Model geometry determined contrast in the feedback between compositionally originating LLSVPs and dynamically generated plates

Nearly three decades of investigation has made steady progress towards self-consistently generating multiple features of plate tectonics from global mantle convection models.  Accordingly, the modelling of dynamic plates with migrating boundaries and evolving areas has become commonplace in both 2-D and 3-D geometries. Investigating the properties required for obtaining durable deep mantle formations similar to the Large Low Shear-wave Velocity Provinces (LLSVPs) has received similar attention. In this study, we model LLSVPs by assuming their composition is persistent  (i.e, we assume steady-state chemistry). To this end, we incorporate a Compositionally Anomalous Intrinsically Dense (CAID) mantle component comprising 2–3.5 per cent of the total mantle volume. We explore the impact of both an intrinsic contrast in density and viscosity for the CAID component, in an effort to stimulate the formation of a pair of LLSVP-like structures and a surface that exhibits the principle features of terrestrial plate tectonics; including recognizable and narrowly focused divergent, convergent and (in 3-D) transform plate boundaries that separate 8–16 distinct plate interiors. Although we find that a pair of CAID material provinces can be readily obtained in 2-D calculations while maintaining a surface exhibiting plate-like behaviour, specifying the same system parameters in 3-D calculations does not yield a pair of enduring provinces for any values of the parameters investigated.  In addition, CAID component inclusion in the calculations can affect global geotherms, so that in comparison to the surface behaviour obtained for the initial condition isochemical model, the cases incorporating the dense component do not yield surfaces that simulate plate tectonics. In general, CAID material components that are 3.75–5 percent denser than the surrounding mantle (at surface temperatures), and up to a factor of 100 times greater in intrinsic viscosity, form layers populated by voids, or nodes connected by ridges that reach across the core–mantle boundary (CMB), rather than distinct piles resembling the morphology of the LLSVPs. However, due to their temperature, we find the CAID material forms masses on the CMB that are relatively less dense (0.625–1.5 per cent) and viscous than the adjacent mantle material, in comparison to the percentage differences obtained at common temperatures. By adjusting our yield stress model to account for the influence of the CAID material on the geotherm, we find a highly satisfactory plate-like surface can be re-attained. Nevertheless, the formation of a pair of LLSVP-shaped masses remains elusive in 3-D calculations with plate-like surface behaviour and we suggest that caution is required if inferring the physical properties of the LLSVPs from 2-D models.