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International audience; Si and C are cosmochemically abundant elements soluble in hcp Fe under pressure and temperature, and could therefore be present in the Earth’s inner core. While recent ab initio calculations suggest that the observed inner core density and velocities could be matched by an Fe-C-Si alloy, the combined effect of these two elements has only recently started to be investigated experimentally. We therefore carried out synchrotron X-ray diffraction measurements of an hcp Fe-C-Si alloy with 4 at% C and 3 at% Si, up to ∼150 GPa. Density functional theory calculations were also performed to examine different incorporation mechanisms. These calculations suggest interstitial C to be more stable than substitutional C below ~350 GPa. In our calculations, we also find that the lowest energy incorporation mechanism in the investigated pressure range (60-400 GPa) is one where two C atoms occupy one atomic site; however this is unlikely to be stable at high temperatures. Notably, substitutional C is observed to decrease the volume of the hcp Fe, while interstitial C increases it. This allowsus to use experimental and theoretical equations-of-state to show unambiguously that C in the experimental hcp Fe-C-Si alloys is not substitutional, as is often assumed. This is crucial since assuming an incorrect incorporation mechanism in experiments, leads to incorrect density determinations of ~4%, undermining attempts to estimate the concentration of C in the inner core. In addition, the agreement between our experiments and calculations, support Si and C as being light elements in the inner core.

In this paper we report a computational study of the structural and vibrational properties of the Mg-end members forsterite, wadsleyite and ringwoodite of Mg2SiO4, and akimotoite, majorite and the perovskite phase of MgSiO3. Our calculations have been carried out in the framework of Density Functional Theory (DFT) using a plane wave basis set and the Projector-augmented Wave (PAW) method to account for the core electrons. All structures have been fully relaxed at a series of volumes corresponding to the pressure range relevant to the transition zone in the Earth's mantle, and at each volume the phonon frequencies have been obtained and classified. Using the quasi-harmonic approximation, we have estimated a series of thermodynamic properties for each structure, including the Gibbs free energy, from which we have computed approximate phase diagrams for Mg2SiO4 and MgSiO3. In spite of our reliance on the quasi-harmonic approximation, which is expected to break down at high temperatures, our calculated phase diagrams qualitatively reproduce the main features expected from diagrams fitted to experimental data. For example, from the computed phase diagram for Mg2SiO4 we obtain a post-spinel boundary at P=22.1GPa at T=1873K, with a slope of -3.4MPa/K. This supports experimental results suggesting a relatively large slope rather than those favouring a much flatter one. It also suggests that vertical deflections of the 660km discontinuity due to thermal signatures from plumes and slabs should be similar to those at the 410km, and that a deflection of 35km as seen in recent seismic studies could be caused by a thermal anomaly as small as 330K. We also identify the triple point between the ringwoodite, ilmenite (plus periclase) and perovskite (plus periclase) phases to be at P=22.9GPa and T=1565K. Our results clearly illustrate the stringent requirements made on theoretical models in order to extract predictions compatible with the available experimental data., ERH thanks the Leverhulme Trust for the award of a Leverhulme Visiting Professorship to UCL, during which this work was started, and the Spanish Research and Innovation Office through project No. FIS2012-31713.

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.

In this article we describe the eMinerals mini grid, which is now running in production mode. Thisis an integration of both compute and data components, the former build upon Condor, PBS and thefunctionality of Globus v2, and the latter being based on the combined use of the Storage ResourceBroker and the CCLRC data portal. We describe how we have integrated the middleware components,and the different facilities provided to the users for submitting jobs within such an environment. We willalso describe additional functionality we found it necessary to provide ourselves.

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.

In order for existing grid middleware to remain viable it is important to investigate their potentialfor integration with emerging grid standards and architectural schemes. The Open Grid ServicesArchitecture (OGSA), developed by the Globus Alliance and based on standard XML-based webservices technology, was the first attempt to identify the architectural components required tomigrate towards standardized global grid service delivery. This paper presents an investigation intothe integration of Condor, a widely adopted and sophisticated high-throughput computing softwarepackage, and OGSA; with the aim of bringing Condor in line with advances in Grid computing andprovide the Grid community with a mature suite of high-throughput computing job and resourcemanagement services. This report identifies mappings between elements of the OGSA and Condorinfrastructures, potential areas of conflict, and defines a set of complementary architectural optionsby which individual Condor services can be exposed as OGSA Grid services, in order to achieve aseamless integration of Condor resources in a standardized grid environment.

On 16 and 17 January 2002 The Royal Society held a meeting to discuss one of the most contentious issues in the Earth sciences, which has generated heated debate between scientists studying the Earth's interior for the last 20 years or so. In its simplest and earliest form the debate centred on whether the Earth's upper and lower mantles are mixing. Several compelling geochemical arguments required (at least) two distinct mantle reservoirs that have preserved chemical heterogeneity for billions of years. The boundary between the two reservoirs has traditionally been placed at 670 km from the Earth's surface, the depth where the major phase transition of ringwoodite to perovskite and magnesiowustite may act to impede flow. Recent seismological studies, however, provide what seems to be overwhelming evidence that some subducting slabs do penetrate deep into the lower mantle (i.e. deeper than 670 km) and thereby directly contradict the model of geochemically isolated upper and lower mantles. This evidence seems so strong that, in fact, nobody at this Discussion Meeting seriously proposed that the simple model of separately convecting isolated upper and lower mantles is appropriate for today's Earth (although Professor McKenzie, during a discussion period, did give a word of warning that the tomographic images only show seismically fast areas: they do not in themselves give direct evidence for mass transport). Although the simple model of two-layered convection appears (for the moment at least) to be out of favour, many of the arguments that supported it are still difficult for a whole-mantle-convection model to explain. Plenty of exciting alternative scenarios were presented at the meeting.