Routes for Roots: Do continental keels find their way into mantle plume sources?

Overview

Geodynamic modelling and, more tentatively, seismic evidence suggest that most hotspots are the result of mantle plumes that are thought to predominantly origin at the core-mantle boundary (CMB). Each hotspot produces OIBs with their own chemical flavour, which are clearly distinct from Mid-Ocean Ridge Basalts (MORB). The leading paradigm is that (ancient) oceanic crust and its sedimentary package subduct to form a compositionally distinct stable layer at the CMB, and that this layer is feeding mantle plumes that deliver material to the source region of OIBs. However, this theory is not without its problems, and, for decades, many studies have suggested that some of the OIB source rocks might have a more direct, shallow origin. In particular, it has been proposed that mixing of CMB plume material with continental subcontinental mantle lithosphere or lower crust in the shallow upper mantle may occur (Workman et al., 2004; O’Reilly et al., 2009; Konter and Becker, 2012, and references therein).

This project will quantify the mechanisms by which continental material is incorporated in the source of OIBs. We will develop geodynamical flow models of potential OIB source material, and test those models against observations from geochemical analyses and seismic imaging. We hypothesize that eroded continental keel material will feed the OIB source region through asthenospheric flow. Results will profoundly improve our knowledge about the nature, extent and timescales for plate tectonic cycling of material.

The target area is the South Atlantic. Ocean island basalts from the intraplate volcanic islands of Tristan de Cuhna and Gough Island, have a source rock component for which a shallow continental origin has been proposed (O’Reilly et al., 2009). The Tristan mantle plume dynamics is well-studied (e.g. Gassmaller et al., 2016), and International Ocean Discovery Program (IODP) Expedition 391 (Dec. 2021 – Feb 2022) will drill and recover igneous material from the Walvis Ridge in the South Atlantic. Co-supervisor Julie Prytulak is part of the seagoing science party for the expedition, and will have direct access to the samples and data, which will provide a very significant benefit for this project. The South Atlantic has also been extensively studied using geophysical techniques.

Click on an image to expand

Image Captions

Preliminary modelling results for continental lithospheric erosion. From Hastie and van Hunen, unpublished.

Methodology

There are a number of potential end member pathway scenarios for continental material to move from its lower crust or mantle lithosphere origin to the OIB source region:
1. Pelagic sediments on the ocean floor subduct to the CMB, and ultimately arrive at the OIB in a mantle plume;
2. Continental mantle lithosphere delaminates (i.e. erodes from the base of the lithosphere), travels to the CMB, and then travels to the IOB in a mantle plume;
3. Eroded or dislocated lithospheric root material travels to the OIB directly through the upper mantle.
Method 1 is the presently most accepted model, and, from a geodynamical point of view, the most straightforward mechanism. This project will focus on the much less well studied Methods 2 and 3. Delamination of continental roots can take a number of forms, including during continental breakup (O’Reilly et al., 2009), through dislocation of the entire mantle lithosphere below the Mid-Continental Discontinuity (Wang et al., 2017), or via small-scale or edge-driven convection (Kaislaniemi and van Hunen, 2014). We will investigate these scenarios using the the community-supported code ASPECT (https://aspect.geodynamics.org).

Project Timeline

Year 1

Literature review, including compilation of geochemical data, training in numerical modelling and the software package ASPECT, IAPETUS2 DTP training, 9-month progress report. Academic secondment to Glasgow University.

Year 2

Model development and testing; international academic secondment; 21-month progress report; preparation for publication of first key results in a peer-reviewed journal.

Year 3

Key stage in hypothesis testing; continuation of writing towards scientific publications; major international conference attendance.

Year 3.5

Finalizing further publications of research outcomes; thesis completion and submission.

Training
& Skills

The student will join a vibrant research culture in the department of Earth Sciences, in which ~70 postgraduate students work on a wide range of Earth Science research projects. The student will closely collaborate with academic staff, postdoctoral researchers and fellows, and postgraduate students in the geodynamics and geochemistry research groups.

A geodynamical background, affinity with code development, and an interest in geochemical processes are desirable. Training will be provided in geodynamical modelling (programming, code development, model setup, and usage), data management of high-performance computing systems, and interpretation and integration of key geochemical data and techniques, notably those collected during the IODP project 391. The project allows the student to become proficient in computer programming and large dataset analysis, with support from an enthusiastic ASPECT community. The code is open source with an importance placed on member participation in development (which is done in the open at https://github.com/geodynamics/aspect), allowing for worldwide collaboration and education (e.g., through Hackathons and public meetings). In addition, the student will receive training in general and transferable skills.

The student will have the opportunity to attend national and international conferences to disseminate research results and to spend time away from Durham to collaborate with some of the project partners at the partner institutes. Additionally, the student will be encouraged to participate in post-expedition IODP Exp.391 Science party meetings, which will be actively occurring through the duration of the PhD.

References & further reading

Gassmaller et al.(2016), Major influence of plume-ridge interaction, lithosphere thickness variations, and global mantle flow on hotspot volcanism. The example of Tristan, G3, 17, 1454-1479, https://doi.org/10.1002/2015GC006177
Kaislaniemi & van Hunen (2014). Dynamics of lithospheric thinning and mantle melting by edge-driven convection: Application to Moroccan Atlas mountains. G3 15: 3175-3189.
Konter & Becker (2012). Shallow lithospheric contribution to mantle plumes revealed by integrating seismic and geochemical data. G3, 13, Q02004. https://doi.org/10.1029/2011GC003923
O’Reilly et al. (2009). Ultradeep continental roots and their oceanic remnants: A solution to the geochemical “mantle reservoir” problem? Lithos, 112, 1043-1054.
Wang et al. (2017). Ancient continental lithosphere dislocated beneath ocean basins along the mid-lithosphere discontinuity: A hypothesis. GRL, 44, 9253-9260. https://doi.org/10.1002/2017GL074686
Workman et al. (2004). Recycled metasomatized lithosphere as the origin of the Enriched Mantle II (EM2) end-member: evidence from the Samoan Volcanic Chain. G3, 5 , 1-44, . https://doi.org/10.1029/2003GC000623
IODP Exp 391 Scientific Prospectus: https://iodp.tamu.edu/scienceops/expeditions/walvis_ridge_hotspot.html

Further Information

For any information on the project, the tectonics or geodynamics research groups, the department of Earth Sciences or, more generally, matters related to doing a PhD in Durham, please contact Jeroen van Hunen (jeroen.van-hunen@durham.ac.uk).

Apply Now