NOBLE-GASES AND EARTHS ACCRETION

Earth probably accreted in two stages corresponding to conditions befo re and after the removal of gas from the nebular disk. Helium and neon in the mantle primarily record a solar like component likely incorpor ated during the first stage by solution into a magma ocean blanketed u nder a massive protoatmosphere of molecular hydrogen and helium. The h eavier noble gases primarily reflect a meteoritic (planetary) componen t trapped in accreted planetesimals. This model provides a possible ex planation 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.