Details of Award
NERC Reference : NE/C510983/1
The fate of subducted crust in the Earth's mantle: constraints from Li isotopes.
Grant Award
- Principal Investigator:
- Dr T Elliott, University of Bristol, Earth Sciences
- Co-Investigator:
- Professor S A Kasemann, University of Bremen, Geosciences
- Grant held at:
- University of Bristol, Earth Sciences
- Science Area:
- Earth
- Overall Classification:
- Earth
- ENRIs:
- Global Change
- Science Topics:
- Planetary science
- Mantle & Core Processes
- Abstract:
- The plates that cover the surface of the planet are returned or 'recycled' to the Earth's interior (mantle) by subduction. The upper part of the plates (crust) are chemically distinct from the mantle and can potentially be used to track what happens to the plates once they are subducted. Currently there is considerable disagreement as to the basic manner in which the mantle moves (convects). Tracking the fate of the recycled crust would allow us to resolve some of these fundamental arguments by providing a record of movement in the Earth's interior. This has been a long held goal of geological sciences but has remained elusive. Although chemically distinct, it is still difficult to distinguish clearly the chemical 'signature' of recycled crust from other possibilities. A new method, using Li isotope ratios, can circumvent many of the previous problems. Exciting new data on mantle derived melts suggest that the expected signature of recycled crust cannot be detected. This is contrary to many models of the Earth's interior and suggests that subducted crust ponds inaccessibly at the bottom of the mantle. As with most new approaches, there remain some outstanding questions that need to be answered before the data can be unequivocally interpreted. The aim of this work is to comprehensively address these remaining uncertainties to enable the new approach to be used with confidence. Most importantly we need to thoroughly document how the Li isotope ratios are changed (fractionated) during the process of subduction. Initial data suggests that the Li isotope ratios become significantly lighter (lower 7Li/6Li). This is key to the inference that Li isotope variations seen in the mantle are not the result of the subducted crust itself. A more complete assessment of the behaviour of Li during subduction will allow this major implication to be substantiated. We will achieve this by measuring Li isotopes on a wide range of samples believed to come from different stages of subduction (and then fortuitously exposed on the Earth's surface by mountain building events). It is also vital to be certain that Li isotope variations are from the mantle itself and not a result of contamination of mantle derived material on the way to the surface. Although initial indications suggest this is not the case, this proposal will rigorously assess this possibility. We will measure the elements B and Cl in samples that show extreme Li isotope compositions. Possible contaminants that could influence the Li isotope ratios will also change B and Cl concentrations. However, B and Cl are effectively lost from the subducted plate during subduction, and so mantle derived Li isotopic variations should not be associated with changing B and Cl concentrations. This provides an effective test to resolve mantle signatures from contamination. Finally it is possible that in some scenarios a process of diffusion could produce variations in Li isotopes in the mantle, unrelated to recycled material. We will develop an additional procedure (measuring Sr isotope ratios) to directly measure this effect in samples which show large Li isotope variations. These three themes exhaustively test the potential of Li isotopes to track subducted crust. This will then permit us to fully interpret the significance of the variations of Li isotopes in mantle derived materials and ultimately determine the mechanics of the Earth's interior.
- NERC Reference:
- NE/C510983/1
- Grant Stage:
- Completed
- Scheme:
- Standard Grants Pre FEC
- Grant Status:
- Closed
- Programme:
- Standard Grant
This grant award has a total value of £224,723
FDAB - Financial Details (Award breakdown by headings)
| Total - T&S | Total - Staff | Total - Other Costs | Total - Indirect Costs |
|---|---|---|---|
| £10,066 | £105,908 | £60,034 | £48,718 |
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