Investigating the stability and composition of LLSVP-like material in mantle convection models

The Large Low Shear Velocity Provinces (LLSVPs) are basal mantle structures, located beneath the Pacific and Africa, which are defined by their negative anomaly in δV_s. Since the first detection of LLSVPs, the reason for their seismic signature has been questioned, whether it is purely thermal, chemical or thermo-chemical in nature. The origin and age of LLSVPs have also been interrogated in the context of mantle dynamics as plumes seem to be associated with these structures and correlate with intraplate volcanism locations. The LLSVPs are often invoked as a potential reservoir to store primitive mantle in order to explain primitive He ratios observed in oceanic island basalts. Such a scenario would suggest that at least some part of the LLSVPs are long-lived, quasi-stable structures. Previous 3D geodynamic experiments suggest that LLSVP longevity is achieved through replenishment of the constituent material [1], potentially disqualifying them as a reservoir of primordial material. However, 2D experiments have shown that remnants of a primordial layer may become trapped within accumulations of recycled, dense oceanic crust for extended periods of time [2]. It remains to be seen if a similar process may occur in 3D simulations.

Using the 3D spherical mantle convection code TERRA [3] and seismic conversion tables [1], we investigate the ability of geodynamic models to generate such seismic structures and the preservation of primordial material within them. We explore various mantle viscosities, densities of material (buoyancy of primitive and enriched material) and concentrations of heat-producing elements. We track the core-mantle boundary coverage and volume of the detected structures to evaluate their stability as a function of time and geodynamical context. Results focus on the composition of these structures, the amount of primitive and early enriched material stored within them and how they evolve with time.

[1]  James Panton, J. Huw Davies, and Robert Myhill. “The Stability of Dense Oceanic Crust Near the Core-Mantle Boundary”. In: Journal of Geophysical Research: Solid Earth 128.2 (2023).

[2]  T. D Jones, N Sime, and P. E van Keken. “Burying Earth’s Primitive Mantle in the Slab Graveyard”. In: Geochemistry, geophysics, geosystems : G3 22.3 (2021).

[3]  John R. Baumgardner. “A Three-Dimensional Finite Element Model for Mantle Convection”. PhD thesis (1983).