Diffusion in the DAC: Probing the physical state of the Earth's inner core

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

The inner core is the deepest and most inaccessible layer within the Earth. It is a sphere of solid iron, alloyed with some nickel and one or more 'light' elements (such as silicon, sulfur and carbon) and is 2500 km in diameter and grows by 1 mm every year. The conditions within it are unimaginably extreme, with pressures up to 3.6 million times the pressure at the surface of the Earth, and temperatures near 6000 C, similar to the surface of the sun. The inner core is central to the Earth system. As it crystallises it produces latent heat that helps drive convection in the liquid iron outer core above it, which drives the Earth's heat engine. Because some light elements such as oxygen prefer liquid iron to the solid, the growth of the inner core also changes the chemistry of the outer core, which may in turn effect the chemistry of the whole Earth. Many numerical models of the Earth's geodynamo - the mechanism whereby the convecting outer core produces the Earth's magnetic field - require the presence of the inner core for several reasons, including the extra heat produced by its growth. The geomagnetic field shields us from the harmful effects of the solar wind. Thus the inner core is important to the way in which life has developed on our planet and for maintaining the clement conditions on its surface today.

Yet there is much we do not understand about its structure and evolution. Much of what we do know comes from the study of seismic waves that pass through the inner core. These studies tell us that it is a complex place, with seismic waves travelling faster from pole to pole than they do through its equator. There is also a so-called 'hemispheric dichotomy' where seismic waves in the surface of the inner core travel faster in the eastern hemisphere than the western hemisphere. There is also evidence that there is an 'innermost inner core' around 1000 km in diameter with a different seismic signature to the rest of the inner core. Some studies have even suggested that the whole inner core may be rotating faster than the rest of the Earth.

Understanding and interpreting these surprising discoveries requires knowledge of the physical properties of the iron alloy from which the inner core is made. One of the most important of these properties is viscosity, for which no direct measurements have yet been made. Just such a measurement is the aim of this research.

Viscosity can be determined by measuring how fast iron atoms diffuse through crystals of iron. This will be done in two ways. Firstly, using the laser-heated diamond anvil cell at the School of Earth Sciences, University of Bristol. This equipment consists of two opposing gem-quality diamonds with flat tips, between which discs of iron, less than the diameter of a human hair (around 100 microns) and around 5 microns thick, are compressed to enormous pressures up to 200 million bar. The discs will be coated with a layer of iron, enriched in one of its isotopes, to act as a tracer. While at high-pressure, the sample is heated to temperatures up to 4000 C using infrared lasers and causing the tracer atoms to diffuse through the iron. Using a technique known as secondary ion mass spectrometry (SIMS) at the NERC ion microprobe facility, University of Edinburgh, we can strip away the iron, a few atomic layers at a time, measuring the tracer concentration at each stage. How far the tracer atoms managed to diffuse during a certain heating time tells us the diffusion rate, from which the viscosity can be determined.

Even with this technology, it will be difficult to reach the extreme conditions of the inner core so we will use a second method, ab initio computer simulation at the Department of Earth Sciences, UCL. In this method, a box of iron atoms is simulated within a computer using quantum mechanical methods. Hypothetical tracer atoms can be followed as they diffuse through the box, again allowing us to calculate the rate of diffusion and the viscosity.

Planned Impact

The proposed research has potential benefits for those outside the immediate scientific field of the geosciences. The techniques and equipment supported by research such as that proposed here are often of utility to other scientific groups, especially materials scientists. Over the past year I have been working with members of the Interface Analysis Centre at the University of Bristol, using the diamond anvil cell and Raman spectroscopy to understand the behaviour, under applied stress, of the thermal barrier coatings used on turbine blades. This work has been very successful and papers are being prepared for submission; the results have a potential for direct economic benefit to industry. This synergy between high-pressure experimental petrology and the materials sciences is still to be exploited to its fullest; I intend to continue to develop these links during the lifetime of the proposed project (see Pathways to Impact document for details).

