The Origin of Volatiles in the Earth ’ s Mantle

10 The Earth’s deep interior contains significant reservoirs of volatiles such as H, C, and N. Due 11 to the incompatible nature of these volatile species, it has been difficult to reconcile their 12 storage in the residual mantle immediately following crystallization of the terrestrial magma 13 ocean (MO). As the magma ocean freezes, it is commonly assumed, very small amounts 14 of melt is retained in the residual mantle, limiting the trapped volatile concentration in the 15 primordial mantle. In this article, we show that inefficient melt drainage out of the freezing 16 front can retain large amounts of volatiles hosted in the trapped melt in the residual mantle 17 while creating a thick early atmosphere. Using a two-phase flow model, we demonstrate that 18 compaction within the moving freezing front is inefficient over time scales characteristic of 19 magma ocean solidification. We employ a scaling relation between the trapped melt fraction, 20 the rate of compaction, and the rate of freezing in our magma ocean evolution model. For 21 cosmochemically plausible fractions of volatiles delivered during the later stages of accre22 tion, our calculations suggest that up to 77% of total H2O and 12% of CO2 could have been 23 trapped in the mantle during magma ocean crystallization. The assumption of a constant 24 trapped melt fraction underestimates the mass of volatiles in the residual mantle by more 25 than an order of magnitude. 26