Details of Award
NERC Reference : NE/J017981/1
Paradoxes, conundrums and gaps: ancient Pb and Os in refractory upper mantle
Fellowship Award
- Fellow:
- Dr J Harvey, University of Leeds, School of Earth and Environment
- Grant held at:
- University of Leeds, School of Earth and Environment
- Science Area:
- Earth
- Overall Classification:
- Earth
- ENRIs:
- Global Change
- Science Topics:
- Mantle & Core Processes
- Properties Of Earth Materials
- Volcanic Processes
- 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.
- NERC Reference:
- NE/J017981/1
- Grant Stage:
- Completed
- Scheme:
- Advanced Fellow (FEC)
- Grant Status:
- Closed
- Programme:
- Advanced Fellow
This fellowship award has a total value of £586,063
FDAB - Financial Details (Award breakdown by headings)
| DI - Other Costs | Indirect - Indirect Costs | DI - Staff | DA - Estate Costs | DA - Other Directly Allocated | DI - T&S |
|---|---|---|---|---|---|
| £116,429 | £149,659 | £211,232 | £65,329 | £4,118 | £39,293 |
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