G. David Price

Possible thermal and chemical stabilization of body-centred-cubic iron in the Earth's core

The nature of the stable phase of iron in the Earths solid inner core is still highly controversial. Laboratory experiments suggest the possibility of an uncharacterized phase transformation in iron at core conditions and seismological observations have indicated the possible presence of complex, inner-core layering. Theoretical studies currently suggest that the hexagonal close packed (h.c.p.) phase of iron is stable at core pressures and that the body centred cubic (b.c.c.) phase of iron becomes elastically unstable at high pressure. In other h.c.p. metals, however, a high-pressure b.c.c. form has been found to become stabilized at high temperature. We report here a quantum mechanical study of b.c.c.-iron able to model its behaviour at core temperatures as well as pressures, using ab initio molecular dynamics free-energy calculations. We find that b.c.c.-iron indeed becomes entropically stabilized at core temperatures, but in its pure state h.c.p.-iron still remains thermodynamically more favourable. The inner core, however, is not pure iron, and our calculations indicate that the b.c.c. phase will be stabilized with respect to the h.c.p. phase by sulphur or silicon impurities in the core. Consequently, a b.c.c.-structured alloy may be a strong candidate for explaining the observed seismic complexity of the inner core.

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.

Ab initio elasticity and thermal equation of state of MgSiO3 perovskite

We have used high-temperature ab initio molecular dynamic simulations to study the equation of state of orthorhombic MgSiO3 perovskite under lower mantle pressure^temperature conditions. We have determined the Gru « neisen parameter, Q, as a function of volume. Our state-of-the-art simulations, accurate to within 10%, resolve the long-standing controversy on thermal expansion (K) and Gru « neisen parameter of MgSiO3 perovskite. Under ambient conditions we find the values for K and Q of 1.86U10 35 K 31 and 1.51, respectively, in excellent agreement with the latest experimental studies. Calculated elastic constants and the static equation of state at 0 K agree well with previous simulations. We have found no evidence for the high-temperature phase transitions of orthorhombic MgSiO3 perovskite to cubic or tetragonal phases at mantle temperatures. fl 2001 Elsevier Science B.V. All rights reserved.

Impact induced melting and the development of large igneous provinces

Abstract We use hydrodynamic modelling combined with known data on mantle melting behaviour to examine the potential for decompression melting of lithosphere beneath a large terrestrial impact crater. This mechanism may generate sufficient quantity of melt to auto-obliterate the crater. Melting would initiate almost instantaneously, but the effects of such massive mantle melting may trigger long-lived mantle up-welling that could potentially resemble a mantle hotspot. Decompression melting is well understood; it is the main method advocated by geophysicists for melting on Earth, whether caused by thinned lithosphere or hot rising mantle plumes. The energy released is largely derived from gravitational energy and is outside (but additive to) the conventional calculations of impact modelling, where energy is derived solely from the kinetic energy of the impacting projectile, be it comet or asteroid. The empirical correlation between total melt volume and crater size will no longer apply, but instead there will be a discontinuity above some threshold size, depending primarily on the thermal structure of the lithosphere. We estimate that the volume of melt produced by a 20 km diameter iron impactor travelling at 10 km/s may be comparable to the volume of melt characteristic of terrestrial large igneous provinces (∼106 km3); similar melting of the mantle beneath an oceanic impact was also modelled by Roddy et al. [Int. J. Impact Eng. 5 (1987) 525]. The mantle melts will have plume-like geochemical signatures, and rapid mixing of melts from sub-horizontal sub-crater reservoirs is likely. Direct coupling between impacts and volcanism is therefore a real possibility that should be considered with respect to global stratigraphic events in the geological record. We suggest that the end-Permian Siberian Traps should be reconsidered as the result of a major impact at ∼250 Ma. Auto-obliteration by volcanism of all craters larger than ∼200 km would explain their anomalous absence on Earth compared with other terrestrial planets in the solar system.

Ab initio lattice dynamics and structural stability of MgO

Using density-functional perturbation theory, we have studied lattice dynamics, dielectric and thermodynamic properties, and P–T stability fields of the NaCl- (“B1”) and CsCl- (“B2”) structured phases of MgO. The results compare well with available experiments and resolve the controversy between earlier theoretical studies of the phase diagram of MgO. We predict that at all conditions of the Earth’s mantle the B1 structure is stable. Static calculations predict the B1–B2 transition to occur at 490 GPa; zero-point vibrations lower this pressure by 16 GPa. The B2-structured phase is dynamically unstable below 110 GPa, but becomes dynamically stable at higher pressures. On the contrary, the B1 phase does not display soft modes at any of the studied pressures. MgO remains an insulator up to ultrahigh pressures: we predict metallization of the B2-structured phase of MgO at 20.7 TPa.

