carbonate (magnesite) and (2) graphite with mixed potassium and magnesium carbonate at pressures and temperatures above 9.5 GPa, 1600 degrees C and 9 GPa, 1650 degrees C, respectively. At these conditions (1) the pure magnesite is solid, whereas (2) the mixed carbonate exists as a melt. In this pressure range, graphite seems to be partially transformed into hexagonal diamond. Measured carbon isotope delta(13)C values for all the materials suggest that the origin of the carbon source to form cubic diamond was the initial graphite powder, and not the carbonates.

Experimental determination of Mn-Mg mixing properties in garnet, olivine and oxide

We have measured the mixing properties of Mn-Mg olivine and Mn-Mg garnet at 1300° C from a combination of interphase partitioning experiments involving these phases, Pt-Mn alloys and Mn-Mg oxide solid solutions. Activity coefficients of Mn dilute in Pt-Mn alloys at 1300° C/1 atm were measured by equilibrating the alloy with MnO at known fO2. Assuming that the log fO2 of the Mn-MnO equilibrium under these conditions is-17.80 (Robie et al. 1978), we obtain for γMn: logγMn = −5.25 + 3.67 XMn + 11.41X2Mn Mixing properties of (Mn,Mg)O were determined by reversing the Mn contents of the alloys in equilibrium with oxide at known fO2. Additional constraints were obtained by measuring the maximum extent of immiscibility in (Mn,Mg)O at 800 and 750° C. The data are adequately described by an asymmetric (Mn,Mg)O solution with the following upper and lower limits on nonideality: (a) WMn = 19.9kj/Mol; WMg = 13.7kj/Mol; (b) WMn = 19.9kj/Mol; WMg = 8.2kj/Mol; Olivine-oxide partitioning was tightly bracketed at 1300° C and oxide properties used to obtain activity-composition relations for Mn-Mg olivine. Despite strong M2 ordering of Mn in olivine, the macroscopic properties are adequately described by a symmetric model with: Wol = 5.5 ± 2.5 kj/mol (1-site basis) Using these values for olivine, garnet-olivine partitioning at 27 kbar/1300° C leads to an Mn-Mg interaction parameter in garnet given by: Wgt = 1.5 ± 2.5kJ/mol (1-site basis) Garnet-olivine partitioning at 9 kbar/1000° C is consistent with the same extent of garnet nonideality and the apparent absence of excess volume on the pyrope-spessartine join indicates that any pressure-dependence of WGt must be small. If olivine and garnet properties are both treated as unknown and the garnet-olivine partitioning data alone used to derive WOl and WGt, by multiple linear regression, best-fit values of 6.16 and 1.44 kJ/mol. are obtained. These are in excellent agreement with the values derived from metal-oxide, oxide-olivine and olivine-garnet equilibria.