Primordial-material preservation and Earth lower mantle structure: the influence of recycled oceanic crust

Abstract. The compositional structure of the Earth's lower mantle holds the key to understand the evolution of the coupled interior-atmosphere system, but remains elusive. Geochemical observations point to long-term preservation of primordial materials somewhere in the lower mantle, but the relationship of these reservoirs to geophysical anomalies is still debated. It has been shown that bridgmanitic material formed during magma-ocean crystallization can resist convective entrainment over geologic timescales to be preserved as “Bridgmanite-Enriched Ancient Mantle Structures” (BEAMS). BEAMS may host primordial geochemical reservoirs, but their style of preservation needs further testing. Using global-scale geodynamic models, we here explore how the physical properties of recycled oceanic crust (ROC) affect the style of primordial-material preservation. We show that significant BEAMS preservation is only obtained for ROC accumulation in the deep mantle as thermochemical piles, or a global ROC layer, due to high intrinsic ROC density. High intrinsic ROC viscosity also enhances BEAMS preservation, especially in the thermochemical piles regime. We find that primordial and recycled domains have a mutually protective effect. The coupled preservation of BEAMS-like structures in the mid-mantle and ROC piles in the lowermost mantle is consistent with the diverse isotopic record of ocean-island basalts, reconciling the preservation of distinct geochemical reservoirs in a vigorously convecting mantle.