Crystallization of a basal magma ocean recorded by Helium and Neon

Interpretation of the noble gas isotopic signature in hotspots is still controversial. It suggests that relatively primitive material remains untapped in the deepest mantle, even while mantle convection and sub-surface melting efficiently erase primordial heterogeneities. A recent model suggests that significant differentiation and fractionation affects the deepest mantle following the formation of a dense basal magma ocean (BMO) right after core segregation (Labrosse et al., 2007). Here we explore the consequences of the crystallization of a BMO for the noble gas evolution of the mantle. The crystals extracted from a BMO upon cooling generate dense chemical piles at the base of the mantle. We show that if the solid–melt partition coefficients of He and Ne are > 0.01 at high pressure and temperature, He and Ne isotopic ratios in pile cumulates can be pristine like. Hence, the entrainment of modest amounts of BMO cumulate in mantle plumes (< 10%) potentially explains the primitive-like He and Ne signatures in hotspots. Because pile material can be depleted in refractory elements while simultaneously enriched in noble gasses, our model forms a viable hypothesis to explain the complex relationship between He and refractory isotopic systems in Earth's interior.

Research highlights
► We propose a model for the origin of primitive-like isotopic ratios in mantle plumes.
► We compute the evolution of noble gases for the crystallizing basal magma ocean (BMO).
► Cumulates from the BMO have primitive-like He and Ne ratios.
► Cumulate entrainment impacts the noble gases but not refractory elements.
► Our results depend on high pressure partitioning of He and Ne and refractory elements.