The principal source of information concerning the outgassing history of the atmosphere is the isotopic composition of helium, argon and xenon in the modern atmosphere and emerging from the upper mantle. Helium provides the best indication of current noble-gas outgassing rates which are seen to be extremely low in comparison to the mean rates over geological time. Extremely high (40Ar/36Ar) ratios in MORB (Mid-Ocean Ridge Basalt) indicate conclusively that the major release of primordial volatiles occurred before there had been significant growth of radiogenic 40Ar in the mantle, i.e. within the first 500 Ma or so of Earth history. High (129Xe/130Xe) ratios in MORB combined with the argon data can be used to indicate that 80% or more of the release occurred in the first 50 Ma of Earth history, though alternative interpretations are possible. The data can be used to constrain a narrow range of acceptable mathematical models. However, the major conclusion that outgassing occurred early in Earth history is inescapable and model independent. Further information on the detailed dependence of outgassing rates with time may come from a study of the isotopic composition of palaeoatmospheres.

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