Paradoxes, conundrums and gaps: ancient Pb and Os in refractory upper mantle

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

We live on a planet that comprises several different layers, each with very different chemical compositions and physical properties. Through our day-to-day experience we are familiar with the atmosphere and the outermost solid layer on which we live - the crust. However, these two layers comprise only 1% of the volume of the Earth; the remainder being the mantle and the core, both of which are effectively inaccessible to us.
The mantle is the largest of Earth's layers and accounts for more than two thirds of the Earth's volume. It is therefore very important in terms of our understanding of how elements are distributed within the Earth. However, the composition of the mantle has changed over time - the crust was formed by the melting and extraction of part of the mantle, with this melt erupted at the surface of the Earth as lava. So, the removal of the lava changed the chemical composition of the mantle. Over the last 50 years it has been assumed that the composition of the usually inaccessible mantle can be determined by looking at the abundance and ratios of isotopes of certain elements, e.g. neodymium (Nd), strontium (Sr), lead (Pb) and osmium (Os) in the crust. Although the crust and mantle have different chemical compositions it is assumed that the relative proportions of the isotopes of these elements in the mantle are faithfully transferred from the mantle to the crust when the crust is formed. This assumption has underpinned our understanding of mantle composition.
However, recent information derived from rare samples of mantle rocks, combined with increasingly sensitive means of analysing small differences in chemical and isotopic compositions, means that it is becoming increasingly clear that the assumptions made so far about the shared isotopic signatures of the crust and the mantle are not entirely correct. Some elements, for example lead (Pb) and osmium (Os) have ranges of isotope ratios in the mantle which are not observed in the crust. Similarly, there are portions of the crust that have highly variable Pb and Os isotope ratios over relatively short distances - something that up until now has been difficult for geochemists to account for.
Over the past few years, my research and that of my colleagues has involved looking at isotopes of Pb and Os in mantle samples and working out how these elements move around during melting, i.e. how they may be transferred from the mantle to the crust. So far we have found that these elements can be found predominantly in small grains of sulphide within the mantle. Also, there are different types of sulphide that have different isotopic signatures for Os and Pb. What we suspect is that the behaviour of these different types of sulphide will provide the reason for the difference in Pb and Os isotope ratios between the crust and the mantle. During the early stages of crust generation one type of sulphide, with a distinctive isotopic signature, becomes incorporated into the melt, which will be erupted at the surface, much more than the other type of sulphide which is left behind in the mantle. Under these circumstances, the melt erupted at the surface will have a different Pb and Os isotope composition than the mantle that remains. However, although we have some data on Os isotopes that would appear to support this theory (i) we have not made very many measurements and (ii) we have only recently discovered that Pb and Os appear to move in very similar ways so we would like to investigate the implications of these recent, albeit preliminary findings.
This is particularly important not just because for the past half-century the distribution and behaviour of Pb in the Earth has been imperfectly understood but also because a better understanding of (i) how material is transferred from the mantle to the crust and (ii) over what sorts of scales it varies may radically alter the assumptions that we have made about the Earth's structure until now.

Planned Impact

The nature of this project means that the greatest impact will be within the geosciences academic community. This is mainly because the subject matter concerns issues of mantle composition and heterogeneity that, despite nearly half a century of research still remains equivocal. The opportunities for impact for geoscientists are significant: (i) a solution to the long outstanding problem of the Pb paradox and more recent, but related, Os isotopic gap and (ii) the implications for a different understanding of how partial melt in the mantle is generated. Both of these areas potentially involve fundamental changes to how geoscientists think about the composition of the mantle and how it imparts an isotopic fingerprint onto the basalt that it produces. These breakthroughs in our understanding will be communicated via conference attendance and through peer-reviewed journal articles (as detailed in the attached pathways to impact document).

That is not to say that this subject area is inaccessible to a more general audience. However, unlike climate change and the scarcity of natural resources, some areas of scientific research which address fundamental, Earth-system science, planetary-scale processes can often be perceived as not being particularly relevant to the general public and prone to be regarded as deriving little impact. In instances such as this it is even more important to ensure that results from research related to these areas is communicated in as wide a range of methods, to reach the greatest number of people and hence have the greatest impact possible. For the general public to derive a societal benefit from the results of this project requires imagination and ingenuity on my part, not least because the subject matter is not at the forefront of mainstream news or culture. The societal impact of this project will be directly related to the number of people that are exposed to my research and in the "Pathways to Impact" attachment to this document I have outlined the various means in which my research can be disseminated amongst the scientifically-curious general public.

