Volatile trapping by the freezing front hygrostat in the Hadean mantle

Abstract:

The Earth's deep interior contains significant reservoirs of volatiles such as H, C, and N. Due to the incompatible nature of these volatile species, it has been difficult to reconcile their storage in the residual mantle immediately following crystallization of the terrestrial magma ocean. As the magma ocean freezes, it is commonly assumed, very small amounts of melt is retained in the residual mantle, limiting the trapped volatile concentration in the primordial mantle. In this presentation, we show that inefficient melt drainage out of the freezing front can retain large amounts of volatiles in the residual mantle while creating a thick early atmosphere. Using a two-phase flow model, we demonstrate that compaction within the moving freezing front is inefficient over time scales characteristic of magma ocean solidification. We employ a scaling relation between the trapped melt fraction, the rate of compaction, and the rate of freezing in our magma ocean evolution model. For cosmochemically plausible fractions of volatiles delivered during the later stages of accretion, our calculations suggest that up to 76% of total H2O and 11% of CO2 could have been trapped in the mantle during magma ocean crystallization. The assumption of a constant trapped melt fraction underestimates the mass of volatiles in the residual mantle by up to a factor of 15. Further, owing to particularly rapid magma ocean crystallization rates, the deepest mantle and the transition zone likely accumulate excess volatiles.