Mantle Degassing and Origin of the Atmosphere

This paper first reviews noble gas isotopic data and mantle degassing models. The different models for mantle degassing and origin of the atmosphere are then critically evaluated. All noble gas constraints are presented in a model-independent form so that different models can be examined. Common features and differences of most degassing models are discussed. He-Ne systematics are shown to be more consistent with the notion that Ne degassed more rapidly relative to He. That is, the systematics are more consistent with a solubility-controlled mantle degassing model. However, due to relatively large uncertainties in the isotopic and concentration ratios of various reservoirs, the systematics do not rule out other degassing models at present. Whether the atmosphere originated entirely from mantle degassing is more complicated. Radiogenic nuclides of noble gases (⁴⁰Ar and ²¹Ne^*) almost certainly originated from mantle degassing. Nonradiogenic nuclides of noble gases in the atmosphere may be entirely from mantle degassing if Loihi mantle is 71% degassed. They cannot be entirely from mantle degassing if Loihi mantle is truly undegassed. Fractionation of nonradiogenic Xe isotopes between air/mantle Xe and U-Xe most likely occurred before Xe accreted on earth.