Rheological control of Wadati-Benioff zone seismicity

Intermediate and deep focus earthquakes in Wadati-Benioff zones are thought to occur in the cold interiors of downgoing slabs which are significantly stronger than the warmer mantle. Given that earthquakes in oceanic lithosphere appear restricted by an isotherm, and hence a given value of lithospheric strength, we investigate whether a similar formulation is useful for subducting plates. Strength in downgoing slabs should be affected by both pressure and temperature, an effect previously treated using a depth‐dependent limiting temperature for seismicity [Wortel, 1982]. We find this limiting temperature implies that a possible limiting strength increases strongly with depth, unless either the temperatures were too low or the activation volume too large. Comparison of the analytic model used by Wortel with numerical thermal models appears to exclude the first possibility. We explore the second possibility by using the numerical thermal model to compute strength contours for flow law constants reported from laboratory experiments, and find that the expected pressure strengthening is large enough that the slab should have considerable strength well below the deepest seismicity. We conclude that if laboratory results are applicable to these conditions, either a strongly depth‐dependent limiting strength exists or factors in addition to strength control the distribution of subduction zone earthquakes

Ab initio simulations of iron-nickel alloys at Earth's core conditions

We report ab initio density functional theory calculations on iron–nickel (FeNi) alloys at conditions representative of the Earth's inner core. We test different concentrations of Ni, up to ∼39 wt% using ab initio lattice dynamics, and investigate the thermodynamic and vibrational stability of the three candidate crystal structures (bcc, hcp and fcc). First of all, at inner core pressures, we find that pure Fe transforms from the hcp to the fcc phase at around 6000 K. Secondly, in agreement with low pressure experiments on Fe–Ni alloys, we find the fcc structure is stabilised by the incorporation of Ni under core pressures and temperatures. Our results show that the fcc structure may, therefore, be stable under core conditions depending on the temperature in the inner core and the Ni content. Lastly, we find that within the quasi-harmonic approximation, there is no stability field for FeNi alloys in the bcc structure under core conditions

High temperature elastic anisotropy of the perovskite and post-perovskite polymorphs of Al2O3

Finite temperature ab initio molecular dynamics calculations were performed to determine the high temperature elastic and seismic properties of the perovskite and post-perovskite phases of pure end-member Al2O3. The post-perovskite phase exhibits very large degrees of shear-wave splitting. The incorporation of a few mole percent of Al2O3 into MgSiO3 is predicted to have little effect on the perovskite to post-perovskite phase transition pressure and seismic properties of the post-perovskite phase; although a small difference in shear-wave splitting may be observable

Leveraging HTC for UK eScience with very large Condor pools: demand for transforming untapped power into results

We provide an insight into the demand from the UK eScience community for very large HighThroughput Computing resources and provide an example of such a resource in current productionuse: the 930-node eMinerals Condor pool at UCL. We demonstrate the significant benefits thisresource has provided to UK eScientists via quickly and easily realising results throughout a rangeof problem areas. We demonstrate the value added by the pool to UCL I.S infrastructure andprovide a case for the expansion of very large Condor resources within the UK eScience Gridinfrastructure. We provide examples of the technical and administrative difficulties faced whenscaling up to institutional Condor pools, and propose the introduction of a UK Condor/HTCworking group to co-ordinate the mid to long term UK eScience Condor development, deploymentand support requirements, starting with the inaugural UK Condor Week in October 2004

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

Water distribution in the lower mantle: Implications for hydrolytic weakening

The presence of water in lower mantle minerals is thought to have substantial effects on the rheological properties of the Earth's lower mantle in what is generally known as “hydrolytic weakening”. This weakening will have profound effects on global convection, but hydrolytic weakening in lower mantle minerals has not been observed experimentally and thus the effect of water on global dynamics remains speculative. In order to constrain the likelihood of hydrolytic weakening being important in the lower mantle, we use first principles methods to calculate the partitioning of water (strictly protons) between mineral phases of the lower mantle under lower mantle conditions. We show that throughout the lower mantle water is primarily found either in the minor Ca-perovskite phase or in bridgmanite as an Al3+–H+ pair. Ferropericlase remains dry. However, neither of these methods of water absorption creates additional vacancies in bridgmanite and thus the effect of hydrolytic weakening is likely to be small. We find that water creates significant number of vacancies in bridgmanite only at the deepest part of the lower mantle and only for very high water contents (>1000 ppm). We conclude that water is thus likely to have only a limited effect on the rheological properties of the lower mantle

Elastic properties of ferropericlase at lower mantle conditions and its relevance to ULVZs

The elasticity of FexMg1 − xO was examined under lowermost mantle temperature and pressure conditions using density functional theory (DFT). The addition of iron decreases the shear modulus of MgO but has varying effects on the bulk modulus depending on the spin state of the iron. The spin state of iron in FexMg1 − xO is dependent on pressure and temperature but also on the concentration of iron. At 136 GPa, Fe in low concentrations (75%) it is nearly entirely in the high spin state. There is, as expected, a large decrease in seismic velocities with iron substitution. However, the effect of Fe is greater at high-temperatures than at low-temperatures, meaning it is difficult to extrapolate low-temperature experimental results. We cannot simultaneously match the density and seismic velocities of ULVZs with Fe-enriched ferropericlase. This is reflected in (dln Vs/dln Vp)T,P, which in ULVZs is generally observed to be about 3, but does not exceed about 1.5 for Fe-enriched periclase. A mixture of ferropericlase and ferrous perovskite can cause Vs decreases of up to 45%, which allows the range of ULVZ Vp, Vs and densities to be matched. We also find that (dln Vs/dln Vp)T,P increases up to as much as 3 but this value is strongly dependent on the bounds of the mixing geometry. We conclude, therefore, that the properties of ULVZs can be readily explained by a lower mantle with a single phase that is heavily enriched in Fe

The pressure medium as a solid-state oxygen buffer

We present a simple method to buffer oxygen fugacity at high pressures and high temperatures where the traditional 'double capsule' method is inappropriate. The pressure medium is doped with a metal which partially reacts with the free oxygen in the pore spaces of the, cell. The resultant finely intergrown metal-metal oxide assemblage buffers the oxygen fugacity in the sample as long as the capsule and furnace materials are oxygen permeable