Primordial atmosphere, outgassing

Figure 11. Evolution of terrestrial Xe, Kr, Ar and Ne from primordial atmospheric distributions to present-day compositions by Giant Impact (Gl)-driven fractionation, addition of outgassed (OG) solar Kr, Ar and Ne, a later stage of solar EUV-powered Ne escape fractionation, and subsequent outgassing of radiogenic and fissiogenic Xe and nucleogenic Ne (after Pepin 1997). The 6 Xe and 5 Kr representations are defined by 5 = 1000 [(R/Rref) - 1] %o here R = Xe/ °Xe and Kr/ Kr respectively, and the references Rj-ef are the corresponding isotope ratios in the nonradiogenic Earth atmosphere for Xe (Pepin 1991) and the present Earth atmosphere (Basford et al. 1973) for Kr. Xe data from Table 1 solar and atmospheric Kr from Wider (2002, Table 5, refs. 2 and 4) solar Ar/ Ar = 5.80 0.06, °Ne/ Ne = 13.84 0.04, and Ne/ Ne = 0.0334 0.0003 from Palma et al. (2002).

It seems imlikely that the Earth kept much of its earliest atmosphere drrring early accretion, thus the primordial atmosphere would have been derived from outgassing of the planet s interior, which is thought to have occurred at temperatures between 300 to 1500° C. Modem volcanoes emit a wide range of gas mixtures, most of which is CO2 and SO2, rather than CH4 and H2S. However, it seems likely that most of the gases released today are from the reactions of reworked crastal material and water, and do not represent components of the Earth s deep interior. Thus modem volcanic gases may tell us little about the early Earth s atmosphere. 

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