Understanding noble gases in the context of mantle dynamics

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.

This project is focussed on fundamental research that will transform our understanding of the evolution of Earth's interior and its manifestation at Earth's surface. The combined geophysical, geochemical and geological expertise across complimentary work packages means that we will generate benefits spanning a range of topics. We identify five key arenas of societal and economic impact.

Short-term beneficiaries (1-5 years and beyond):

[1] School children, teachers, anyone with an interest in science: Geosciences (including Earth surface topics and plate tectonics) is taught as part of UK school curricula from early years to school leavers (Key Stages 1-3). Earth's interior and related surface phenomena are also popular topics during outreach events.

[2] Exploration industries: The exploration industry (e.g. hydrocarbon, minerals) makes use of geological histories and seismology to understand and predict distribution of natural resources (see Letters of Support, LoS).

[3] Infrastructure and sub-surface imaging: Academia and industry have made recent advances in seismic imaging (e.g. full waveform tomography) and in using derivatives to understand dynamical processes (see LoS).

Longer-term beneficiaries:

[4] Sea level and climate change: The development of reliable glacio-eustatic and climatic baselines requires information about how the Earth's interior has evolved on a range of timescales.

[5] Natural Hazards: Most natural hazards (e.g. volcanoes, earthquakes) are ultimately linked to deep-Earth processes.

How might they benefit from this research?

[1] Enhancing the quality of geosciences in school curricula can be achieved by introducing new materials and activities to spark interest of school children and teachers. A focus will be on developing and disseminating 3D printed globes depicting Earth's interior. Project partners, including UCL's GeoBus, reach hundreds of school children every year and will distribute these products widely. We will also build on successful outreach programmes at the universities involved in this consortium to deliver interactive workshops in outreach events.

[2] Crucial constraints for explorationists include histories of uplift, subsidence, basal heat-flow and landscapes evolution. Our project will provide significant new insights into the history of lithospheric dynamic support, which is a fundamental concern at frontier settings and in re-evaluation of exploration targets. Most leading exploration support companies employ sequence stratigraphic frameworks that contain little/no information about vertical motions generated by sub-plate processes, which we will directly address in this project.

[3] The recent explosion in the use of microseismic data and seismic imaging to map the sub-surface will benefit from quantifying uncertainties in seismic imaging, which is a cornerstone of this project, and from the transfer of knowledge and skills developed to mine and model large volumes of data.

[4] Estimates of sea-level change tend to be based on simplifying assumptions about the history of lithospheric vertical motions and the viscosity structure of the mantle. This project will provide insight into histories of dynamic support, and modern and historical estimates of mantle viscosity, which can be used to improve models of glacio-eustasy. The evolution of Earth's surface also plays a crucial role in regulating and perturbing Earth's climate. The histories of dynamic support and mantle temperatures we will generate will provide important constraints for these rapidly evolving fields.

[5] Mantle and lithospheric temperatures and stresses play crucial roles in determining the distribution of natural hazards (e.g. volcanism, seismicity). This project will generate new insights into the thermal and temporal evolution of Earth's convecting interior, which will provide insights into the evolution and origins on natural hazards.