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Understanding noble gases in the context of mantle dynamics


Cardiff School of Earth and Environmental Sciences

About the Project

Project Background:
Mantle convection drives critical processes such as seismicity, volcanoes and mountain building. Noble gases provide key constraints on this poorly understood, but fundamental, mantle dynamics. Models have struggled to fully incorporate these constraints.

These constraints include:
- Ocean Island Basalts have higher 3He/4He ratios than mid-ocean ridge basalts, and so must be supplied by a reservoir with much higher ratios of primordial 3He to U+Th. This is generally interpreted to be due to retention of high concentrations of primordial gases (Porcelli and Elliott, 2008).
- The amount of 40Ar in the atmosphere and upper mantle appears to account for only half of the total produced by 40K, requiring another, deep storage reservoir for Ar.
- Variations in the ratios of Xe isotopes produced by short-lived nuclides only present during early Earth history require early separation of noble gas reservoirs.

Establishing the necessary reservoirs for noble gases within the convecting mantle has been difficult, and the nature and location of these reservoirs has been hotly debated. Possibilities include domains within the convecting mantle, convective isolation of mantle layers, a separate mantle reservoir at the Core Mantle Boundary (CMB), and the core. This studentship will engage in using the convecting mantle models developed in the MC^2 NERC Large Grant project to explore how the noble gas observational data can be successfully explained.
The approach is to expand numerical mantle circulation models (MCM) to include the tracking of noble gases (van Heck et al., 2016). Like other chemical and isotopic characteristics, the noble gases will be tracked on particles that are advected by the flow. The model is capable of partitioning the noble gases into melts and outgassing them to external reservoirs, they can also be input from the core. The student will be able to adjust and investigate the result of all such choices. Tomography identifies two large low shear wave velocity provinces (LLSVP) above the CMB. They are hypothesized to be poorly resolved thermal upwellings, or a primordial layer left over from earliest Earth history, or a graveyard of subducted lithosphere, or a combination thereof. The LLSVP could be the separate reservoir above the CMB mentioned above that can help to reconcile the enigmatic noble gas observations. The MCMs can address all these hypotheses.

In addition, the studentship will take the novel approach of explicitly supplementing these expensive dynamical models with analytical models. These analytical models will allow the student to interrogate a wider range of possible implications of the convection models for the noble gases.

Tasks:
- Incorporate He, Ar, and Xe into numerical runs to determine to what extent the observational constraints can be satisfied within the convecting mantle, and how this can be optimized with different CMB properties.
- Use analytical models to determine how noble gases will evolve in the CMB layer generated by the numerical models.
- Evaluate the role and rates of inputs from the core, linking possible inputs of noble gases with those of metals where evidence for core inputs to Pt group elements has been growing (W and Os isotopes).
- Assess the feasibility of retaining possibly gas-rich early material from early in Earth history within this layer.
Candidate Requirements: The candidate would ideally have a background either in the methods of modelling or Earth sciences. The student would also need to have a desire to develop numerical modelling skills and relish the prospect of interacting with high performance computing, and multiple disciplines (geodynamics, geochemistry). The student could therefore come from a broad range of discipline backgrounds, including for example geophysics, mathematics, physics, engineering, geology, chemistry or computing.
Training : The student will join the Cardiff mantle geodynamics research group and learn about the fields of mantle dynamics and geochemistry. This project will provide ample opportunities to develop high-level skills in computing, programming, numerical modelling and high-performance computing, e.g. using the National Supercomputer ARCHER 2. The student will also have access to a very wide range of University and NERC GW4 DTP provisioned courses which will further enhance research and transferable skills. The student would be expected to attend at least one international conference, e.g. AGU, San Francisco; and visit expert colleagues associated with the linked MC2 NERC Large Grant project.



Funding Notes

The studentship is open to UK and Irish nationals. International and EU students are also eligible to apply but should note that they may have to pay the difference between the home UKRI fee (View Website)) and the institutional International student fee (View Website).


References

This studentship is funded as part of the NERC Large Grant “Mantle Circulation Constrained (MC^2): A multidisciplinary 4D Earth framework for understanding mantle upwellings”. It is fully funded for 3.75 years with Research Council stipend, for 2021/22 this will be at least £15,285 pa. It is available to students from anywhere in the world. The studentship though funds tuition fees at the rate for UK students only, therefore non-UK applicants would need to find other funding sources to cover the extra needed for the International tuition fees. The studentship will be affiliated with the NERC GW4 Doctoral Training Programme. There are also funds to cover project and training costs, including presenting at International Conference (e.g. AGU, San Francisco), Project related workshops, e.g. CIDER at Santa Barbara, California and visits to Consortium project partners.

Barry, T, Davies JH, et al. Whole-mantle convection with tectonics plates preserves long-term global patterns of upper mantle geochemistry, Scientific Reports, 7, 1870, 2017.
Porcelli, D. and T Elliott, The evolution of He isotopes in the convecting mantle and the preservation of high 3H/4He ratios, Earth and Planetary Science Letters, 269, 175-185, 2008.
van Heck, HJ, JH Davies, T Elliott, and D Porcelli, Global scale modelling of melting and isotopic evolution of Earth’s mantle, Geosci. Model Dev., 9, 1399-1411, doi:10.5194/gmd-9-1399-2016, 2016.

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