Computer Science
Scientific paper
Aug 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..94...45m&link_type=abstract
Earth and Planetary Science Letters, Volume 94, Issue 1-2, p. 45-56.
Computer Science
31
Scientific paper
The isotopic composition of neon and xenon measured in MORB glasses confirm significant deviations from atmospheric values. These are (1) 21Ne excesses which are attributed to nucleogenic reactions in the mantle: (2) 20Ne/22Ne ratios higher than the air ratio interpreted as an evidence for the occurrence of solar-type Ne at depth: (3) 129Xe and 131-136Xe excesses, attributed to both extinct (129I and 244Pu) and present (238U) radioactivities.
Ne and Xe isotopic signatures in the mantle can hardly be explained in the framework of classical models for the atmospheric evolution (which postulate a mantle origin for atmospheric gases) and appeal for at least two sources of gases. Ne isotopic differences between air and MORB appear too large to be accounted for by any reasonable fractionation process in the mantle. They imply either fractionation of neon during hydrodynamic escape of a primary atmosphere or different degrees of mixing between primordial Ne components, which, in turn imply isolation of the surface reservoir (air) and deep reservoir (mantle) from the accretional period (except for mantle outgassing through volcanism, the contribution of which is 41% at best for 20Ne).
129I-129Xe, 244Pu-238U-136Xe systematics for atmospheric and MORB-type xenon suggest that either atmospheric gases derived from a source whose formation was delayed (>= 17 Ma) with respect to the mean accretion time of the mantle source and/or atmospheric gases and MORB-type gases derived from chemically distinct sources. These features are consistent with heterogeneous accretion models for the Earth. Volatile degassing was probably contemporaneous to accretional events, following impact degassing, and might have been most efficient during the late stages of Earth formation.
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