We have seen in the previous sections that the radiogenic, nucleogenic and fissiogenic isotopic ratios of helium, neon, argon, and xenon suggest contrasting mantle sources of MORB and OIB. Such differences in 4He/3He, 21Ne/22Ne, 40Ar/36Ar and 129–136Xe/130Xe require chemical fractionation processes to have occurred over geological time scales because the parental isotopes (232Th, 235U, 238U and 40K) have long half-lives (>700 Ma). For the rare gas systems, fractionation of lithophile parents from atmophile (volatile) daughters is readily related to degassing processes, which we...

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