In summary though, I am by no means unambitious regarding the degree of impact that I can achieve with non-academic beneficiaries. With the range of methods available for engaging the public, outlined in my "Pathways to Impact" document, there is enormous potential for generating interest in a wider audience and therefore optimizing the impact of my research. The greatest societal impact will come from providing the public with a better understanding of the planet on which they live through popular science articles related to my research. The periodical "New Scientist" has a weekly circulation of >135,000, providing a tremendous platform for disseminating information about the significance of my research to a wider audience. I will endeavour to publish at least two articles in New Scientist that deal with the broader issues connected with the transfer of material between the mantle and the crust. The broadsheet Sunday newspapers (Times, Observer, Independent, Telegraph) each have a science section and a combined weekly circulation in excess of 2 million copies (not including online subscriptions and users of not-paid-for content). In collaboration with an excellent university press office at the University of Leeds, and combined with NERC's own efforts to publicize the research they fund, a significant proportion of the public could be exposed to aspects of my research.

It is my intention to publicize any major breakthrough that my research reveals in our understanding of planetary-scale processes as vigorously in the popular science press and broadcast media as I will through conferences and peer-reviewed journal articles. This will ensure that my research is exposed to the widest possible audience and therefore maximizing the societal impact through a better understanding of our planet.
 
Description I have gain a valuable insight into how metals associated with sulphides in the Earth's mantle are distributed amongst sulphides of varying composition. I have also ascertained what happens to the those sulphides (and the metals that they contain) when the host portion of the mantle melts to form magma, which may ultimately be transported to the Earth's surface
Exploitation Route The distribution and behaviour of sulphides in the mantle can provide insights into how the elements that they carry become concentrated in economically viable ore deposits
Sectors Education,Other

 
Description Europlanet 2020
Amount € 18,750 (EUR)
Funding ID 16-EPN2-009 
Organisation Europlanet 
Sector Charity/Non Profit
Country France
Start 06/2017 
End 06/2017
 
Description MIneralogical Society of Great Britain and Ireland - senior bursary
Amount £500 (GBP)
Organisation The Mineralogical Society of Great Britain and Ireland 
Sector Charity/Non Profit
Country United Kingdom
Start 03/2014 
End 08/2014
 
Description Blaustein visiting professorship - Stanford University 
Organisation Stanford University
Country United States 
Sector Academic/University 
PI Contribution The Blaustein program is funded entirely by Stanford University with the aim of promoting collaborations between their scientists and leading researchers in fields of interest. I had the opportunity to interact with Prof. Jessica Warren's research group and offer my expertise in the setting up of some of their analytical procedures
Collaborator Contribution I received a stipend towards my living and analytical costs, learnt how to make Pb isotope measurements on single sulphide grains by thermal ioonization mass specrtrometry (not currently possible in the UK) and had the opportunity to travel to other US institutions to investigate the possibility of further collaborations with Stanford and Leeds universities
Impact Alteration of mantle sulfides: the effects of oxidation and melt infiltration in a Kilbourne Hole harzburgite xenolith. Barrett, T., Harvey, J., Warren, J., Klein, F., Walshaw, R. AGU Fall Meeting (2013), San Francisco, USA. Detecting mantle heterogeneity at a grain scale with improvements in high precision neodymium isotope (NdO+) analysis. Honn, D., Harvey, J., Warren, J., Baxter, E.F. AGU Fall Meeting (2013), San Francisco, USA. The effects of oxidation and melt infiltration in a Kilbourne Hole harzburgite xenolith: Alteration of mantle sulfides. Harvey J., Warren, J., Klein, F., Walshaw, R., Barrett, T. Geochemistry Group of the Geological Society of London / Mineralogical Society of Great Britain and Ireland Research in Progress meeting (2014), Milton Keynes, UK. Grain scale Nd isotope heterogeneity in peridotite: implications for basalt petrogenesis and its genetic link with the mantle. Harvey, J., Honn, D., Baxter, E.F., Warren, J.M., Hammond, S. J., Walshaw, R.D. AGU Fall Meeting (2014), San Francisco, USA.
Start Year 2013
 
Description visiting investigator 
Organisation Carnegie Institution for Science (CIS)
Country United States 
Sector Charity/Non Profit 
PI Contribution My host at DTM (Prof. Steven B. Shirey) are co-authors on a book chapter a for Mineralogical Society of America volume.
Collaborator Contribution My project partners offered me visiting investigator status for the purposes of producing a book chapter
Impact output is pending for this collaboration
Start Year 2014