ORCID Profile
0000-0003-3121-3255
Current Organisation
University of Oxford
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Publisher: California Digital Library (CDL)
Date: 21-08-2019
Publisher: IOP Publishing
Date: 06-2007
Publisher: Springer Science and Business Media LLC
Date: 28-03-2008
Publisher: California Digital Library (CDL)
Date: 24-07-2020
Publisher: Springer Science and Business Media LLC
Date: 10-2017
Publisher: Elsevier BV
Date: 09-2023
Publisher: California Digital Library (CDL)
Date: 27-09-2018
Publisher: IEEE
Date: 12-2006
Publisher: Mineralogical Society of America
Date: 08-2008
DOI: 10.2138/AM.2008.2896
Publisher: Royal Society of Chemistry (RSC)
Date: 2010
DOI: 10.1039/C0JM01550D
Publisher: IEEE
Date: 12-2010
Publisher: Royal Society of Chemistry (RSC)
Date: 2008
DOI: 10.1039/B802158A
Publisher: Mineralogical Society
Date: 10-2011
DOI: 10.1180/MINMAG.2011.075.5.2597
Abstract: The thermal diffusivity of diopside, jadeite and enstatite were measured at simultaneous pressures and temperatures of up to 7 GPa and 1200 K using the X-radiographic Ångström method. The measurements herein show that the pressure dependency of thermal diffusivity in pyroxenes is significantly greater than in olivine or garnet and that in the MORB-layer of a subducting slab the thermal diffusivity of pyroxenes are a factor of 1.5 greater than that of olivine. The temperature dependence of all the data sets is well described by a low-order polynomial fit to 1/ K and the pressure dependence is exponential in 1/ K , formulations which are consistent with the d ed harmonic oscillator model for thermal properties.
Publisher: International Union of Crystallography (IUCr)
Date: 11-09-2008
DOI: 10.1107/S0021889808025417
Abstract: The ability to perform neutron diffraction studies at simultaneous high pressures and high temperatures is a relatively recent development. The suitability of this technique for determining P –V – T equations of state has been investigated by measuring the lattice parameters of Mg 1− x Fe x O ( x = 0.2, 0.3, 0.4), in the range P 10.3 GPa and 300 T 986 K, by time-of-flight neutron powder diffraction. Pressures were determined using metallic Fe as a marker and temperatures were measured by neutron absorption resonance radiography. Within the resolution of the experiment, no evidence was found for any change in the temperature derivative of the isothermal incompressibility, ∂ K T /∂ T , with composition. By assuming that the equation-of-state parameters either varied linearly or were invariant with composition, the 60 measured state points were fitted simultaneously to a P –V – T – x equation of state, leading to values of ∂ K T /∂ T = −0.024 (9) GPa K −1 and of the isothermal Anderson–Grüneisen parameter δ T = 4.0 (16) at 300 K. Two designs of simultaneous high- P / T cell were employed during this study. It appears that, by virtue of its extended pressure range, a design using toroidal gaskets is more suitable for equation-of-state studies than is the system described by Le Godec, Dove, Francis, Kohn, Marshall, Pawley, Price, Redfern, Rhodes, Ross, Schofield, Schooneveld, Syfosse, Tucker & Welch [ Mineral. Mag. (2001), 65 , 737–748].
Publisher: Springer Science and Business Media LLC
Date: 20-07-2016
DOI: 10.1038/SREP29981
Abstract: It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.