In addition to fostering multidisciplinary research, the proposed project also has the potential to enthuse the general public, especially school age pupils. The nature of this research, with its 'extreme conditions' and high-tech experimental and computational approach to exploring a seemingly remote and unimaginable place can inspire awe in those with curious minds. The potential for interest in the results generated from this project from the popular scientific press and the wider media is significant and is something I intend to pursue alongside the usual pathways of publication in the peer-reviewed scientific literature. More specifically, this kind of research is ideally suited for use as a method of encouraging pupils in school to take an active interest in STEM (Science, Technology, Engineering and Mathematics) subjects and ultimately to study them. This is especially true of GCSE and A-level students who are actively deciding on the next step in their future educational path. I intend to visit local schools and educate students about not just the nature of the Earth and the findings of the proposed project but also to try and actively encourage students to study science, especially the geosciences at A-level and university (see the Pathways to Impact plan). I am currently involved in a project at UCL, entitled 'box-office blunders' which aims to educate pupils at local schools about the nature of the Earth through a 'myth-busting' exercise. The idea is to show disaster movies that have a geophysical aspect to the plot (e.g.: "The Core" and the recent "2012") and have the students draw up a list of scientific flaws. Afterwards, a workshop session will be held to explain how the Earth really works, how geoscientists go about studying it, and what academic research is really like.

Publications

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Briggs R (2017) High-pressure melting behavior of tin up to 105 GPa in Physical Review B

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Dobson D (2016) The phase diagram of NiSi under the conditions of small planetary interiors in Physics of the Earth and Planetary Interiors

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Drewitt J (2019) The fate of carbonate in oceanic crust subducted into earth's lower mantle in Earth and Planetary Science Letters

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Lord O (2014) The melting curve of Ni to 1 Mbar in Earth and Planetary Science Letters

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Lord O (2014) The NiSi melting curve to 70GPa in Physics of the Earth and Planetary Interiors

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Lord OT (2015) The equation of state of the phase of NiSi. in Journal of applied crystallography

 
Description This award allowed me the independence to work on a wide range of projects and collaborate with other researchers in Bristol and elsewhere. As a result there are many different findings that are a direct consequence of this award beyond those already described in the narrative impact section concerning collaborations with material scientists on industrial thermal barrier coatings and nuclear reactor graphite. 1. Transition metal melting: I have determined the melting curves of Ni, NiSi and Sn as well as the sub-solidus phase diagram and equation of state of NiSi using a combination of thermal signal processing and in situ x-ray diffraction during laser heating in the diamond anvil cell. In the case of Ni and Sn, these measurements have helped to eliminate a major controversy resulting from a dramatic mismatch between melting temperatures determined by computer simulations and experiment. 2. Phase relations in the MgO-Al2O3-SiO2-H2O system. This study showed that it is possible to transport water, disolved in solid minerals, throughout the Earths mantle all the way to the boundary with the core, which is a significant contribution to our understanding of the Earth's water cycle. 3. Melting of carbonates. This study determined the melting temperatures of carbonate mixtures of the sort that are subducted into the mantle and showed theat their melting curves have maxima, which might suggest that the liquids become more dense than the solids, which has implications for whether these melts would remain within the mantle or percolate up to the surface. As well as these findings I have developed new software for 2-D temperature measurement in the diamond anvil cell, and used novel microfabrication processes to build fully-encapsulated samples in the DAC, to overcome significant problems with existing experiments, namely extreme temperature gradients and reactions between samples and their surroundings.
Exploitation Route The findings on Ni, NiSi, Sn, hydrous accessory phases and carbonate melting have all led to subsequent studies by other groups. For example, the work on Ni has led others to make similar measurements using X-ray Absorption Spectroscopy, which are in good agreement with my results, and indicate that a previously disparity between the two techniques when applied to iron may be the result of contamination. The technical developments either have been published (in the case of the temperature measurement software) or will be soon, and I expect other experimentalists to use these methods.
Sectors Other

