IAP-24-126

How to peel a slab: Insights into slab weakening and slab-plume interactions in the Afar region

Systematic geochemical trends from the Red Sea Basalts show that helium concentrations in the Afar mantle plume are significantly lower than those of the upper mantle (Balci et al. 2024). This is suggests that the upwelling mantle is contaminated by oceanic crust subducted within the last 100 million years. A potential source for the younger slab material could be the Tethyan slab, currently sinking beneath southeastern Iran (Balci et al., 2024). However, lead isotope analyses indicate a considerably older subduction age (>300 million years). Current hypotheses struggle to reconcile the geochemical conundrum and provide a satisfactory geodynamic mechanism to supply the regional mantle transition zone with required amounts of basalt.

A possible plume contamination mechanism might be ‘slab peeling’, wherein some slabs through sufficient weakening at the top of the lower mantle, lose some of their basaltic and volatile components (e.g., van Keken et al., 1996, Tauzin et al., 2024; Yu et al., 2023). Evidence of long-lived subduction in the Afar region suggests the regional mantle transition zone is volatile-rich and potentially basaltic-enriched. As the Afar plume journeys towards the surface it can pick up these anomalous signatures from the mantle transition zone and the present-day subducting slab thus, explaining the low helium ratios observed at the surface (Balci et al., 2024, Figure 1). However, the mechanism by which slabs may peel off is unclear. Mineral physics and whole scale mantle convection models suggest that grain size dynamics and the presence of water may induce significant weakening at the top of the lower mantle (Dannberg et al., 2017, Mohiuddin et al., 2022, van Driel, 2022,). Furthermore, seismic tomography models suggest that the Afar plume might pond at bottom of the upper mantle before branching in the mantle transition zone (Chang et al., 2016). Plume ponding has the potential to warm up the mantle transition zone and the slab, inducing the latter to further weakening. The Afar deep-seated plume could also source geochemical signatures from the core-mantle boundary which might explain the unusual lead signatures observed at the Red Sea Basalts. This suggests that the anomalous Afar plume signatures at the surface are indicative of a complex slab-plume-mantle interaction at the top of the lower mantle, informed by a long history of subduction.

Project Objectives:

This project will use numerical modelling to explore the interactions between subducted slabs and mantle plumes, incorporating water and grain-size dynamics in both 2D and 3D subduction models. Key research questions include:

1. What is the influence of grain size dynamics and the presence of water on slab weakening, and peeling at the top of the lower mantle?
2. What is the role of plume pooling at the bottom of mantle transition zone in slab weakening and peeling?
3. Can the Afar geochemical signatures be explained by the entrainment of peeled-off slab material vs. deep seated entrainment from the core-mantle boundary?

The project will involve thermo-chemical-mechanical subduction models with evolving grain size using the state of the art, open-source, adaptive mesh refinement, finite element software ASPECT (Heister et al., Geophys. J. Int., 2017) with a free surface, spatially variable internal heating, and visco-plastic convection in 2-D and 3-D.

The project will involve the implementation and application of the following main components:

1. Grain size dynamics into subduction models
2. Coupling grain size evolution, water availability and water weakening rheologies with slab dynamics
3. Applying recent ASPECT implementations for fluid transport in subduction zones to the Afar region

Year 1

(1) Literature review,
(2) Familiarisation with code,
(3) Training in HPC techniques, modelling and data analysis
(4) Running of setup and boundary condition tests locally

Year 2

(1) Implementation and testing of grain size dynamics within a 2 and 3D subduction model
(2) Application of volatile dehydration and recycling to the Afar case study to investigate water weakening effects on slab peeling
(3) Author a publication
(4) Attend and present at an international workshop e.g., Ada Lovelace workshop on Mantle and Lithosphere dynamics.

Year 3

(1) Combine fluid and grain size dynamics in 2 and 3D subduction models to test various subduction parameters (e.g., slab age, subduction angle, crust thickness, water content)
(2) Model slab-plume interactions and compare this with geochemical evidence from the Afar plume
(3) Attend and present at an international conference e.g., AGU or EGU.
(4) Writing of further manuscript for publication

Year 3.5

Completion of manuscripts for publication and thesis writing

The student will receive training in writing, testing, running, processing and analysing numerical models. This will include essential skills like good programming practices (including version control), GitHub skills, code development and usage of high-performance computing systems (HPCs)

Training in a wider range of important skills (e.g., networking, presentation skills, paper/thesis writing) will be provided by the College of Science and Engineering and the Research and Innovation Services at the University of Glasgow, and the student will also benefit from cross-disciplinary training provided as part of the IAPETUS DTP.

The student will have opportunities to work with collaborators at the SUERC and the GEOMAR Helmholtz Centre for Ocean Research. The student will also have several opportunities to attend national and international scientific meetings to present research results and to participate in the annual ASPECT Hackathon where they can contribute to and develop new code. We aim to see all students publish at least two papers in leading scientific journals during their PhD. Upon completion, the student will be well equipped for a career in academia or in a range of industries.