Publisher: Wiley
Date: 18-04-2021
Publisher: Elsevier BV
Date: 02-2011
Publisher: Mineralogical Society of America
Date: 07-2014
DOI: 10.2138/AM.2014.4532
Publisher: Elsevier BV
Date: 2009
Publisher: Informa UK Limited
Date: 09-2010
Publisher: Elsevier BV
Date: 08-2006
Publisher: Springer Science and Business Media LLC
Date: 04-01-2019
Publisher: American Geophysical Union (AGU)
Date: 04-2021
DOI: 10.1029/2020JE006726
Abstract: Modeling the planetary heat transport of small bodies in the early Solar System allows us to understand the geological context of meteorite s les. Conductive cooling in planetesimals is controlled by thermal conductivity, heat capacity, and density, which are functions of temperature ( T ). We investigate if the incorporation of the T ‐dependence of thermal properties and the introduction of a nonlinear term to the heat equation could result in different interpretations of the origin of different classes of meteorites. We have developed a finite difference code to perform numerical models of a conductively cooling planetesimal with T ‐dependent properties and find that including T ‐dependence produces considerable differences in thermal history, and in turn the estimated timing and depth of meteorite genesis. We interrogate the effects of varying the input parameters to this model and explore the nonlinear T ‐dependence of conductivity with simple linear functions. Then we apply non‐monotonic functions for conductivity, heat capacity, and density fitted to published experimental data. For a representative calculation of a 250 km radius pallasite parent body, T ‐dependent properties delay the onset of core crystallization and dynamo activity by ∼40 Myr, approximately equivalent to increasing the planetary radius by 10%, and extend core crystallization by ∼3 Myr. This affects the range of planetesimal radii and core sizes for the pallasite parent body that are compatible with paleomagnetic evidence. This approach can also be used to model the T ‐evolution of other differentiated minor planets and primitive meteorite parent bodies and constrain the formation of associated meteorite s les.
Publisher: Springer Science and Business Media LLC
Date: 10-2003
Publisher: American Geophysical Union (AGU)
Date: 17-11-2022
DOI: 10.1029/2022GL100692
Abstract: Earth's core has produced a global magnetic field for at least the last 3.5 Gyrs, presently sustained by inner core (IC) growth. Models of the core with high thermal conductivity suggest potentially insufficient power available for the geodynamo prior to IC formation ∼1 Ga. Precipitation of silicon from the liquid core might offer an alternative power source for the ancient magnetic field, although few estimates of the silicon partition coefficient exist for conditions of the early core. We present the first ab initio determination of the silicon partition coefficient at core‐mantle boundary conditions and use these results to confirm a thermodynamic description of partitioning that is integrated into a model of coupled core‐mantle thermal evolution. We show that models including precipitation of silicon can satisfy constraints of IC size, mantle convective heat flux, mantle temperature and a persistent ancient geodynamo, and favor an oxygen poor initial core composition.
Publisher: Elsevier BV
Date: 12-2012
Publisher: Elsevier BV
Date: 12-2021
Publisher: California Digital Library (CDL)
Date: 27-09-2018
Publisher: Springer Science and Business Media LLC
Date: 19-05-2015
Publisher: Informa UK Limited
Date: 12-2007
Publisher: Elsevier BV
Date: 07-2007
Publisher: California Digital Library (CDL)
Date: 18-07-2019
Publisher: Wiley
Date: 03-09-2010
Publisher: IEEE
Date: 12-2006
Publisher: Royal Society of Chemistry (RSC)
Date: 2021
DOI: 10.1039/D1NR02179F
Abstract: Redox-responsive NPs, delivering DTX in combination with TUBB3 siRNA, increased DTX activity in lung cancer (LC) cells. After local administration in LC mice models, NPs were retained into the lungs thus exerting high siRNA silencing efficacy.
Publisher: Royal Society of Chemistry (RSC)
Date: 2005
DOI: 10.1039/B505612H
Abstract: Dislocations influence many properties of crystalline solids, including plastic deformation, growth and dissolution, diffusion and the formation of polytypes. Some of these processes can be described using continuum methods but this approach fails when a description of the structure of the core is required. To progress in these types of problems, an atomic scale model is essential. So far, atomic scale modelling of the cores of dislocations has been limited to systems with rather simple crystal structures. In this article, we describe modifications to current methodology, which have been used for strongly ionic materials with simple structures. These modifications permit the study of dislocation cores in more structurally complex materials.