Molecular dynamics simulations of CaCO3 melts to mantle pressures and temperatures: implications for carbonatite magmas

Carbonatite magmas have been suggested to be important agents of metasomatism of the lithospheric mantle. However, the structures and properties of this important class of melt have been only poorly constrained at mantle pressures and temperatures. In the present study a molecular dynamics approach is adopted to constrain carbonatite magma properties and structure, since experimental difficulties preclude the direct study of alkaline carbonate melts at pressure. Simulation results suggests that CaCO3 melt densities increase from 2000 kg m−3 at P ≈ 0.1 GPa to 2900 kg m−3 at P ≈ 10.0 GPa. Estimates of the constant pressure heat capacity of 1.65–1.90 J g−1 K−1, isothermal compressibilities of 0.0120-0.002 kbar−1 and thermal expansivities of 1.886-0.589 × 10−4 K−1 for CaCO3 melts to mantle pressures and temperatures are also provided from simulation results. Self-diffusion coefficients, calculated from simulation results, qualitatively suggest that CaCO3 melts have very low viscosities even at high pressures. The molecular dynamics simulations suggest octahedral Ca2+ coordination in carbonatite melts up to at least 11.5 GPa and the presence in the melt phase of associative metal-carbonate clusters. Fluid-flow calculations, based on the derived density data, suggest carbonatite ascent rates of 20–65 m s−1, which imply that mantle derived carbonatites should be capable of transporting mantle xenoliths of dimensions up to 0.25 that of their conduits.

Elastic anisotropy of FeSiO3 end‐members of the perovskite and post‐perovskite phases

The athermal elastic constants of the perovskite and post-perovskite polymorphs of pure end-member FeSiO3 were calculated from ab initio calculations. We predict that incorporating ten mole percent FeSiO3 together with four mole percent Al2O3 into MgSiO3 reduces the perovskite to post-perovskite phase transition pressure by 5 GPa. Small changes in the seismic properties of the post-perovskite phase due to the incorporation of iron and alumina are compatible with observations for the lower mantle. MgSiO3 post-perovskite enriched in fifty percent or more iron may be responsible for ultra-low velocity zones at the base of the mantle.

CaSiO3 perovskite at lower mantle pressures

We investigate by first-principles the structural behavior of CaSiO3 perovskite up to lower mantle pressures. We confirm that the cubic perovskite modification is unstable at all pressures. The zero Kelvin structure is stabilized by SiO6 octahedral rotations that lower the symmetry to tetragonal, orthorhombic, rhombohedral, or to a cubic supercell. The resulting structures have comparable energies and equation of state parameters. This suggests that relatively small deviatoric/ shear stresses might induce phase transformations between these various structures softening some elastic moduli, primarily the shear modulus. The seismic signature accompanying a local increase in CaSiO3 content should be a positive density anomaly and a negative V-S anomaly.

An infrared and Raman study of carbonate glasses: implications for the structure of carbonatite magmas

place constraints on the structures of these glasses and natural carbonatite magmas. The activity of the fundamental modes of the carbonate ion indicate that at least two structural populations of CO:- exist in carbonate glass structure, one of which, by virtue of the large vibrational splitting of its v) mode, is suggested to occupy a highly asymmetric site. The spectral activity of the O-H stretching region suggests that water exists both as molecular Hz0 and OH, interacting variably with carbonate ions and as metal complexes occupying relatively high symmetry sites in these glasses. The presence of bicarbonate groups, however, is prohibited by the absence of characteristic O-H stretching frequencies. It is suggested on the basis of the vibrational spectra that carbonate glass structures represent “flexible” frameworks constructed by the “bridging” of carbonate ions by strongly interacting metal cations and that the flexible framework is supported by framework modifying cations and molecular groups. The presence of at least two structural populations of CO:- in carbonate glasses implies a level of medium range order and the existence of extended structural units in carbonate melts and it is suggested that such groups represent metal-carbonate complexes. The possible effects of such complexes on the geochemical behaviour of elements in carbonatite melts is discussed and a general model by which variations in elemental solubility could be understood is proposed.

Oxygen in the Earth's core: a first-principles study

Abstract First-principles electronic structure calculations based on DFT have been used to study the thermodynamic, structural and transport properties of solid solutions and liquid alloys of iron and oxygen at Earths core conditions. Aims of the work are to determine the oxygen concentration needed to account for the inferred density in the outer core, to probe the stability of the liquid against phase separation, to interpret the bonding in the liquid, and to find out whether the viscosity differs significantly from that of pure liquid iron at the same conditions. It is shown that the required concentration of oxygen is in the region 25–30 mol%, and evidence is presented for phase stability at these conditions. The Fe/O bonding is partly ionic, but with a strong covalent component. The viscosity is lower than that of pure liquid iron at Earths core conditions. It is shown that earlier first-principles calculations indicating very large enthalpies of formation of solid solutions may need reinterpretation, since the assumed crystal structures are not the most stable at the oxygen concentration of interest.