Noble Gases and Earth's Accretion

Abstract

Earth probably accreted in two stages corresponding to conditions before and after the removal of gas from the nebular disk. Helium and neon in the mantle primarily record a solarlike component likely incorporated during the first stage by solution into a magma ocean blanketed under a massive protoatmosphere of molecular hydrogen and helium. The heavier noble gases primarily reflect a meteoritic (planetary) component trapped in accreted planetesimals. This model provides a possible explanation for the reduced and oxidized accretion stages inferred from siderophile elements and may explain why the present mantle appears to be too oxidized to be in equilibrium with the core.