The oxygen fugacity of core segregation and the redox evolution of the mantle: constraints from iron and chromium isotopes

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

Abstract

We know very little about the origin of our own planet, how it has evolved through time and how it came to be suitable for life. There is considerable controversy surrounding issues like climate change, and the search for new planets around stars other than the Sun and missions to planets in our own solar system and it is currently a very exciting time to be an Earth scientist. The Earth formed about 4600 million years ago, from particles of dust and rocky material present in the early solar system. Only a few million years later, the Earth underwent a massive reorganisation from an enormous mass of unsorted primitive material into a planet composed of a metal core in its centre and a surrounding rocky outer part. We still do not understand precisely how this happened, as the Earth has undergone a wide range of geological events which hide much of the evidence. One possibility is that when the Earth was still hot and molten, liquid metal separated from the rest of the planet and descended to the centre of the Earth, forming its core. During this process, most of the Earth's iron and many other elements were distributed preferentially into the metal core. This imparted a characteristic chemical signal to the outer rocky parts of the Earth, which we can sample through rocks erupted from volcanoes. However, there are many aspects of this part of Earth history that we do not understand. For example, we do not know the exact conditions of core formation and how it could have affected other aspects of the planet's chemistry, such as the amount of oxygen present in the rocky interior of the planet. This is an important question to answer if we are to understand how life originated on Earth, as it is likely that the Earth's oceans and much of the Earth's atmosphere originated from gases leaking out of planet's interior when the Earth was still very young. My project is aimed at understanding what kind of conditions the Earth's core formed under and how this affected the amount of oxygen present in the rocky interior of the Earth. It uses experiments which simulate the very high pressures and temperatures that would have been present in the Earth's interior when the core formed, combined with very precise chemical analyses of these experiments. From these results I will learn how certain chemical elements distributed themselves between the metal core and the rocky outer part of the Earth, and whether this distribution behaviour changes with different conditions and with the amount of oxygen present. By comparing the results I get from the experiments with the chemical compositions of rocks from the Earth and very primitive meteorites we will be able to understand better how the Earth's core formed, and how this may have affected the chemistry of our planet and the development of its atmosphere and oceans.

Publications


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Armytage R (2011) Silicon isotopes in meteorites and planetary core formation in Geochimica et Cosmochimica Acta
Horner TJ (2015) Persistence of deeply sourced iron in the Pacific Ocean. in Proceedings of the National Academy of Sciences of the United States of America
Williams H (2012) Isotopic evidence for internal oxidation of the Earth's mantle during accretion in Earth and Planetary Science Letters

Related Projects

Project Reference Relationship Related To Start End Award Value
NE/F014295/1 31/03/2009 31/10/2010 £476,042
NE/F014295/2 Transfer NE/F014295/1 01/01/2011 31/12/2015 £317,422
 
Description The Earth's mantle is currently oxidised and out of chemical equilibrium with the core. The reasons for this and for the relatively oxidised state of Earth's mantle relative to the mantles of other terrestrial planets are unclear. It has been proposed that the oxidised nature and high ferric iron (Fe3 +) content of Earth's mantle was produced internally by disproportionation of ferrous iron (Fe2 +) into Fe3 + and metallic iron by perov- skite crystallisation during accretion. Here we show that there is substantial Fe isotope fractionation between experimentally equilibrated metal and Fe3+-bearing perovskite (_0.45‰/amu), which can account for the heavy Fe isotope compositions of terrestrial basalts relative to equivalent samples derived from Mars and Vesta as the latter bodies are too small to stabilise significant perovskite. Mass balance calculations indicate that all of the mantle's Fe3 + could readily have been generated from a single disproportionation event, con- sistent with dissolution of perovskite in the lower mantle during a process such as the Moon-forming giant impact. The similar Fe isotope compositions of primitive terrestrial and low-titanium lunar basalts is consis- tent with models of equilibration between the mantles of the Earth and Moon in the aftermath of the giant impact and suggests that the heavy Fe isotope composition of the Earth's mantle was established prior to, or during the giant impact. The oxidation state and ferric iron content of the Earth's mantle was therefore plausibly set by the end of accretion, and may be decoupled from later volatile additions and the rise of ox- ygen in the Earth's atmosphere at 2.45 Ga.
Exploitation Route Use of triple spike techniques to recover equilibrium fractionation factors from experimental data. Use of stable isotopes in general to recover conditions of planetary core formation.
Sectors Other
 
Description This project has involved the development of high-precision techniques for analysing transition metal stable isotopes. These have been employed by a wide range of sectors, from mineral exploration to bio-medical.
First Year Of Impact 2011
Sector Other
Impact Types Societal,Economic
 
Description ERC Starting Grant proposal
Amount £1,600,000 (GBP)
Organisation European Research Council (ERC) 
Sector Public
Country European Union (EU)
Start 01/2013 
End 01/2018
 
Title Metal-perovskite multi-anvil press isotope fractionation experiments 
Description Use of 3-isotope method as applied to multi-anvil experiments. Involved developing method for creating an isotopically labelled starting material and for separating the run products. 
Type Of Material Improvements to research infrastructure 
Year Produced 2012 
Provided To Others? Yes  
Impact Citation of published paper. Take up of separation methods by other research groups. 
 
Description Collaboration Dr. Hannah Hughes 
Organisation Camborne School of Mines
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution Intellectual, analytical
Collaborator Contribution Intellectual, samples
Impact samples
Start Year 2017
 
Description Collaboration with Dr Hanika Rizo 
Organisation University of Quebec (Université du Québec)
Country Canada 
Sector Academic/University 
PI Contribution Intellectual and analytical expertise
Collaborator Contribution Intellectual input, sample provision
Impact Samples provided
Start Year 2017
 
Description Collaboration with Dr Michael Bizimis 
Organisation University of South Carolina
Country United States of America 
Sector Academic/University 
PI Contribution Fe isotope analysis, manuscript writing
Collaborator Contribution Provision of characterised samples, Hf isotope analysis, data modelling
Impact Published paper: Williams and Bizimis EPSL 2014 - please see publications list
Start Year 2011
 
Description Collaboration with Dr. Igor Puchtel 
Organisation University of Maryland
Country United States of America 
Sector Academic/University 
PI Contribution Intellectual, analysis
Collaborator Contribution Intellectual, samples
Impact Samples
Start Year 2017
 
Description Collaboration with Imperial College London 
Organisation Imperial College London (ICL)
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Academic/University 
PI Contribution Collaboration with Imperial College London to investigate Zn isotope fractionation during core formation
Start Year 2012