Interaction Between Mantle Plumes, Subducted Oceanic Crust and Primordial Reservoirs at Earth's Lowermost Mantle

Trace element chemistry of ocean island basalts (OIBs) suggests that they are sourced from multiple compositional reservoirs, including a less-degassed primordial reservoir, recycled ocean crust, and depleted mantle. Geodynamic models have shown that dense primordial material could be swept into thermochemical piles, with mantle plumes forming from their tops. In addition, oceanic crust has been shown to subduct from the surface to the lowermost mantle. However, an important question centers on how these compositional components interact with each other and how mantle plume successively sample each of them. Furthermore, it is unclear why the recycled crustal component in OIBs shows large variable ages ranging from Archean to relatively recent. Here, we performed high resolution thermochemical calculations to explore the interaction between mantle plumes, subducted oceanic crust, and primordial reservoirs. We model the entire mantle system which include three compositional components: subducted oceanic crust, primordial material, and depleted mantle. We found that in the lowermost mantle, some subducted oceanic crust is entrained directly into mantle plumes, but a significant fraction of oceanic crust enters primordial reservoirs, some of which is later entrained into plumes with the primordial material. As a result, mantle plumes arising from the tops of primordial reservoirs contain a variable combination of primordial material, depleted mantle material, and subducted oceanic crust of varying age. Cycling of oceanic crust through mantle reservoirs can therefore explain the multiple compositional components of OIBs and reconcile the different recycled oceanic crustal ages observed in OIBs. In addition, oceanic crust that episodically enters the primordial reservoir also causes multi-scale compositional heterogeneity within the LLSVPs, which may explain seismic discontinuity and/or heterogeneity within LLSVPs.