Publisher: California Digital Library (CDL)
Date: 09-06-2023
DOI: 10.31223/X5R67Z
Abstract: Shear-wave velocity anisotropy is present throughout the earth. The strength and orientation of anisotropy can be observed by shear-wave splitting (birefringence) accumulated between earthquake sources and receivers. Seismic deployments are getting ever larger, increasing the number of earthquakes detected and the number of source-receiver pairs. Here, we present a new software package, SWSPy, that fully automates shear-wave splitting analysis, useful for large datasets. The software is written in python, so it can be easily integrated into existing workflows. Furthermore, seismic anisotropy studies typically make a single-layer approximation, but in this work we describe a new method for measuring anisotropy for multi-layered media, which is also implemented. We demonstrate the performance of SWSPy for a range of geological settings, from glaciers to Earth's mantle. We show how the package facilitates interpretation of an extensive dataset at a volcano, and how the new multi-layer method performs on synthetic and real-world data. The automated nature of SWSPy and the discrimination of multi-layer anisotropy will improve the quantification of seismic anisotropy, especially for tomographic applications. The method is also relevant for removing anisotropic effects, important for applications including full-waveform inversion and moment magnitude analysis.
Publisher: Springer Science and Business Media LLC
Date: 24-05-2013
Publisher: Springer Science and Business Media LLC
Date: 27-10-2009
Publisher: Geological Society of London
Date: 03-03-2020
DOI: 10.1144/JGS2019-114
Publisher: The Royal Society
Date: 16-12-2008
Abstract: We describe RMCS as one of the first tools for grid computing that integrates data and metadata management into a single job submission system. The system is easy to use, with client tools that are easy to install. Although the RMCS system was developed as a prototype, it is now in production use and a number of scientific studies have been completed using it.
Publisher: Schweizerbart
Date: 09-11-2006
Publisher: California Digital Library (CDL)
Date: 26-03-2023
DOI: 10.31223/X50M2C
Publisher: California Digital Library (CDL)
Date: 27-03-2020
Publisher: Elsevier BV
Date: 08-2014
Publisher: Elsevier BV
Date: 08-2016
Publisher: The Royal Society
Date: 16-12-2008
Abstract: We review the work carried out within the e Minerals project to develop eScience solutions that facilitate a new generation of molecular-scale simulation work. Technological developments include integration of compute and data systems, developing of collaborative frameworks and new researcher-friendly tools for grid job submission, XML data representation, information delivery, metadata harvesting and metadata management. A number of erse science applications will illustrate how these tools are being used for large parameter-sweep studies, an emerging type of study for which the integration of computing, data and collaboration is essential.
Publisher: Springer Science and Business Media LLC
Date: 25-07-2022
Publisher: Elsevier BV
Date: 02-2019
Publisher: IOP Publishing
Date: 05-2016
Publisher: Springer Science and Business Media LLC
Date: 16-06-2013
DOI: 10.1038/NGEO1844
Publisher: Oxford University Press (OUP)
Date: 27-08-2015
DOI: 10.1093/GJI/GGV304
Publisher: Wiley
Date: 24-11-2020
Publisher: Wiley
Date: 02-2021
Publisher: Elsevier BV
Date: 08-2006
Publisher: McGill University Library and Archives
Date: 19-10-2023
Publisher: Informa UK Limited
Date: 10-2006
Publisher: Elsevier BV
Date: 06-2020
Publisher: California Digital Library (CDL)
Date: 14-07-2023
DOI: 10.31223/X59W86
Abstract: Pallasite meteorites contain evidence for vastly different cooling timescales: rapid cooling at high temperatures (K/yrs) and slow cooling at lower temperatures (K/Myrs). Pallasite olivine also shows contrasting textures ranging from well-rounded to angular and fragmental, and some s les record chemical zoning. Previous pallasite formation models have required fortuitous changes to the parent body in order to explain these contrasting timescales and textures, including late addition of a megaregolith layer, impact excavation, or parent body break-up and recombination. We investigate the timescales recorded in Main Group Pallasite meteorites with a coupled multiscale thermal diffusion modelling approach, using a 1D model of the parent body and a 3D model of the metal-olivine intrusion region, to see if these large-scale changes to the parent body