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 From Hastie and van Hunen, unpublished.