Tracing pollution of the mantle with isotopically anomalous Mo and U
Lead Research Organisation:
University of Bristol
Department Name: Earth Sciences
Abstract
Over the 4.5 billion year course of Earth History most of the silicate portion of the Earth (i.e. all the solid planet except the core) has likely been processed through the plate tectonic cycle at the Earth's surface. While at the Earth's surface, alteration by seawater imparts a distinctive chemical signature to the top of the plate before it is returned by subduction back to the mantle. Tracking such chemical traces of old plates is an important task. If we can identify these signatures in material erupted again at the surface it gives us a means to understand how the mantle has been stirred. Whilst there are improving models of mantle convection, it is a complex problem and needs to be calibrated with observations. A major question remains as to how well the upper and lower portions of the mantle are mixed. It used to be thought that they were isolated from one another. Recent work from seismology has made this picture untenable, but it is nevertheless unsure as to just how much communication there is between top and bottom. We are attempting to solve this problem by looking at two elements (Mo and U) that are returned to the mantle with a distinctive isotopic label. They are also 'recycled' in sufficient abundance to be able to result in a different isotopic signature in the upper and lower mantle, if they are not well mixed . We can effectively sample upper and lower mantle using melts that come from ocean ridges and oceanic island respectively. If the whole mantle is well mixed they will have identical isotopic signatures of Mo and U, if not we predict small but significant differences. This work is new in that it has only recently become possible to measure the isotopic ratios of suitable elements for this problem with high enough accuracy. Our initial work suggests that distinct differences in Mo and U between upper and lower mantle are present, but these results need to be investigated in much greater detail to have full confidence in their significance and implications. This is the work we propose.
Organisations
People |
ORCID iD |
Timothy Elliott (Principal Investigator) |
Publications
Willbold M
(2015)
High-Precision Mass-Dependent Molybdenum Isotope Variations in Magmatic Rocks Determined by Double-Spike MC-ICP-MS
in Geostandards and Geoanalytical Research
Willbold M
(2017)
Molybdenum isotope variations in magmatic rocks
in Chemical Geology
Willbold M
(2016)
High-Precision Mass-Dependent Molybdenum Isotope Variations in Magmatic Rocks Determined by Double-Spike MC-ICP-MS
in Geostandards and Geoanalytical Research
Skora S
(2017)
An experimental study of the behaviour of cerium/molybdenum ratios during subduction: Implications for tracing the slab component in the Lesser Antilles and Mariana Arc
in Geochimica et Cosmochimica Acta
Hin R
(2019)
Molybdenum isotope fractionation between Mo4+ and Mo6+ in silicate liquid and metallic Mo
in Chemical Geology
Hin R
(2022)
The influence of crustal recycling on the molybdenum isotope composition of the Earth's mantle
in Earth and Planetary Science Letters
Freymuth H
(2016)
Tracing subducted black shales in the Lesser Antilles arc using molybdenum isotope ratios
in Geology
Freymuth H
(2019)
Uranium isotope fractionation during slab dehydration beneath the Izu arc
in Earth and Planetary Science Letters
Freymuth H
(2015)
Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc
in Earth and Planetary Science Letters
Freymuth H
(2016)
Thorium isotope evidence for melting of the mafic oceanic crust beneath the Izu arc
in Geochimica et Cosmochimica Acta
Description | We have discovered that the upper mantle (the outermost third of the Earth's interior) has a U isotope ratio notably different (some 50 parts per million higher in 238U/235U) from that of material derived from deeper portion of the Earth. The latter have compositions akin to meteorites that we believe make a good analogue for the Earth as a whole. The composition of the upper part of the mantle has been shifted by its interaction with the surface environment, through subduction of tectonic plates. Moreover, we believe the isotopic signature brought from surface to upper mantle has only been produced since surface conditions changed in response to the last increase in atmospheric oxygen at ~600 million years ago. This then sets a timescale for the mixing of this surface derived material into the upper mantle. Finding these differences in U isotopic composition of the Earth's interior is in itself remarkable, the isotope ratio of U having long been assumed to be constant. We have also documented widespread variability in the Mo isotopic composition of the mantle. The variations in Mo isotopic ratios of mantle derived materials are larger than U (some 300 parts per million) but show less systematic variations). A notable finding, however, is that nearly all samples are isotopically lighter than expected from a model of the Earth based on meteorite compositions. |
Exploitation Route | There are many exciting ways of moving this research forward. Firstly it will be interesting to examine further detail in the variability of the U isotopic signature to try to map out the pattern of mantle mixing. Finding an explanation for the 'non-chondritc' Mo isotopic composition of the Earth is an important goal. It is not apparently compatible with core formation. The role of sulphide needs to be investigated, as too does a better quantification of net recycling processes through refining the composition of the continental crust. |
Sectors | Aerospace, Defence and Marine,Energy,Environment |
URL | http://vimeo.com/30501446 |
Description | The findings from this project have formed the basis of a number of public and academic presentations (keynote lectures, departmental seminars). We anticipate some further impact from the publication shortly to appear in Nature. The ability to make such precise U measurements also has potential in nuclear forensics. We have made some initial exploration of this possible use. |
First Year Of Impact | 2014 |
Sector | Education,Environment |
Impact Types | Cultural |
Description | Tracing mantle evolution with novel isotopic systems |
Form Of Engagement Activity | A talk or presentation |
Part Of Official Scheme? | Yes |
Geographic Reach | International |
Primary Audience | Participants in your research or patient groups |
Results and Impact | Talk given as part of European Association of Geochemistry Distinguished Lecturer Tour of Eastern Europe. This talk given in Warsaw (Poland) and Cluj (Romania) This was the second of two talks I presented in my Distinguished Lecturer tour of Eastern Europe, based on work done during this grant. This porgramme, run by the European Association of Geochemistry, is designed to disseminated modern geochemical resear |
Year(s) Of Engagement Activity | 2012 |