 
Description This fellowship allowed me to develop collaborations with a range of researchers at Bristol and elsewhere, including materials scientists working on industrial materials. These included both thermal barrier coatings used to protect turbine blades from oxidation during high temperature operation and graphite of the sort used in the UK fleet of nuclear reactors. I provided the expertise and equipment to fabricate micron-scale cantilevers for mechanical properties testing of the graphite material, and to perform high pressure experiments on the thermal barrier coatings to improve our understanding of how they behave during their lifetime. This latter collaboration resulted in a new Raman spectroscopy method for determining the stress that the thermal barrier coatings are under and a model of their microstructural properties
First Year Of Impact 2014
Sector Aerospace, Defence and Marine,Energy
Impact Types Economic

 
Description Laser heating at I15 
Organisation Diamond Light Source
Country United Kingdom 
Sector Private 
PI Contribution I have been providing support and advice to the staff at beamline I15 (extreme conditions) as they design and install an in situ laser heating system which will be of great utility to a broad range of geoscientists and materials scientists in the UK, and will make Diamond competitive with other synchrotrons which already have this capability. This system is now operational; I was also part of the team which performed the first laser heating experiment with in situ X-ray diffraction at I15 in December 2015.
Collaborator Contribution The staff ay I15 have designed and built the system; we expect to use it regularly from now onwards through the usual beam-time application process.
Impact The laser heating system at I15 is now operational and available to all to use.
Start Year 2015
 
Description Studies of nuclear reactor graphite with Oxford Materials 
Organisation University of Oxford
Department Department of Materials
Country United Kingdom 
Sector Academic/University 
PI Contribution I have provided the expertise and equipment to perform high-pressure Raman spectroscopy analyses of graphite materials used in the construction of nuclear reactors. I have also used my micro-fabrication expertise to manufacture 'micro-cantilevers' on the scale of tens of microns for the in situ testing of mechanical properties at the National Physical Laboratory.
Collaborator Contribution My research partner (Dong Liu) has provided the materials for study as well as the initial idea to study these materials. She has also planned and performed the mechanical tests, performed additional finite-element modelling and is preparing the results for publication.
Impact This collaboration is multi-disciplinary, combining the facilities for high-pressure experiments, sample microfabrication and Raman spectroscopy used by the petrology group in the School of Earth Sciences at the University of Bristol as well as the expertise of materials scientists at Oxford and the National Physical Laboratory. This partnership has already yielded novel data, which is being prepared for publication in 2016, as well as ideas for new studies on a broader range of materials which are being pursued actively by myself and Dong Liu, my research partner at Oxford.
Start Year 2012
 
Description The melting properties of Tin 
Organisation Diamond Light Source
Country United Kingdom 
Sector Private 
PI Contribution I performed high-pressure melting experiments on pure Tin in the School of Earth Sciences, University of Bristol.
Collaborator Contribution My project partners devised the study, performed an earlier set of synchrotron based experiments and are preparing the paper for publication.
Impact A novel dataset has been collected and is in the final stages of being prepared for publication.
Start Year 2014
 
Description The melting properties of Tin 
Organisation University College London
Department Department of Chemistry
Country United Kingdom 
Sector Academic/University 
PI Contribution I performed high-pressure melting experiments on pure Tin in the School of Earth Sciences, University of Bristol.
Collaborator Contribution My project partners devised the study, performed an earlier set of synchrotron based experiments and are preparing the paper for publication.
Impact A novel dataset has been collected and is in the final stages of being prepared for publication.
Start Year 2014
 
Description Transition metal melting with the Institute of Mineralogy, physical materials and Cosmochemistry, Paris 
Organisation National Center for Scientific Research (Centre National de la Recherche Scientifique CNRS)
Department Institute of Mineralogy, physical materials and Cosmochemistry IMPMC, Paris
Country France 
Sector Public 
PI Contribution I have been performing experiments to try to determine the high-pressure melting lines of various transition metals, which are highly controversial, due to dramatic differences in the predictions made by ab initio computer simulations and earlier experiments.
Collaborator Contribution My collaborators (Guillaume Morard and others) have provided equipment time; specifically, access to Focussed Ion Beam technology which has yielded new data, as well as expertise
Impact This collaboration is only in the early stages; it has not yielded any formal outcomes so far.
Start Year 2014