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.