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The Oxidation State of the Earth’s Mantle


Project Description

Project Background
The mantle acts as the tape recorder of Earth’s history, preserving a record of early continent formation and the subsequent plate tectonic recycling of crustal material. Hence, in order to reconstruct Earth’s dynamic history, it is of critical importance to determine the composition and compositional heterogeneity of the mantle. Recycled materials introduce volatile elements which oxidise the mantle. Therefore, one of the key ways of tracking the recycling process is to determine the oxidation state of the mantle. This project will target the depleted mantle: the mantle domain that is residual after continental crust extraction and now underlies the global mid-ocean ridge system. Conventionally, the oxidation state of the depleted mantle is measured through its melting product: mid-ocean ridge basalts (MORB). However, work from our group has recently shown that: (i) MORB is homogenised in the crust and is therefore not a good proxy for the compositional heterogeneity of the underlying mantle. Instead, this heterogeneity is preserved in crystals formed in lower crustal magma storage regions (Lambart et al., 2019, Nature Geoscience); (ii) melt inclusions, particularly those hosted in plagioclase, preserve samples of melt present in the deep part of these magma systems (Bennett et al. (2019, Nature). These melt inclusions should therefore enable a more rigorous determination of the oxidation state of the depleted mantle, as well as its variability, than has been possible hitherto.


Project Aims and Methods
In this project, the student will determine, for the first time, the oxidation state of the depleted mantle using the melt inclusion record. This will be achieved by measuring the oxidation state of Fe using a technique recently developed in Bristol (Hughes et al., 2018). Given that lower crustal minerals are far more sensitive recorders of mantle heterogeneity than MORB (Lambart et al., 2019), and that melt inclusions originate from the deep lower crust (Bennett et al., 2019), the melt inclusion record is hypothesised to contain an improved record of mantle oxidation state. The student will test this hypothesis. The results will have far-reaching implications for our understanding of mantle composition, its relationships to plate recycling, and magma genesis.

Candidate Requirements
This project will be of interest to students keen on the petrology and geochemistry of igneous systems. Analytical experience is not required, but an affinity for in-situ analytical techniques would be required.

CASE or Collaborative Partner

Training
The student will receive an extensive training programme in petrology/geochemistry. The project comprises a wide range of techniques, with a particular focus on in-situ microanalysis (SEM, EPMA, Ion microprobe). In addition to project-specific training and the DTP training courses, the student will have access to the large range of Cardiff University Student Development courses, to maximise transferable skills. The student is also expected to present project results to national and international conferences. Finally, the student will have the opportunity to demonstrate both in the classroom and in the field. Combined, the training package of the project will give the student an excellent basis for a career in academia or industry.

How to apply:

You should submit an application for postgraduate study via the Cardiff University Online Application Service, including:

an upload of your CV
a personal statement/covering letter
two references (applicants are recommended to have a third academic referee, if the two academic referees are within the same department/school)
current academic transcripts.

You should apply to the Doctor of Philosophy in Earth and Ocean Sciences with a start date of October 2020.

In the research proposal section of your application, please specify the project title and supervisors of this project and copy the project description in the text box provided. In the funding section, please select ’I will be applying for a scholarship/grant’ and specify that you are applying for advertised funding from NERC GW4+ DTP.

If you wish to apply for more than one project please email .

The deadline for applications is 16:00 on 6 January 2020.

Shortlisting for interview will be conducted by 31 January 2020.

Shortlisted candidates will then be invited to an institutional interview. Interviews will be held in Cardiff University between 10 February and 21 February 2020

Funding Notes

Residency:

UK Research Council eligibility conditions apply. Please contact us for further details

Full UK/EU tuition fees

Doctoral stipend matching UK Research Council National Minimum
Additional funding to the value £11,000 is available over the course of the programme for conference attendance, fieldwork allowance, travel allowance and other project costs. A further £3,250 is available in the form of as a training credits over the course of the programme for specialist training courses and/or opportunities (plus £750 ringfenced for travel and accommodation on compulsory cohort events).

References

References / Background reading list
• Bennett, E. N., F. E. Jenner, M.-A. Millet, K. V. Cashman, and C. J. Lissenberg (2019), Deep roots for mid-ocean-ridge volcanoes revealed by plagioclase-hosted melt inclusions, Nature, 572(7768), 235-239.
• Berry, A. J., G. A. Stewart, H. S. C. O'Neill, G. Mallmann, and J. F. W. Mosselmans (2018), A re-assessment of the oxidation state of iron in MORB glasses, Earth and Planetary Science Letters, 483, 114-123.
• Frost, D. J., and C. A. McCammon (2008), The redox state of Earth's mantle, Annu. Rev. Earth Planet. Sci., 36, 389-420.
• Hughes, E. C., B. Buse, S. L. Kearns, J. D. Blundy, G. Kilgour, H. M. Mader, R. A. Brooker, R. Balzer, R. E. Botcharnikov, and D. Di Genova (2018), High spatial resolution analysis of the iron oxidation state in silicate glasses using the electron probe, American Mineralogist: Journal of Earth and Planetary Materials, 103(9), 1473-1486.
• Lambart, S., J. M. Koornneef, M.-A. Millet, G. R. Davies, M. Cook, and C. J. Lissenberg (2019), Highly heterogeneous depleted mantle recorded in the lower oceanic crust, Nature Geoscience, 12(6), 482.

Related Subjects

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