The oxygen fugacity of core segregation and the redox evolution of the mantle: constraints from iron and chromium isotopes
Lead Research Organisation:
Durham University
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.
People |
ORCID iD |
Helen Williams (Principal Investigator) |
Publications
Armytage R
(2011)
Silicon isotopes in meteorites and planetary core formation
in Geochimica et Cosmochimica Acta
Bonnand P
(2016)
Stable chromium isotopic composition of meteorites and metal-silicate experiments: Implications for fractionation during core formation
in Earth and Planetary Science Letters
Bridgestock L
(2014)
Unlocking the zinc isotope systematics of iron meteorites
in Earth and Planetary Science Letters
Debret B
(2019)
Shallow forearc mantle dynamics and geochemistry: New insights from IODP Expedition 366
in Lithos
Debret B
(2016)
Isotopic evidence for iron mobility during subduction
in Geology
Debret B
(2018)
Carbonate Transfer during the Onset of Slab Devolatilization: New Insights from Fe and Zn Stable Isotopes
in Journal of Petrology
Gall L
(2012)
Determination of mass-dependent variations in nickel isotope compositions using double spiking and MC-ICPMS
in J. Anal. At. Spectrom.
Gall L
(2013)
Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic
in Earth and Planetary Science Letters
Hibbert K
(2012)
Iron isotopes in ancient and modern komatiites: Evidence in support of an oxidised mantle from Archean to present
in Earth and Planetary Science Letters
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
McCoy-West A
(2018)
The Fe and Zn isotope composition of deep mantle source regions: Insights from Baffin Island picrites
in Geochimica et Cosmochimica Acta
Millet M
(2016)
Titanium stable isotope investigation of magmatic processes on the Earth and Moon
in Earth and Planetary Science Letters
Pons M
(2020)
Precise measurement of selenium isotopes by HG-MC-ICPMS using a 76-78 double-spike
in Journal of Analytical Atomic Spectrometry
Pons ML
(2016)
Zinc isotope evidence for sulfate-rich fluid transfer across subduction zones.
in Nature communications
Savage P
(2013)
Silicon isotopes in granulite xenoliths: Insights into isotopic fractionation during igneous processes and the composition of the deep continental crust
in Earth and Planetary Science Letters
Savage P
(2013)
The silicon isotope composition of the upper continental crust
in Geochimica et Cosmochimica Acta
Savage P
(2012)
The silicon isotope composition of granites
in Geochimica et Cosmochimica Acta
Tang F
(2019)
Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo.
in Proceedings of the National Academy of Sciences of the United States of America
Wang S
(2012)
Magnesium isotopic variations in cratonic eclogites: Origins and implications
in Earth and Planetary Science Letters
Williams H
(2012)
Isotopic evidence for internal oxidation of the Earth's mantle during accretion
in Earth and Planetary Science Letters
Williams H
(2014)
Iron isotope tracing of mantle heterogeneity within the source regions of oceanic basalts
in Earth and Planetary Science Letters
Williams H
(2011)
Copper stable isotopes as tracers of metal-sulphide segregation and fractional crystallisation processes on iron meteorite parent bodies
in Geochimica et Cosmochimica Acta
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 stable isotope approach in high pressure experiments; prediction of bridgemanite signatures in mantle melts |
Sectors | Other |
Description | Since publication of the Fe-Cu isotopic fractionation dataset (Williams and Archer 2011), a number of other groups have started to explore the use of Fe and Cu isotopes as tracers of planetary differentiation (metal-sulphide differentiation). Although take-up has been primarily in the academic sector, this work has also prompted interest in the potential of these isotope systems as tracers in exploration geology. Further studies (e.g. Williams et al., 2012) have highlighted the importance of constraining these processes experimentally and this approach has also been taken up by a number of other groups. |
Sector | Other |
Impact Types | Societal Economic |
Description | ERC Advanced Grant 'EarthMelt' |
Amount | € 3,500,000 (EUR) |
Organisation | University of Cambridge |
Sector | Academic/University |
Country | United Kingdom |
Start | 11/2021 |
End | 10/2026 |
Description | ERC Starting Grant proposal |
Amount | £1,600,000 (GBP) |
Organisation | European Research Council (ERC) |
Sector | Public |
Country | Belgium |
Start | 01/2013 |
End | 01/2018 |
Description | Marie Curie Fellowship (Mentored) |
Amount | £120,000 (GBP) |
Organisation | Marie Sklodowska-Curie Actions |
Sector | Charity/Non Profit |
Country | Global |
Start | 11/2015 |
End | 10/2017 |
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. |
Title | Use of Fe and Zn isotopes to track sulfate-bearing fluids |
Description | We have documented that Fe and Zn isotopes in subducted serpentinites are highly sensitive to the loss of sulfate-bearing fluids |
Type Of Material | Improvements to research infrastructure |
Year Produced | 2016 |
Provided To Others? | Yes |
Impact | Methodology adopted by other groups |
Description | Collaboration Dr. Hannah Hughes |
Organisation | Camborne School of Mines |
Country | United Kingdom |
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 |
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 |
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 |
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 |
Country | United Kingdom |
Sector | Academic/University |
PI Contribution | Collaboration with Imperial College London to investigate Zn isotope fractionation during core formation |
Start Year | 2012 |
Description | "Pint of Science" - general science outreach talk to public and debate forum |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | No |
Geographic Reach | Regional |
Primary Audience | Public/other audiences |
Results and Impact | "Pint of Science" - general science outreach talk to public and debate forum |
Year(s) Of Engagement Activity | 2019 |
Description | European Science Open Forum Copenhagen |
Form Of Engagement Activity | A formal working group, expert panel or dialogue |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Policymakers/politicians |
Results and Impact | Seminar given to press and policymakers on the factors governing the evolution of the Earth as a habitable planet. Given as part of the European Science Open Forum, Copenhagen. |
Year(s) Of Engagement Activity | 2014 |
Description | Outreach event with local female state-school students |
Form Of Engagement Activity | Participation in an open day or visit at my research institution |
Part Of Official Scheme? | No |
Geographic Reach | National |
Primary Audience | Schools |
Results and Impact | Outreach event with local female state-school students |
Year(s) Of Engagement Activity | 2017 |
Description | Presentation of research to secondary-school students visiting Jesus College, Cambridge |
Form Of Engagement Activity | Participation in an open day or visit at my research institution |
Part Of Official Scheme? | No |
Geographic Reach | National |
Primary Audience | Schools |
Results and Impact | Presentation of research to secondary-school students visiting Jesus College, Cambridge |
Year(s) Of Engagement Activity | 2019 |