are necessary. We test a range of intrusion volumes and aspect ratios, metal-to-olivine ratios, and initial temperatures for both the background mantle and the intruded metal. We find that the contrasting timescales, textural heterogeneity, and preservation of chemical zoning can all occur within one simple ellipsoidal segment of an intrusion complex. These conditions are satisfied in 13% of our randomly generated models (2200 model runs), with small intrusion volumes (with a mean radius ≲100 m) and colder background mantle temperatures (≲1200 K) favourable. Large rounded olivine can be explained by a previous intrusion of metal into a hotter mantle, suggesting possible repeated bombardment of the parent body. We speculate that the formation of pallasitic zones within planetesimals may have been a common occurrence in the early Solar System, as our model shows that favourable pallasite conditions can be accommodated in a wide range of intrusion morphologies, across a wide range of planetesimal mantle temperatures, without the need for large-scale changes to the parent body. We suggest that pallasites represent a late stage of repeated injection of metal into a cooling planetesimal mantle, and that heterogeneity observed in micro-scale rounding or chemical zoning preservation in pallasite olivine can be explained by erse cooling rates in different regions of a small intrusion.
Publisher: American Geophysical Union (AGU)
Date: 03-2017
DOI: 10.1002/2016GC006705
Publisher: Springer Science and Business Media LLC
Date: 19-03-2019
Publisher: Wiley
Date: 10-08-2008
DOI: 10.1002/CPE.1290
Publisher: IGI Global
Date: 2012
DOI: 10.4018/978-1-61350-116-0.CH021
Abstract: This chapter describes the key principles and components of a good data management system, provides real world ex les of how these can be successfully integrated with scientific research processes and enable successful data sharing, provides an outlook on future developments, and discusses lessons learned. We conclude with a short section on how to get started for those whose interest has been piqued.
Publisher: Elsevier BV
Date: 03-2014
Publisher: Mineralogical Society
Date: 06-2010
DOI: 10.1180/MINMAG.2010.074.3.381
Abstract: Recent advances in computer simulation at the atomic scale have made it possible to probe the structure and behaviour of the cores of dislocations in minerals. Such simulation offers the possibility to understand and predict the dislocation-mediated properties of minerals such as mechanisms of plastic deformation, pipe diffusion and crystal growth. In this review the three major methods available for the simulation of dislocation cores are described and compared. The methods are: (1) cluster-based models which combine continuum elastic theory of the extended crystal with an atomistic model of the core (2) dipole models which seek to cancel the long-range elastic displacement caused by the dislocation by arranging for the simulation to contain several dislocations with zero net Burgers vector, thus allowing a fully periodic super-cell calculation and (3) the Peierls-Nabarro approach which attempts to recast the problem so that it can be solved using only continuum-based methods, but parameterizes the model using results from atomic-scale calculations. The strengths of these methods are compared and illustrated by some of the recent studies of dislocations in mantle silicate minerals. Some of the unresolved problems in the field are discussed.
Publisher: California Digital Library (CDL)
Date: 17-06-2022
DOI: 10.31223/X5H34M
Abstract: Earth's core has produced a global magnetic field for the last 4 Gyrs, presently sustained by inner core growth.Models of the core with high thermal conductivity suggest potentially insufficient power available for the geodynamo prior to inner core formation ~1 Ga.Precipitation of SiO2 from the liquid core might offer an alternative power source for the magnetic field before inner core growth, however, no estimates of partition coefficient exist for conditions of the early core.We present the first determination of the silicon partition coefficient at core-mantle boundary conditions and use these results to build a thermodynamic model that is integrated into a model of coupled core-mantle thermal evolution.We show that models including precipitation of silicon can satisfy constraints of inner core size, mantle convective heat flux, mantle temperature and a persistent ancient geodynamo whilst those excluding fail.Successful power from precipitation favours an oxygen poor initial core composition.
