Inefficient Loss of Moderately Volatile Elements From Exposed Planetesimal Magma Oceans

Abstract

Some melted and differentiated planetesimals, such as the parent bodies of angrites and howardite-eucrite-diogenite meteorites, are severely depleted in moderately volatile elements (MVEs). The origins of these depletions are critical for understanding early solar system evolution but remain topics of debate. Numerous previous studies have invoked evaporation from magma oceans as a potential mechanism for producing these depletions, yet this process is poorly explored. In this study, we examine the efficiency of MVE loss from planetesimal magma oceans. Upon heating from short-lived ${}^{26}Al$, internal magma oceans can develop beneath insulating crusts. The magma oceans may be exposed to the surface by collisional disruption of the crusts, but would be rapidly cooled by the cold environments. The exposed surface would be quenched to solid/glass; even if the quenched skin can be recycled by convection such that the magma ocean can be continuously resurfaced, only a small portion of the surface can remain molten. In the convection boundary layer, “vertical” advection is suppressed, energy and element transports toward the surface occur via thermal and chemical diffusion (if MVEs do not exsolve as bubbles). As chemical diffusivity is much smaller than thermal diffusivity, MVE transport is much less efficient than heat transport, and MVE loss during magma ocean cooling is likely minimal ($\lesssim 1\%$ the total inventory). Therefore, MVE depletions may not be easily explained by evaporation from ${}^{26}Al$-heated planetesimal magma oceans.