SPECULATIONS ABOUT THE COSMIC ORIGIN OF HE AND NE IN THE INTERIOR OF THE EARTH

Recent developments in rare gas geochemistry suggest that the terrestrial atmosphere was generated by degassing part of the Earth's mantle. Neon, however, does not fit into this picture because the Ne-20/Ne-22 ratios in MORB (between 10 and 13) and in air (9.8) are very different, and there exists no nuclear reaction which could be responsible for the high Ne-20/Ne-22 ratios of up to 13 in upper mantle derived basalts. We have therefore previously proposed that cosmic He and Ne implanted in micrometeorites were introduced into the mantle by subduction after the major part of mantle degassing was achieved. Here we explore a simple neon input-output steady-state budget for the shallower degassed mantle, which shows that most of the Ne-20 in this reservoir can be of extraterrestrial origin, whereas the same source could be responsible only for a small fraction of the He-3 there.