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Reconstructing the plumbing systems of mid-ocean ridge volcanoes: olivine vs plagioclase-hosted melt inclusions


Project Description

Project Background
Interconnected magma reservoirs are the key driver of the dynamics of volcanic systems around the world. Understanding their nature, therefore, is an important step towards understanding how volcanoes are supplied with magma, and, ultimately, how they erupt. Melt inclusions are key tools for reconstructing the vertical extent and architecture of magma plumbing systems. These inclusions (Fig. 1) contain dissolved CO2 and H2O, the concentrations of which enable a calculation of their entrapment pressure. Along with their major- and trace element composition, this enables a reconstruction of the depth distribution of magma chambers and the magma evolution occurring within them. Traditionally, olivine has been the mineral of choice for melt inclusion studies. However, in a recent study (Bennett el al., Nature, 2019), we have shown that, at volcanoes at the Gakkel mid-ocean ridge in the Arctic Ocean, plagioclase provides a significantly extended depth record relative to olivine (Fig. 2). This suggests that the melt inclusion data currently available for mid-ocean ridge volcanoes, all of which have been determined using olivine-hosted inclusions, significantly underestimate the extent of their magma plumbing system. This has far-reaching implications for our understanding of magma transport, evolution and crustal growth at mid-ocean ridges.

Project Aims and Methods
The recent discovery that Gakkel Ridge plagioclase-hosted melt inclusions record higher depths than those hosted in olivine raises two significant questions:
(i) What is the significance of the dichotomy in the depths recorded by the two minerals? An answer to this question is required to place the existing olivine-hosted melt inclusion data into context, and to determine ways of reconstructing the full depths of magma plumbing systems of mafic volcanoes.
(ii) What is the true extent of mid-ocean ridge plumbing systems? Do the existing olivine-hosted melt inclusion records underestimate the depth range of these systems?
The student will address these questions by undertaking a comparative study of olivine- and plagioclase-hosted melt inclusions in lavas from the Mid-Atlantic Ridge and East Pacific Rise. This will comprise a detailed petrological reconstruction of the crystallisations histories of the olivine and plagioclase phenocrysts and their melt inclusions, followed by a determination of melt inclusion compositions. These data will enable the student to determine the significance of the volatile record of both inclusion types, and lead to significant new constraints on the extent of mid-ocean ridge magma plumbing systems.

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 geochemical analytical techniques would be required.

Training
The student will receive an extensive training programme in petrology/geochemistry. This comprises a wide range of techniques, with a particular focus on in-situ microanalysis (SEM, EPMA, laser ablation ICP-MS, SIMS, Raman spectroscopy). Furthermore, the student will have the opportunity to join a research cruise to explore the Pacific ocean floor, thus acquiring key field skills. The student will also have access to the large range of DTP and Cardiff University Student Development courses, is expected to present project results at national and international conferences, and 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

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).

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

References

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.
Maclennan, J. (2017), Bubble formation and decrepitation control the CO2 content of olivine‐hosted melt inclusions, Geochemistry, Geophysics, Geosystems, 18(2), 597-616.
Wanless, V. D., and A. M. Shaw (2012), Lower crustal crystallization and melt evolution at mid-ocean ridges, Nature Geosci, 5(9), 651-655.
Wanless, V. D., and M. D. Behn (2017), Spreading rate‐dependent variations in crystallization along the global mid‐ocean ridge system, Geochemistry, Geophysics, Geosystems, 18(8), 3016-3033

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