Mantle Degassing and Origin of the Atmosphere

摘要

This pnper first reviews noble gas isotoplc 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 Nc degassed more rapidly relative to Me. 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 ("'Ar and 21Nc') nlmost 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. Fracllonation of nonradiogenic Xe isotopes between air/mantle Xe nnd U-Xe most likely occurred before Xc accreted on earth.