Publisher: American Chemical Society (ACS)
Date: 14-02-2006
DOI: 10.1021/CM052260R
Publisher: American Geophysical Union (AGU)
Date: 05-2007
DOI: 10.1029/2006JB004620
Publisher: Springer Science and Business Media LLC
Date: 28-03-2022
Publisher: Elsevier BV
Date: 08-2018
Publisher: Springer Science and Business Media LLC
Date: 22-02-2018
Publisher: Elsevier BV
Date: 02-2012
Publisher: American Physical Society (APS)
Date: 23-06-2021
Publisher: Springer Science and Business Media LLC
Date: 07-09-2004
DOI: 10.1038/NMAT1213
Publisher: California Digital Library (CDL)
Date: 07-04-2023
DOI: 10.31223/X5BT0S
Abstract: We examine magnesium and potassium solubility in liquid Fe mixtures, representative of Earth’s core composition, in equilibrium with liquid silicate mixtures representative of an early magma ocean. Our study is based on the calculation of the chemical potentials of MgO and K2O in both phases, using density functional theory. For MgO, we also study stability against precipitation of the solid phase. We use thermal evolution models of the core and mantle to assess whether either radiogenic heating from 40K decay or Mg precipitation from the liquid core can resolve the new core paradox by powering the geodynamo prior to inner core formation. Our results on K show that concentrations in the core are likely to be small and the effect of 40K decay on the thermal evolution of the core is minimal, making it incapable of sustaining the early geodynamo alone. Our results also predict small concentrations of Mg in the core although these might be sufficient to power the geodynamo prior to inner core formation, depending on the process by which it is transported across the core mantle boundary.
Publisher: Elsevier BV
Date: 07-2023
Publisher: The Royal Society
Date: 16-12-2008
Abstract: A collaborative environmental eScience project produces a broad range of data, notable as much for its ersity, in source and format, as its quantity. We find that extensible markup language (XML) and associated technologies are invaluable in managing this deluge of data. We describe F o X, a toolkit for allowing Fortran codes to read and write XML, thus allowing existing scientific tools to be easily re-used in an XML-centric workflow.
Publisher: Royal Society of Chemistry (RSC)
Date: 2005
DOI: 10.1039/B505716G
Abstract: In an accompanying article, we have described a methodology for the simulation of dislocations in structurally complex materials. We illustrate the applicability of this method through studies of screw dislocations in a structurally simple ionic ceramic (MgO), a molecular ionic mineral (forsterite, Mg2SiO4), a semi-ionic zeolite (siliceous zeolite A) and a covalent molecular crystalline material (the pharmaceutical, orthorhombic paracetamol-II). We focus on the extent of relaxation and the structure of the dislocation cores and comment on similarities and points of disparity between these materials. It is found that the magnitude of the relaxation varies from material to material and does not simply correlate with the magnitude of the principal elastic constants in an easily predictable fashion, or with the size of the cohesive lattice energy or length of the Burgers vector, which emphasises the need to model the non-linear forces and atomic structure of the core.
Publisher: Springer Science and Business Media LLC
Date: 2012
DOI: 10.1038/481153A
Publisher: ACM
Date: 18-06-2021
Publisher: American Geophysical Union (AGU)
Date: 2014
DOI: 10.1002/2013GC005032
Publisher: Oxford University Press (OUP)
Date: 10-12-2012
DOI: 10.1093/GJI/GGS068
Publisher: California Digital Library (CDL)
Date: 26-07-2018
Abstract: Dislocation glide is an important contributor to the rheology of olivine under conditions of high stress and low to moderate temperature, such as occur in mantle wedges. Interactions between point defects and dislocation core may alter the Peierls stress,
Publisher: Elsevier BV
Date: 11-2017
Publisher: Elsevier BV
Date: 02-2012
Publisher: Springer Science and Business Media LLC
Date: 15-03-2018
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Andrew Walker.